Electrical machines

1. Explain with neat diagram the working principle of D.C. Motor.A DC motor in simple words is a device that converts direct current(electrical energy) into mechanical energy. In Simple words working principle of motor is when any current carrying conductor placed in magnetic field experience a force.

The very basic construction of a dc motor contains a current carrying armature which is connected to the supply end through commutator segments and brushes and placed within the north south poles of a permanent or an electro-magnet as shown in the diagram.

There are two conditions which are necessary to produce a force on the conductor. The conductor must be carrying current, and must be within a magnetic field. When these two conditions exist, a force will be applied to the conductor, which will attempt to move the conductor in a direction perpendicular to the magnetic field. This is the basic theory by which all DC motors operate. The force exerted upon the conductor can be expressed as follows.

F= B I l Newton (1) where B is the density of the magnetic field, l is the length of conductor, and I the value of current flowing in the

conductor. The direction of motion can be found using Fleming’s Left Hand Rule.

Fleming’s left hand rule is used to determine the direction of force acting on the armature conductors of dc motor. Fleming’s left hand rule says that if we extend the index finger, middle finger and thumb of our left hand in such a way that the current carrying conductor is placed in a magnetic field (represented by the index finger) is perpendicular to the direction of current (represented by the middle finger), then the conductor experiences a force in the direction (represented by the thumb) mutually perpendicular to both the direction of field and the current in the conductor.

1 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

2. Explain constructional details of DC motor.

A DC motor like we all know is a device that deals in the conversion of electrical energy to mechanical energy and this is essentially brought about by two major parts required for the construction of dc motor, namely.

1) Stator – The static part that houses the field windings and receives the supply and,

2) Rotor – The rotating part that brings about the mechanical rotations.

3) Yoke of dc motor.

4) Poles of dc motor.

5) Field winding of dc motor.

6) Armature winding of dc motor.

7) Commutator of dc motor.

8) Brushes of dc motor. All these parts put together configures the total construction of a dc motor.

Yoke: The magnetic frame or the yoke of dc motor made up of cast iron or steel. Its main function is to form a protective covering and provide support to the armature.

Poles and pole shoe of DC Motor : Its function is to just hold the field winding . whereas the pole shoe spreads the flux produced over the air gap between the stator and rotor to reduce the loss due to reluctance.

Field Winding of DC Motor: The field winding of dc motor are made with field coils (copper wire) wound over the slots of the pole shoes in such a manner that when field current flows through it, then magnetic field is produced.

Armature Winding of DC Motor: The armature winding of dc motor is attached to the rotor, The construction of armature winding of dc motor can be of two types:- Lap Winding : In this case the number of parallel paths between conductors A is equal to the number of poles P.i.e A = P Wave Winding: Here in this case, the number of parallel paths between conductors A is always equal to 2 irrespective of the number of poles..

Armature core: To reduce the magnetic losses like hysteresis and eddy current loss laminated steel sheets are stacked together to form the cylindrical structure of the armature core.

Commutator of DC Motor :The commutator of dc motor is a cylindrical structure made up of copper segments stacked together, but insulated from each other by mica. Its main function is to convert AC to DC.

Brushes of DC Motor : The brushes of dc motor are made with carbon or graphite structures, making sliding contact over the rotating commutator.

2 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

3. Discuss different types of D.C. motor with suitable circuit diagram, voltage equation and application.

The DC motor has a lot of application in today’s field of engineering and technology. Starting from an electric shaver to parts of automobiles, in all small or medium sized motoring applications DC motors come handy. And because of its wide range of application different functional types of dc motor are available in the market for specific requirements. The types of DC motor can be listed as follows   

3 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

Separately exited dc motor:

Applications Of DC motor :

Types of motors

characteristics Applications

Shunt motor 1. Almost constant speed2. Adjustable speed3. Starting torque is medium

For driving constant speed line shafting, lathes, centrifugal pumps, reciprocating pumps ,. Machine tools.

