JADEER LEARNING MANUAL PDF - Electrical Area 5

Classification: General Business Use JADEER LEARNING MANUAL ELECTRICAL AREA 5 Classification: General Business Use 0 255 4...

Classification: General Business Use JADEER LEARNING MANUAL ELECTRICAL AREA 5 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 CONTROL PAGE DISCIPLINE JOB / AREA # DESCRIPTION ASSIGNMENT Electrical Technician Area 5 Plant Electrical Maintenance-Intermediate DOCUMENT REFERENCE # CONTROLS JAD-LC -A5-YP-EMT-LM MODULE # REVISION # ISSUE DATE: NEXT REVIEW DATE: 2024 2024.01 05 Nov 2024 01 Oct 2027 DEVELOPED BY: REVIEWED BY APPROVED BY: (SUBJECT MATTER EXPERTS): (SUBJECT MATTER EXPERTS): (AREA OWNERS/LEADERS): Marwan Owaidhah (12943) Electrical Ali Fallatah (12610) Electrical Specialist Bandar Al-Mesawi (13107) Electrical Trainer Specialist Ahmed Haresi (33423) Electrical Specialist Page | 2 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 ELECTRICAL MOTORS Page | 3 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 LEARNING OBJECTIVES SN OBJECTIVE 1 Working Principle, construction, and type of AC motors 2 AC motors maintenance and its protections 3 DC motors basic, construction, and protection 4 Basic principle of motor operated valve (MOV) 5 Troubleshooting of AC/DC motors Page | 4 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 TABLE OF CONTENTS SN SUBJECT PAGE NUMBER 1 INTRODUCTION 2 WORKING PRINCIPLE, CONSTRUCTION AND TYPES OF AC MOTORS 3 MOTOR NAME PLATE DESCRIPTION (ANSI, IEEE IP PROTECTION, AREA / ZONE CLASSIFICATION) 4 AC MOTOR PROTECTION BASICS 5 MAINTENANCE OF AC MOTORS 6 TROUBLESHOOTING OF AC MOTORS 7 DC MOTOR BASIC, CONSTRUCTION AND OPERATION 8 TROUBLESHOOTING OF DC MOTORS 9 MAINTENANCE OF DC MOTORS Page | 5 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 MOTOR INTRODUCTION Rotating Electrical Machines can be divided into two parts. 1) Motor: Which converts electrical energy into mechanical energy. 2) Generator: Which converts mechanical energy into electrical energy. Both types operate through the interaction between a magnetic field and a set of windings. There are different types of Motors. Motor Types: Page | 6 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 TYPES OF MOTORS There are two basic types of AC motors, depending on the type of rotor used: 1. synchronous motor 2. Induction motor Synchronous motors Synchronous motors rotate at exactly the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet. Induction motors Induction motors rotate slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current. Single phase and three phase motors Another division of AC motors is that of single phase and three phase motors. The supply for single phase AC electricity is typically used in home applications. Three phase electrical power is used in industrial applications. Page | 7 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 AC MOTORS AC Motors A Motor is a device which uses electrical energy to produce motion. AC motors are used worldwide in many residential, commercial, Industrial and utility applications. Motors may be part of pumps, fans or may be connected to other equipment such as winders, conveyers, and mixers. Page | 8 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 NEMA / IEC The National Electrical Manufacturers Association (NEMA) sets standards for a wide range of electrical products, including motors. NEMA is primarily associated with motors used in North America. The Standards represent general industry practices and can be found in NEMA Standard publication No MG 1. Some large AC motors may not fall under NEMA Standards and are built to meet the requirements of a specific application. These are referred to as “above NEMA” Motors. The International Electrotechnical Commission (IEC) produces a group of recommended electrical practices developed by committees from participating IEC countries. These Standards differ from NEMA Standards, and they are associated with motors used in many countries, including North America. Motors which meet or exceed these standards are called IEC motors. Page | 9 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Force: In order to understand AC motors, It is necessary to understand some basic concepts associated with motor operation. Many of these are familiar from other contexts. In simple terms, a force is a push or pull. Force may be caused by electromagnetism, gravity, or a combination of physical means. Net force is the vector sum of all forces that act on an object, including friction and gravity. When forces are applied in the same direction, they are added. For example, if two 10 pound forces were applied in same direction, the net force would be 20 pounds. Page | 10 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Torque: Torque is a twisting of turning force that causes an object to rotate. For example, a force applied at the end of a lever causes a torque at the pivot point. Torque is calculated by computing the product of the force and the radius, or lever distance. It can be seen that increasing either the force or the lever distance will increase the torque. Page | 11 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Inertia: Mechanical system subject to law of inertia, which states that an object will tend to remain in its current state of rest or motion unless acted upon by an external force. For example, a soccer ball remains at rest until a player applies a force by kicking it. It will then remain in motion until another force, such as friction from the goal net, stops it. Page | 12 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Because friction removes energy from a mechanical system, a continual force must be applied to keep an object in motion. The law of inertia is still valid, since the force applied is needed only to compensate for the energy lost. In the following illustration, a motor runs a conveyor. A large amount of force must be applied to overcome inertia and start the system. Once it is in motion, only the energy required to compensate for various losses need be applied to keep the conveyor in motion. Friction in the conveyor bearing. Friction within the motor Friction between the conveyor belt & rollers. Page | 13 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Speed An object in motion travels a given distance in a given time. Speed is the ratio of the distance travelled to the time it takes to travel that distance. Speed=Distance / Time A Car, for example, may travel 60 miles in an hour. The Speed of the car would be 60 miles per hour. Page | 14 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Speed of a Rotating Object Speed also applies to rotating objects, such as the tires of a car or the shaft of a motor. The speed of a rotating object is a measurement of how long it takes a given point on the object to make one complete revolution. It is generally in RPM, or revolutions per minutes. An object that makes 10 revolutions in one minute has a speed of 10RPM. Page | 15 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Acceleration and Deceleration: An object can change speed. An increase in the speed of an object is called acceleration. Acceleration only Occurs when there is a change in the net force acting on the object. For example, if a car increases in speed from 30MPH to 60MPH, the change