Brushless DC motors - gotta study and learn

Does anyone know of a good start with basic and get to advanced reading material on brushless DC motors.

Why I ask
I need to learn more. Have a in wheel hub motor off a scooter that we wanted to test and see if would rotate.

Phase to phase reads approx 0.3 ohm
phase to ground 1M or better.
rotates freely. -- So motor looks OK so far.

Tried running for one or two second on
1. AB non vector VFD no luck although it was harder to turn tire when energized
2. Old six step inverter motor would turn approx 1/8 turn
3. Applied DC to two of 3 phase leads motor would turn approx 1/8 turn did on all three pairs and turned each time

SO we conclude
the motor is OK it will turn when energized
move on to the controller and replace both capacitors - one is blown out.
try controller and see if can make run

IF that fails students decide to either
scrap project
buy new controller

From another source we see the pulse shape may be special for this not a PWM and not a full six step. So we do not know what wave form a controller must have to make motor run.

Scooter is
1. owned by a group of EV enthusiasts
2. Donated by an electric vehicle shop who gave up on it and is a basket case
3 We are doing this project as Inependent Study in association with high school students and is THEIR project all I do is mentor and advise they make own decisions
4. complete mystery no markings not even a serial number anywhere on scooter. Students were able to track down US representative who is helpful but gave wrong manual - working on that issue
5. This is a great lesson to students on reality of working in industry with equipment ie "do we have a manual" etc etc.
6. These are good hard working high achieving kids who are worthy of your help.

Any help study info on what the waveform may be would be great.

Dan Bentler

If its a brushless DC motor the back-EMF waveform will be either sinusoidal or trapezoidal. The easiest way to determine this is to hook an o-scope to two phases and spin the motor. You will want to spin the thing at a fairly constant speed so the waveform isn't too distorted. This little exercise should give you the voltage waveform, the number of poles and the voltage constant, all in one neat test.

The difference between a trapezoidal and sinusoidal waveform comes down to winding design. Generally speaking a trapeziodal back-EMF motor is driven by a six-step commutation and used Hall Effect sensors for commutation feedback. A sinusoidal back-EMF motor is driven with sinusoidal commutation and uses either encoder or resolver commutation feedback. In practice either motor type can be driven by either commutation type with an increase in torque ripple and attendant decrease in efficiency.

You can't operate a permanent magnet AC motor using most drives built in vector control algorithms. While the basic concept of field oriented control applies to these motors also the control algorithms found in most drives are designed for vector control of induction motors. As such these drives need to attempt to control two current components 90 electrical degrees apart; one to produce torque and one to produce the rotor field. With the permanent magnet motors the rotor field is already there so a vector controlled drive will not be able to interpret the armature feedback correctly.

A third option is to operate the motor as if it is a syncronous AC motor, which in effect it is. The downfall to this is that if you become magentically uncoupled between the stator and rotor fields you may not get coupled back up without stopping. I know the AB PowerFlex 70 and 700 as well as the AB 1305 drives will support driving a synchronous motor. Just be careful of voltage level.

Keith

Hi Dan, this may help a little or you may need a little more advanced.... but since you brought it up, I decided to read up a little on them as I knew nothing and it helped me

http://en.wikipedia.org/wiki/Brushless_DC_electric_motor