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
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.