in speed of 30MPH is bought about by the force of the engine. The speed of an object can also decrease. This is called deceleration and applies to rotating object as well. Page | 16 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Work and power Whenever a force of any kind cause motion, work is accomplished. Work is calculated by computing the product Of force and distance, and is expressed in foot-pounds. Work =Force X Distance W=F X D You can see from the formula that if twice is applied, twice the work is done. If the object is moved twice the Distance, twice the work is also done. Power is the rate of doing work, computing by diving the work done by the time taken to do it. Power= Force X Distance / Time Power= Work / Time Page | 17 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Horse Power Power can be expressed in foot-pounds per second but is often expressed in horsepower (HP). This unit was defined in the 18th century by James watt. Watt sold steam engines and was asked how many horses one steam engine would replace. He had horses walk around a wheel that would lift a weight. He found that horses averaged about 550 foot-pounds of work per second. Page | 18 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Electrical Energy: In an electrical circuit, voltage applied to a conductor will cause electrons to flow. Voltage is the force and electron flow is motion, so work is done. The rate at which this work is done is called power. Power is measured in watts. The watt is defined as the rate work is done in a circuit when 1 amp flows with 1 volt applied. Page | 19 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Power Consumed: Power consumed in a resistor depends on the amount of current that passes through it. For a given voltage, the amount of current will depend on the resistance value of the resistor. Power consumed is calculated by Multiplying voltage (E) and the Current (I), as in the following example. Page | 20 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Power in AC circuit: Power consumed by a resistor is called true power. True power is the rate at which energy is consumed and is measured in watts (W). In AC circuits, inductance and capacitance also affect power. Current in AC circuit rises to peak values and returns to zero many times per second. Energy stored in inductors or capacitors is returned to the source when current changes direction. Energy is not consumed. The power in such a circuit is called reactive power and is measured in voltamps reactive (VAR). The overall power in an AC circuit is the vector sum of true power and reactive power. It is called apparent power and is measured in voltamps (VA). Power factor is the ratio of true power to apparent power. Page | 21 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Horsepower & Kilowatts: AC motors manufactured in the United States are generally rated in horsepower. European equipment is Generally rated in Kilowatts. One kilowatt is equal to one thousand watts. Horse power can be converted to Kilowatts with the following formula: KW=0.746 X HP For example, 25HP is equivalent 18.65KW. Kilowatts can be converted to horse power with the following formula: HP=1.341 X KW Page | 22 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Three-Phase Power: Power is single phase when it is produced by one voltage source. Single-Phase power is used for small electrical demands such as are found in the home. Three-phase power is a continuous series of overlapping AC voltages. Each voltage wave is a phase, and is offset by 120 electrical degrees. Three phase power is used where a large quantity of electrical power is required, such as in commercial and industrial applications. Page | 23 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Magnetism: Magnetism plays an important role in the operation of AC Motors. All magnets have two characteristics. First, they attract and hold metal objects like iron and steel. Secondly, when free to move, like a compass needle, magnets assume a roughly north-south orientation. Page | 24 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Magnetic Lines of flux: A magnet attracts iron or steel by an invisible force. We draw lines of flux to represent the magnet’s invisible magnetic field. Every magnet has two poles. Flux lines leave the north pole and enter the south pole, while flux lines are invisible. We can see the effects of magnetic fields. When a sheet of paper is placed on a magnet, and iron fillings loosely scattered over it, they will arrange themselves along the invisible lines of flux. Page | 25 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Drawing Lines of Flux: By drawing lines the way the iron filings have arranged themselves, the following diagram can be obtained. The lines and arrows indicate the paths and directions of magnetic flux lines. Though not shown, these lines exist inside the magnet also, and form closed loops which leave the north pole and enter the south pole. They return to the north pole through the magnet. Like and Unlike Poles: When two magnets are brought together, the magnetic flux fields around interact. Unlike poles attract each other, while like poles, when brought together, repel each other. Page | 26 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Electromagnetism: When current flows through a conductor, a magnetic field is produced around it. This filed is defined by lines of Flux, similarly to a natural magnet. The size and strength of the magnetic field increases with increased current flow and decreases with decreased current flow. Page | 27 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Left-Hand rule for conductors: The left hand rule shows the relationship between the direction of current flow in a conductor and the direction of the magnetic field around the conductor. Page | 28 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Electromagnets: An Electromagnet can be made by winding a conductor into a coil and applying a DC voltage. The lines of flux, formed by current flow through the conductor, combine to produce a larger and stronger magnetic Field. The center of the coil is know as the core. Page | 29 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Changing Polarity: The field of an electromagnet has the same characteristics as that of a natural magnet, including north and south poles. However, when the direction of current flow through an electromagnet changes polarity of the magnetic field will also change. The polarity of an electromagnet connected to an AC source will change at the same frequency as the AC source. Page | 30 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Induced Voltage: A moving magnetic field will induce a voltage in a conductor. In the illustration an electromagnet is connected to a power source. Another electromagnet is placed above it. At Time 1, voltage and current are zero in both circuits. At Time 2, voltage and current are increasing in the bottom electromagnet, and the magnetic field created cuts across the conductors in the top electromagnet, including a current in it. At time 3 current flow has reached its peak in both circuits. The magnetic field around the bottom coil continues to build up and collapse, following the AC voltage source. As the filed moves through space, lines of