# Induction motor regen mode question



## broadcastcentral (Jul 6, 2008)

Hello. I have been working on designing a VFD for a 3 phase squirrel cage induction motor. Power source is a battery bank of lithium ion. An encoder will be used to get real time rotor speed.

In motor operation, power is delivered from the batteries, through an IGBT inverter to the motor.

When in regen mode (from pressing a brake pedal) I want a system that will allow the excess energy produced to recharge the batteries.

Regen mode is simply when the rotating magnetic stator field is a lower RPM than the actual rotor RPM.

If the regen current is too high or the batteries are full, the excess power will be dumped onto resistors to dissipate the power as heat.

I need help making a modified schematic that would allow this system to work. If it's already been done by someone and I can simply look at how they did it, I'd love to see a link!

I know the tesla motor company has done it, but I haven't been able to find their schematics anywhere.. Here's some info: http://www.teslamotors.com/forum/forums/regenerative-braking
Hope someone can help me out.


----------



## major (Apr 4, 2008)

broadcastcentral said:


> Hello. I have been working on designing a VFD for a 3 phase squirrel cage induction motor. Power source is a battery bank of lithium ion. An encoder will be used to get real time rotor speed.
> 
> In motor operation, power is delivered from the batteries, through an IGBT inverter to the motor.
> 
> ...


There is no modification needed to the schematic of the motor drive to implement regenerative braking. It is done with the standard 3 phase bridge inverter by the control logic. And there is no need to add extra hardware like a braking resistor to electric cars. If the situation arises where energy from regeneration cannot be stored in the battery, then the electric braking torque is decreased and the friction brakes are employed to dissipate the energy as heat. Afterall, this is the primary function of the standard automotive braking system so there is no need for additional hardware to accomplish the task. Again the control logic handles the monitoring of the battery condition and controls the level of braking torque appropriately. No circuit modifications are needed except perhaps for an input command sensor linked to the brake pedal.

There are some applications where regeneration cannot be fed back to the source and then a braking resistor is added to the inverter circuit. This resistor is placed across the DC bus and has a control module (essentially a buck converter) placed in series with it. The control module allows current to flow thru the resistor when the DC bus voltage exceeds an upper limit set point. This action causes the DC bus voltage to decrease. Allowing for hysteresis, the control modulates the current thru the resistor keeping the bus voltage level safe and dissipating the energy in the resistor. 

These braking resistors and modules are common equipment for industrial VFDs and you should be able to easily find application notes and information about them. In fact, they are so common, that many IGBT modules intended for VFD use are designed with 7 switches instead of the 6 needed for the 3 phase bridge. The extra switch is included specifically for the braking resistor modulation. But like I said above, on EVs, braking resistors are redundant because all cars have the primary service braking system.


----------



## broadcastcentral (Jul 6, 2008)

Thanks for the information. So what you're saying is that the battery bank will be able to absorb the energy created during regen mode no batter what the voltage or current levels are? I thought I would have to add in some sort of voltage / current regulation so that the batteries would charge properly. Also, when in regen mode, if I have the IGBT's switching in a way to create a stator frequency RPM slightly less than that of the actual rotor RPM, will the excess energy flow into the batteries during the time when the IGBT's are on? I guess I'm asking how I should have the IGBTs switching during regen mode to allow the batteries to charge properly. Thanks


----------



## major (Apr 4, 2008)

broadcastcentral said:


> I guess I'm asking how I should have the IGBTs switching during regen mode to allow the batteries to charge properly.


How do you have them switching for motoring mode?


----------



## broadcastcentral (Jul 6, 2008)

major said:


> How do you have them switching for motoring mode?


I will have them switching in the usual VFD manner.. A sine wave of the proper frequency is overlaid with a triangular waveform at 8khz or similar. Where the two waves intersect is where the IGBT will be triggered to turn on or off. This gets rid of harmonics, auditory hum, etc. Here is a sample diagram of this type of PWM. It isn't the exact one I'll be using, it's just a quick picture I pulled off google to illustrate the process.










So this _should _work in regen mode too without any additional hardware from what I've been told..

I am using Marathon electric 'bluemax' 3HP 3 phase squirrel cage induction motors. I've got two of them and they're brand new. I got them both for 80$ total. I got a really good deal. I will eventually make a bigger controller for a bigger motor, but this size I feel is perfect for my first controller. I'll be able to experiment and fine tune the system before scaling up to a system with much more expensive components.

