Electrical Motor Control Chapter 1 PDF

1M chapter Electrical Motors Electrical Motor Control  Electrical Motor Control Course Content 1. M1 Electrical Motor definition 2. M2 Electrical Motor types 3. M3 Ele...

1M chapter Electrical Motors Electrical Motor Control  Electrical Motor Control Course Content 1. M1 Electrical Motor definition 2. M2 Electrical Motor types 3. M3 Electrical Motor protection 4. M4 Load Protection 5. M5 Motor Selection and Installation Considerations 6. M6 Motor Preventative Maintenance 1. Electrical Motor definition The drive system abstracted here as a block diagram integrates all major components that will be described and explained in this course. Electrical and mechanical energy flow toward the operative element (normal). The reverse direction is also possible, however, such as during braking or in regenerative mode (generally speaking). A system controlling the machine, section of the plant or plant. Example a Process control systems (PCS), sometimes called industrial control systems (ICS), function as pieces of equipment along the production line during manufacturing that test the process in a variety of ways and return data for monitoring and troubleshooting. Many types of process control systems exist, including supervisory [Date] 2  Electrical Motor Control control and data acquisition (SCADA), programmable logic controllers (PLC), or distributed control systems (DCS), and they work to gather and transmit data obtained during the manufacturing process. Actuating component is part of the High- Medium- and low voltage distribution network. Switch gear and electrical protection. Drive component is a combination of a motor control system and various types of electrical motors. The transfer component connects the electrical motor to the load. Example gearbox The Operative Element consists of the process or part of the process. Example A conveyer belt to move crushed rock from point A to point B. Electrical Motor Control System Electric motor: Direct current, asynchronous or synchronous motor, motor/generator operation. Mechanical energy transfer Gearbox, rotary/linear transfer of motion. Load: Process or machine to deliver a product. Energy conversion: The energy conversion chain for electrical drive systems leads from the supply system, from which the electrical energy is drawn, through a final control element to the electrical machine. There, the electrical energy is converted to mechanical energy. The mechanical energy is then fed to the active process via a mechanical energy converter. Supply Alternating current (AC) and 3-phase systems, 50 or 60 Hz Typical voltage levels: 230 V, 400 V, 690 V Final control element Switches, contactors, converters, or frequency converters Electrical machine Direct current (DC), asynchronous or synchronous machine, motor/generator operation. Mechanical energy conversion Gearbox, rotary/linear motion The individual drive components are typically described using characteristic values and characteristic curves. For example, the efficiency curve of a motor is presented and the efficiency η is stated. [Date] 3  Electrical Motor Control Magnet The principles of magnetism are an integral part of electricity. Electromagnets are used in some direct current circuits. Alternating current cannot be understood without first understanding magnetism. Every magnet has two poles, one north pole and one south pole. This is the point at which maximum attraction occurs. Invisible magnetic lines of flux leave the north pole and enter the south pole. While the lines of flux are invisible, the effects of magnetic fields can be made visible. When a sheet of paper is placed on a magnet and iron filings loosely scattered over it; the filings will arrange themselves along the invisible lines of flux. Broken lines indicate the paths of magnetic flux lines. The field lines exist outside and inside the magnet. The magnetic lines of flux always form closed loops. Magnetic lines of flux leave the north pole and enter the south pole, returning to the north pole through the magnet. Interaction Between Magnets [Date] 4  Electrical Motor Control When two magnets are brought together, the magnetic flux field around the magnet causes some form of interaction. Two unlike poles brought together cause the magnets to attract each other. Two like poles brought together cause the magnets to repel each other. Left-hand rule for conductors An electromagnetic field is a magnetic field generated by current flow in a conductor. Whenever current flows a magnetic field exists around the conductor. Every electric current generates a magnetic field. A definite relationship exists between the direction of current flow and the direction of the magnetic field. The left-hand rule for conductors demonstrates this relationship. If a current-carrying conductor is grasped with the left hand with the thumb pointing in the direction of electron flow, the fingers will point in the direction of the magnetic lines of flux. Coil [Date] 5  Electrical Motor Control A coil of wire carrying a current act like a magnet. Individual loops of wire act as small magnets. The individual fields add together to form one magnet. The strength of the field can be increased by adding more turns to the coil. The strength can also be increased by increasing the current. A left-hand rule exists for coils to determine the direction of the magnetic field. The fingers of the left hand are wrapped around the coil in the direction of electron flow. The thumb points to the north pole of the coil. An electromagnet is composed of a coil of wire wound around a core. The core is usually a soft iron which conducts magnetic lines of force with relative ease. When current is passed through the coil, the core becomes magnetized. The ability to control the strength and direction of the magnetic force makes electromagnets useful. As with permanent magnets, opposite poles attract. An electromagnet can be made to control the strength of its field which controls the strength of the magnetic poles. A large variety of electrical devices such as motors, circuit breakers, contactors, relays and motor starters use electromagnetic principles. Singe Phase Motor Single-Phase AC Induction Motor with Start Mechanism The single-phase induction motor is not self-starting. When the motor is connected to a single-phase power supply, the main winding carries an alternating current. This current produces a pulsating magnetic field. Due to induction, the