Synchronous versus asynchronous?

[PaulM]

I am working on converting an alternator to a motor and am in the process of testing right now.

http://endless-sphere.com/forums/viewtopic.php?f=10&t=8420

What I would like to know is if this "motor" is considered synchronous or not. Are all BLDC motors synchronous? It is my impression that they are, however, this would imply that increasing voltage would not increase speed, which we know is incorrect. Is the controller increasing the output frequency in order to increase the BEMF to match the increased input voltage?

On my alternator, changing the field strength has a drastic effect on speed. Here is what I think happens when the field voltage is decreased, weakening the rotating magnetic field:

1. The BEMF decreases due to the weaker field
2. The stator current increases due to the decreased BEMF
3. The increased current results in a stronger stator field, causing torque, causing acceleration
4. The ESC senses the acceleration as the BEMF pulse occurs sooner, and increases the output frequency to maintain synchronization
5. This continues until the BEMF increases enough to form an equilibrium

Please correct me if I am wrong!

This leads to another question:

What would happen if a BLDC motor is run from a constant frequency source, like say from an industrial VFD? Is it possible for the rotor poles to slip relative to the stator poles if enough load is applied? I envision a controller for this a being very simple. There would be no need to monitor rotor position, just apply a specific frequency with an appropriate voltage for that frequency. If the load is to high (which the controller could maybe sense through current draw?), simply reduce the frequency. Please tell me why this won't work; surely it doesn't, or else they would exist. :?

 

[olaf-lampe]

The behavior you described sounds more like a SEPEX motor to me, but I'm no expert in that.

-Olaf

 

[fechter]

1. The BEMF decreases due to the weaker field
2. The stator current increases due to the decreased BEMF
3. The increased current results in a stronger stator field, causing torque, causing acceleration
4. The ESC senses the acceleration as the BEMF pulse occurs sooner, and increases the output frequency to maintain synchronization
5. This continues until the BEMF increases enough to form an equilibrium

Please correct me if I am wrong!

This is correct. The strength of the field and resulting BEMF will determine the speed at which the BEMF equals the applied voltage. When the BEMF reaches the applied voltage, the current will be zero, so the motor can't go any faster. Due to friction and other losses, the motor requires some current to just spin at a particular speed, so you never quite reach the matching voltage.

With a fixed frequency drive, it is possible for the rotor to slip. Unlike an induction motor, when slip occurs with a BLDC motor, you loose all torque and the motor stalls. If you never exceed the torque needed to slip, the motor will stay synchronous. It's kind of all or nothing.

With a VFD, you might be able to simply short out the rotor windings to make it behave like an induction rotor. I sort of tried this once, but I didn't have a very good drive. My impression was that it didn't make a very efficient induction rotor, but it was very simple and it did work. If you installed an induction rotor taken from an AC motor (assuming you could find one that fits), it might work quite well.