flux cut across the top coil. As current flows in the top coil, it produces its own magnetic field. Page | 31 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Electromagnetic Attraction: The polarity of the magnetic field induced in the top electromagnet is opposite that of the bottom electromagnet. Since opposite poles attract, the top electromagnet will follow the bottom when it is moved. Page | 32 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 ASYNCHRONOUS AC INDUCTION MOTORS Introduction The AC induction motors are well suited to applications requiring constant speed operation and are commonly used in industrial applications. An asynchronous motor is one in which the rotor speed differs from the speed of the rotating magnetic field. In general, for AC induction motors this means that the rotor speed will be less than the synchronous speed of the rotating field, due to slip. Induction motors are the workhorses of industry. In general, the induction motor is cheaper and easier to maintain compared to other alternatives. Working Principle: In AC induction motor, one set of electromagnets is formed in the stator because of the AC supply connected to the stator windings. This alternating supply induces an electromagnetic force in the rotor as per Lenz’s laws, thus generating another set of electromagnets, interaction between these magnetic field of these electromagnets generates twisting force or torque. As a result, the motor rotates in the direction of the resultant torque. Page | 33 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 DESCRIPTION The induction motor is made up of the stator, or stationary windings, and the rotor. The stator consists of a series of wire windings of very low resistance permanently attached to the motor frame. As a voltage and a current are applied to the stator winding terminals, a magnetic field is developed in the windings. By the way the stator windings are arranged, the magnetic field appears to synchronously rotate electrically around the inside of the motor housing. The rotor is comprised of a number of thin bars, usually aluminum, mounted in a laminated cylinder. The bars are arranged horizontally and almost parallel to the rotor shaft. At the ends of the rotor, the bars are connected together with a “shorting ring.” The rotor and stator are separated by an air gap which allows free rotation of the rotor. The magnetic field generated in the stator induces an EMF in the rotor bars. In turn, a current is produced in the rotor bars and shorting ring and another magnetic field is induced in the rotor with an opposite polarity of that in the stator. The magnetic field, revolving in the stator, will then produce the torque which will “pull” on the field in the rotor and establish rotor rotation. Page | 34 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 AC MOTOR BASIC CONSTRUCTION AND WORKING PRINCIPLE Induction is another characteristic of magnetism. It is a natural phenomena which occurs when a conductor (aluminum bars in the case of a rotor) is moved through an existing magnetic field or when a magnetic field is moved past a conductor. In an induction motor the current flow in the rotor is not caused by any direct connection of the conductors to a voltage source, but rather by the influence of the rotor conductors cutting across the lines of flux produced by the stator magnetic fields. In other words, in an induction motor the current flow in the rotor is not caused by any direct connection of the conductors to a voltage source, but rather by the influence of the rotor conductors cutting across the lines of flux produced by the stator magnetic fields. The induced current which is produced in the rotor results in a magnetic field around the rotor conductors as shown in Figure 2. This magnetic field around each rotor conductor will cause each rotor conductor to act like the permanent magnet as shown in Figure 1. As the magnetic field of the stator rotates, due to the effect of the three-phase AC power supply, the induced magnetic field of the rotor will be attracted and will follow the rotation. The rotor is connected to the motor shaft, so the shaft will rotate and drive the connection load. Page | 35 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Start : It is easier to visualize a rotating field is a start time is picked when no current is flowing through one phase. In the accompanying illustration, at the start time phase A has no current flow. Phase B has current flow in negative direction, and Phase C has current flow in the positive direction. At this time B1 and C2 are south poles and B2 and C1 are north poles. Magnetic lines of flus leave the B2 north pole and enter the nearest south pole, C2. Similarly, Flux lines leave C1 and enter B1. The resultant magnetic field is indicated by the arrow. Page | 36 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Time 1 & Time 2: At time 1 after start, the field has rotated 60 degrees. Phase C has no current flow. Phase A has positive current flow, and Phase B has negative current flow. Winding A1 and B2 are north poles, and windings A2 and B1 are south poles. Page | 37 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 360 Degree Rotation: At the end of Six such time interval the magnetic field will have rotated one full revolution, or 360 degrees. The process will repeat 60 times a second with a 60 hertz power supply. Page | 38 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 AC MOTOR BASIC CONSTRUCTION AND WORKING PRINCIPLE Construction: Stator: The Stator is made up of several thin laminations of aluminum or cast iron. They are punched and clamped together to form a hollow cylinder with slots as shown in figure. Coils of insulated wires are inserted into these slots. Each grouping of coils, together with the core it surrounds, form an electromagnet of the application of AC supply. The number of poles of an AC induction motor depends on the internal connection of the stator winding. The stator windings are connected directly to the power source. Internally they are connected in such a way, that on applying AC supply, a rotating magnetic field is created. Page | 39 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 STATOR OF INDUCTION MOTOR The stator core is made up of several hundred thin laminations of aluminum or cast iron. They are punched and clamped together to form a hollow cylinder (the stator core) with slots as shown in the diagram below. Coils of insulated wire are inserted into slots of the stator core, usually along with stiff paper insulation. Each grouping of coils, together with the core it surrounds, forms an electromagnet (a pair of poles) on the application of AC supply. Page | 40 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 STATOR CONTINUE The stator’s 'rotating magnetic fields' phenomenon can be better understood by examining the following diagram. This shows the relationship of the coils in an AC motor. In this example the stator has six magnetic poles (coils) and the rotor has two poles. The six stator coils used are, two for each of the three phases. These coils, referred to as motor windings, operate in pairs. Each coil is wrapped around the soft iron core material of the stator and acts as a separate electromagnet. The coils are wound in such a way that when