Hello,
Can someone explain to me how electric motor re-gen braking works when the motor is an induction type motor (squirrel cage rotor)?
I understand that when the motor is being driven via the stator magnetic field, an opposing magnetic field is induced in the rotor.
When you start re-gen braking where does the magnetic field come from? I would imagine that the rotor field would very quickly dissipate once the stator field is turned off?

Here is a reference on the subject:

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

Walt

Thanks Walt, I have looked at a few articles in Wikipedia and I have a few books on Electric motors and generators but I still have not found an explanation as to where the magnetic field comes from when a sqirrel cage 3 phase induction motor stops being driven and is used as a generator for braking? This is a common system on heavy plant machinery trains and cranes which use the motor for braking the drive wheels and/or hoist pulleys on crains? The machines and motors that I am referring to are all controlled by frequency converters/ vector drive systems?

If the rotor is traveling at a different speed than the stator field, there are currents and torques induced in the rotor a direction which make it tend to catch up with the stator field.

We are familiar with this principle in the normal case of a running induction motor under load. The stator field spins and the rotor spins slightly slower, with torque in the direction to try to help it catch up.

The simplest form of braking is to apply a dc current to the stator. Then the stator field is stationary. Now the rotor is spinning faster than the stator field. Again there is a torque on the rotor which makes it want to travel at the same speed as the stationary stator field - in this case that amounts to a decelerating torque on the rotor.

Hi there,

This is regen-braking in a nutshell.

You are probably familiar with the fact that a rotor in a squirrel cage induction motor is pulled along by the rotating field in the stator. The rotor never ever actually catches up to the stator rotation and the difference is called slip. It is this slip ( difference ) where we get our torque.

Now imagine that the rotor is moving along in its merry way at a given speed when suddenly our VFD reduces ( not shut off ) the frequency to the motor so that the rotor is actually rotating faster than the field in the stator. Now the slip is reversed and the motor will attempt to deliver current back to the VFD while also trying to slow down to return to a positive slip condition. Ofcourse, the VFD will continue to ramp down the frequency.

Current flows back into the motor bridge and through it to arrive at DC bus capacitors within the VFD. The capacitor bus voltage will quickly begin to rise and eventually overcharge unless the energy is dissipated. The VFD then reconfigures the DC link ( switching on some transistors to bypass the blocking diodes ) to permit current to flow through the input section of the VFD ( which ofcourse is now an output temporarily ). The input brige is fired sequentially to send current back into the mains supply. The reduction of frequency is controlled so that the current generated by the motor does not exceed the ratings of the internal components of the VFD.

Many higher end VFDs will also have some backup systems to facilitate braking in the event of a power outage. The DC bus within the drive can keep the VFD electronics 'alive' long enough to inject DC current into a single phase of the motor ( DC injection was discussed earlier by Electricpete ). Sometimes there is even a large bleeder resister on standby that can be shorted across the DC bus capacitors to dissipate the excess energy as heat.

Ofcourse, true regen just sends power back into the mains supply as a form of energy savings.

I tried to keep it simply. Hope this helps.

DanS

We have two good explanations by EP and DanS. I will add another application; for the hybrid vehicle the motor inertia “pumps up” the DC bus and is used to charge the batteries as the batteries become the load in regen mode.

Electricpete and DanS gave a perfect explanation. The largest regen braking system I saw was in Andes, a 1okms conveyor system with a vertical drop more than 1,000 m. Motors were used for starting the conveyor. Once fully loaded it would feed the grid with almost 20MW of energy! For safety there were also large resistor banks for braking and dissipating power in case the connection to grid was lost…fascinating application….
The new modern mining truck operate on similar principles the difference being that the braking energy is dissipated into a resistor bank (you cannot feed back the generator ).

Thanks very much, this is food for thought. The application that I am familiar with is the same as on mining trucks, but these are on cranes and straddle carriers. Siemens or Bosch Rexroth controlled VFD's with resistor banks.

When you are at it, can you also explain how the VFD perform the magic so there are almost no slip when running at steady state at defined speed? It´s a pain when trying to look at rotor bar problems, on the other hand, VFD drives don´t have them anyway. I guess it´s a part or result of the COS fi adjustment feature? Olov

Oli,
I don't think VFDs eliminate slip. Slip is what produces torque. VFDs modify the AC frequency and voltage to allow motors to run at speeds other than 60 hz. They are capable of producing high amounts of torque at low speeds by the same action.

J-

I agree with Wally on that.

And I agree with Oli, vfd motors are much less likely to have rotor bar problems. Rather than applying full line frequency instantly, the frequency can be ramped up with the rotor speed trailing not too far behind. That way the rotor does not see the tremendous heating that it sees during a "normal" across the line start.

Can you get me a standard MCA spectra from a modern VFD motor with Cos fi compensation that have a normal slip compared to the same w/o VFD? Are there other reasons that I don´t see the slip freq. or it´s extremely close to the grid freq. or am I blind? Eg. compared to the synthesised freq. a VFD use as grid freq. those I have seen lately have been very close to syncronous. It may be that the load has been abnormal light but that is also uncommon these days. Or to turn it around, how do they do the cos fi comp? To balance/compensate for the inductive load they need to add some fake capacitive stuff? Always wondered, never knew who to ask :-). Olov

Keep in mind that VFDs are not exclusively used with induction motors. Synchros and servos are also frequently used with VFDs which would ofcourse mean no rotor bars ( and no slip ) but rather permanent magnets or other such craziness....

Dan