Electrical Machines PDF

Subject Code - 23RO2005 Subject Name - Electrical Machines 1  MODULE 1 – DC MOTOR Definition, Working Principle, Applications DC Motor – Construction DC Motor Working Torque Speed Characteristics Separately Excited DC Motor Self-Excited DC Motor Frequently asked...

Subject Code - 23RO2005 Subject Name - Electrical Machines 1  MODULE 1 – DC MOTOR Definition, Working Principle, Applications DC Motor – Construction DC Motor Working Torque Speed Characteristics Separately Excited DC Motor Self-Excited DC Motor Frequently asked Questions 2  DC MOTOR Definition : A DC motor is defined as an electrical machine that converts electrical energy into mechanical energy. Input : Direct current (Electrical) Output : Mechanical Rotation Working principle of DC motor When a current carrying conductor is placed in a magnetic field, it develops a mechanical force (torque) and develops a tendency to move. In short, when electric fields and magnetic fields interact, a mechanical force arises. 3 Applications : Elevators, electric trains, hoists and heavy metal rolling mills are all examples of huge motor usage. Automobile motors, robotics, hand power tools and food blenders are all examples of small motor applications. Micro Machines are electric machines with parts the size of red blood cells that have a wide range of medical applications. 4  FLEming’s left hand rule Fleming’s Left Hand Rule states that if we arrange our thumb, forefinger and middle finger of the left-hand perpendicular to each other, then the thumb points towards the direction of the force experienced by the conductor (motion), the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the electric current. 5  Fleming’s Left-Hand Rule It is used for electric motors The purpose of this rule is used for finding the direction of the magnetic force acting in an electric motor. The magnitude of this force is given by Lorentz force. The middle finger represents the direction of the current Lorentz force is defined as the combination of the magnetic and electric force on a point charge due to electromagnetic fields. It is used in electromagnetism and is also known as electromagnetic force. In the year 1895, Hendrik Lorentz derived the modern formula of Lorentz force. The magnetic force is zero if the charge is not moving or no current flows through the conductor. 6 Construction of a DC Motor Parts : 1. Yoke 2. Magnetic Field System 3. Armature Core 4. Armature Winding 5. Commutator 6. Brushes 7 1. Yoke ✔ The outer frame of a DC motor. ✔ It is a hollow cylinder. ✔ It acts as a protecting cover to the machine. ✔ It provides mechanical support for the poles. 2. Magnetic Field System (pole core, pole shoe, field winding) ✔ It is the stationary part of the machine (stator). ✔ It consists of an even number of pole cores bolted to the yoke and field winding wound around the pole core. ✔ The pole shoe supports the field winding. ✔ It produces the magnetic flux in the motor. 8  3. Armature Core ✔ It is the rotating part of the machine (rotor). ✔ It is mounted on the shaft and rotates between the field poles. ✔ It has slots on its outer surface ✔ The armature conductors are put in these slots. ✔ These conductors are insulated and connected to each other. 4. Armature Winding ✔ The connected arrangement of conductors is known as armature winding. ✔ There are two types of armature windings are used – wave winding and lap winding. 9  5. Commutator ✔ It consists of two split rings. ✔ It is used to reverse the direction of current. ✔ A commutator is a mechanical rectifier. ✔ The ends of the armature windings are connected to each segment of the commutator. 6. Brushes ✔ The brushes are mounted on the commutator. ✔ The brushes are made of carbon and is supported by a metal box called brush holder. ✔ The current flows from the external DC source to the armature winding through the carbon brushes and commutator. 10  Working of DC Motor Armature winding is Perpendicular to the magnet… 11 Direction of current… 12 To find the Direction of force… 13 14 Armature winding is Parallel to the magnet… N S 15  Half rotation… N S For every half rotation, the direction of current is reversed the direction of force is same (anti-clockwise) 16 DC Motor Working When a current carrying conductor is placed in a magnetic field, it develops a mechanical force (torque) and a tendency to move. 1. Magnetic Field Creation 2. Magnetic Field orientation 3. Current Flow in Armature Conductors 4. Force on Conductors 5. Torque Generation 6. Continuous Rotation 17 Magnetic Field Creation: When the field coil (stator) of the DC motor is energized, it generates a magnetic field. This magnetic field flows through the air gap between the stator and the armature (rotor). Magnetic Field Orientation: The magnetic field lines extend radially through the armature. They enter the armature from the North pole of the field coil and exit through the South pole. Current Flow in Armature Conductors: When current flows through the conductors of the armature, it interacts with the magnetic field created by the stator. 18 Force on Conductors: According to Fleming's left-hand rule, a force is exerted on each conductor of the armature due to the interaction between the current and the magnetic field. The direction of the force depends on the direction of the current and the orientation of the magnetic field. Torque Generation: The conductors located on one side of the armature experience a force in one direction, while the conductors on the opposite side experience a force in the opposite direction. These two opposing forces create a torque that causes the armature to rotate. Continuous Rotation: The commutator and brushes ensure that the current direction in the armature windings is switched appropriately as the armature rotates, maintaining continuous rotational motion in a single direction. 19 Electrical circuit of a dc motor : There are two properties of this type of dc motor: 1. The armature and field windings are separated from each other, electrically. 