Series motor 1. Variable speed2. Adjustable varying speed3. Starting torque is very high

For traction ( Electric trains locomotives)Lift, crane, hoists, conveyor belts, trolleys car.

Cumulative motor

1. variable speed2. adjustable variable speed3. higher starting torque

Punch, shears, rolling mills, planning machines in workshop, elevators.

4. Explain working principle of three phase induction motor with its applications.

An electrical motor is such an electromechanical device which converts electrical energy into a mechanical energy. In case of three phase AC operation, most widely used motor is Three phase induction motor as this type of motor does not require any starting device or we can say they are self starting induction motor.

In an induction motor the stator winding is fed with an three phase AC supply.

Alternating flux is produced around the stator winding due to AC supply. This alternating flux revolves with synchronous speed. The revolving flux is called as "Rotating Magnetic Field" (RMF).

The relative speed between stator RMF and rotor conductors causes an induced emf in the rotor conductors, according to the Faraday's law of electromagnetic induction. The rotor conductors are short circuited, and hence rotor current is produced due to induced emf. That is why such motors are called as induction motors.

(This action is same as that occurs in transformers, hence induction motors can be called as rotating transformers.) 

Now, induced current in rotor will also produce alternating flux around it. This rotor flux lags behind the stator flux. The direction of induced rotor current, according to Lenz's law, is such that it will tend to oppose the cause of its production. 

As the cause of production of rotor current is the relative velocity between rotating stator flux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same direction as that of stator flux to minimize the relative velocity. However, the rotor never succeeds in catching up the synchronous speed. This is the basic working principle of induction motor of either type, single phase of 3 phase. 

4 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

v=Eb+ I aRa

I L=I a

I f=VRSH

Synchronous speed: The rotational speed of the rotating magnetic field is called as synchronous speed.

where, f = frequency of the supply            P = number of poles

Slip: Rotor tries to catch up the synchronous speed of the stator field, and hence it rotates. But in practice, rotor never succeeds in catching up. If rotor catches up the stator speed, there wont be any relative speed between the stator flux and the rotor, hence no induced rotor current and no torque production to maintain the rotation. However, this won't stop the motor, the rotor will slow down due to lost of torque, the torque will again be exerted due to relative speed. That is why the rotor rotates at speed which is always less the synchronous speed.

The difference between the synchronous speed (Ns) and actual speed  (N) of the rotor is called as slip.

5. Explain types of three phase induction motors with constructional details.

Induction Motors are the most commonly used motors in many applications. These are also called as Asynchronous Motors, because an induction motor always runs at a speed lower than synchronous speed. Synchronous speed means the speed of the rotating magnetic field in the stator.

3-phase induction motor also consists of a stator and a rotor. Basically there are two types of 3 phase IM - 1. Squirrel cage induction motor and

2. Phase Wound induction motor (slip-ring induction motor).

Both types have similar constructed stator, but they differ in construction of rotor.

Stator : Stator core : The main function of the stator core is to carry alternating flux. In order to reduce the

eddy current losses the stator core is laminated.. The stamping are generally made up of silicon steel, which reduces the hysteresis loss. Slots are the present to hold the three phase windings.

Stator winding : The stator is wound with a 3 phase winding which is fed from a 3 phase supply. It is wound for a defined number of poles, and the number of poles is determined from the required speed. For greater speed, lesser number of poles is used and vice versa.

5 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

Rotor: As described earlier, rotor of a 3 phase induction motor can be of either two types, squirrel cage rotor and phase wound rotor (or simply - wound rotor).

Squirrel cage rotor

Most of the induction motors (upto 90%) are of squirrel cage type. Squirrel cage type rotor has very simple construction. This type of rotor consist of a cylindrical laminated core, having parallel slots on it. These parallel slots carry rotor conductors. In this type of rotor, heavy bars of copper, aluminum or alloys are used as rotor conductors instead of wires.Rotor slots are slightly skewed to achieve following advantages -

1. it reduces locking tendency of the rotor, i.e. the tendency of rotor teeth to remain under stator teeth due to magnetic attraction.

