--- Jeff Major <
[email protected]> wrote:
>
> Hey Marty,
>
> Pushing the wheelbarrow over to the side, I'll give
> you a couple of my impressions.
>
> First off I do not have the motor curves you refer
> to.
> Please send them and I'll look them over. But as
> for now, there are many different designs possible
> for
> the two different motors. Just how they would
> interact depends a lot on the particular design of
> each.
>
> You mention "having a good torque over wider RPM
> range, like an AC motor." I have done both, DC and
> AC, motors. When you push an AC motor to its
> limits,
> the resultant speed/torque map shape closely
> resembles
> that of a series wound DC motor. At the higher
> frequencies (RPM) with the AC motor, you end up
> field
> weakening (or reducing the volts/hertz), so you lose
> torque. On the other end, the AC motor (induction)
> will have a torque limit (breakdown) and the DC
> motor
> a controller current limit. The shape of the
> speed/torque maps are about the same, just that the
> AC
> motor is higher speed/less torque that the DC. But
> with proper gearing, they become nearly equal, some
> torque advantage to the DC.
>
> So, I guess I have to question your motivation here.
>
> What you have proposed, in my opinion, will work.
> But, again in my opinion, is likely to less
> effective
> than two equal motors. That is for a given total
> mass
> of iron and copper, two equal motors will give you
> better performance and thermal suitability than two
> different sized motors.
>
> I realize that you have to work with what is
> available, but if your goal is to shape the
> speed/torque curve to suit a particular need, this
> can
> be done by altering design in a single motor. Or by
> the control method of the motor. A simple method
> comes to mind when you say "counteracting a big
> motor's tendency to go flat as RPM rises because of
> back EMF limiting current". Switch in a field
> weakening resistor.
>
> Another point, you say "As RPM rises, the current,
> and
> heat, migrates to the small motor because of lower
> back EMF". Now, both motors have the same RPM.
> Both
> have the same applied voltage. Why do you think the
> Eg is less in the small motor? That is saying that
> the equivalent motor resistance times the current in
> the small motor is larger than in the big motor. It
> might be. Might not be. The motor currents are not
> necessarily equal. And their resistance is unlikely
> equal.
>
> But, I encourage you to pursue this. You might come
> up with something to work well for your application.
>
> Certainly, plotting out the motor curves and summing
> the torques vs RPM will give you a good look.
>
> I guess I just worry about the little guy going
> under
> the knife for a rewind.
>
> Good luck,
>
> Jeff
>
>
> --- Marty Hewes <
[email protected]> wrote:
>
> > I am yet unconvinced. NetGain lists red line for
> > all their motors at the
> > same RPM, so I'm not worried about overrevving the
> > bigger motor. It's not
> > about total torque produced over a narrow band,
> it's
> > about having good
> > torque over a wider RPM range, like an AC motor
> with
> > lower cost. The curves
> > show that at low RPM, the big motor dominates, but
> > as RPM goes up, a smaller
> > motor produces more torque than a big one unless
> the
> > pack voltage is pretty
> > high. Just look at the torque and horsepower
> curves
> > for the motors for
> > voltages of 144 or less. Big motors (11" or
> bigger)
> > crap out below 3000
> > RPM, small ones then out pull them to 5500. The
> sum
> > of the two might be
> > interesting. I need to draw some composite curves
> > before I rule the idea
> > out. The combination wouldn't produce more peak
> > power at the point the
> > controller is just coming out of current limiting,
> > because that's determined
> > mostly by the battery and controller capabilities
> > anyway, not the motor, but
> > I really think it might provide decent power over
> a
> > wider RPM range,
> > counteracting a big motor's tendency to go flat as
> > RPM rises because of back
> > EMF limiting current, which might benefit
> > eliminating the trans. Our
> > problem with running without a trans is that the
> > controller current limit
> > limits power at low RPM when current is high, and
> > the pack voltage limits
> > power at higher RPM. I think that with two
> > different size motors in
> > parallel, the big one would automatically dominate
> > at low RPM, and as RPM
> > rises, the smaller one would take over. I suspect
> > it would act a little
> > like a CVT in terms of matching battery capability
> > to road speed. The
> > motors would have to stay electrically in
> parallel,
> > or the power would
> > always go to the less capable motor instead.
> >
> > Jim is absolutely correct on one point, that the
> > smaller motor could easily
> > be overworked. As RPM rises, the current, and
> heat,
> > migrates to the small
> > motor because of lower back EMF, so the small
> motor
> > would have to have
> > enough capacity to do most of the work at higher
> > RPM. This wouldn't be a
> > setup for Bonneville speed record attempts or
> > highway speed mountain driving
> > without a trans. Here in Illinois? Maybe.
> >
> > ----- Original Message -----
> > From: "Jim Husted" <
[email protected]>
> > To: "Electric Vehicle Discussion List"
> > <
[email protected]>
> > Sent: Thursday, August 16, 2007 10:13 AM
> > Subject: Re: [EVDL] twin, different motors?
> >
> >
> > > Hey Jeff
> > >
> > > I couldn't have said it better 8^) BTW I'm not
> > > scrawny I consider it more like being bulk
> > challenged
> > > 8^o
> > >
> > > I will say some of those here are not overly
> > concerned
> > > about having to warranty a motor 8^P
> > >
> > > Anyway I feel the wheelbarrow stories help
> people
> > to
> > > see how things work duty cycle wise. Maybe we
> > should
> > > write a wheelbarrows guide to motor usage, LMAO.
> > >
> > > Had fun
> > > Cya
> > > Jim Husted
> > > Hi-Torque Electric
> > >
--- Jeff Major <
[email protected]> wrote:
> > >
> > >> Hey Marty,
> > >>
> > >> Instead of talking volts and amps, I'll try it
> > like
> > >> some other guys explain stuff.
> > >>
> > >> Say Dad has a big pile of dirt in the yard he
> > needs
> > >> moved. He gets his twin sons, Bill and Ted,
> out
> > >> there
> > >> to move half of it. But he has just one
>
=== message truncated ===
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