Re: DC torque

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

> A DC motor without a controller is about as useful as an ICE
> engine without a throttle plate.

Ah, but there are lots of applications that use electric motors without
controllers (and very few for ICE's without throttles :-)

The difference is that electric motor characteristics can be tailored to match
 many loads *without* a controller. Pure series motors head for infinite speed
at no load; so add compounding (a smaller shunt field) to limit maximum no
load rpm. Likewise, rotor construction in an AC motor can be altered to
produce almost any desired torque-speed characteristic to suit the load.

> IMO, the motor's functionality is so intertwined with its controller
> that talking about the motor alone doesn't give a picture of what
> they'll do as a system.  That's especially true with AC motors.

The motor, controller, and battery are each links in a chain. The weakest link
sets the system's overall limits. For example, there's nothing a controller
can do to overcome fundamental limitations in a motor or battery. Thus, some
parts of the performance "envelope" are set by battery weaknesses, some by the
controller, and some by the motor. It is necessary to understand each
component to know where these limits are.

> designing a controller is a one-time engineering project.

Except that designers don't share their work. It is difficult and expensive;
why should they give it away for free? So each new group of designers has to
start from scratch, gleaning whatever hints and free advice other designers
will reveal, or what can be learned by reverse engineering existing products.

> Some sort of current limiting is needed *anyway* to provide streetability.

What I was referring to is that different motor types have fundamentally
different characteristics that cannot be overcome by the controller.

Series DC motors deliberately trade off high speed performance to get
excellent low speed performance. Specifically, they can deliver high torque at
low currents, even at stall. That's because torque is current squared. 10
battery amps (100 motor amps) can deliver plenty of torque to smoothly start a
2-ton EV moving from a dead stop. The PWM controller only needs a 10:1 step
down ratio to achieve this (120vdc in to 12vdc out), which is fairly easy to achieve.

In contrast, a low slip EV grade AC induction motor delivers a torque
proportional to current. Thus you need much higher currents (like 500 motor
amps) to produce the same torque. Its controller might need a 50:1 step-down
ratio (120vdc to 2.4vac) to produce enough current to match that series
motor's torque. This is much more difficult design challenge.

> Could you do the same feeding a locked-rotor AC synchronous motor?

This highlights a problem in AC synchronous and DC PM motors. If you want high
torque, you need high current. But the high currents can demagnetize the
magnets. So the ultimate maximum torque they can generate is always lower than
a wound-field motor.

> look up "Magic sinewaves", you'll find some interesting answers there.

It is well known that you can produce any periodic waveform from a fundamental
sinewave and its harmonics. But an infinite number of other possible waveforms
can do this, too. For instance, electronic organs also use sawtooth, square,
and pulse waveforms to synthesize other waveforms (sounds) to a high degree of
accuracy.
In a digital world, it is much easier to make square waves than sine waves.
The "magic waveforms" are just one method to synthesize a desired waveform by
summing up a square wave and its harmonics. These waveforms don't look good
(on an oscilloscope) or sound good (for music synthesis). But they are pretty
good for motors and transformers because they minimize the energy in lower
harmonics. For instance, you can produce a 60 Hz waveform with no harmonics up
to the 9th.

Lee Hart                     If you would not be forgotten
4209 France Ave. N.          Soon as you are dead and rotten
Robbinsdale, MN 55422 USA    Either write things worth the reading
phone (612) 533-3226         Or do things worthy of the writing
e-mail XURQ03A@prodigy.com   (Ben Franklin, Poor Richard's Almanac)