Rich Grise said:
Well, it _is_ doable, albeit admittedly probably kinda wasteful. I once
saw a 25 HP 3-phase motor on one of those controller thinngies that
produced full torque from zero all the way up to rated RPM (which wasn't
very fast - whatever the standard 3-phase 60 Hz induction motor runs at).
It sounded kinda weird while running up the RPM - it was like the PWM was
at some harmonic of the drive frequency or something.
Cheers!
Rich
PWM VF controllers can use very sophisticated feedback techniques to
monitor current in each leg of a bridge circuit to produce maximum torque
at virtually any RPM, and they now have sensorless vector controllers that
use the back EMF to determine rotor position and velocity. You can program
the controller for different modulation frequencies to reduce audible beat
frequencies and resonances which is probably what you heard. Induction
motors can be designed with tradeoffs for low end or high end torque,
efficiency, and variable speed. At zero RPM, the excitation frequency is
just about the "slip" speed, usually about 20-50 RPM, which may be 0.5 to
several Hertz, depending on the number of poles and other factors.
A two pole motor runs at just under 3600 RPM at 60 Hz. There are 4 pole, 6
pole, 8 pole, and 12 pole motors, which run correspondingly slower but with
higher torque, so your motor might have been a 12 pole at about 600 RPM. It
can be driven up to 400 Hz, at which it will run at 4000 RPM, but the
torque will be much less because you do not have a corresponding increase
in voltage above nominal line voltage. You can probably run a 240 VAC motor
on 480 VAC at 120 Hz and get twice its rated HP.
Much higher voltages would probably overstress the insulation, and push the
limits of available IGBTs and MOSFETs used in the bridge circuits. I am
researching the practicality of winding motors for much lower voltages and
then pushing the HP by high frequency PWM. From what I have heard from
professional motor engineers, magnetic losses will limit this boost to
about 2.5 to 3x. Higher frequencies may require thinner laminations of
higher grade steel, or other changes. An induction motor is much like a
rotating transformer, and most power transformers work quite nicely up to
about 1000 Hz. It would be very nice to make a motor almost 20 times
smaller than its 60 Hz counterpart of the same HP. Of course, a 2 HP frame
could not handle the torque of a 40 HP motor, and unless it was wound with
more than the usual maximum of 12 poles, it would spin at a possibly
dangerous 10,000 RPM.
Returning to the original question of this post, the complexity is the cost
of making something efficient and practical. You could bolt wheels on a
couple of big series wound motors and control the speed by selecting 6V,
12V, 18V, etc., but it would be hard to drive safely and would not be
practical or efficient. Adding sophisticated electronics, using BLDC or
induction motors, adding regeneration, incorporating a multispeed or
infinitely variable transmission, and maintaining optimum torque on all
wheels under all conditions, is a very complicated task but results in an
efficient, practical, and elegant design.
Then there is the most serious limitation of the capabilities of batteries,
which limit range, increase cost and weight, and require regular
maintenance and replacement. Hybrid vehicles overcome some of these
limitations, but greatly increase the complexity, especially for those that
combine the output of the ICE and electric motors to drive the wheels.
Whew! 'nuff said.
Paul