current flows in them one coil becomes a north pole (A1) while the other becomes a south pole (A2). When current reverses direction, the polarity of the poles also reverses. Page | 41 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 STATOR CONTINUE Applied AC: Phase windings A, B, and C are placed 120 degrees apart. The number of poles is determined by how many times a phase winding appears. In this example, each winding appears twice, so this is a two-pole stator. If each phase winding appeared four times, it would be a four-pole stator. When AC voltage is applied to the stator, current flows through the windings. The magnetic field developed in a phase winding depends on the direction of the current flow through that winding. The accompanying chart is shown for illustrative purposes. It assumes that positive current flow in the A1, B1 and C1 windings results in north poles in those windings. Page | 42 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 AC MOTOR BASIC CONSTRUCTION AND WORKING PRINCIPLE Rotor: The Rotor is made up of several thin steel lamination with evenly spaced bars, which are made up of aluminum or copper, along the periphery. In the most popular type of rotor (Squirrel Cage rotor), these bars are connected at ends mechanically and electrically by the use of rings. This Total assembly resembles the look of squirrel cage, which gives the rotor its name. The rotor slots are not exactly parallel to the shaft. Instead, they are skew for two reasons. 1. To make motor run quietly by reducing magnetic hum and to decrease slot harmonics. 2. To reduce the locking tendency of the rotor. Even the Stator slots and Rotor slots are not equal to avoid locking between stator and rotor teeth. The rotor is mounted on the shaft using bearing on each end, one end of the shaft is normally kept longer than the other for driving the load. Page | 43 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Design The rotor consists of a stack of steel laminations with evenly spaced conductor bars placed around the circumference. The steel laminations are stacked together to form the rotor core. Aluminum is die cast in the core slots to form a series of conductor bars around the perimeter of the rotor. Current flow through conductor bars Current flow through the conductor bars, which are mechanically and electrically connected by the end rings, produces the magnetic field. The entire core is mounted on a steel shaft to form the rotor assembly. Page | 44 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 How a rotor works In order to see how a rotor works, picture a permanent magnet mounted on a shaft inside a stator winding. When the stator is energized with AC, a rotating magnetic field is established. The permanent magnet's field interacts with this rotating field. The rotating north pole attracts the south pole of the permanent magnet, and the rotating south pole attracts the permanent north pole. This causes the permanent magnet to rotate. A motor designed this way is called a permanent magnet synchronous motor. Induced Voltage in the Rotor The squirrel cage rotor acts in essentially the same way as the permanent magnet synchronous motor. When power is applied to the stator, current flows through the winding, causing an expanding electromagnetic field that cuts across the rotor bars. This includes a magnetic field in the rotor that causes it to behave similarly to a permanent magnet. Page | 45 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 When the stator's moving magnetic field cuts across the rotor's conductor bars, it induces voltage in them. This voltage produces current, which circulates through the bars and around the rotor end ring. This current in turn produces a magnetic field around each rotor bar. The continuously changing stator magnetic field results in a continuously changing rotor field. The rotor becomes an electromagnet with continuously alternating poles, which interacts with the stator poles. The accompanying diagram shows one instant in time. At this instant, current flow through winding A1 produces a North pole. The expanding field of this North pole cuts across an adjacent rotor bar, inducing a voltage in it. The resultant magnetic field in the rotor produces a South pole, which is attracted to the stator's North pole, causing the rotor to rotate. As the stator magnetic field rotates, the rotor follows. The rotor has moved to align itself with the stator's field. Inertia will tend to carry it past this point. Page | 46 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SQUIRREL CAGE ROTOR The rotor is made up of several thin steel laminations with evenly spaced bars, which are made up of aluminum or copper, along the periphery. In the most popular type of rotor (squirrel cage rotor), these bars are connected at ends mechanically and electrically using rings. Most industrial squirrel cage motors are foot mounted Totally Enclosed Fan Cooled (TEFC), in which the motor internals are isolated from the surrounding environment by shaft seals etc., thus minimizing the ingress of dust and moisture. The inevitable heat generated by I2R losses in the internal windings is transferred to the surrounds by air circulation within the casing together with cooling air blown by an integral fan along fins on the casing's exterior. Design advantages Almost 90% of induction motors have squirrel cage rotors. This is because the squirrel cage rotor has a simple and rugged construction. The rotor consists of a cylindrical laminated core with axially placed parallel slots for carrying the conductors. Each slot carries a copper, aluminum, or alloy bar. These rotor bars are permanently short-circuited at both ends by means of the end rings. Page | 47 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SPEED OF AN INDUCTION MOTOR Induction motors may contain multiple field windings. For example, a 4-pole and an 8-pole winding corresponding to 1800 and 900 rpm synchronous speeds. Definition The speed with which the stator magnetic field rotates, which will determine the speed of the rotor, is called the Synchronous Speed (SS, also abbreviated as Ns). The SS is a function of the power source frequency and the number of poles (pole pairs) in the motor. The relationship to calculate the SS of an induction motor is Ns = 120f/P where; Ns = SS = Synchronous Speed (RPM), f = frequency (cycles / second) = 60, P = number of poles (pole pairs) Ns = 120f/P = 120*60/4 = 1800 rpm (4-pole) Ns = 3600 rpm (2-pole) Page | 48 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SLIP OF THE INDUCTION MOTOR Difference in speed The rotor in an induction motor cannot turn at the synchronous speed. In order to induce an EMF in the rotor, the rotor must move slower than the SS. If the rotor were to somehow turn at SS, the EMF could not be induced in the rotor and therefore the rotor would stop. However, if the rotor stopped or even if it slowed significantly, an EMF would once again be induced in the rotor bars and it would begin rotating at a speed less than the SS. The difference in speed is called 'slip.' Load dependent Slip is load dependent and is necessary to produce motor torque. An increase in load will cause the rotor to slow down, that is increase the slip. A decrease in load will decrease the slip. Slip calculation: Slip is