1. INTRODUCTION
A 3-phase induction motor cannot be operated at variable speed.
1-phase induction motor is not self starting.
Thus, A.C. commutator motors have been developed to provide improved starting torque, good power factor and better speed control.
A.C. Commutator Motor
1-Phase Motor Phase Motor
A.C. Series Motor Φ Series Comm. Motor
Universal Motor Φ Shunt Comm. Motor
Repulsion Motor Schrage Motor

2. AC SERIES MOTOR
If A.C. supply is given to a d.c. series motor it can start and run because the torque produced is proportional to the product of the flux and the armature current.
During positive half cycle, the flux and the armature current are in positive direction thus torque is in positive direction.
During negative half cycle, both the flux and the armature current reverse but its product is positive. So, torque remains positive.
This kind of motor is known as A.C. Series motor.

3. PROBLEMS IN A.C SERIES MOTOR
The alternating flux would cause excessive eddy current loss in the yoke and field cores which will become extremely heated.
Vicious sparking will occur at brushes because of the huge voltage and current induced in the short-circuited armature coils during their commutation period.
Power factor is low because of high inductance of field and armature circuits.
Torque developed is not of constant magnitude, but pulsates between zero and maximum value each half-cycles.

4. MODIFICATION IN DESIGN OF AC SERIES MOTOR
The eddy current loss has been reduced by laminating the entire iron structure of the field cores and yoke by silicon steel.
In order to improve power factor ,reactance of the field winding and the armature winding should be reduced.
Field reactance is reduced by reducing the number of turns on the field windings. For a given current, it will reduce the field m.m.f. which will result in reduced air- gap flux.
This will increase the speed but reduce motor torque. For same torque, the number of armature turns should be increase proportionately.

5. Increasing armature turns will increase inductive reactance of the armature, so the overall reactance of the motor will not be decreased.
Increased armature m.m.f can be neutralized by compensating winding .
On the basis of the compensating winding there are two types of connection :
Conductively compensated motor
Inductively compensated motor

6. Conductively compensated motor:
The compensated winding is connected
in series with the armature.
Inductively compensated motor:
The compensated winding is short-
circuited and has no interconnection
with the motor circuit.
The compensating winding act as a short-circuited secondary ,for which the armature winding acts as a primary.
The current in the compensating winding will be proportional to the armature current and 180º out of phase with it.

7. Unless the transformer voltage is neutralized
In A.C Series motor,the field winding associated with the armature coil undergoing commutation acts as primary and the armature coil during its commutating period acts as a short-circuited secondary.
This transformer action produces heavy current in the armature coil as it passes through its commutating period and results in vicious sparking.
Unless the transformer voltage is neutralized

8. Speed- Torque Characteristics

9. UNIVERSAL MOTOR
There are small capacity series motors which can be operated on d.c. supply or single phase alternating supply of same voltage with same characteristics, called universal motors.
They are of two types.
i) Projected pole - Non compensated type
ii) Distributed field-Compensated type

10. Cross-section of non compensated universal motor
Non-compensated Type: in this the pole and yoke are not separate but there is single lamination for the two.
Series field winding is wound over the poles.
Armature winding is placed in the armature slots.
Ends of armature coils are connected to the risers of the commutator bars.
Cross-section of non compensated universal motor
Non Compensated Universal Motor

11. Compensated type Universal Motor
Compensated Type: There are no projected pole in this type.
Distributed type field winding similar to single phase induction motor is placed in the stator.
Armature winding is placed in the armature slots.
Ends of armature coils are connected to the risers of the commutator bars.
Compensated type Universal Motor

12. Working of Universal Motor
Supply is given
Flux is produced due to field winding carrying current.
Torque is produced on the armature
When the current flows through armature winding
Armature rotates
Direction of torque remains same during the negative half cycle of a.c supply as the direction of both the field & armature current are reversed.

13. Speed –Torque Characteristic

14. Speed Control of Universal Motor
Resistance Method
Tap Field Method
Centrifugal Mechanism
The field coil number of turns are adjusted by tapping method using switch S.
Switch S is controlled by external lever. S open, R is introduced to reduce speed.
The variable resistance connected in series with the motor used in sewing machine
S close, R is shorted increasing speed.The C reduces the sparking.
Used in domestic machines such as mixers.

15. Reversal of Direction of Rotation
By reversing the flow of current through the armature or field windings, the direction of rotation can be reversed for salient pole non compensated type universal motor. This is possible by interchanging the terminals on brush holders .
In case of compensated type, the armature or field loads are interchanged and brushes are shifted against the direction of rotation of motor, to achieve the reversal of direction.

16. Application of Universal Motor
The universal motors are used for domestic applications like
vacuum cleaners
food processor
Mixers
hair driers
coffee grinders
electric shavers etc.
Their other applications are blowers, portable tools like
drilling machines and small drivers.