rotor is energized. As the main magnetic field is pulsating, the torque necessary for the motor rotation is not generated. This will cause the rotor to vibrate, but not to rotate. Hence, the single-phase induction motor is required to have a starting mechanism that can provide the starting kick for the motor to rotate. [Date] 6  Electrical Motor Control The starting mechanism of the single-phase induction motor is mainly an additional stator winding (start/auxiliary winding) as shown in Figure. The start winding can have a series capacitor and/or a centrifugal switch. When the supply voltage is applied, current in the main winding lags the supply voltage due to the main winding impedance. At the same time, current in the start winding leads/lags the supply voltage depending on the starting mechanism impedance. Interaction between magnetic fields generated by the main winding and the starting mechanism generates a resultant magnetic field rotating in one direction. The motor starts rotating in the direction of the resultant magnetic field. Permanent split-capacitor motors This is a modified split-phase motor with a capacitor in series with the start winding to provide a start “boost.” Like the split-phase motor, the capacitor start motor also has a centrifugal switch which disconnects the start winding and the capacitor when the motor reaches about 75% of the rated speed. Since the capacitor is in series with the start circuit, it creates more starting torque, typically 200% to 400% of the rated torque. And the starting current, usually 450% to 575% of the rated current, is much lower than the split-phase due to the larger wire in the start circuit. Shaded pole motor [Date] 7  Electrical Motor Control The shaded-pole motor has many positive features, but it also has several disadvantages. It’s low starting torque is typically 25% to 75% of the rated torque. It is a high slip motor with a running speed 7% to 10% below the synchronous speed. Generally, efficiency of this motor type is exceptionally low (below 20%). The low initial cost suits the shaded-pole motors to low power or light duty applications. Perhaps their largest use is in multi-speed fans for household use. But the low torque, low efficiency and less sturdy mechanical features make shaded- pole motors impractical for most industrial or commercial use, where higher cycle rates or continuous duty are the norm. Single Phase Electrical Motor Torque Characteristics Capacitor Start/Capacitor Run AC Induction Motor. This motor has a start type capacitor in series with the auxiliary winding like the capacitor start motor for high starting torque. Like a PSC motor, it also has a run type capacitor that is in series with the auxiliary winding after the start capacitor is switched. The split-phase motor is also known as an induction start/induction run motor. It has two windings: a start and a main winding. The start winding is made with smaller gauge wire and fewer turns, relative to the main winding to create more resistance, thus putting the start winding’s field at a different angle than that of the main winding which causes the motor to start rotating. The main winding, which is of a heavier wire, keeps the motor running the rest of the time. [Date] 8  Electrical Motor Control Three Phase Motors Rotating Field Shaft Rotation 60° Rotating Field Shaft Rotation 120° Three-phase current consists of three electrical phases within the Stator alternating current flowing in each one. In each phase, the sinusoidal current is displaced by one third of the cycle (120°). If three-phase current is applied to the stator windings of the motor, a rotating magnetic field is generated. [Date] 9 Electrical Motor Control Rotating Field Shaft Rotation 360° Rotating Magnetic Field Induction Motor [Date] 10  Electrical Motor Control An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor can therefore be made without electrical connections to the rotor. Asynchronous servomotors are suitable for use in applications in which high external inertias have to be moved in plants and machines and controlled safely. Induction Motor Components The stator mainly consists of punched sheet metal with slots, the laminated rotor core. The three phases of the windings are contained in the slots. At the first glance, the rotor looks like a metal cylinder. Its most important component is the cage: which consists of aluminum bars with shorting rings on both sides.The bars form the current-carrying conductor in the magnetic field. [Date] 11  Electrical Motor Control The remaining volume is made up of the laminated rotor core. It is for the mechanical connection to the shaft and for guiding the magnetic lines of force. Asynchronous Motor as flux machine Rotor If the rotor's speed is lower than the magnetic rotating stator field, a changing magnetic field is created in the rotor. Comparison with Stator and rotor are comparable to the windings of a transformer. Transformer Here too, a changing magnetic field is created in the secondary-side windings: Current is induced, and the current starts to flow. Cage Rotor. The other magnet pole is acting on the opposing side of the rotor. The currents there are flowing in the opposite direction. The currents can equalize via the shorting rings and therefore flow in a circle. This electrical condition is really a short-circuit, hence the name short-circuited rotor bars. These currents now generate the torque - the rotor starts to rotate. The rotor accelerates for as long as more torque is generated than required by the load. Slip Prerequisite for the torque producing current in the rotor is a torque difference between the rotating stator field and the rotor speed, the slip. The rotor must always have slip compared to the rotating stator field if torque is required. The rotor runs [Date] 12  Electrical Motor Control asynchronously. The amount of slip incurred will depend on the mechanical load. The slip increases if the load increases. Generator Negative slip is incurred with braking (only possible with converter). Operation The rotor then revolves faster than the rotating field generated by the converter. Duty Cycle For an electrical motor, the number of starting and braking per unit of time have a large incidence/frequency on the internal temperature. The IEC standard : Rotating electrical machines - Part 1: Rating and performance (IEC 60034-1:2004) gives the service factors which allow to calculate the heat generated ad size correctly a motor