Key Facts about Thyristor DC Motor Drives Operation

For motors up to a few kilowatts the armature converter draws power from either a single phase or 3-phase utility supply. For large motors, 3-phase power supply is preferred because the waveforms are much smoother although traction uses single-phase with a series inductor to smooth the current. A separate thyristor or diode rectifier is used to supply the field of the motor: the power is much lower than armature power, the inductance is much higher and so the supply is often single-phase as shown in the diagram below:

The above figure shows a typical closed-loop dc motor drive speed control. The main power circuit consists of a six-thyristor bridge circuit which rectifies the incoming ac supply to produce a dc supply to the motor armature. By altering the firing angle of the thyristors the mean value of the rectified voltage can be varied thereby allowing the motor speed to be controlled.

Note that, the controlled rectifier produces a crude form of dc with a pronounced ripple in the output voltage. This ripple component gives rise to pulsating currents and fluxes in the motor, and to prevent excessive eddy-current losses and commutative problems, the poles and frame should be of laminated construction.

Motors supplied with thyristor drives usually have laminated construction, also dc motors for variable speed operation are normally supplied with an attached ‘’blower’’ motor as a standard. This provides continuous through ventilation and allows the motor to operate continuously at full torque without overheating even down to the lowest speeds.

Low-power control circuits are used to monitor the principle variables of interest (usually motor current and speed), and to generate appropriate firing pulses so that the motor maintains constant speed despite variations in the load. The speed reference, historically an analogue voltage varying from 0 to 10 V obtained from a manual setting potentiometer or from elsewhere in the plant, or in their more typical current digital forms.

The combination of power, control and protective circuits constitutes the converter. Standard modular converters are usually available as off-the-shelf items in sizes from 100 W up to several kW, while larger drives are tailored to individual requirements. Individual converters may be mounted in enclosures with isolators, fuses, etc. or groups of converters may be mounted together to form a multi-motor drive.