Re: Small Curtis as transistor driver?

[XURQ03A@prodigy.com ( LEE A HART)]

Jean-Claude Touzin wrote:
> For my lectric Honda 3wheelers... I have scrounged some more parts in
> addition to the 200A Aircraft generator... I am thinking of a 36V system.

These generators don't need much of a controller. The ultimate in simplicity
is to just switch the armature directly across the battery, and control speed
with field current. This is only a few amps; a rheostat can do it. You get
automatic, full-power regen, too!

The motor starts with a "jerk" -- 0 to its minimum speed (max field current)
in a fraction of a second. You can either use the clutch to start smoothly, or
include a starting resistor. The resistor is connected in series with the
armature to get the motor started, then shorted out once it is running -- all
further speed control is via the field.

But, you can use a PWM controller as well -- either for the armature, or
field, or both.

> are the problems described more or less manageable at lower voltage?

Lower voltages make things easier, because you don't need as many devices to
do the switching, and can afford larger safety margins.

> since lower voltage mosfets are cheap, I was thinking to put in a lot...

Would you replace a 1/2" bolt with sixteen #6 screws? In theory, they would
have the same strength. But in practice, it's a lot more work, and they won't
share the load equally. You're better off using the fewest number of parts,
even if it appears to cost more.

> I can start somehow the above Aircraft thing with 720A of mosfets?

The tiny R/C aircraft controllers use a pile of MOSFETs because -- well,
because I think they are "duct tape" designs. Built by people with little
real-world experience. Also, they are toys; they don't have to work more than
5 minutes at a time before the batteries are dead, and nobody measures
efficiency, EMI, reliability, etc.

> one amp per gate (ouch!)... how do you calculate when you have enough
> amp so that you do not lose too much time in between off to fully on?

The MOSFETs's data sheet will tell you the gate capacitance and amount of
charge needed to turn it on and off. Most gate driver ICs can deliver about 1
amp of peak current. The recommended circuits suggest a small resistor (10
ohms or so) in series with the gate so the chips don't get overloaded and to
suppress ringing due to the fast rise/fall times. Most circuits will have
rise/fall times in the range of 10-100 nanoseconds.

As for calculating what is "too much" time: Calculations will only get you so
far. It is better to measure what actually happens in a real circuit. Then you
have to use your judgement to decide what is "too much". Designers generally
aim to have half the MOSFET's temperature rise due to switching losses, and
the other half due to conduction (resistance) losses, but that is just a rule
of thumb.

- Also since this is a generator, during off duty cycle it will continue to
"generate", so I will have to "kill it"? How is this done?

To prevent generated current from flowing back into the controller, you can
include a diode in series with the armature. This guarantees no regen current.

But... MOSFETs have a parasitic (inevitable) diode between source and drain.
If the armature voltage exceeds the battery voltage, these MOSFET diodes
conduct, and regen current flows into the batteries. This is OK if the MOSFET
diodes are big enough to handle it. But remember there is no current limit in
this mode (except by what you are doing to the field current).

If you try to drive the armature with a PWM controller, you'll need to add
some series inductance. Otherwise, the peak currents will be too large. The
armature has too little inductance by itself to adequately limit current rate-of-change.

> maybe that I will try to control the fan heater motor of my car
> as a first project after all!

Good idea. You'll know you're ready when nothing gets hot and you can't blow
up your controller no matter what you do.

> do some of you have a ready made diagram for a clean triangle wave at
> 15-20KHz? Just asking before going to the design board :-)

The lowly 555 timer is an easy way to get started. Or, any switchmode power
supply controller chip.

Lee Hart                     If you would not be forgotten
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Robbinsdale, MN 55422 USA    Either write things worth the reading
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