according to the operation. The following information is an overview of these service factors. Additional information will be found in the relevant IEC standard and the manufacturers' catalogues. S1 : Continuous Running Duty [Date] 13  Electrical Motor Control S2 :Short-time Duty Operation at constant load, for a period of time which is less than that required to reach thermal equilibrium, followed by a rest and motor de- energised period of sufficient duration for the machine to re-establish temperatures to within 2oC of the ambient or the coolant temperature. The values 10min, 30min, 60min and 90min are recommended periods for the rated duration of the duty cycle. S3 :Intermittent Periodic Duty not affected by the Starting Process Series of identical cycles, each with a period of operation and a pause. The starting current in this type of duty is such that it has no significant effect on heating. [Date] 14  Electrical Motor Control S4 : Intermittent Periodic Duty affected by the Starting Process Series of identical cycles, each with a significant starting period, a period of constant-load operation and a pause. S5 : Intermittent Periodic Duty affected by the Starting Process and also by Electric Braking Series of duty cycles, each with a starting period, a period of constantloadoperation, a period of electrical braking and a pause. [Date] 15  Electrical Motor Control S6 : Continuous Operation, Periodic Duty with Intermittent Load S7 : Uninterrupted Periodic Duty, affected by the Starting Process and also by Electric Braking Series of identical duty cycles, each with a starting period, a period of constant-load operation and a period of electrical braking. There is no pause. [Date] 16  Electrical Motor Control S8 : Uninterrupted Periodic Duty with Recurring Speed and Load Changes Series of identical duty cycles, each with a period of constant-load operation at a preset rotation speed, followed by one or more periods of constant-load operation at other speeds (e.g. by changing the number of poles). There is no pause. Motor Definitions Normal motor rating has a service factor of 1.0. Some applications may require motor to exceed the rated power. In these 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 the motor’s nameplate horsepower. Motors with a service factor of 1.15 are recommended for use with AC drives. It is important to note, however, that even though a motor has a service factor of 1.15 the values for current and horsepower at the 1.0 service factor are used to program a variable speed drive. The National Electrical Manufacturers Association (NEMA) 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. Class A is seldom used. Before a motor is started, its windings are at the temperature of the surrounding air. This is known as ambient temperature. NEMA has standardized on an ambient temperature of 40° C, or 104° F for all motor classes. AC motor efficiency is expressed as a [Date] 17  Electrical Motor Control percentage. It is an indication of how much input electrical energy is converted to output mechanical energy. The nominal efficiency of this motor is 93.0%. IEC 60034-1 classifies the temperature rise limits of insulation materials and specifies the maximum permissible temperatures that the various classes of insulation materials could withstand. The temperature rise of the induction machine is the permissible increase in temperature, above this maximum ambient, to allow for the losses in the motor when running at full load. Motor Temperature rise according to Insulation Class Temperature rises in the motor as soon as it is started. 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. The maximum winding temperature is 145° C (40° ambient plus 105° rise). A margin is allowed for a point at the center of the motor’s windings where temperature is higher. This is referred to as the motor’s hot spot. The operating temperature of a motor is important to efficient operation and long life. Operating a motor above the limits of the insulation class reduces the motor’s life expectancy. A 10° C increase in the operating temperature can decrease the life expectancy of a motor as much as 50%. [Date] 18  Electrical Motor Control Motor Equivalent Circuit V = Stator Terminal Voltage IS = Stator Current RS = Stator Effective Resistance XM = Stator Leakage Reactance XS = Stator Leakage Reactance RC = Iron Losses ES = Counter EMF (generated by the air gap flux) IR = Rotor Current RR = Rotor Resistance sXR = Rotor Leakage Reactance sER =Rotor EMF The simplified Equivalent Circuit [Date] 19  Electrical Motor Control V = Stator Terminal Voltage IS = Stator Current RS = Stator Effective Resistance XM = Stator Leakage Reactance XS = Stator Leakage Reactance RC = Iron Losses IR = Rotor Current X’R = Rotor Leakage Inductance referred to the stator R’R = Rotor Resistance referred to the stator S = Slip Not all the electrical input power emerges as mechanical output power. A small portion of this power is lost in the stator resistance and the core losses and the rest crosses the air gap to do work on the rotor. An additional small portion is lost in the rotor. The balance is the mechanical output power PM of the rotor. The mechanical power is described on the motor nameplate, but the true shaft power is unknown except if measured with a dynamometer or "dyno" for short. A dynamometer is a device used for simultaneously measuring the torque and rotational speed (RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dynamometer itself as kW or bhp. At no-load, the power factor is low, which reflects the high component of magnetising current. As mechanical load grows and slip increases, the effective rotor resistance falls, active current increases, and power factor improves. P = 3 x V x I S x Cos φ kW [Date] 20  Electrical Motor Control Motor Losses The power in the machine differs, depending on the operating point. During stand still and no-load operation, mechanical power P2 =0. Copper losses are losses in the windings and the cage rotor due to electrical resistance. Iron losses Additional losses occur in the cores due to the rotating field, the saturation of the magnetic material, and the presence of eddy currents. Mechanical losses occur as a result of air resistance of rotor and fan as well as friction, e.g., in the bearings. [Date] 21  Electrical Motor Control Power Factor /efficiency High efficiencies are only achievable in the small-slip range From the graph, it is apparent that the peak efficiency ŋ and cos Ф occur at different speeds. The maximum efficiency is therefore not reached if the maximum active power is drawn. This influences the design of the motor, depending on which factor is more important for the application. [Date] 22  Electrical Motor Control Motors Construction Motor Cooling NB1 Low-speed applications needs an external cooling device (forced or water cooling). Duty cycle must be considered for non-ventilated motors. Commonly Used Types of Construction [Date] 23  Electrical Motor Control Standardized Type of Construction and Mounting Arrangement The types of construction and mounting arrangements designate the arrangement of the machine components with regardto fixings, bearing arrangement and shaft extension, as standardized in IEC 60034-7, DIN 42950 and NEMA MG 1-4.03. Standard IEC 60072 determines the location of the terminal box on the motor that shall be situated with its centre-line within a sector ranging from the top to 10º below the horizontal centre-line of the motor on the right-hand side, when looking at the Drive-end of the motor. Motor Modular add on modules. Holding Brake Encoder Forced Cooling Fan Motor Gearbox Combination [Date] 24 2M chapter Electrical Motors Electrical Motor Control  Electrical Motor 2. Electrical Motor Types 2.1 DC Motor 2.2 Synchronous motors 2.2.1 Wire wound synchronous motor 2.2.2 Permanent Magnet motors 2.2.3 Synchronous reluctance motors (SynRM) 2.2.4 Servo Motors 2.2.5 Linear Motors 2.2.6 High torque motors 2.3 Asynchronous motors 2.3.1 Squirrel cage induction motors 2.3.2 Conical motor 2.3.3 Slip ring motor Motor Electrical Mechanica l Electrical energy flows from the supply to the load, or output side, via the energy converter. The forward running of the motor is defined when facing the drive end(D-end). Forward running means that the shaft is rotating in the clockwise direction. In generator operation, when mechanical energy flows via the motor shaft in the direction of the supply side, it is quite possible that the final control element (e.g., a rectifier) does not permit this energy flow back into the supply system. [Date] 2  Electrical Motor In this case, steps must be taken to ensure that this energy flow does not damage the final control element or, alternatively, generator operation must limit or prevented. A simple process is the conversion of this regenerative energy in the final control element to heat in a braking resistor. In terms of improving energy efficiency, it is often better to use regenerative frequency converters. In both electrical and mechanical systems, power is described by the product of two primary quantities: Pel = U · I and Pm = M · ω = M · 2p · n / 60 Because each primary quantity can take on positive or negative values, there are four possible combinations. These four possibilities define four operating modes. 2.1 DC Motor Basic DC Motor Magnetic Fields You will recall from the previous section that there are two electrical elements of a DC motor, the field windings and the armature. The armature windings are made up of current carrying conductors that terminate at a commutator. DC voltage is applied to the armature windings [Date] 3  Electrical Motor through carbon brushes which ride on the commutator. In small DC motors, permanent magnets can be used for the stator. However, in large motors used in industrial applications the stator is an electromagnet. When voltage is applied to stator windings an electromagnet with north and south poles is established. The resultant magnetic field is static (nonrotational). For simplicity of explanation, the stator will be represented by permanent magnets in the following illustrations. DC Motor A DC motor is any of a class of rotary electrical motors that converts direct current electrical energy into mechanical energy. The most common types rely on the forces produced by magnetic fields. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor. DC motors were the first form of motor widely used, as they could be powered from existing direct-current lighting power distribution systems. A DC motor's speed can be controlled over a wide range, using either a variable supply voltage or by changing the strength of current in its field windings. Small DC motors are used in tools, toys, and appliances. The Larger DC motors are currently used in propulsion of electric vehicles, elevator and hoists, and in drives for steel rolling mills. The advent of power electronics has made replacement of DC motors with AC motors possible in many applications. DC motors have been used in industrial applications for years. Coupled with a DC drive, DC motors provide very precise control. DC motors can be used with conveyors, elevators, extruders, marine applications, [Date] 4  Electrical Motor material handling, paper, plastics, rubber, steel, and textile applications to name a few. DC Motor Construction Construction DC motors are made up of several major components which include the following: Frame Shaft Bearings Main Field Windings (Stator) Armature (Rotor) Commutator Brush Assembly Of these components, it is important to understand the electrical characteristics of the main field windings, known as the stator, and the rotating windings, known as the armature. An understanding of these two components will help with the understanding of various functions of a DC Drive. [Date] 5  Electrical Motor DC Motor Construction Field windings are mounted on pole pieces to form electromagnets. In smaller DC motors the field may be a permanent magnet. However, in larger DC fields the field is typically an electromagnet. Field windings and pole pieces are bolted to the frame. The armature is inserted between the field windings. The armature is supported by bearings and end brackets (not shown). Carbon brushes are held against the commutator. Armature The armature rotates between the poles of the field windings. The armature is made up of a shaft, core, armature windings, and a commutator. The armature windings are usually form wound and then placed in slots in the core. Brushes ride on the side of the commutator to provide supply voltage to the motor. The DC motor is mechanically complex which can cause problems for them in certain adverse environments. Dirt on the commutator, for example, can inhibit supply voltage from reaching the armature. A certain amount of care is required when using DC motors in certain industrial applications. Corrosives can damage the commutator. In addition, the action of the