Performance of DC Motors Operating on Phase Controlled Converters:
The nature of the output voltage and output current of a phase controlled converters impains the performance of the dc motor operating on these converters. The output of the converter is in the form of pulses. The average voltage is superimposed by an ac ripple which results in a deterioration of the operation. The motor current also has ripple content. The torque produced is given by the average current whereas the rms current influences the copper loss and heating. The rms to average value of the motor current is greater than one. So, for a given armature heating the torque capability of the motor decreases. The ratio of peak value to average value of the current is also greater than unity. The peak value affects the commutating capability of the motor. The ripple content also affects the commutation of the motor. The motor current is not always continuous; the ripple amplitude makes it discontinuous. Under this condition both load voltage and load current are zero. There is loss of torque and the ‘performance deteriorates because of poor speed regulation. Discontinuous conduction also affects the commutation. The performance improvement can be obtained by a proper choice of the converter, e.g., using a converter with increased pulses. Sometimes the motor design can be altered.
Discontinuous Armature Current and its Effects on the Performance of the Motor
When a dc motor is fed from a phase controlled converters the current in the armature may flow in discrete pulses or it may flow continuously with an average value superimposed on by a ripple. The former is called discontinuous conduction and the latter is called continuous conduction. In the case of discontinuous conduction the load voltage is back emf and load current is zero. The converter output voltage (average value) is also more than what would occur if the conduction were continuous for the same firing angle. The control characteristics are valid only for continuous conduction. The voltage waveform must be known to determine the average dc voltage when there is discontinuous conduction. The discontinuous conduction has the following effects on the motor performance:
1.The motor has a large speed drop as it is loaded. Speed falls very fast and speed regulation is very poor. As the current flows in pulses there is a loss of The determination of the speed-torque characteristic is involved. For impact loads there is a substantial drop in the speed. An additional inductance in the load circuit improves the performance.
2.The range of speed control is limited and speed oscillations are present.
3.The ratios of peak to average and rms to average currents become more. The former deteriorates the commutating capability whereas the latter results in increased heating of the motor. For rated armature current to develop rated torque, the motor gets overheated; for normal heating there is loss of torque.
4.It affects the commutation by increasing bar to bar voltage of the commutator.
5.The dynamic response of the motor is very poor.
6.A sufficient value of inductance in the load circuit improves the performance of the motor.