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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.
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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.
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