ACIM theory question

[fourteh]

I keep seeing all this information talking about really good regen on ACIMs. What I learned about induction motors is that running at negative slip draws power just like running at positive slip. I'm looking for a technical explenation of how an ACIM is forced to act as a generator(trying to make my own controller), or are ACIMs just self excited synchronous motors and the language is screwing with me?

[drmiller100]

yeah, I'm in also. I don't understand it either.

[JasonG]

I'll try.
Slip as you are using it applies to DC motors, if I understand correctly.
When an AC motor is overdriven it turns into a generator. There are no major differences from an AC generator and motor.
Good test:
Find a fan around 30-50HP
Turn the contactor off.
During the 2 min or so it coasts down, put your meter on the bottom leads of the contactor. There will be voltage present. This is why (before VFDs were reasonably priced) you always had to wire a delay relay into the control circuits. If you closed the contactor while it was spooling down, BOOM, blown fuses and sometimes the whole contactor.
This is the way VFDs working in "common bus" configuration work.
Example:
A conveyor bringing large ore down hill to a crusher spins its motor through gravity.
That is fed to the common DC bus of its VFD to the bus of other VFDs that send the crushed ore to the top of a pile, or into rail cars

Make sense ?

[fourteh]

I understand the concept of induced voltage due to magnetic lines of flux moving relative to a conductor, but a ACIM has no permanent field. If you remove the excitation voltage it will "idle" producing no voltage at the terminals, similar to a sepex dc motor with the field windings in parallel. I'm looking for a physics with calculus explanation of how to make this construction act as a generator.

[JasonG]

Try here first :

You will need to excite the field first, but that is already happening if you have been running and are now going into coastdown.

All About Circuits usually has good answers as well

The chart 3/4 of the way down the page should be what you're looking for.

For those looking for the simple / practical answer, when the voltage applied to the stator is les than the syncro speed the shaft is turning, there will be induced current back into your controller. Be carefull, quickly lowering the "drive" voltage is like slamming on the brakes. Think "downshifting" with your voltage curve.


Just reread your origional post. Yes, they are just self excited syncro motors, the language is screwing with you.
Where you went wrong is that with ACIMs, negative slip induces current not consumes it.

Google "induction braking" as well for formulas on how much you will need to absorb/dissipate.