carbon brush against the commutator causes sparks which may be problematic in hazardous environments. [Date] 6  Electrical Motor Series Connected DC Motor In a series DC motor the field is connected in series with the armature. The field is wound with a few turns of large wire because it must carry the full armature current. A characteristic of series motors is the motor develops a large amount of starting torque. However, speed varies widely between no load and full load. Series motors cannot be used where a constant speed is required under varying loads. Additionally, the speed of a series motor with no load increases to the point where the motor can become damaged. Some load must always be connected to a series-connected motor. Series-connected motors generally are not suitable for use on most variable speed drive applications. [Date] 7  Electrical Motor Shunt Motors In a shunt motor the field is connected in parallel (shunt) with the armature windings. The shunt-connected motor offers good speed regulation. The field winding can be separately excited or connected to the same source as the armature. An advantage to a separately excited shunt field is the ability of a variable speed drive to provide independent control of the armature and field. The shunt-connected motor offers simplified control for reversing. This is especially beneficial in regenerative drives. [Date] 8  Electrical Motor Compound Motor Compound Motors Compound motors have a field connected in series with the armature and a separately excited shunt field. The series field provides better starting torque and the shunt field provides better speed regulation. However, the series field can cause control problems in variable speed drive applications and is generally not used in four quadrant drives. Point Of Equilibruim [Date] 9  Electrical Motor Speed/Torque Curves The following chart compares speed/torque characteristics of DC motors. At the point of equilibrium, the torque produced by the motor is equal to the amount of torque required to turn the load at a constant speed. At lower speeds, such as might happen when load is added, motor torque is higher than load torque and the motor will accelerate back to the point of equilibrium. At speeds above the point of equilibrium, such as might happen when load is removed, the motor’s driving torque is less thanrequired load torque and the motor will decelerate back to the point of equilibrium. Armature Speed Armature Speed, Typically armature voltage is either 400 VDC.The speed of an unloaded motor can generally be predicted for any armature voltage. For example, an unloaded motor might run at 1200 RPM at 400 volts. The same motor would run at approximately 600 RPM at 250 volts. The base speed listed on a motor’s nameplate, however, is an indication of how fast the motor will turn with rated armature voltage and rated load (amps) at rated flux (Φ). The maximum speed of a motor may also be listed on the nameplate. This is an indication of the maximum mechanical speed a motor should be run in field weakening. If a maximum speed is not listed the vendor should be contacted prior to running a motor over the base speed. [Date] 10  Electrical Motor DC Motor Transfer Function Va = (KtΦn) + (IaRa)(Spannings val oor Where: Va = Applied Armature Voltage Kt = Motor Design Constants Φ = Shunt Field Flux n = Armature Speed Ia = Armature Current Ra = Armature Resistance DC Motor Equations In a DC drive voltage applied (Va) to the armature circuit is received from a variable DC source. Voltage applied to the field circuit (Vf) is from a separate source. The armature of all DC motors contains some amount of resistance (Ra). When voltage is applied (Va), current (Ia) flows through the armature. You will recall from earlier discussion that current flowing through the armature conductors generates a magnetic field. This field interacts with the shunt field (Ф) and rotation results. Armature Voltage The following armature voltage equation will be used to demonstrate various operating principles of a DC motor. Variations of this equation can be used to demonstrate how armature voltage, CEMF, torque, and motor speed interact with each other. [Date] 11  Electrical Motor CEMF We have already learned that rotation of the armature through the shunt field induces a voltage in the armature (Ea) that is in opposition to the armature voltage (Va). This is counter electromotive force (CEMF). CEMF is dependent on armature speed (n) and shunt field (Ф) strength. An increase in armature speed (n) or an increase of shunt field (Ф) strength will cause a corresponding increase in CEMF (Ea). Ea = Kt Ф n or Ea = Va - (IaRa)Ra = Armature Resistance DC Motor Design Excitation winding Erreger- wicklung Armature Anker- winding wicklung S n IA IA N Brushes Bürsten (Excitation) flux of the machine: summation of all field lines Laminated Anker- iron Φf = w * B * A Blechpaket Number of windings [Date] 12  Electrical Motor The excitation winding generates the magnetic field in the machine. With electrical excitation, the exciting field can be set using the field current. The sum of all “field lines” that establish the armature is measured in total over the (excitation) flux. This quantity is used for machine equations (flux constant). In two-pole machines there is a north pole and a south pole. In multi-pole machines, the configuration is repeated p-times on the circumference. Accordingly, the flux is distributed among multiple poles, and the yoke height (thickness of the stator iron) can be reduced. However, the magnetization frequency of the rotor iron is higher for the same speed, and, as a result, the iron losses go up. The armature winding carries the armature current that generates the armature field. Armature field and excitation field are perpendicular to each other. Excitation losses: Erreger- Current wicklung heat losses Current heat losses Anker- wicklung In the armature conductors S n IA IA N Brush losses: Friction Bürsten and current heat losses Iron losses Anker- with rotating armature Blechpaket DC Motor Desighn [Date] 13 Brush losses: Fri  Electrical Motor Transfer of power in motor operation and losses There are several sources of loss in an electrical machine: Armature circuit: Ohmic losses (Joule heat) in the armature conductors: 2 PVA = RA · I A The losses at the sliding contact (brushes / commutator) are included in the armature resistance RA. Armature: The laminated core of the armature passes through the excitation