Re: Permanent Magnet Motor Questions -> for E-Bike

["PETER VANDERWAL" <peterv@peoplepc.com>]

> Perhaps we need to restate our assumptions -snip-
> Something like an automobile fan motor. We want to use them on a small
> EV; something between a bicycle and a golf cart. Cost and simplicity are
> important, so we want to restrict ourselves to inexpensive controller.

Aboslutely, bicycle with cheap (to reasonably priced) controller

> >     To reiterate:
> >         1) Regen braking with a contactor controller will only be
effective
> > at relatively high speeds.
>
> Are you assuming a) fixed gearing, b) fixed battery voltage, and c) no
> series/parallel switching of motor(s) or batteries? If so, then

Yes fixed ratio, I said that in the previous post.  One motor, series
parallel switching of two batteries (at least that's what we were talking
about).

> generated voltage is a function of speed, and you only get a reasonable
> charging voltage at one speed. If everything is set up for motoring,
> then I agree, you will only get regen at higher than normal speeds.
>

Thank you.

> On the other hand, if you can switch your motors and/or batteries in
> series for motoring and parallel for braking, you can get regen braking
> down to half your motoring speed. If *both* batteries and motors can be

Not quite half,  you can get (some) regen to about 2/3 to 3/4 of top
cruising speed.
A real world example was recently posted by Scott Hull:  series/parallel
batteries, 8mph low / 16mph cruising,  Regen down hill = 10-12mph low /
20-25 high.

> >         2) Dynamic braking can burn out the motor unless you have a
constant
> > current load and still won't brake all the way to a stop.
>
> Of course I agree; any uncontrolled voltage or current can burn out
> anything. This just means you need to include appropriate protective
> devices, for *any* form of motoring or generating.
>

Thank you again.  Of course adding all of the protective devices will
increase cost/complexity/weight.

> Braking force is proportional to current, so you can pick a load
> resistor for any desired current at any desired rpm. At low speeds, the

Yes but if you want it to work over a wide range of rpms, then you need to
add additional completexity/cost/weight above that previously mentioned.

> The main problem is that the heat is being generated in the external
> resistor and the motor. The types of motors we are talking about here
> have negligible cooling, so dumping extra heat in the motor is bad.
>

Again, this was one of my original points.

> >         3)  Plug braking can definitely burn out the motor unless you
only
> > use it a low speeds and it still won't brake all the way to a stop.
>
> Again, any excessive unlimited voltage or current is bad. You just need
> to limit it safely. Fuses, circuit breakers, resistor, and diodes are
> not expensive or complex.
>

They aren't free, and if you are depending on the motor to provide braking
and the fuse blows...no braking.  This is why I recommended against removing
the rear brake and depending on the motor for braking.

> Plug braking means running the motor in reverse while it is spinning in
> the forward direction. Since current is torque, reverse current is

My missunderstanding.  I had thought from reading previous postings about
this that shorting the motor was called "plug braking".

> >         4)  A PWM 2 quadrant (or 4 quadrant) controller can brake all
the
> > way to a stop, but you have to be carefull because they buck the voltage
up
> > and current down and at low RPMs the motor current can get VERY high
which
> > could get dangerous to the motor.
>
> Again, I agree. But by the time you get to a PWM 4-quadrant controller,
> it's going to have some form of current limit to protect itself and the
> motor.

Some cheap controllers don't limit current at all and many that limit
motoring current don't limit regen.

> > Possibly, but this would be true when you are using the motor for regen
too.
> > Unless you expect us to put the brushes to a nuetral position and loose
> > efficiency as a motor (where it spends most of it's time).
>
> Brush advance doesn't have much effect on efficiency. The manufacturer

Well that's not true for my motor, but my motor doesn't neccesarily indicate
a trend.  On it if I adjusted the brush setting more than a couple degrees,
efficiency started to fall.  Current went up and rpms fell.

> >> surplus 12v 12amp motors... shaft-to-shaft as a motor-generator...
>
> >     So what was the voltage out of the generator when load down to it's
> > rated voltage?   Possibly around 4V?
>
> No-load is 12v 4amps in, 11v 0amps out. Full load for the motor is 12v
> 12amps in, 5v 8amps out. As you can see, they are fairly inefficient
> (50-60%). Shorting the generator makes the motor pull over 30amps at 10v
> (the most my test power supply could deliver).
>

    Now you are making MY point...the one you were arguing against.
    Your example clearly agrees with my statement that you must increase a
motors rpms about 3 times when running as a generator in order to get the
rated voltage and current output.
    The drive motor is running at rated voltage and current so obviously
it's running at rated rpm.  The generator is only producing 42% rated
voltage and 67% rated current.  In order to increase this to the rated
voltage we must increase rpms 2.4 times and further increase rpms to bring
the current up.  By the time you get the current AND voltage up to the rated
levels the motors rpms will be close to 3 times the rated rpms!!!

    Since your example clearly agrees with mine, I think that pretty much
obliviates any further arguement.