Regeneration from induction motor with DC?

[DennisCNC]

What if DC voltage is applied to two phases what will the third phase output if the induction motor is spun by another source?

Thanks

[Geof]

Where is Al the Man when you need him?

I think the third phase will output nothing, and all you will do is make your rotor hot.

Three phase AC motors can be braked by applying DC to the windings; this creates a stationary magnetic in which the rotor is turning and that creates eddy currents in the rotor that give the braking effect. Or at least that is how it was explained to me; I have no idea if it is right or wrong.

[DennisCNC]

The question came to me after seeing this deal:
http://www.pentadyne.com/site/our-pr...echnology.html

How do they get energy out of a induction motor? they apply single phase ac?

[vger]

How do they get energy out of a induction motor? they apply single phase ac?

They are using a "Synchronous reluctance motor" to spin the flywheel and regenerate power. Not sure exactly how they excite the rotor to draw power back off the flywheel, but it's not a standard incuction motor. Here is a link about using an induction motor as a generator:
http://www.qsl.net/ns8o/Induction_Generator.html

Not very efficiant for sure, but it does work.

Steve

[Geof]

Originally Posted by DennisCNC .....How do they get energy out of a induction motor? they apply single phase ac?

You do not have to do anything to the motor.

Do you have any machines that run three phase motors from a variable frequency drive (VFD) that is capable of regenerative braking?

In other words do you have any CNC machines with perfectly ordinary spindle drives?

A VFD uses fancy electronics and synthesises three phase AC from a DC supply and varies the frequency and voltage to run the motor at different rpm. For each frequency there is what is called a synchronous rpm which depends on how many poles the motor has. You have probably seen standard AC motors rated at 3450 rpm and 1475 rpm (I think these are the numbers); these correspond to a synchronous rpm of 3600 and 1800, but the motors run slightly slower. This is called 'slip' and is necessary for these motors to operate; the slip is how they convert the power they are drawing from the supply into rotational energy and as the load increases the slip increases (they slow down slightly further below the synchronous speed) and they draw more power. Electrical energy is converted into mechanical energy whenever the motor rpm is slipping behind the synchronous rpm; the VFD or CNC controller ramps up the frequency and the motor rpm follows. The reverse is also the case that when the VFD ramps the frequency down the motor also follows and slows down; the reason is that when the motor rpm is ahead of (leading) the synchronous rpm it acts as a generator. When a motor 'leads' it converts the mechanical energy in the motor rotation back into electrical energy and delivers that through the VFD back to the DC supply; this is regenerative braking. Normally the DC supply is a capacitor bank charged from the incoming AC which is rectified and regenerative braking increases the DC voltage on the capacitors so at a certain level a relay brings the Regen Resistors into the circuit to bleed of the excess energy and dissipate it as heat. But with more fancy electronics it is possible to synthesize AC and feed it back into the power supply; this is done in flywheel storage back up power supplies.

[Al_The_Man]

I believe the effect of using a 3 phase motor with DC into two phases may not be practical, The field produced by one winding would be canceled by the field produced in the second due to both being opposed windings.
Although I have not tested this theory.
BTW to expand on Geoffs statement, the reason a standard induction motor cannot run at synchronism to the applied frequency is at synchronism there would be no current induced into the rotor due to the rotor running at exact sync with the applied Fr.
What is done to make it a synchronous motor, is to have the rotor winding brought out via slip rings or some other coupling method, and when the induced slip freq. reaches around 3~5hz, a DC current is injected into the winding and the rotor clamps to the applied FR, this used to be a common method.
Al.