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Vijay Balu Raskar – Electrical Maintenance – Polyphase Circuit Junior Science College ( Std – XI ) - Syllabus

Suggestions or difficulties are most welcome at [email protected]

Theory “ 1ph Motors” Its basically used in home appliances, shops, offices etc. It is true that 1ph motors are less efficient than 3ph motors. 3ph power is normally not available except in large commercial and industrial use. 1phase motors are less efficient than 3phase motors. 1phase motors are not self starting. Alternating magnetic flux cannot move the rotor because of no rotational torque. Single

phase motors produced only alternating flux which is not rotating type. The field polarity reverses after each half cycle but the field does not rotate. Consequently, alternating flux cannot produce rotation in a stationary squirrel-cage rotor. However, if the rotor of a single phase motor is rotated in one direction by some

mechanical means, it will continue to run in the direction of rotation. The rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though 1ph current is

flowing through the stator winding. Types of single phase motors:-

Single phase motors are generally built in the fractional horsepower range. As a 1ph I.M. has no inherent starting torque and therefore special means must be used to make it self starting.

There are 4 types / methods:- Single phase Induction Motor

a) Split Phase Type b) Capacitor Type c) Shaded pole type

A.C. Series motor or Universal motor Repulsion Motors

I. Repulsion start induction run motor II. Repulsion Induction motor

Synchronous Motors A. Reluctance motor B. Hysteresis motor

Single phase Induction motor:- 1phase motors works only on 1phase power supply. 1ph motor is similar to a 3phase squirrel cage induction motor except that the stator has

distributed 1ph winding. Basic Construction of 1phase I.M. :- Rotor:- A rotating cylindrical part is called as Rotor. Stator:- A stationary part is called as Stator.

→ The single phase stator winding produces a magnetic field that pulsates in strength in a

sinusoidal manner.



→ The field polarity reverses after each half cycle but the field does not rotate. → Consequently, the alternating flux cannot produce rotation in a stationary squirrel cage

rotor. → If the rotor of 1phase motor rotates by applying some mechanical force then rotor rotates

and quickly accelerates upto below the synchronous speed. → When it reached synchronous speed then motor runs continuously after receiving 1phase. → Hence, 1phase motor is not self starting. → To make a single phase motor self-starting then we have to produce a revolving stator

magnetic field. → This can be achieved by converting a single phase supply into two phase supply through

additional winding. → After getting sufficient speed, this additional winding removed depending upon type of

motor. → Single phase motor can be classified as,

1) Split phase motors – started by starting winding/ auxiliary winding 2) Capacitor motors – started by starting winding and capacitor. 3) Shaded pole motors – motion of magnetic field produced by means of a shading coil

around a portion of the structure.

1) Split phase Induction motors:-

Introduction:- The stator of a Split – phase induction motor is provided with an auxiliary or starting

winding S in addition to the main or running winding M. The starting winding is located 90 degree electrical from the main winding. This main winding operates during the brief period when the motor starts up. Starting winding has high resistance and relatively small reactance. Main winding has low resistance and relatively high reactance. Consequently, there is 25 to 30 degree phase shift between two windings.

Construction of Split Phase Induction Motor:- → Stator – it is made up of silicon steel.



→ Stator having slots in which two windings are kept. → There are two windings called running (Main) winding & Starting winding. → Running Winding is with high reactance & low resistance. → Starting winding is with high resistance & low reactance. → Rotor :- It is a cylindrical structure having bars in the slots. → Rotor is made up of silicon steel and rotor windings are round copper bars embedded in the

rotor slots and permanently short circuited. → Centrifugal switch is connected in series with starting winding. → Starting winding is located 90 degree electrical from the main winding and operates only

during the brief period when the motor starts up. Working of Split Phase Motor:-

→ When an a.c. supply is given to the motor then current splits into two parts say mI and SI

→ mI is flowing through main winding and SI is flowing through starting winding.

→ mI is lags by greater angle due to more inductive & SI is lags by smaller angle due to less

inductive. → Since, main winding is made highly inductive while the starting winding highly resistive, the

currents have reasonable phase angle ( 25 to 30 )

→ Main windings creates flux m & Starting winding creates flux S

→ Both the flux having phase angle difference which set up r.m.f. → Due to r.m.f., rotor starts rotating.

→ Starting Torque = sin... smS IIkT

→ When the rotor speed reaches 70-80% of the normal speed then centrifugal switch is operated.

→ Centrifugal switch disconnected electrically from the circuit. → Motor runs continue till power on. → Hence, it is also called as Resistance Start Induction Run Motor.

