# Regenerative Braking - 3ph Induction machine



## mizlplix (May 1, 2011)

Greetings: 

It my understanding that the motor is isolated from the battery voltage during regen by the rectifier bridge (Diodes). 

It is also my understanding that it is not so much voltage but amperage (current) that does the actual recharging. My controller has a percentage of regen setting to signal the controller how much regen you want. Which tells me it does not rely on solely a voltage signal like a regular automotive charging system/

I may be partially wrong in this but these are my beliefs.

I know there are MANY RKI's here, someone will eventually post.

TY, Miz


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## major (Apr 4, 2008)

Stiive said:


> Just wondering how regen works when you got a battery pack thats higher potential than the rating of your induction machine.


Hi Stiive,

I know this is difficult to visualize. Think of the 3 phase inverter bridge as 3 half bridge units (2 switches and 2 diodes). And then the 3 motor phases, one connected to each half bridge. Each can be operated as a boost converter. Taking a low voltage generated in the motor coil and boosting it to a level above the battery potential.

This is much the same as a DC controller capable of regeneration using a PM or SepEx motor. In the normal mode, it functions as a buck converter providing a lower output (to the motor). In the regen mode, it functions as a boost converter providing a higher voltage (to the battery).

Regards,

major


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## Stiive (Nov 22, 2008)

major said:


> Hi Stiive,
> 
> I know this is difficult to visualize. Think of the 3 phase inverter bridge as 3 half bridge units (2 switches and 2 diodes). And then the 3 motor phases, one connected to each half bridge. Each can be operated as a boost converter. Taking a low voltage generated in the motor coil and boosting it to a level above the battery potential.
> major


Hi Major, thanks for your reply.
After some thinking, I think I know where my intial thoughts went wrong; I was thinking that if the motor produced the 300V it would start breaking down the insulation, but really it’s output is being PWM’d so the RMS voltage in the motor is much lower. So i think what really happens is you set the terminal voltage to 200V RMS, it will start regenerating at 200V RMS (which is fine for the motor) but each pulse of the PWM going back into the batteries will peak at (or above) 300V, and wallah potential difference and current flow. 



Pretty much the same principal as driving the motor, but in reverse.
This means the motor can provide regen to a battery pack using any terminal voltage/frequency and can therefore be controlled to not saturate (my previous concern)


Now im trying to think how this voltage build up occurs. Obviously as you said, the inductance of the coil acts like a boost controller, but i'm finding it hard to visualise how the current is first built up.


Perhaps you can finish this for me:




1. Assume the motor is spinning at high RPM (say 5250RPM 175Hz) with no connections and not magnetised. 

2. You start firing IGBTs and supplying reactive power at an arbitrary voltage (say 200V RMS, 160Hz) from a 300V DC bus (inc batteries and capacitors/snubbers). 
3. The machine magnetises from the reactive power supplied and the machine starts to generate back EMF/negative torque due to being spun above the base speed. 

4. Back emf voltage starts to build up. (In opposite polarity to the reactive power??)



From here im not sure....


Perhaps 



5. Reactive current is circulated through the phases until magnetisation current is adequate
6. Gates are opened to the DC bus
7. Inductance says current needs to keep going so it increases its voltage until its greater than the DC bus
8. Current flows to DC bus charging batteries/caps etc




Cant seem to find any decent literature on it, perhaps im completely wrong then...I think ill have to sleep on it...






Rgds,
Stive


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