A circuit that converts AC to DC or from DC to AC or acts as AC frequency changer. Modern electric drive systems, in which speed control of the electric motor is required, use power supply converters. These converters adapt the voltage and frequency of the power supply to the electric motor as required for the desired motor speed. The basic components of the converters are diodes, transistors and thyristors.

Cyclo-converter – A cyclo-converter is a single-stage (AC-AC) converter and converts AC with a constant frequency directly to an AC with a varying frequency, as required for the desired motor speed.

Cyclo-converters are used to power and control the speed of synchronous motors. Motor speed is adjusted by changing the frequency of the motor’ s supply and allows full torque over the speed range in either direction. As cyclo-converters produce relatively low frequencies, they are more associated with direct drive low-speed motors.

Pulse Width Modulation (PWM) converter – PWM converter has a dual conversion process (AC-DC-AC) and uses a DC link. PWM converters are used to power and control the speed of asynchronous motors. Pulse width modulation (PWM) uses a rectifier to create DC voltage in the same way as a synchro-converter. On the inverter side it uses forced commutation to give a series of pulses of common voltage both positive and negative. In this way the output voltage can be made to approximate AC, while varying of the pulse number and width can increase or decrease the frequency. In marine propulsion terms they are at the low end of the power band (up to 8MW) and as frequency increases, the output wave becomes increasingly distorted.

Synchro-converter – Synchro-converter is AC-DC-AC converter : it converts three phase AC with a constant voltage and frequency to a DC with a varying voltage and then again to a three phase AC with varying voltage and varying frequency. It can be used only in combination with a synchronous motor. To increase the motor speed, the current is increased, which creates higher magnetic forces and torque. This, in turn, causes the rotor to move faster which commutates the thyristors more quickly, increasing the AC frequency until the required speed is reached.

Synchro-converters can produce frequencies in excess of 100Hz and are suited to high-speed motors.

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