D.O.R:- (Direction Of Rotation) DOR can be changed by interchanging the connection of either starting winding or running

winding but not both the winding.

Application a. Small machine tools b. Duplicating machines c. Oil burners

2) Capacitor Motors:- Capacitor motors started by two phase motor action through the use of an auxiliary winding and capacitor. Capacitor Motor :-

1) Capacitor Start Motor 2) Capacitor Start – Capacitor Run Motor 3) Permanent Capacitor Motor



a) Capacitor Start Induction Motor:-

Construction of Capacitor Start Induction Motor:- → Stator – it is made up of silicon steel and stator windings are enameled Cu. → Stator having slots in which two windings are kept. → There are two windings called running (Main) winding & Starting winding. → Running Winding is with high reactance & low resistance. → Starting winding is with high resistance & low reactance. → Rotor :- It is a cylindrical structure having bars in the slots. → Rotor is made up of silicon steel and rotor windings are round copper bars embedded in the rotor slots and permanently short circuited. → Capacitor is connected in series with starting winding. Working of Capacitor Start Induction Motor:-

→ This motor is similar to Split phase induction motor except that the starting winding has as many turns as the main winding.

→ Capacitor (C) is connected in series with the starting winding.

→ Capacitor value will be selected that SI leads mI by about 80 degree.

→ Starting Torque = sin... smS IIkT

→ Starting torque is more than Split phase Induction Motor. → When the rotor speed reaches 70-80% of the normal speed then centrifugal switch is

operated. → Centrifugal switch disconnected electrically from the circuit. → Motor runs continue till power on.

D.O.R:- (Direction Of Rotation) DOR can be changed by making starting winding as running winding and vice versa.

Application:- Capacitor start Induction Motors are used where high torque required.

1) Refrigerator 2) Drilling machine



3) Lathe machine 4) Pumps 5) Printing press 6) Compressor 7) Large fans 8) High-inertia Loads

b) Capacitor Start – Capacitor Run Induction Motor:- → This motor is identical to a capacitor start motor except that starting winding is not opened

after starting. → So that both the windings remain connected to the supply when running as well as at

starting. → There are two designs. Constructions of both the designs are same as Capacitor start but

difference is as follows.

i. Design – 1 (Permanent Capacitor Motor)

A single capacitor C is used for both starting and running winding.

This design eliminates the need of a centrifugal switch and at the same time improves the power factor and efficiency of the motor.

D.O.R. – DOR can be changed by making starting winding as running and vice versa. Application:- Permanent Capacitor Induction Motors are used Where low torque is required Ceiling Fan, Table fan etc

ii. Design – 2 ( Capacitor Start Capacitor Run Induction Motor)



Two capacitor 1C and 2C are used in the starting winding.

1C is very small capacitor and permanently connected in the circuit.

1C helps for optimum running conditions.

2C is very large capacitor connected in parallel with 1C

2C helps for optimum running conditions and remains in the circuit during

starting.

When motor reaches about 75% of synchronous speed then 2C will disconnected

Motor runs continue till power is ON. D.O.R. – DOR can be change by making starting winding as running winding and vice versa.

Application:- Room Cooler, Refrigerator, Compressor etc

c. Shaded Pole Motor:-

→ The shaded pole motor is very popular for ratings below 0.05H.P. (app. 40W). → Because of its simple construction. → It has salient poles on the stator excited by single phase supply and squirrel cage rotor. → A portion of each pole is surrounded by a short circuited turn of copper strip called Shading

Coil and made up of copper (cu coil). → Because of small starting torque developed in the motor and relatively large power loss in

the shading coil. Construction of Shaded Pole Motor:-

Stator:- it is made up of silicon steel stamping having salient poles.



A slot is cut in the pole and a copper.

Coil is placed at one corner.

Copper coil is known as shading coil. This part is of the pole is known as shaded part.

Stator Winding is made up of good quality enameled copper and wound on poles.

Rotor: it is round cylindrical part made up of silicon steel stamping having slots for rotor bars.

Rotor winding is made up of round copper bars which are placed in rotor slots and are permanently short circuited (Squirrel Cage).

Working of Shaded Pole Motors:- When the supply is switched ON then current flowing in the coil produces alternating flux

in the poles. Voltage is induced in each of the shading coils which act as short circuited transformer

secondary windings.

(A) (B) (C)

According to transformer action emf is induced in the shading coil and heavy current circulate it (according to Lenz’s law) which opposes the magnetic flux. Hence, Magnetic Axis (M.A.) shifts towards the unshaded part.