expressed as a percentage, and is calculated using the following formula: For example, a four-pole motor operated at 60 Hz has a synchronous speed (Ns) of 1800 RPM. If the rotor speed at full load is 1765 RPM (NR), then slip is 1.9%. If the motor slip is known, the rotor speed can be expressed by the equation: N R = (1 – S) X Ns Example (using the information from above): Ns = 1800 RPM, %S = 1.9% (or S = 0.019), NR = (1 – S) X Ns NR = (1 – 0.019) X 1800 RPM, NR = 1765 RPM Page | 49 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SQUIRREL CAGE ROTOR CONTINUE Disadvantages: Squirrel cage motors are not without their drawbacks, notably during starting when current drain is high. Mechanical aspects will be stressed here - we are interested mainly in how to select a motor to drive a given mechanical load - but it is essential to appreciate in broad terms the thermal -electrical behavior when selecting a motor, since the winding temperature dictates the life that results, as suggested by the sketch. It is the motor's heat dissipation capability dictated by heat transfer and the integral fan which to a large extent determines the motor's maximum continuous mechanical power rating for winding temperatures commensurate with an acceptable life. Page | 50 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SINGLE PHASE AC INDUCTION MOTORS Introduction A three phase motor may be run from a single phase power source. (Figure below) However, it will not self-start. It may be hand started in either direction, coming up to speed in a few seconds. It will only develop 2/3 of the 3-φ power rating because one winding is not used. The single coil of a single phase induction motor does not produce a rotating magnetic field, but a pulsating field reaching maximum intensity at 0o and 180o electrical. (Figure below) Another view is that the single coil excited by a single phase current produces two counter rotating magnetic field phasors, coinciding twice per revolution at 0o (Figure above-a) and 180o (figure e). When the phasors rotate to 90o and -90o they cancel in figure b. At 45o and -45o (figure c) they are partially additive along the +x axis and cancel along the y axis. An analogous situation exists in figure d. The sum of these two phasors is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is developed. Page | 51 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Construction Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of steel laminations, typical of poly-phase induction motors. Permanent-split capacitor motor One way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current displaced 90o in time. This is called a permanent split capacitor motor in Figure below. This type of motor suffers increased current magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to minimize losses. The losses are less than for a shaded pole motor. This motor configuration works well up to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor. Single phase induction motors may have coils embedded into the stator as shown in Figure above for larger size motors. Though, the smaller sizes use less complex to build concentrated windings with salient poles. Page | 52 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Capacitor start induction motor: In Figure below a larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower (multi-kilowatt) sizes. Capacitor-run motor induction motor A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large capacitor for high starting torque but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors. Page | 53 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Resistance split-phase motor induction motor: If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. (Figure below) With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads. This motor has more starting torque than a shaded pole motor (next section), but not as much as a two phase motor built from the same parts. The current density in the auxiliary winding is so high during starting that the consequent rapid temperature rise precludes frequent restarting or slow starting loads. Page | 54 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 BASICS OF WOUND ROTOR MOTOR Design The conductors of the wound rotor consist of wound coils, instead of the bars found in the squirrel cage rotor. These coils are connected through slip rings and brushes to external resistors. However, no power is applied to the slip rings, currents are induced in the windings just as they would be in shorted turns. However, the advantage of using windings is that the wires can be brought out through slip rings so that resistance and the current through the windings can be controlled. Page | 55 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Principle of operation: The sole purpose of the slip rings is to allow resistance to be placed in series with the rotor windings while starting. The rotating magnetic field induces a voltage in the rotor windings. Increasing the resistance of the windings using the variable resistors causes less current to flow in them. This decreases the motor speed. Decreasing the resistance allows more current, increasing motor speed. This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart. Page | 56 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Placing resistance in series Resistance is placed in series with the rotor to decrease the starting current and increase the motor torque. Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting. While this is not a severe problem for small motors, it is for large (10's of kW) motors. Placing resistance in series with the rotor windings not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT). The figure below shows that by increasing the rotor resistance from R0 to R1 to R2, the breakdown torque peak is shifted left to zero speed. Note that this torque peak is much higher than the starting torque available with no rotor resistance (R0). Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting. Page | 57 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Advantages This motor is suited for starting high inertial loads. A high starting resistance makes the high pull out torque available at zero speed. For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its' synchronous speed. Disadvantages The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor. The initial purchase price is more than that of a squire cage. Page | 58 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Starting Characteristics: Locked rotor torque and locked rotor current Induction motors, at rest, appear just like a short circuited transformer and if connected to the full supply voltage, draw a very high current known as the 'Locked Rotor Current'. They also produce torque which is known as the 'Locked Rotor Torque'. Locked Rotor Torque (LRT) and Locked Rotor Current (LRC) are a function of the terminal voltage of the motor and the motor design. As the motor accelerates, both the torque and the current will tend to alter with rotor speed if the voltage is maintained constant. Starting current Some induction motors can draw over 1000% of full load current during starting, although a few hundred percent is more common. Small motors of a few kilowatts or smaller can be started by direct connection to