One of the drawbacks of a converter-fed d.c. drive is that the supply power factor is very low when the motor is operating at high torque (i.e. high current) and low speed (i.e. low armature voltage), and is less than unity even at base speed and full load. This is because the supply current waveform lags the supply voltage waveform by the delay angle a,as shown (for a 3-phase converter) in Figure 4.9, and also the supply current is approximately rectangular (rather than sinusoidal).
Figure 4.9 Supply voltage and current waveforms for single-phase converter-fed d.c. motor drive.
It is important to emphasise that the supply power factor is always lagging, even when the converter is inverting. There is no way of avoiding the low power factor, so users of large drives need to be prepared to augment their existing power factor correcting equipment if necessary.
The harmonics in the mains current waveform can give rise to a variety of interference problems, and supply authorities generally impose statutory limits. For large drives (say hundreds of kilowatts), filters may have to be provided to prevent these limits from being exceeded.
Since the supply impedance is never zero, there is also inevitably some distortion of the mains voltage waveform, as shown in Figure 4.10 which indicates the effect of a 6-pulse converter on the supply line-to-line voltage waveform. The spikes and notches arise because the mains is momentarily short-circuited each time the current commutates from one thyristor to the next, i.e. during the overlap period discussed earlier.
Figure 4.10 Distortion of line voltage waveform caused by overlap in three-phase fully-controlled converter.(The width of the notches has been exaggerated for the sake of clarity.)
For the majority of small and medium drives, connected to stiff industrial supplies, these notches are too small to be noticed (they are greatly exaggerated for the sake of clarity in Figure 4.10); but they can pose a serious interference problem for other consumers when a large drive is connected to a weak supply.
Coming up in Part 3: Control arrangements for DC drives.
Printed with permission from Newnes, a division of Elsevier. Copyright 2005. "Electric Motors and Drives" by Austin Hughes. For more information about this title and other similar books, please visit www.elsevierdirect.com.
Related links:
Basics of the Electric Servomotor and Drive - Part 1: Basic Magnetics and Motor Control Overview | Part 2: Permanent-Magnet Brush Motors | Part 3: Brushless PM Motors
Tutorial: Motion-control architectures adapt to technology changes
Implementing Embedded Speed Control for Brushless DC Motors: Part 1 | Part 2 | Part 3
Open-Source Robotics and Process Control Circuit Examples - Part 3: Speed-controlled DC motor
