الثلاثاء، 14 يونيو 2011

Single Phase A.C. Series Motor

1. Single Phase Commutator Motors
       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.

1.1 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 modification 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 reactnaces. Field reactnace can be decreased by reducing the number of turns. But this reduces the field flux. But this reduction in flux (N α 1/Φ), increases the speed and reduce 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 produce 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.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 short circuited on its self as shown in the Fig. 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.
Fig. 1
       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 motors is as shown in the Fig.2.
Fig . 2  Speed-torque characteristic of a.c. and d.c. series motor
Applications : Because of high starting torque it is used in electric traction, hoists, locomotives etc.

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