Special Features of Thyristor Drive Motors:

The most commonly used dc motors for thyristor drive are separately excited and series excited dc motors. The thyristor drive motors usually differ from conventional dc motors in construction. Before discussing the special features of thyristor drive motors, discussion on the effects of thyristor power supply on performance of a dc motor is necessary.

Effects of Thyristor Power Supply on the Performance of a DC Motor

1. The output voltage from thyristor converter consists of a dc component and ac harmonic components.
2. The output voltage can change very rapidly in com­parison to that of a motor-generator set owing to the absence of field time constants associated with the generator.
3. An abnormally high value of armature current may rise in the event of thyristor fault when operating in the inverter mode.
4. The magnitude of harmonics decreases with the increase in frequency of harmonic for all converters. The magnitude of the harmonics increase as the firing angle is increased and dc voltage is reduced for all bidirectional converters. The higher the armature inductance, the lesser will be the harmonic currents.
5. Torque is developed by the dc component of the current whereas heating is developed by the effective (or rms) value of current. The form factor (ratio of effective value to average value) for half-wave three-phase thyristors may be taken as 1.2 while for full-wave three-phase thyristors it is 1.1. This increases electrical losses and, there­fore, heating is 5 to 7 per cent more for three-phase full-bridge converters while for three-phase half-bridge converters it is from 15 to 20 per cent.
6. The commutating ability is seriously affected by the presence of harmonic currents. The peak value of the current is increased, interpole flux will be reduced in magnitude and a time lag will he introduced between the interpole current and the flux due to eddy currents generated in the iron path of the inter polar flux.

The other effects of thyristor power supply on motor performance are heating of interpole winding, saturation of interpole magnetic circuit, transformer voltage at the brushes and increase in voltage commutator segment.

Special Features of Thyristor Drive Motors

The thyristor drive dc motors are designed with the following special features in order to improve their performance.

1. The thyristor drive dc motors are made with larger diameter armature and larger size poles of reduced height.
2. The commutators are made larger in order to provide extra insulation to withstand larger and rapid voltage fluctuations.
3. The yoke as well as the main and commutating poles are laminated to reduce the eddy current effects.
4. Loss inertia armatures are employed for improving the response.
5. Compensating windings are used in large motors to reduce armature reaction effect.
6. Split brushes of good commutating quality are used for reducing the effect of transformer voltage in the coils undergoing commutation.
7. The use of a laminated yoke instead of a solid yoke improves the commutation to a greater extent.
8. The use of a large number of commutator bars reduces the voltage between commutator segments and improves the commutation.
9. The use of an octagonal, rather than circular, shape for the frame accommodates more material and gives a larger rating for the same frame size.
10. The current densities used for the armature and interpole winding are reduced compared to conventional dc motors of the same frame size and rating in order to reduce the effect of heating armature and interpoles.
11. A better class of insulation (class F materials as standard insulation) is used to allow higher temperature rise and dissipation of more losses from a given frame.
12. The ratio pole arc/pole pitch is reduced in order to reduce the ratio of commutating zone to neutral zone.
13. Armature inductance is increased to reduce the ripple current. It may also increase reactance voltage. The number of turns per armature coil is kept at the minimum since the reactance voltage is proportional to the square of the turns per coil. Use of dummy coils is avoided.
14. Forced cooling by an auxiliary ac motor is widely used for improving cooling of the motor at reduced speeds.
15. Great care is required to be taken for accurate con­struction: spacing of brushes, poles and manufacture of commutators.