field where it experiences an alternating magnetization that leads to the so-called iron losses. With increasing speed and increasing field, the iron losses increase. The iron losses are composed of eddy-current losses in the armature laminations and hysteresis loss (passing through the hysteresis loop of the iron). Excitation circuit: Ohmic losses occur in the excitation winding because of the excitation current. Friction: Friction losses occur at the sliding contact of brush/commutator, at the bearings. In addition, ventilation losses are generated because of the moving air at the armature surface. Motor operation In motor operation, the friction losses must be covered from the electromagnetically generated torque of the machine – the so- called internal torque. That is, the torque available at the shaft is less than what the machine generates internally. Bf BA Maximum torque (Counterclockwise direction) [Date] 14  Electrical Motor BA Bf No torque Maximum torque (Clockwise direction) Basic types of electric machines: - Machines with a stationary magnetic field (DC machines) - Machines with a rotating magnetic field (polyphase machines) The torque is defined by the vector or cross product of excitation field BF and armature current (resp. –field) BA. Thus, the torque depends on the angle between these components. The design of the machine makes sure that this angle is 90 degrees, making the cross product the maximum possible. DC motor design Armature winding: Lap winding [Date] 15  Electrical Motor Commutator and service Brushes must be installed and maintained well. They wear down rather quickly, particularly if the wrong material (too soft) is used or the commutator is too cold (should be around 95 degrees C). The commutator must be checked for the condition of the brush path (dark but glossy patina and no deep grooves). Partial load If a motor is run in partial load most of the time, some parallel brushes must be removed to increase current density and brush/commutator temperature. If necessary, even air flow direction and throughput must be changed (in the latter case also the excitation current must be reduced because the field winding is not sufficiently cooled anymore). Power conversion Energy transfer and operating modes Four Quadrant Operation From the equation Pel = Ui * I or Pmech = M * w, it is possible to derive the operating mode of the machine - whether motor or generator - and the direction in which energy is flowing in the actuator: These possibilities define four operating modes, which are called four- quadrant operation and cover the whole area in the speed-torque diagram. If there is only one current direction possible, this defines either a single quadrant drive (motor operation only) or a two quadrant drive (motor and generator). [Date] 16  Electrical Motor Block circuit diagram of the converter-fed DC machine Controlled bridges (B6C) are used for voltage adjustment for the armature circuit of the DC machine. The mean value of the DC voltage Udia determines the operating point of the DC machine: For four-quadrant operation, two anti-parallel bridge circuits (B6C)A(B6C) are required for the armature circuit. Example of DC Motor Nameplate (courtesy SIEMENS) [Date] 17  Electrical Motor 2.2 Synchronous Motors 2.2.1 Wire Wound Synchronous Motors The stator windings are connected to the three-phase star (U, V and W). Connecting a three-phase supply causes the stator winding to produce a revolving field. The rotor in a synchronous motor has either an electromagnet (current conducting winding arrangement) or a permanent magnet. The rotor field is generated "actively“ The high energy-density of new, extremely high-performance permanent magnets increases the motor's performance while simultaneously reducing the mass. This results in increased drive dynamics and smaller motor sizes. Optimized concentricity enables high-precision positioning. Synchronous motor with rotor windings and rotor slipring Two common approaches are used to supply a DC current to the field circuits on the rotating rotor: 1. Supply the DC power from an external DC source to the rotor by means of slip rings and brushes; 2. Supply the DC power from a special DC power source mounted directly on the shaft of the machine. [Date] 18  Electrical Motor Slip rings are metal rings completely encircling the shaft of a machine but insulated from it. One end of a DC rotor winding is connected to each of the two slip rings on the machine’s shaft. Graphite-like carbon brushes connected to DC terminals ride on each slip ring supplying DC voltage to field windings regardless the position or speed of the rotor. 2.2.2 Permanent Magnet Synchronous Permanent magnet synchronous motors have the rotor winding replaced by permanent magnets. These motors have several advantages over synchronous motors with rotor field windings, including: Elimination of copper loss Higher power density and efficiency Lower rotor inertia Larger airgaps possible because of larger coercive force densities. SPM and IPM motors The magnets of the surface mounted PM (SPM) motor are attached on the surface of the rotor, whereas those of the interior buried PM (IPM) motor are buried inside.The magnets of SPM motors need to be fixed on the rotor surface using adhesive, thus mechanical strength is weaker than IPM motors especially in the high speed region. The rotor magnetic flux of the induction motor is induced by the rotation of the stator magnetic field. The IPM motor has high efficiency and high torque because it utilizes both magnet and reluctance torques caused by the magnetic saliency. [Date] 19  Electrical Motor Permanent magnet synchronouse motor equivalent circuit PM Motor Equivalent Circuit Shows the definition of the motor dq-axes. The north pole of the rotor is defined as d-axis. The PM motor electrical circuit equations are given by: (6) [Date] 20  Electrical Motor where all the AC quantities are transformed into DC quantities for the convenience of analysis, i.e., the synchronous reference frame is used. The PM motor back-emf voltage waveform. The equivalent circuits are useful to analyze the motor voltage and power factor according to the motor current and speed. Commissioning Parameters: 1. Rated motor power (kW), 2. RMS motor current (A) 3. Motor pole number (poles) 4. Motor stator resistance per phase (ohm) Measure motor terminal resistances R(u-v), R(v-w), and R(w-u) respectively using an ohm meter that has 10−3ohm range resolution or better. Take an average of the terminal resistances. 