Confusion regarding AC and DC Motors

[carbon.nanotube.capacitor]

Hi everybody!

Well, I've found some information that suggests AC motor controllers can slide the torque of the motor to any rpm, making the motor perform with full strength at all speeds. I'm a little confused though as to what the difference between the windings is between AC and DC motors. Is it just a matter of how the controller is designed? Could a 3 phase AC controller drive a 3 phase BLDC motor?

 

[fechter]

There could be some confusion in the terminology here. There are many types of AC motors.
I think what you are referring to is an AC induction motor. These have a very wide power band compared to a BLDC motor.

A BLDC motor is actually an AC motor too, but uses permanent magnets and is driven by a DC powered inverter (controller).

The control scheme for an induction motor is a bit different than a BLDC, and the same controller would not work for both unless it had software settings for each type of motor. The power stage and most of the hardware is the same. Most AC induction motor controllers are very expensive. Curtis and Sevcon make nice ones.

Any motor with permanent magnets is going to be limited in speed by the voltage applied. This is not really the case with an induction motor.

I tried using a Crystalyte controller to run an induction motor by leaving it connected to its BLDC motor, and tapping into the 3 phase wires. It sort of worked but the electrical rpm of the BLDC motor was too high for the induction motor. It had little torque, but got going so fast that I had to back off on the power because I was afraid the rotor would fly apart.

 

[carbon.nanotube.capacitor]

Hmm. Interesting - I kind of wondered why DC brushless motors were referred to as DC... So from what you've told me, a controller could work with either BLDC or AC motors as long as the software (assuming a digital controller) were designed for it. Did I understand that correctly? How is an AC induction motor different from a BLDC motor? Are there no permanent magnets in an induction motor? This is all fascinating stuff! Maybe a car with four hub motors should have AC induction motors instead of BLDC. And maybe an AC induction motor would make a better bicycle hub motor.
Oh, just re-reading some of your post - if the top speed of an induction motor is not so dependent on voltage, then that would lend itself very well to avoiding reduced top speed due to tired, drained batteries. Right?

 

[Mr. Mik]

How can you determine what type of motor you have in front of you?

I ask because I would like to add a motor controller to the two battery cooling fans in my Vectux (=out of warranty Vectrix).

I know that they run well with a straight 12V DC supply, using about 2 amps each.

The motor controller would allow me to turn the noise down when the full cooling power is not needed.

I have a kit for building a 12V DC (10A max) motor controller that works by pulse width modulation. If I understand it correctly, it just turns the 12V DC supply on and off very rapidly.

No markings or specs on the motors.
I believe the motors are brushless, because they run very quietly when not at full speed, and they take a long time to stop spinning. Very little friction and no noise at low speed.

Is there some way of figuring out if a particular controller would work with those motors, without hooking them up?

And how can I determine what the risk of frying the motor would be if I test it out with a PWM controller?

 

[fechter]

The guys at Electric Motorsport have built a number of motorcycles using induction motors. Since they have a wider power band, you can do nicely with a single gear ratio. Todd mentioned that as the batteries drained, the top speed was largely unaffected by the voltage drop. The Tesla roadster uses an induction motor.

An induction motor uses essentially the same windings and stator as a BLDC motor, but instead of a magnetic rotor, the rotor is made of iron laminations with copper or aluminum 'shorting bars' embedded in it. Since there are no magnets, it can withstand very high rpm and temperature.

It would be hard to make an induction hub motor unless is was geared. It might be a bit heavier than a PM motor of the same power level.

On the cooling fans, they are likely brushless motors that have the controller built in. You can reduce the voltage going to these to slow them down. Your speed control kit may not work properly driving a brushless motor, but I don't think it would hurt to try it. With fans that small, you might get away with just a big resistor. You could also wire the two fans with a series/parallel switch to run them at half speed.

 

[Mr. Mik]

On the cooling fans, they are likely brushless motors that have the controller built in. You can reduce the voltage going to these to slow them down. Your speed control kit may not work properly driving a brushless motor, but I don't think it would hurt to try it. With fans that small, you might get away with just a big resistor. You could also wire the two fans with a series/parallel switch to run them at half speed.

Thanks!

I was incorrect about no label on the fan motors...sorry.

When I looked through my photos I found that they do indeed have stickers on them. At the time I had the fans (or more correctly the "impellers") out of their housing, there was a lot of other stuff going on around me. I quickly took some photos, put them back together in a rush and stored something like: "cannot make sense out of the label" in my memory...and that memory changed later into "there is no label"!

Well, here are the pictures:


And here a datasheet: http://charcroft.com/site/pdf/data/mftr_data/comair_rotron/Comair%20Rotron%20Diplomat%20Motorised%20Impellers.pdf

It is the DD523612B1A , with an added "X" behind it: DD523612B1AX , whatever that might mean.

They describe the motor series as:

"The Diplomat® Product Family is constructed using Brushless, Electronically commutated DC Motors, giving excellent control whilst minimising electrical noise."

That's another way of saying: "Built in motor controller", right?

What would be the most promising approach to try to control the speed of these impellers?

I am envisaging something like this: A thermistor in the intake airstream and another thermistor in the exhaust airstream. If the temperature is equal (or intake is warmer than output), then the impeller speed should be so low that it just allows a bit of airflow to keep the temp measurements accurate. If the exhaust air is a lot warmer than the inlet air, then the impellers should run at maximum speed.(I don't need to worry much about cold air where I live, but in cold climates it would probably be best to turn the impellers off altogether when the battery temperature is below 16°C or so.)

In the datasheet linked above it also says:

The following can be independently or collectively added to the basic product offering:
• TTL or Open Collector Tachometer Output –
• Discrete Alarms –
• Harness Assemblies –
• Speed Control – Via thermistor, voltage or Pulse Width Modulation, the speed of the blower can be
controlled, to minimise noise and power consumption.

I hope this is not too fat off topic - sorry if it is. Let me know, then I'll start a new thread for this impeller taming project!

 

[fechter]

I guess this is getting pretty off topic and a new one might be a good idea as others with a Vectrix may be interested.

Many brushless motors have a wire coming out of them for speed control. You might take a look at where the wires come out of the motors to see if there are any unused connections.

Here's more info from the manufacturer. I didn't see an "AX" suffix, but the speed control option would be a number after the A.
http://www.comairrotron.com/cgi-bin...ure_unit=inh2o&diameter_val=&diameter_unit=in
http://www.comairrotron.com/fan_speed_control.shtml

If your fans don't have the built-in speed control option, you can always just reduce the supply voltage to slow them down.