Thyristor DC Drives,DC motors, fault finding, working, troubleshooting, common faults and solutions.

Speed does not become zero even when potentiometer is made zero....

Some times, too much of gain in the Speed Amplifer makes the motor to rotate very slowly even at zero speed reference signal. The only method to prevent this from happening, if at all it is critical, is to reduce the gain without compromising on performance at higher speed levels. If the drive has a provision to apply a biasing signal of opposite polarity as that of the main speed reference signal, applying a bias may also solve this problem. This may add a small non-linearity in the overall performance.

Unstable or erratic Speed control.

Speed variation can be gradual over time or erratic and unpredictable. If the variation is gradual it could be arising from heating of some component, especially in the feedback circuit. Sudden or erratic variations can be caused by several reasons. Dry solder in reference path, leaky capacitors in Ramp circuit are just two of many reasons.

Motor starts only after reference voltage is increased substantially, but the control is okay once the motor is started...

This is a very peculiar situation. As the drive reference voltage is slowly increased, the motor does not repspond for quite some voltage. The motor starts running at a much higher speed. However, once started, the reference voltage has good control and the speed can be reduced almost till zerp without problem. This is caused by unequal supply voltages to the opertional amplifiers in RAMP circuit. Normally the voltages are +15 and -15 volts. If these voltages are unequal, e.g. +14.3 V and – 16.5 volts, then the situation described above is arises. Given below is a typical arrangement of generating +ve and -ve voltages without using 3 pin voltage regulators. The selection of zener diode in one of the sections may reqire trial and error method to get exact equal voltages at the output of NPN and PNP transistors.