Half Wave vs. Full Wave
Half wave thyristor DC motor drives are used for bidirectional and four quadrant control of DC brush motors. This type of drive performs best in applications requiring rapid or frequent speed and direction changes. Typical applications include positioning, bidirectional actuators, start-stop material feeders or power servo type applications.

For a typical 90VDC rated PM motor rated at 1HP output, at 85% efficiency, armature resistance is 1.8 . With a 115VAC input and the theoretical maximum conduction angle, a peak output of 1.62HP can be obtained (disregarding demagnetization consequences) with a full wave drive. The average armature current consists of two current pulses per line cycle.

For this same specific motor operated from a 115VAC source driven by a half wave drive, the maximum output at nominal base speed will only be 0.81HP. Why? The maximum power that can be applied to the motor is a function of line voltage and armature resistance. For a half wave circuit, power is applied to the motor only once each half cycle.

It can be seen that the lower duty cycle produced by the half wave drive requires higher peak currents to produce the same output horsepower as an equivalent full wave drive. This phenomenon may be quantitatively expressed as a "form factor", or waveform dependent term, relating the RMS voltage and current to average equivalents. A higher than unity form factor is a relative disadvantage. However, when analyzed in the total context of overall system costs, performance, and reliability, the overall benefits of the half wave drive generally outweigh the higher form factor cost.

Four-Quadrant Analysis   Optimum Motor Selection