2. increases the effective transformation ratio between stator and rotor

3. increases rotor resistance due to increased length of the rotor conductor

The rotor bars are brazed or electrically welded to short circuiting end rings at both ends. Thus this rotor construction looks like a squirrel cage and hence we call it. The rotor bars are permanently short circuited, hence it is not possible to add any external resistance to armature circuit.

Phase wound rotor: Phase wound rotor is wound with 3 phase, double layer, distributed winding. The number of poles of rotor are kept same to the number of poles of the stator. The three phase rotor winding is internally star connected. The other three terminals of the winding are taken out via three insulated sleep rings mounted on the shaft and the brushes resting on them. These three brushes are connected to an external star connected rheostat. When motor is running at its rated speed, slip rings are automatically short circuited by means of a metal collar and brushes are lifted above the slip rings to minimize the frictional losses.

6 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

6. Why Single Phase Induction Motor is not Self Starting?We know that the ac supply is a sinusoidal wave and it produces pulsating magnetic field in uniformly distributed stator winding. Since pulsating magnetic field can be assumed as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting and due to this the motor does not run.

After giving the supply, if the rotor is made to rotate in either direction by external force, then the motor will start to run.

How to make single phase induction motor self starting?

As explained above, single phase induction motor is not self-starting. To make it self-starting, it can be temporarily converted into a two-phase motor while starting. This can be achieved by introducing an additional 'starting winding' also called as auxiliary winding.

Hence, stator of a single phase motor has two windings: (i) Main winding and (ii) Starting winding (auxiliary winding). These two windings are connected in parallel across a single phase supply and are spaced 90 electrical degrees apart. Phase difference of 90 degree can be achieved by connecting a capacitor in series with the starting winding. 

Hence the motor behaves like a two-phase motor and the stator produces revolving magnetic field which causes rotor to run. Once motor gathers speed, say up to 80 or 90% of its normal speed, the starting winding gets disconnected form the circuit by means of a centrifugal switch, and the motor runs only on main winding.

7. Explain construction and working principle of single phase induction motor. And write types of motor.

Construction of a single phase induction motor is similar to the construction of three phase induction motor having squirrel cage rotor, except that the stator is wound for single phase supply. Stator is also provided with a

7 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

'starting winding' which is used only for starting purpose.

Working principle of single phase induction motor

When the stator of a single phase motor is fed with single phase supply, it produces alternating flux in the stator winding.  The alternating current flowing through stator winding causes induced current in the rotor bars (of the squirrel cage rotor ) according to Faraday's law of electromagnetic induction. This induced current in the rotor will also produce alternating flux. Even after both alternating fluxes are set up, the motor fails to start (the reason is explained below). However, if the rotor is given a initial start by external force in either direction, then motor accelerates to its final speed and keeps running with its rated speed.

Methods for Making Single Phase Induction motor as Self Starting Motor

For producing this rotating magnetic field we require two alternating flux, having some phase difference angle between them. When these two fluxes interact with each other they will produce a resultant flux. This resultant flux is rotating in nature and rotates in space in one particular direction only.

Rotating fluxes are cut by rotor conductor and emf is induced in rotor conductor. As rotor conductors are closed current flows in rotor conductor. And When current carrying conductor placed in magnetic field experience the force. And rotor starts rotating.

Once the motor starts running, the additional flux can be removed. The motor will continue to run under the influence of the main flux only. Depending upon the methods for making single phase induction (asynchronous) motor as Self Starting Motor, there are mainly four types of single phase induction motor namely,

1. Split phase induction motor,2. Capacitor start induction motor,3. Capacitor start capacitor run induction motor,4. Shaded pole induction motor.

