Single Phase Commutator Motors

Single Phase Commutator Motors AU : May-03, 04, 08, 13, 16,17,18, Dec.-06, 10, 11, 13, 16

The commutator is a feature of d.c.motors. But a.c. motors having wound rotor with brushes and commutator arrangements, are called commutator motors which work on single phase a.c. supply. The commutator arrangement present in these motors is similar to the armature of a d.c. motor. In this section we will discuss two types of single phase commutator motors namely a.c. series motors and the universal motors which are widely used in practice.

1. Single Phase A.C. Series Motor

In a normal d.c. motor if direction of both field and armature current is reversed, the direction of torque remains unchanged. So when normal d.c.series motor is connected to an a.c. supply, both field and armature current get reversed and unidirectional torque gets produced in the motor hence motor can work on a.c. supply.

But performance of such motor is not satisfactory due to the following reasons :

i) There are tremendous eddy current losses in the yoke and field cores, which causes overheating.

ii) Armature and field winding offer high reactance to a.c. due to which operating power factor is very low. 

iii) The sparking at brushes is a major problem because of high voltage and current induced in the short circuited armature coils during the commutation period.

Some modifications are required to have the satisfactory performance of d.c.series motor on a.c. supply, when it is called a.c. series motor. The modifications are :

i) To reduce the eddy current losses, yoke and pole core construction is laminated.

ii) The power factor can be improved by reducing the magnitudes of field and armature reactances. Field reactance can be decreased by reducing the number of turns. But this reduces the field flux. But this reduction in flux (N ∝ 1/ ϕ) increases the speed and reducing the torque. To keep the torque same it is necessary to increase the armature turns proportionately. This increases the armature inductance.

Now to compensate for increased armature flux which produces severe armature reaction, it is necessary to use compensating winding. The flux produced by this winding is opposite to that produced by armature and effectively neutralizes the armature reaction.

If such a compensating winding is connected in series with the armature as shown in the Fig. 9.4.1 (a),

the motor is said to be 'conductively compensated'. For motors to be operated on a.c. and d.c. both, the compensation should be conductive. If compensating winding is shortcircuited on itself as shown in the Fig. 9.4.1 (b),

the motor is said to be 'inductively compensated'. In this compensating winding acts as a secondary of transformer and armature as its primary. The ampere turns produced by compensating winding neutralise the armature ampere turns.

To reduce the induced e.m.f. due to transformer action in the armature coils while commutation period, the following measures are taken : 

i) The flux per pole is reduced and number of poles are increased.

ii) The frequency of supply used is reduced.

iii) Preferrably single turn armature coils are used.

The characteristics of such motor are similar to that of d.c. series motor. The torque varies as square of the armature current and speed varies inversely as the armature current. The speed of such motor can be dangerously high on no load condition and hence it is always started with some load. Starting torque produced is high which is 3 to 4 times the full load torque. The speed-torque characteristics of such type of motor is as shown in the Fig. 9.4.2.

Applications : Because of high starting torque it is used in electric traction, hoists, locomotives etc.

2. 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 similar characteristics, called universal motors. The general construction of such motor is similar to that of a.c. series motor as discussed in last article. It is manufactured in two types.

i) Non-compensated, low h.p.

ii) Compensated type, high h.p.

Non-compensated type pole has 2 poles, having entire magnetic path as laminated.

Armature is wound type similar to the normal d.c. motor. Such noncompensated construction is shown in the Fig. 9.4.3.

While in compensated type, the motor has distributed field winding consisting of main field and compensating winding. This is somewhat similar to the stator of split phase single phase induction motor type construction. This also has a wound armature similar to the normal d.c. motor. Fig. 9.4.4 shows the connection diagrams for both the types of universal motor.

Speed-Torque characteristics : The speed - torque characteristics for both the types of universal motor are shown in the Fig. 9.4.5.

Compensated type universal motor has better speed-torque characteristics i.e. the characteristics are same for the operation of motor on a.c. or d.c. supply. The motors are generally designed for full load operating speeds ranging between 3000 to 20000 r.p.m.

Applications : Though compensated type characteristics are better, the non-compensated type are more preferred for low h.p. applications. While compensated type of universal motors are preferred for high h.p. applications. High starting torque is the important feature of universal motors.

The universal motors are used for domestic applications like vacuum cleaners, food processors and mixers, hair driers, coffee grinders, electric shavers etc. Their other applications are blowers, portable tools like drilling machines and small drives.

3. Phasor Diagram of A.C. Series Motor

Consider the equivalent circuit of a.c. series motor as shown in the Fig. 9.4.6.

It is conductively coupled a.c. motor.

When armature current Ia flows through it, there is voltage drop across each winding impedance and induced e.m.f. Eb(a.c.), when a.c. supply is given to it

Thus adding the drops Ia R and Ia X to Eb(a.c.) vectorially, the voltage V can be obtained. As Eb(a.c.) is produced due to current Ia, it is proportional to Ia and in phase with it. Thus the phasor diagram with Ia as reference is shown in the Fig. 9.4.7. The angle between V and Ia is and cos is the power factor of the motor.

Example 9.4.1 A universal motor has a resistance of 30 ohm and an inductance of 0.5 H. When connected to a 250 volt d.c. supply and loaded to 0.8 amp, it runs at 2000 r.p.m. Determine the speed, torque and power factor when connected to 250 volt, 50 Hz A.C. supply and loaded to take same amount of current.

Solution :

On d.c. supply,

E_b(d.c.) = V - I_a R_a = 250 - 0.8 × 30 = 226 V

N_dc = 2000 r.p.m.

The inductance does not play any role in d.c. operation. 

On a.c. supply,

The phasor diagram is shown in the Fig. 9.4.8.

From the phasor diagram, consider triangle OCD

(OC)² = (OD) ² + (CD) ² = (OA + AD² + (CD) ²

Review Questions

1. Write a brief note on a.c. series motor.

2. What modifications are necessary in a d.c. series motor so that it may work satisfactorily on a.c. supply? Explain its operation. State its applications.

3. Explain the construction and working of a.c.series motor.