For dc motor control, controlled dc power from a constant voltage ac supply is obtained by means of controlled rectifiers or converters employing thyristors and diodes. The control of dc voltage is achieved by firing the thyristors at an adjustable angle with respect to the applied voltage. This angle is known as the firing angle and the scheme of control is called the phase control. Another basic method of control is known as the integral-cycle control. Here the current is gated to flow from the ac supply for a number of complete cycles and is then quenched for a further few cycles, the process being repeated continuously. Control is applied by adjusting the ratio of on and off durations. This method is suitable for the control of fractional-kW dc motors. Line commutation is easily adopted for both these control schemes.
Phase-control and integral-cycle control methods are also applicable for ac motors wherein the converter circuitry is not required.
Control of dc motors fed from dc supply is achieved by means of a thyristorswitching circuit called the chopper. The chopper controller periodically opens and closes wherein the control of the average voltage is achieved by varying on and off durations. This gives an efficient and stepless control of de motors. The motor can also be made to operate in the regenerative braking mode. The chopper controller requires forced commutation of the thyristor.
For the control of ac motors fed from dc supply, thyristor, transistor or MOSFET based inverters are employed. These switching circuits transfer energy from dc supply to ac load of variable frequency and/or variable voltage. Because of the switching operation, the ac voltage waveforms are stepped, the harmonic contents of which are filtered out by the ac motor. As the power supply is normally ac, the complete scheme for obtaining variable-voltage and frequency power involves the use of both an inverter and a converter.
A cycloconverter is a control unit for obtaining variable-voltage and frequency-power directly from a fixed-frequency supply without the necessity of an intermediate dc stage. The mechanism of voltage and frequency control is a combination of those used in the phase-controlled converter and the pulse-modulated inverter. Cycloconverters, in spite of their attraction of direct ac to ac conversion, suffer from certain disadvantages because of which they have not gained popularity. Some of these disadvantages are: cycloconverters can produce only a sub-frequency output, they produce output with high harmonic content and have a low input power factor. Cycloconverters are used for low-speed drives and for controlling linear motors in high-speed transportation systems.