the power line. Starting larger motors can cause line voltage sag, affecting other loads. Motor-start rated circuit breakers (analogous to slow blow fuses) should replace standard circuit breakers for starting motors of a few kilowatts. This breaker accepts high over-current for the duration of starting. Page | 59 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Starting torque The starting torque of an induction motor starting with a fixed voltage will drop a little to the minimum torque, known as the pull-up torque, as the motor accelerates and then rises to a maximum torque, known as the breakdown or pull-out torque, at almost full speed and then drop to zero at the synchronous speed. The curve of the start torque against the rotor speed is dependent on the terminal voltage and the rotor design. Running Characteristics: Full load slip Once the motor is up to speed, it operates at a low slip, at a speed determined by the number of the stator poles. Typically, the full-load slip for the squirrel cage induction motor is less than 5%. The actual full-load slip of a particular motor is dependent on the motor design. The typical base speed of the four pole induction motor varies between 1420 and 1480 RPM at 50 Hz, while the synchronous speed is 1500 RPM at 50 Hz. Page | 60 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Speed Torque Curve for an Induction Motor: Starting torque Starting torque is also referred to as locked rotor torque. This is the torque developed with the rotor held at rest and rated voltage and frequency applied, which occurs upon start. During this instant before the rotor starts turning, the motor develops approximately 150 % of the full load torque. Accelerating torque As the motor accelerates, torque decreases slightly and then continues to increase until it reaches a maximum of approximately 200 %. This is referred to as accelerating or pull up torque. Breakdown torque This the maximum torque a motor can produce. If the motor were overloaded beyond its maximum torque, it would stall or abruptly slow down at this point which is referred to as breakdown or pull out torque. Page | 61 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Full-load torque Torque decreases rapidly as speed increases beyond breakdown torque, until it reaches full load torque at a speed slightly less than 100 % of synchronous speed. Full load speed is less than 100 % due to the slip necessary to produce torque. Note that torque is zero at 100 % of synchronous speed. Torque and Speed The complete torque -speed characteristic for a typical squirrel cage motor shown here demonstrates this increase in torque as the slip increases from zero i.e., as the motor speed decreases from synchronous. At starting the speed is zero, the slip is unity and the starting torque is Ts. The torque of a small motor may decrease monotonically from starting, without a distinct minimum or maximum. Full load refers to the maximum continuous torque Tf that a motor can generate without overheating - a motor can operate continuously only at points on the characteristic between full load and synchronism Page | 62 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 DIRECT-ON-LINE STARTING The simplest way to start a three-phase induction motor is to connect its terminals to the line. This method is often called "direct online" and abbreviated DOL. In an induction motor, the magnitude of the induced emf in the rotor circuit is proportional to the stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor, and the rotor current depends on this emf. When the motor is started, the rotor speed is zero. The synchronous speed is constant, based on the frequency of the supplied AC voltage. So the slip speed is equal to the synchronous speed, the slip ratio is 1, and the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains. Disadvantage When an induction motor starts DOL, a very high current is drawn by the stator, in the order of 5 to 9 times the full load current. This high current can, in some motors, damage the windings; in addition, because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, several other strategies are employed for starting motors. Page | 63 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 STAR DELTA STARTER Induction motor's windings can be connected to a 3-phase AC line in two different ways: Wye (star in Europe), where the windings are connected from phases of the supply to the neutral; delta (sometimes mesh in Europe), where the windings are connected between phases of the supply. A delta connection of the machine winding results in a higher voltage at each winding compared to a wye connection (the factor is the square root of 3). A star-delta starter initially connects the motor in wye, which produces a lower starting current than delta, then switches to delta when the motor has reached a set speed. Disadvantages of this method over DOL starting are: Lower starting torque, which may be a serious issue with pumps or any devices with significant breakaway torque. Increased complexity, as more contactors and some sort of speed switch or timers are needed. Two shocks to the motor (one for the initial start and another when the motor switches from wye to delta) Page | 64 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SYNCHRONOUS MOTORS Design Synchronous motors are like induction motors in that they both have stator windings that produce a rotating magnetic field. However, unlike an induction motor, the synchronous motor is excited by an external DC source and, therefore, requires slip rings and brushes to provide current to the rotor. In the synchronous motor, the rotor locks into step with the rotating magnetic field and rotates at synchronous speed. If the synchronous motor is loaded to the point where the rotor is pulled out of step with the rotating magnetic field, no torque is developed, and the motor will stop. Achieving synchronous speed A synchronous motor is not a self-starting motor because torque is only developed when running at synchronous speed. Therefore, the motor needs some type of device to bring the rotor to synchronous speed. A synchronous motor may be started by a DC motor on a common shaft or by a squirrel-cage winding imbedded in the face of the rotor poles. Keeping the same load, when the field excitation is increased on a synchronous motor, the motor operates at a leading power factor. If we reduce field excitation, the motor will operate at a lagging power factor. Application Synchronous motors are used to accommodate large loads and to improve the power factor of transformers in large industrial complexes. Page | 65 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SYNCHRONOUS MOTORS CONSTRUCTION Frame: The frame function is supporting and protecting the motor, also housing the lamination core and the stator windings. It can be built considering horizontal or vertical construction and with protection degree adjusted in accordance with the environment requirements. The frame is built in welded steel profiles and sheets, forming a solid and robust structure that is the structural base of the machine. This kind of construction provides excellent structural strength to withstand mechanical stresses arising from potential short circuits and vibration, enabling the motor to meet the