5. PM motor d- and q-axis inductances (mH) In case of SPM motors, terminal inductance doe not changes according to the rotor position ( i.e. d q L = L ) if the saturation effect by the magnets is neglected. But the rotor of the IPM motors have magnetic saliency, and the inductance measurement results will change according to the rotor position. 6. PM motor back-emf voltage. [Date] 21  Electrical Motor 2.2.3 Synchronous reluctance motors Synchronous reluctance motors (SynRM) The alternating current passing through the stator windings creates a rotating magnetic field in the air gap of the electric motor. Torque is created when the rotor attempts to establish its most magnetically conductive axis (d-axis) with an applied field to minimize the reluctance (magnetic resistance) in the magnetic circuit. The amplitude of the torque is directly proportional to the difference between the direct Ld and quadrature Lq inductances. Therefore, the greater the difference, the greater the torque created. The rotor constantly strives to align itself along the magnetic lines of flux. The lines of flux join up at the lowest energy level. This is known as reluctance In the 1FU motor this effect is amplified by permanent magnets. This makes it more efficient. SynRMs work on a very elegant principle that has been known for a long time, but only since the recent rise of sophisticated VSD control electronics did it become possible to exploit these super-efficient electrical machines fully. In SynRMs, the rotor is designed to produce the smallest possible magnetic reluctance (the resistance to the flow of a magnetic field) in one direction and the highest in the direction perpendicular. The rotor turns at the same frequency as the stator field (as in the PM motor). [Date] 22  Electrical Motor 2.2.4 Servo Motors Servo Motors A servomotor is a motor that lets you control the exact position of the motor shaft as well as the speed and/or acceleration. Corresponding sensors and regulation technology are also used for this purpose. Previously, servomotors were auxiliary drives that were designed for use in machine tools. Servomotors consist of either an asynchronous motor, a synchronous motor, or a DC motor. The difference between the motors is therefore not in the drive principle itself, but only in their regulation capabilities. Servomotor technology overview AC induction motors designed for servo operation are wound with two phases at right angles. A fixed reference winding is excited by a fixed voltage source, while a variable control voltage from a servo-amplifier excites the winding. The windings often are designed with the same voltage-to-turns ratio, so that power inputs at maximum fixed phase excitation, and at maximum control phase signal, are in balance. Any motor designed for servo use is typically 25% to 50% smaller than other motors with similar output and the reduced rotor inertia makes for quicker response. For example, AC servomotors are used in applications requiring rapid and accurate response characteristics — so these induction motors have a small diameter for low inertia and fast starts, stops and reversals. High resistance provides nearly linear speed-torque characteristics for accurate control. [Date] 23  Electrical Motor 2.2.5 Linear Synchronous Motors Linear Synchronouse Motor A linear motor is an electric motor that has had its stator and rotor "unrolled" thus instead of producing a torque (rotation) it produces a linear force along its length. However, linear motors are not necessarily straight. Characteristically, a linear motor's active section has ends, whereas more conventional motors are arranged as a continuous loop. [Date] 24  Electrical Motor 2.2.6 High Torque Motors High Torque Permanent Magnet Motors High-Torque Motors The direct high-torque motors are permanent-magnet synchronous motors that can provide high torques at low speeds directly at the driven machine. High efficiencies and good power factors can also be achieved at low speeds as a result of the permanent magnet rotors. The direct high-torque motors are offered as a harmonized system together with drive converters. Example: CHARACTERISTICS AC/DC Type synchronous Voltage 690 V Protection class IP55 Other characteristics water-cooled, low-voltage, high-efficiency, high-torque, low-speed, custom, forced air-cooled, for cranes, food, for marine applications, for the mining industry, permanent magnet Torque Min.: 6,000,000 Nm Max.: 42,000,000 Nm Power 33 kW, 1,000 kW Min.: 150 kW Max.: 2,100 kW Rotational speed Min.: 200 rpm Max.: 800 rpm [Date] 25  Electrical Motor 2.3 Asynchronous Motors Induction motor or Asynchronous motor An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor can therefore be made without electrical connections to the rotor. Asynchronous servomotors are suitable for use in applications in which high external inertias have to be moved in plants and machines and controlled safely. This is the most widely used motor of all. Imagine a motor with the rotor locked and a three phase supply applied to the stator. The net effect is just like a transformer with an airgap in it’s core. Thus the rotor windings have 50 Hz (or what ever the stator frequency is) induced in them. A rotor field is then established and it rotates at the same speed as that of the stator & torque is produced. We call this “synchronous speed” If we now rotate the rotor in the same direction the rotating field and a speed being a fraction of synchronous speed, the current induced in the rotor will have a frequency equal to the difference between synchronous speed and the rotor speed. The rotor current produces a rotating field on the rotor but because the rotor is moving, the net speed of the rotor field as viewed from the stator is still synchronous speed. Because the rotor moves at less than synchronous speed it is usually talked of as “slipping” and hence the idea of defining it’s speed as shaft or “asynchronous speed”. [Date] 26 Electrical Motor Induction Motor Power Induction Motor Losses Copper Rotor Bars [Date] 27  Electrical Motor The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator. A copper rotor is a rotor made of electrical steel (laminations) where the slots and end rings are filled with copper instead of the traditional material (aluminum). Introducing the copper rotor. The use of copper in place of aluminum can lead to improvements in motor energy efficiency due to a significant reduction in I2R losses (the power lost in an electrical device due to the current flowing through the motor’s windings). 