Thyristor DC Drives

DC motors are much easier to control in a variable-speed application. Thyristor-controlled DC drives are capable of controlling high-power applications.

Thyristor Drives used because:

• Eliminates electrical time lag of the field and armature
• Operation is simple and reliable
• Minimal Maintenance
• Operating efficiency above 95%
• Small in size and weight

Disadvantages:

• Higher ripple content of the converter output adds to motor heating and commutation problems. (Might need a reactor in the armature circuit to smooth ripple current)
• The overload capability is relatively low.
• Complex control circuitry needed to achieve regeneration.

Thyristors can be placed in series and parallel for higher voltages and current ratings.

Different control schemes for DC motors:

• Phase Control
• Integral Cycle Control
• Chopper Control (Pulse Width Modulation)

PWM control requires special thyristors because they switch very fast to help eliminate current ripple. Auxiliary circuits are needed to switch the thyristors.

Control system needs to incorporate either digital or analog feedback control. Phase-locked loops are sometimes employed to provide precise speed control with very low speed regulation. Microcontrollers can be employed.

Other ways of implementing variable speed dc drives:
- Resistance control. Essentially a potentiometer. Very inefficient.
- Motor-Generator. Three machines involved. Losses in each machine = inefficient and slow response.

Pulse Width Modulation has efficiency ratings of 95% or better. This is ideal for use in battery-operated vehicles where energy savings is a top consideration when deciding upon a motor controller. PWM can also be used with regenerative braking systems. It is possible that the EV: Snowmobile could incorporate regenerative braking.