Further current in the shading coil decreases, hence opposition to the MA decreases. Hence, MA shifts towards the center of the pole.

The polarity of the induced voltage is such as to produce m.m.f. that opposes the change in the flux linkage of the coil.

Thus when the flux in the pole is increases then shading coil current produces an m.m.f. opposing the increases.

When the pole flux increases, Flux density of the unshaded portion of the pole is greater than that of the shaded portion.

When the pole flux decreases, Flux density of the unshaded portion of the pole is less than that of the shaded portion.

Thus, the effect of the shading coil is to produce a shift in the flux from the unshaded portion to shaded portion.

This shift in flux may be considered to be a partially rotating field and is sufficient to produce a small starting torque.

D.O.R.:- it is not possible to change the DOR of these motor as the position of the copper rings are fixed. To change DOR position of either shading design should changed in the construction. Application:- This type of motor is built in small size. Small fans, Toys, Hair Dryers etc



3) A.C. Series Motors or Universal Motors:-

→ Universal Motors are generally small capacity fractional H.P.Motors. → Mechanical losses are more and having low efficiency. → A d.c. series motor will rotate in the same direction regardless of the polarity of the supply. → Motor can be operated either on a.c. or d.c. supply & the resulting torque-speed curve is

about the same in each case. Hence, its called Universal Motors. Construction of Universal Motors:-

→ Part of Universal Motors are – 1. Armature 2. Field Poles 3. Commutator 4. Carbon Brushes

→ AC Series motors are similar in construction as d.c. series motors. → The field winding is in series with the armature winding. → Less number of turns on field winding and more numbers of turns on Armature winding.

Working of Universal Motors:-

when the motor is connected to an a.c. supply, the same alternating current flows through the field and armature windings.

The field winding produces an alternating flux that reacts with the current flowing in the armature to produce a torque.

Both armature current and flux reverses simultaneously

The torque is always acts in the same direction.

No rotating torque is produced in this type of machines.

The principle of operation is the same as that of a d.c. series motors.

As per theory of current carrying conductor in the magnetic field a force is excerted in the conductor and its direction is given by Flemings left hand rule.

D.O.R.:- DOR can be changed by interchanging the windings. Application of universal Motors:- The fractional horsepower a.c. series motors have high-speed and large starting torque.

1. Drills, Mixer 2. Machine tools



3. Electric shavers 4. Sewing machines 5. High-speed vacuum cleaner

Some changes must be in a d.c. motor that is to operate satisfactorily on a.c. supply. The changes effected are:-

The entire magnetic circuit is laminated in order to reduce the eddy current loss.

Hence, a.c. series motor requires a more expensive construction than d.c. series motors.

Few turns in field winding to reduce the reactance of the field winding to minimum which reduces the voltage drops across field winding.

There is sparking between brushes and commutator when a.c. supply given.

Alternating flux establishes high currents in the coils short-circuited by brushes. When short-circuited coils break contact high-resistance leads to connect the coils to the commutator segments.

A high field flux is obtained by using a low-reluctance magnetic circuit.

4) Repulsion Motor:-

→ A repulsion motor is similar to an a.c. series motor except that – 1. Brushes are not connected to supply but short circuited. Currents 2. The field structure has non-salient pole construction. → By adjusting the position of short circuited brushes on the commutator, the starting torque

can be developed in the motor. → The repulsion motor produces a high starting torque entirely due to repulsion motor action. → When running, it functions through a combination of induction-motor and repulsion motor

action. Construction of Repulsion Motor:- Stator:- it is made up of silicon steel stamping having salient poles. Stator Winding is made up of good quality enameled copper and wound on stator poles. Rotor :- It is a cylindrical structure having bars in the slots. Rotor is made up of silicon steel and rotor windings are round copper bars embedded in the



rotor slots the end of which are connected to the carbon brush and shorted by a jumper.

Commutator consists of alternate layers of copper and mica segments. Carbon or graphite brushes are used.

Working of Repulsion Motor:- → When supply given to stator winding then field is produced. → Considered N-pole at top and S-pole at bottom, Magnetic axis is vertical. → Brush axis is at an angle alpha between two axis as shown in figure. → Alpha angle between two brush axis. → The like poles of rotor and stator repel each other then motor start rotating. → Due to this repulsion force motor start rotates hence called as repulsion motor.

D.O.R.:- DOR can change by shifting the brushes in reversed direction. Application:- Refrigerator, compressor, mixing machine, floor polishing machine etc.

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