2.3.2 Conical Rotor Brake Motors Conical Rotor Brake Motor( Courtesy Demag) A. Spring B. Cone rotor C. Cooling fan D. Stator windings E. Electrical brake A conical rotor brake motor incorporates the brake as an integral part of the conical sliding rotor. When the motor is at rest, a spring acts on the sliding rotor and forces the brake ring against the brake cap in the motor, holding the rotor stationary. When the motor is energized, its magnetic field generates both an axial and a radial component. The axial component overcomes the spring force, releasing the brake; while the radial component causes the rotor to turn. There is no additional brake control required. [Date] 28  Electrical Motor 2.3.3 Slip Ring Motor Slip Ring Motor Rotor Current Control is another effective method of slip control. With full supply voltage on the stator, giving a constant flux , the rotor current IR can be controlled by adjusting the effective rotor resistance RR A Wound Rotor Induction Motor (WRIM), also called a Slipring Motor, is used. The connections to the rotor windings are brought out to terminals via 3 slip- rings and brushes. By connecting external resistance banks to the rotor windings, the rotor current can be controlled. [Date] 29  Electrical Motor Slip Ring Motor Torque Curves Starting current inrush is reduced in direct proportion to the rotor resistance. Starting Torque, for certain values of rotor resistance, is higher than DOL starting torque and can be as high as the breakdown torque. Starting with a high external rotor resistance, as resistance is decreased in steps, the starting torque is progressively increased from a low value up to the breakdown torque. This type of starting is ideal for applications that require a high pull-away torque with a soft start, such as conveyors, crushers, ball mills, etc. [Date] 30  Electrical Motor Motor Comparison The stepper motor runs according to the electrical pulses that power its coils. Depending on the electricity supply, it can be: 1. unipolar if the coils are always powered in the same direction by a single voltage; 2. bipolar if the coils are powered first in one direction then in the other. They create alternating north and south poles. Stepper motors can be variable reluctance, magnetic or both. The minimum angle of rotation between two electrical pulse changes is called a step. A motor is characterized by the number of steps per revolution (i.e. 360°). The common values are 48, 100 or 200 steps per revolution. [Date] 31 Electrical Motor Stepper motors Current Steps in the Stepper motor [Date] 32  Electrical Motor The motor rotates discontinuously. To improve the resolution, the number of steps can be increased electronically (micro-stepping). This solution is described in greater detail in the section on electronic speed control. Varying the current in the coils by graduation results in a field which slides from one step to the next and effectively shortens the step. Some circuits for micro-steps multiply by 500 the number of steps in a motor, changing, e.g. from 200 to 100,000 steps. Electronics can be used to control the chronology of the pulses and count them. Stepper motors and their control circuits regulate the speed and amplitude of axis rotation with great precision. They thus behave in a similar way to a synchronous motor when the shaft is in constant rotation, i.e. specific limits of frequency, torque and inertia in the driven load. Open loop stepper motor control ues a controller witch wil calculate the number of steps for a displacement. An encoder is integrated in a close loop stepper motor for accurate posittioning and displacement. Closed Loop Stepper motor with Driver, NEMA 23, 1.5Nm [Date] 33 Electrical Motor Close Loop Stepper motor Control [Date] 34  chapter 3 Control Mode From the needs, choose a control mythology, then a technology to control a product Electrical Motor Control Electrical Motor Control 3 Control Mode Numerical control and control devices for motors are based on the principle of intervention into energy. A system is defined as an arrangement of devices related to each other as required by the automation task. The information is contained in the value of the physical variables of a signal. Speed feedback from the motor shaft, for example is a signal. Open loop control is a process within a system where one or several input variables is characterized by an open path of action. Close loop control is a process within a system, where a controlled variable, as example speed, is continuously detected, compared, and adjusted according to a command variable. Figure 1 Open Loop Control Figure 2 Close Loop Control u = Input variable x = Controlled variable v = Output variable xA = Final controlled variable w = Command variable r = Feedback variable y = Manipulated variable e = Error value z = Disturbance variable 128 Electrical Motor Control 3.1 Speed Open Loop Control Figure 3 Example of an Open Loop Control System Open Loop System - A system that does not use feedback to verify the desired result, or output, has been reached. Most motor systems are operated open loop. The command system is a command, which is used for many applications such as the conveyor speed control, fan wind amount control, pump flow amount control, etc. The slip at the rated torque depends on the characteristics of a motor. Approximately 3 to 5% speed fluctuation occurs. Figure 4 Variable Speed Drive Open Loop Control Unlike a DC drive, the AC drive frequency control technique uses parameters generated outside of the motor as controlling variables, namely voltage and frequency. Both voltage and frequency reference are fed into a modulator which simulates an AC sine wave and feeds this to the motor’s stator windings. This 129 Electrical Motor Control technique is called Pulse Width Modulation (PWM) and utilizes the fact that there is a diode rectifier towards the mains and the intermediate DC voltage is kept constant. The inverter controls the motor in the form of a PWM pulse train dictating both the voltage and frequency. Significantly, this method does not use a feedback device which takes speed or position measurements from the motor’s shaft and feeds these back into the control loop. Such an arrangement, without a feedback device, is called an “open-loop drive”. Vd max correction V/f V Voltag V f + + RF - + Frequency n f f n + + -f>f f