DIYbanter - View Single Post - How does a generator and a fan motor 'pulser' control a dc motor?

wrote:

These paragraphs are from an article about a homebuilt hybrid car in
Mother Earth News.

"Any project fresh off the drawing board has its share of problems, and

the Opel hybrid was no exception. When David pressed the accelerator
for the first time, he got a 300-amp surge which melted his relays. So
he searched his graduate texts for the answer ... and finally found it
in-of all places-an old high school physics book: A pulser was
necessary to "chop" the current flow and prevent a heavy initial draw
to the drive motor.

As Dave explains it, "The motor will always have full voltage and full
current, but the pulser makes it 'think' the voltage and amperage are
cut down to about 1/4 of what's actually available. With this
gadget-which is simply a combination of a reworked car generator and an

old fan motor-I can keep the draw within limits and effectively control

the car's acceleration . . . without sacrificing the maximum current or

voltage that's necessary for high-speed driving. I could have achieved
the same results with a commercially available FCR control ... but one
of those units would have cut my power slightly, and cost in the
neighborhood of $800! I can build my own device for about $25, and I
can fix it myself if it breaks!"

This is an EXTREMELY PRIMITIVE PWM motor control scheme. It uses
the commutator out of a starter motor (spun by a fan motor) to interrupt the
current to the traction motor. Depending on how many brushes are connected,
he can get 25, 50 or 75% on-time. I built an electric VolksWagen many years
ago. I used series resistance starting for the jet engine starter
motor, and then
controlled it with field control and the usual clutch and gears. With
the field
at very high strength and 48 V on the armature, it made a HUGE whine.
But, it
was amazingly powerful. Even though my wiring was way too light (AWG #2
or something like that) it would leap up steep hills, seeming to be a
lot more
responsive than the old VW engine.

Jon