severest conditions. Page | 66 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Stator: Stator identical to that of a three-phase induction motor called the “armature”. The stator consists of a core of high-quality silicon steel laminations with slots to fit the stator winding, which operates with alternating current power supply in order to generate the rotating magnetic field. Page | 67 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Rotor: The rotor can be designed with cylindrical poles, laminated salient poles or solid salient poles, depending on the construction characteristics of the motor and its application. The complete rotor is formed by the structure that composes or supports the poles, the field windings and the starting cage, in case of laminated salient poles and cylindrical poles, which are the active rotating parts of the synchronous motor. The field poles are magnetized through the direct current coming from the exciter rotor or directly by slip rings and brushes. In operation, the poles are aligned magnetically by the air gap, and they spin in synchronism with the stator rotating field. The shaft is made of forged steel and machined according to each specification. The shaft end is normally cylindrical or flanged. Page | 68 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Bearings Depending on the application, the synchronous motors can be supplied with rolling bearings or sleeve bearings. Rolling Bearings These bearings are normally composed of ball roller bearings or cylindrical roller bearings, depending on the speed and axial and radial loads to which they are subject, and special bearings can be used in some applications. The rolling bearings can be lubricated with oil or grease. Sleeve Bearings The sleeve bearings can have natural lubrication (self-lubrication) or forced lubrication (external lubrication). Page | 69 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 CONSTRUCTION OF EXCITATION Synchronous motors require a source of direct current in order to supply the necessary power to the field winding (rotor winding), which is usually provided by a rotating brushless exciter or by slip rings and brushes (static exciter). Brushless Exciter: Synchronous motors with brushless excitation system present a rotating exciter, normally located in a compartment in the back of the motor. The exciter rotor supplies the necessary power to the motor excitation winding through a rotating, three-phase rectifier bridge as shown in the picture. Page | 70 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Static Exciter (with Brushes) Synchronous motors with static exciter are designed with slip rings and brushes that allow the current supply to reach the rotor poles by means of sliding contact. The direct current must come from an AC/DC static controller and converter located outside the motor. Synchronous motors with static exciter are more often used in applications with variable speed with a frequency drive operation or in applications where the system dynamic response must be extremely fast. Page | 71 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 OPERATION AND WORKING PHILOSOPHY Operation: When the motor is supplied with A.C. power supply, the stator poles get energized. This in turn attracts (opposite) the rotor poles, thus both the stator and rotor poles get magnetically interlocked. It is this interlock which makes the rotor to rotate at the same synchronous speed with the stator poles. The synchronous speed of rotation is given by the expression Ns=120f/P. Page | 72 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 OVERALL EXPLANATION Page | 73 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 SYNCHRONOUS MOTOR THEORY OF OPERATION To understand the working of a Synchronous Motor easily, let us consider only two poles in the stator and rotor. With reference to the figure, the stator has two poles Ns & Ss. These poles when energized, produces a rotating magnetic field, which can be assumed that the poles themselves are rotating in a circular manner. They rotate at a synchronous speed and let us assume the direction of rotation to be clockwise. If the rotor poles are at the position shown in the figure, we all know that “Like poles repel each other”. So, the North Pole in the stator repels the north pole of the rotor. Also, the south pole of the stator repels the south of the rotor. This makes the rotor to rotate in anti-clockwise direction thus, half a period later, the stator poles interchange themselves, thus making them get aligned with “unlike poles” which attracts each other. I.e., the South Pole of the stator & the North Pole of the rotor gets attracted and get magnetically interlocked. At this position the poles Ns attracts S and poles Ss attracts N. These opposite unlike poles of the rotor & stator start rotating in the same direction as the stator poles. This makes the rotor to rotate unidirectional and at a synchronous speed which is the same speed of rotation of the stator poles. Thus, as the position of the stator poles keep changing with a rapid speed and reversal, the rotor poles also rotate and reverse as same as the stator thus causing the rotor to rotate at a constant, synchronous speed and in the same direction. Page | 74 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Synchronous Motor: Synchronous Motor are not induction motors, though they may use induction for starting. Small horse power synchronous motors often have permanent magnet rotors. These motors neither require DC power to operate, nor use induction for starting. There is another type of synchronous motor that does require a DC power supply. It uses induction starting, and has rotor bars similar to squirrel cage, in addition to coil windings. On start, AC is applied to the stator, initiating the rotating magnetic filed. The rotor starts in a manner similar to a squirrel cage rotor. After maximum speed is reached, DC is applied to the rotor coils. This produces a strong and constant filed that locks in step with the stator filed, so the motor runs at synchronous speed. In other words, there is no slip with this type of motor. Page | 75 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 STARTING PROCEDURE All the Synchronous Motors are equipped with “Squirrel Cage winding”, consisting of Cu (copper) bars, short-circuited at both ends. These windings also serve the purpose of self-starting of the Synchronous motor. During starting, it readily starts and acts as induction motors. For starting a Synchronous motor, the line voltage is applied to the stator terminals with the field terminals (rotor) left unexcited. It starts as an induction motor, and when it reaches a speed of about 95% of its synchronous speed, a weak D.C excitation is given to the rotor, thus making the rotor to align in synchronism with the stator.