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Old 04-30-2009, 01:44 PM   #1099 (permalink)
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I spoke to the EV Tech list about the mechanical chopper, and I guess it's been around for quite a long time. Here's Lee Hart's comments:

This idea is the original PWM "chopper" of Tom Edison's time. Before
transistors, before SCRs, even before vacuum tubes there were still

Yes, you can build a PWM controller with mechanical switches. But this
crude version implies that the builder has no idea how a PWM works, and
never studied the old ones that really *did* work every day in the 1900s.

The fundamental problem is arcing. You can't just switch an inductive
load on and off; the inductance insists that the current *must* flow
somewhere, so you get hideous amounts of arcing that will rapidly
destroy the switch.

Nowdays, we put a freewheel diode across the motor to carry the current
while the switch is off. This at least keeps the switch voltage from
rising past the battery pack voltage. This circuit was widely used well
into the 1990's for things like tape players to control the motor speed.
The switch was operated by a flywheel governor to adjust PWM based on
RPM and thus to efficiently regulate motor speed with no electronics.

In the old days before diodes, they developed other techniques. They
used a second switch in place of the freewheel diode. To deal with the
crossover time when both switches might be on or off, they added
inductors and capacitors to form a resonant circuit.

A typical circuit would have an inductor and two switches in series with
the battery. The motor had another inductor in series (or its field
winding if a series motor), and was connected across the lower switch.
Capacitors were connected to resonate with the inductors at the desired
PWM frequency.

The two switches were operated by various mechanical contraptions (lots
of mechanical ingenuity went into them). The switches were arranged so
you could control the percent on-time of each. Sometimes it looked like
a commutator being spun by a motor; and sometimes it was more like a big
relay wired like a buzzer (called a vibrator).

Today, we call these circuits resonant or quasi-resonant mode ZVS or ZCS
kits and boards
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