Optimum Motor Selection
The higher peak currents required by the half wave drive may be obtained in three different ways.

One method to obtain higher peak input current is to derate the motor rating or use a higher nameplate horsepower for a given required shaft output horsepower. This primarily gains the advantage of lower armature resistance, all other parameters being equal. The lower armature resistance allows higher peak currents to flow for a given input voltage and output RPM. In practice, using a 33% to 50% increase in nameplate rating is generally sufficient.
Optimum Motor Selection

A second method of increasing peak input current is to raise the input line voltage above nominal values. When the output horsepower versus line voltage is compared to a standard full wave voltage rating, it can be seen that even modest increases in line voltage sharply raise peak input currents. The largest component of the increase is the larger voltage difference between line voltage and back EMF. A second effect is the moderate increase in thyristor conduction angle. The boost in input voltage may be easily obtained with a small autotransformer (see PowrUps catalog pages 28 and 29).

The third way to increase peak currents is to use motors with a lower Kv factor. This implies a motor with a lower back EMF for a given base speed. For example, if a full wave configuration requires a 90VDC nominal armature voltage, a nominal armature voltage of 70VDC would provide equally satisfactory performance with a half wave drive. Lowering the rated armature voltage essentially accomplishes the same effect as raising the line voltage.

Since armature windings are readily modified for specific applications with minor cost impact, the third solution is generally the most satisfactory for larger production runs. The idea of using armature voltage ratings 70VDC and 140VDC for half wave drives as compared to using 90VDC and 180VDC for full wave drives is simply an expression of optimum impedance matching for a given supply voltage and waveform.

Half wave thyristor DC motor drives are cost effective and dependable devices for bidirectional and four quadrant control of DC brush motors. Because their instantaneous armature currents are higher than those produced by corresponding full wave drives, use of half wave drives with motors optimized for full wave applications can occasionally produce unsatisfactory results.

Four-Quadrant Analysis Half Wave vs. Full Wave