2. Both armature and field windings are supplied by separated DC sources. Vf = voltage applied to the field winding If = current flowing in the field winding Lf = inductance of field winding coils Rf = resistance of field winding coils VT = voltage applied to the armature winding Ia = current flowing in the armature winding La = inductance of the armature winding coils Ra = resistance of the armature winding coils Ea = induced voltage in the armature Tm = torque of the motor 20 Now we are analyzing the circuit in steady-state, thus the d/dt for inductance is zero. In steady state, the voltage in the stator (field): In steady state, the voltage in the rotor (armature): In DC motor, induced voltage Ea is also known as back-emf, Where K1 = armature constant r = speed of the motor Now, If the value of Ra is small enough to be ignored, then 21 The torque needed to rotate the rotor can be calculated from: Where: Tm = torque of the motor (N-m) Pout = output power of the motor (watt) r = speed of the motor (rad/s) Note - DC motor can also act as a DC generator if the armature is moved by external energy, producing electricity in the process. This one can be called a prime mover. 22 Torque-speed characteristics : There are two types of DC motors: 1. Separately excited DC motor 2. Self-excited DC motor The separately excited DC motor has two voltage sources to supply the armature and field windings separately. The self-excited DC motor has only one voltage source to supply both armature and field windings. This type can be divided further into: a. Shunt DC motor b. Series DC motor c. Compound DC motor 23 Separately Excited DC Motor : In a separately excited DC motor, the This motor has characteristics of: field coils are energised from an 1. The armature (rotor) and field (stator) external source of DC supply. windings have to be supplied by voltage separately, thus we need two DC voltage sources electrically separated from each other. 2. The armature and field windings are separated electrically. 24 1. The torque is inversely proportional to the speed of the rotor. 2. Stall torque is the maximum torque but the rotor is not rotating. 3. No-load speed is the maximum speed of the rotor when there is no torque applied to the rotor. Stall torque refers to the maximum torque a motor can generate when the output rotational speed is zero (the motor shaft is not turning). At this condition, the motor is said to be "stalled”. Extended operation at stall torque can lead to overheating and damage. 25 Self-Excited DC Motor : Shunt Wound Motor In a shunt wound motor, the field This motor has characteristics of: winding is connected parallel to the 1. The armature and field windings are armature connected in parallel. 2. Only one voltage source is needed to supply both armature and field windings 3. The total current drawn from the supply will be, IT = If + Ia. The total input power is VTIT 26 1. This motor relatively has constant torque in a wide range of speed. The torque speed characteristics and equation for shunt DC motor is more or less same as the separately excited DC motor. The torque is inversely proportional to the speed of the rotor. 2. Stall torque is the maximum torque but the rotor is not rotating. 3. No-load speed is the maximum speed of the rotor when there is no torque applied to the rotor. 27 Self-Excited DC Motor : Series Wound Motor In a series wound DC motor, the field This motor has characteristics of: winding is connected in series with 1. The armature and field windings are the armature winding connected in series. 2. Only one voltage source is needed to supply both armature and field windings. 3. The current in the field winding is the same as the current in the armature winding. 28 1. From the curve above, the series DC motor is capable of moving a heavy load with low speed. This motor has high torque at low speed and low torque at high speed. Series motor has a special trait that we can apply AC or DC voltage to it. Even if we reverse the supply’s polarity, the motor will always rotate in the same direction. This is why a series motor is called a universal motor. Of course we will use DC voltage here. 29 Self-Excited DC Motor : Compound Wound Motor DC motors having both shunt - connected in parallel to the armature and series field winding - connected in series to the armature is known as Compound DC motor. The compound motor is further divided into: 1. Cumulative Compound Motor 2. Differential Compound Motor In a cumulative compound motor, the magnetic flux produced by both the windings is in the same direction. In a differential compound motor, the flux produced by the series field windings is opposite to the flux produced by the shunt field winding. 30 The torque speed characteristics and equation for shunt DC motor is more or less same as the separately excited and shunt wound DC motor. 1. The torque is inversely proportional to the speed of the rotor. 2. Stall torque is the maximum torque but the rotor is not rotating. 3. No-load speed is the maximum speed of the rotor when there is no torque applied to the rotor. 31 Frequently Asked Questions on DC Motor Q1 Can a DC motor run on AC? Yes, the DC series motor runs on single-phase AC supply. This is because the torque, which varies as the product of the armature and field current, is always positive. Thus, a positive average torque causes the motor to rotate. Q2 What is a Universal motor? A universal motor is a special type of motor that can run on either DC or single-phase AC supply. Q3 When was the DC motor invented? William Sturgeon invented the first DC motor in 1886. Q4 How do you select the right DC motor for Robotics Applications? Three main factors need to be considered when selecting a DC motor type for any robotics application: Motor Speed: both minimum and maximum speed, with acceleration Torque: maximum torque that can be generated and its relation to speed Precision: of the motor’s operation and its repeatability. 32 Q5 DC series motor should never be started without load. Why? Under the no-load condition, the armature current is very less and hence the flux produced will also be less (φ α Ia). We know, the speed is inversely proportional to the flux (N α Eb/φ). So if the flux is less, high speed is produced in the motor. It is very dangerous as it will damage the motor. Thus, a DC series motor should never be started without load. Q6 Why a DC shunt motor is called as a constant speed motor? In a DC shunt motor, the field winding is connected in parallel to the armature winding and the supply voltage. Under a constant supply voltage, the flux in a shunt field winding remains constant. This constant flux will keep the motor rotating at a constant speed. 33