8. Explain construction and working of capacitor start single phase induction motor.

Constructional details : Stator of Single Phase Induction MotorThe stator of the single phase induction motor has laminated stamping to reduce eddy current losses on its periphery. The slots are provided on its stamping to carry stator or main winding. In order to reduce the hysteresis losses, stamping are made up of silicon steel. Single phase winding is placed in slots. When the stator

8 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

winding is given a single phase ac supply, the magnetic field is produced and the motor rotates at a speed

slightly less than the synchronous speed Ns which is given by

Rotor of Single Phase Induction Motor The rotor is cylindrical in shape and has slots. The slots are not made parallel to each other but are bit skewed as the skewing prevents magnetic locking of stator and rotor . The squirrel cage rotor consists of aluminium, brass or copper bars. These aluminium or copper bars are called rotor conductors and are placed in the slots on the rotor. The rotor conductors are permanently shorted by the copper or aluminium rings called the end rings.

 The working principle and construction of Capacitor start inductor motors and capacitor start capacitor run induction motors are almost the same. We already know that single phase induction motor is not self starting because the magnetic field produced is not rotating type. In order to produce rotating magnetic field there must be some phase difference.

So, in capacitor start inductor motor and capacitor start capacitor run induction motor we are using two winding, the main winding and the starting winding. With starting winding we connect a capacitor so the current flowing in the capacitor i.e Ist leads the applied voltage by some angle, φst.

The running winding is inductive in nature so, the current flowing in running winding lags behind applied voltage by an angle, φm. Now there occur large phase angle differences between these two currents which produce an resultant current, I and this will produce a rotating magnetic field. Since the torque produced by these motors depends upon the phase angle difference, which is almost 90°. So, these motors produce very high starting torque.

In case of capacitor start induction motor, the centrifugal switch is provided so as to disconnect the starting winding when the motor attains a speed up to 75 to 80% of the synchronous speed but in case of capacitor start capacitors run induction motor there is no centrifugal switch so, the capacitor remains in the circuit and helps to improve the power factor and the running conditions of single phase induction motor.

Applications:These motors have high starting torque hence they are used in conveyors, grinder, air conditioners etc. They are available up to 6 KW.

9. Give the applications of following motors. a)DC motor b)1 ph I.M. c) 3ph I.M.a) Applications of DC motors

9 Mrs. V. S. PatkiElectronics department, WIT,Solapur.

Types of motors

characteristics Applications

Shunt motor 4. Almost constant speed5. Adjustable speed6. Starting torque is medium

For driving constant speed line shafting, lathes, centrifugal pumps, reciprocating pumps ,. Machine tools.

Series motor 4. Variable speed5. Adjustable varying speed6. Starting torque is very high

For traction ( Electric trains locomotives)Lift, crane, hoists, conveyor belts, trolleys car.

Cumulative motor

4. variable speed5. adjustable variable speed6. higher starting torque

Punch, shears, rolling mills, planning machines in workshop, elevators.

b) Applications of single phase induction motor.Applications of Split phase induction motors have low starting current and moderate starting torque. So these motors are used in fans, blowers, centrifugal pumps, washing machine grinder etc. These motors are available in the size ranging from 1 / 20 to 1 / 2 KW.

Applications of Capacitor Start IM and Capacitor Start Capacitor Run IMThese motors have high starting torque hence they are used in conveyors, grinder, air conditioners etc. They are available up to 6 KW.

Applications of Shaded pole motors induction motor are- Due to their low starting torques and reasonable cost these motors are mostly employed in small instruments, hair dryers, toys, record players, small fans, electric clocks etc. These motors are usually available in a range of 1/300 to 1/20 KW.

c) Application of three phase induction motor.

Squirrel cage induction motor : Squirrel cage induction motors are simple and rugged in construction, are relatively cheap and require little maintenance.  Squirrel cage motors operate at nearly constant speed, high over load capacity, low starting torque and no speed control drives.and operates at better power factor. Hence, squirrel cage induction motors are preferred in most of the industrial applications such as in

1. Lathes2. Drilling machines 3. Agricultural and industrial pumps4. Industrial drives.

Slip ring induction motors :

Slip ring induction motors when compared to squirrel cage motors have high starting torque, smooth acceleration under heavy loads, adjustable speed and good running characteristics. They are used in

1. Lifts2. Cranes3. Conveyors , etc.,

10 Mrs. V. S. PatkiElectronics department, WIT,Solapur.