An ordinary dc series motor, when connected to a single-phase ac supply, operates, but not very satisfactorily. The reasons for this are as follows:
- The torque developed is not of constant magnitude, as in dc series motors, but pulsates between zero and maximum values each half cycle because the field and rotor currents both reverse every half cycle.
- The alternating field flux would induce eddy currents in the solid parts of the field structure, such as the yoke and cores, causing excessive heating and lowering of efficiency.
- There is no inductive coupling between the field(stator) and armature(rotor) winding, as they are placed in space quadrature. As a result, both the windings act as open-circuited primaries of the Transformer, and therefore each winding has large reactance. This results in a small current and abnormal voltage drop and so reduces the output and power factor to such low values as to make the motor impractical.
- When a series motor operates on a dc supply, it develops rotational emf in conductors placed along the brush or q-axis(along AB) due to the armature reaction flux. Reactance voltage i.e. self-induced emf is developed in these conductors due to the reversal of the current in them on crossing the brush. But in the case of an operation, in addition to rotational emf and reactance voltage, transformer emf is also developed which will be maximum for conductors A and B because the armature coil with the greatest emf is the one whose magnetic axis is parallel to the main field. The voltage developed by the transformer action is greatest between segments at the commutator where the brushes are situated called neutral points. This would cause destructive sparking.
- Because of the larger voltage drop in the machine due to its high inductance the speed for ac operation is much less than that for dc operation for a given torque.
- Starting torque is also poor due to the low power factor at starting.
Modification and Improvement in the design of AC series motor
There are several modifications necessary in dc series motor to operate satisfactorily on ac.
- In order to reduce the eddy current losses, the whole magnetic circuit is laminated, the field magnet system as well as the armature core.
- In order to reduce the reactance of the series field,ac series motors are built with as few turns as possible. Fewer filed turns per pole mean lower flux per pole which improves the commutation by reducing the transformer emf in the commutation coils.
- Reduction in the number of turns on the field winding would also reduce the flux for a given current and would tend to increase the speed and reduce the torque available for a given current. Hence to develop the required load torque the number of armature conductors would have to be increased proportionately.
- The operating voltage is kept low in order to reduce the inductance.
- Since inductive reactance is directly proportional to frequency,ac series motor operating characteristics are better at low frequencies.
- Interpoles are provided in ac series motors to eliminate the armature-resistance leads, which occupy valuable space and lower the rating of the motor by their heating.
Reduction in the number of turns on the field winding would also reduce the flux for a given current and would tend to increase the speed and reduce the torque available for a given current. Hence to overcome this problem there is the use of compensating winding i.e.
- Conductively compensated series motor
- Inductively compensated series motor
Conductively compensated series motor
Excessive sparking and poor power factor are prevented by providing compensating winding.
The field flux is produced along the axis CD, and the armature flux is produced along the axis AB. Thus, the compensated winding must produce a flux along AB and in a direction opposite to that of the armature flux at every instant. The compensating winding is placed in the stator slots,and its axis is kept at 90 electrical degrees to the main field axis.
The number of turns per pole in the compensating winding is so chosen that the mmf produced by it is equal to and opposite to that of the armature. The compensating winding carries current equal to that flowing in the armature but in the opposite direction due to a conductive effect called a conductively compensated series motor.
Inductively compensated series motor
Another method is to short circuit the compensated winding,which behaves like the short-circuited secondary of a transformer, of which the primary is the armature winding. So when the compensated winding is excited inductively, the motor with a short-circuited compensating winding is known as an inductively compensated series motor. The current in the compensating winding will be almost directly proportional to the armature current and practically 180 degrees out of phase with it.
All the ac series motors are provided with commutating poles for improving commutation but commutating poles alone don’t produce satisfactory commutation.
The speed emf and the transformer emf in the commutated coils add in quadrature to give a resultant emf. By shunting the interpoles with a non-inductive shunt resistor, the phase of the interpole flux can be made to neutralize the resultant.