( at this moment the stator & rotor poles get interlocked with each other & hence pull the motor into synchronism). Get motor to maximum speed (usually with no load) Energize the rotor with a DC voltage. Page | 76 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Name Plate: The Name plate of the Motor provides important information necessary for selection and application. The following drawing illustrates the nameplate of a sample 30 horsepower AC motor. Specifications are given for the load and operating conditions as well as motor protection and efficiency. Voltage and Amps: AC Motors are designed to operate at standard voltages and frequencies. This Motor is designed for use on 460 VAC systems. Full load current for this motor is 34.9 Amps. Page | 77 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 RPM: Base speed is the nameplate speed, given in RPM, where the motor develops rated horsepower at rated voltage and frequency. It is an indication of how fast the output shaft will turn the connected equipment when fully loaded with proper voltage and frequency applied. The base speed of this motor is 1765 RPM at 60 Hz. It is known that synchronous speed of a 4 Pole motor is 1800 RPM. When fully loaded there will be 1.9% slip. % Slip= (1800-1765/1800) X 100=1.9%. Service Factor: A motor designated to operate at its nameplate horsepower rating has a service factor of 1.0. This means the motor can operate at 100% of its rated horsepower. Some applications may require a motor to exceed the rated horsepower. In this cases a motor with a service factor of 1.15 can be specified. The service factor is a multiplier that may be applied to the rated power. A 1.15 service factor motor can be operated 15% higher than motor’s name plate horsepower for a short duration. Continuously operating the motor above service factor 1 can reduce the life expectancy in addition to performance characteristics such as full load RPM and FLC will be affected. Page | 78 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Class Insulation: The NEMA (National Electrical Manufacturers Association) has established insulation classes to meet motor temperature requirements found in different operating environments. The four insulation classes are A, B, F and H. Class F is commonly used. Temperature will raise in the motor as soon as it is started. Each insulation class has a specified allowable temperature rise. The combination of ambient temperature and allowed temperature rise equals the maximum winding temperature in a motor. A motor with class F insulation, for example, has a maximum temperature rise of 105 ̊C rise. When operated at a 1.0 service factor. The maximum winding temperature is 145 ̊ C (40 C ambient plus ̊ 105 ̊ C rise). Page | 79 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Motor Design: The NEMA (National Electrical Manufacturers Association) has established standard for the Motor construction and performance NEMA design B motors are most commonly used. There are different types of NEMA Design A, B, C and D. NEMA C and D are used for specialized applications. A motor must be able to develop enough torque to start, accelerate and operate a load at rated speed. Based on the Speed Vs Torque characteristic curves these standards are defined. Speed Torque Curve for NEMA B Motors: NEMA B motor develops approximately 150% Starting torque of its full load torque. Page | 80 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Speed Torque Curve for NEMA C Motors: NEMA C motor develops approximately 225% Starting torque. This is used in hard to start applications like Plunger pumps, heavily loaded conveyors and the compressors which requires higher starting torque. Slip and full load torque are same as NEMA B motor. Speed Torque Curve for NEMA D Motors: NEMA C motor develops approximately 280% Starting torque. This is used in very hard to start applications like Punch press, cranes, hoists, oil well pumps etc. Page | 81 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Efficiency: AC Motor efficiency is expressed as a percentage. It is an indication of how much input electrical energy is converted to output mechanical energy. The nominal efficiency of this motor is 93.6%. The higher the percentage the more efficiently the motor converts the incoming electrical power to mechanical horsepower. Derating Factors: Several factors can effect the operation and performance of an AC motor. These need to be considered when applying a Motor. Voltage / Frequency variation: A small variation in supply voltage and frequency can have a dramatic affect on motor performance, especially on starting toque and load torque. Page | 82 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Enclosures: Enclosures provides protection from contaminants in the environment in which the motor is operating. In addition, the type of enclosure affects the cooling of the motor. There are two categories of enclosures: Open and totally enclosed. Open drip proof (ODP): Open enclosure permits cooling air to flow through the motor. The rotor has fan blades that assists in moving the air through the motor. One type of open enclosure is the drip proof enclosure. It prevents liquids and solids falling from above at angles up to 15 ̊ C from vertical direction. Page | 83 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Totally Enclosed Non-Ventilated (TENV): In some cases, air surrounding the motor contains corrosive or harmful elements which can damage the internal parts of a motor. A totally enclosed motor enclosure restricts the free exchange of air between the inside of the motor and the outside. The enclosure is not airtight, however a seal at the point where the shaft. Totally Enclosed Non-Ventilated (TENV) In case where the surrounding air contains corrosive or otherwise harmful elements, a totally enclosed non ventilated enclosure may be needed. Such an enclosure restricts the free flow of air but is not completely airtight. At the point where the motor Shaft exits the housing, a seal is installed to keep out liquids, dust, and other foreign matter. Since there are no ventilating openings, all heat generated by the motor must be dissipated through the enclosure by conduction. For larger Horsepower motors, the enclosure frame is heavily ribbed to help this process. TENV motors, most of which are fractional in horsepower can be used both in and outdoors. Page | 84 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Totally Enclosed Fan cooled (TEFC) The totally enclosed fan cooled motor has an external fan mounted opposite its drive end. This provides additional cooling by blowing air over the motor exterior. A shroud covers the fan to prevent injury. TEFC motors can be used in dirty, moist, or mildly corrosive opening conditions, since no outside air enters the motor. They are widely used for integral horsepower applications, as opposed to TENV motors, which Are mostly found in fractional horsepower's. Page | 85 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 Explosion Proof (XP) The explosion proof motor enclosure is similar in appearance to the TEFC. Most XP enclosures are made of Cast iron and designed for use in hazardous locations. The application of such motors in the United States is subject to regulations and standards set by agencies such as National Fire Protection Association (NFPA) and Underwriter’s Laboratories(UL). It’s important to understand regulations that apply to hazardous locations, though you should never specify a location as hazardous or non- hazardous. It is the user’s responsibility to contact local regulatory agencies and to comply with all applicable codes. Page | 86 Classification: General Business Use 0 255 4 227 77 217 247 159 205 30 82 77 217 247 223 0 66 5 77 217 247 NEMA Dimensions: NEMA set