I haven't assembled the actual controller yet because I'm still in the process of raising the funds for the battery bank, IGBT's and caps. I'm also still partially in the research phase of the project.. Still need to feel better about the regen situation.

I will be programming the micro controller for the IGBT inverter myself. I want to build this thing myself from scratch, and that includes writing all the code. Still haven't decided on a micro controller yet, but will probably go with an ATMega128 or similar.. unless anyone else can suggest something better.

I will be using lithium ion batteries. At the moment I have about 150 blackberry batteries that I'm putting together as a small pack. This should get me up around 120v but the amp hours will be low. That's okay since I'll be using it for testing. I plan to use lithium ion batteries from laptops on the full scale battery bank. Still need the funds for that though.

I don't have a scope yet either.. and this is going to be important once I start testing the controller under different situations and parameters. The scope I used to have got stolen and I'll need to pick up another one sometime soon.

Here's the motors. Aint they pretty?









Ohyea.. I'm from Ontario Canada. somebody asked me that..

Oh one last thing.. I'll be using a variant of FOC control, closed loop with an encoder to get rotor speed, position, etc.

If I left anything out let me know! Thanks again guys for your open arms. I was trying to ask about this type of motor controller build on a wind turbine / solar forum and they insisted that ACIMs wouldn't be worth using regen on because of added cost and complexity without much return for that effort. Those guys were stubborn.


----------



## major (Apr 4, 2008)

broadcastcentral said:


> So this _should _work in regen mode too without any additional hardware from what I've been told..


Yes. But that shows the phase voltage, not the switching. That will be another step in your process.




broadcastcentral said:


> Oh one last thing.. I'll be using a variant of FOC control, closed loop with an encoder to get rotor speed, position, etc.


So you have a ways to go. I suggest that you not get hung up on regeneration at this point. Once you get a motor running with your FOC variant, regen will be obvious and you'll have everything you need already there.




broadcastcentral said:


> Ohyea.. I'm from Ontario Canada. somebody asked me that..


That was me. Thanks. Why not add it to your profile so it shows each time you post and the next reader will better understand you?



broadcastcentral said:


> If I left anything out let me know!


Still wondering about this? 


major said:


> I can't figure out what you mean by feeding back -6 volts to the battery


----------



## dcb (Dec 5, 2009)

fyi that looks like a 118 lb 3hp motor there, at 230/460 volts (lotsa batteries). I don't really understand what limits the output of such a motor though (time at overcurrent?)

re: regen, the thing about acim is they won't generate without being hooked up to a power source, with a dependency on frequency. so maybe that is what the windmill guys were on about. 

If you just spin it you won't get any real power out of it, but if you hook it up to an inverter or the grid then spin it faster than the applied frequency then you are generating. Windmills tend to spin at whatever speed they feel like.


----------



## major (Apr 4, 2008)

dcb said:


> fyi that looks like a 118 lb 3hp motor there, at 230/460 volts (lotsa batteries). I don't really understand what limits the output of such a motor though (time at overcurrent?)


It's a standard NEMA frame motor, TENV, inverter rated obviously. It appears to be cast iron and have a large brake or something attached. All that accounts for a lot of the weight. So it can run continuously 24/7 at 3hp with a temperature rise of 95ºC on the windings. What's not to understand about that 

I find it cool that it has equivalent circuit parameters on the nameplate. Also note stall torque of 9 lb.ft. @ 4A, 2Hz.


----------



## dcb (Dec 5, 2009)

Lol, self explanatory I guess 

I saw the dataplate too, some useful info there, but the basic hp vs power didn't add up, in a strange way.


So at 60hz, it says 3hp, 1745 rpm and 9 ft lbs. That adds up (output).
But on the same line it says 460v and 4.1amp (input), that only adds up to 2.5 hp. Not sure why it is saying 2.5hp input = 3hp output...


----------



## major (Apr 4, 2008)

dcb said:


> So at 60hz, it says 3hp, 1745 rpm and 9 ft lbs. That adds up (output).
> But on the same line it says 460v and 4.1amp (input), that only adds up to 2.5 hp. Not sure why it is saying 2.5hp input = 3hp output...


Better check your math and 3 phase real power equation. I get 2643 Watts real power in, so 84% efficient for 2240 Watts on the shaft.


----------



## dcb (Dec 5, 2009)

K, good to know (as in seriously)  pf=.81


----------



## broadcastcentral (Jul 6, 2008)

major said:


> Yes. But that shows the phase voltage, not the switching. That will be another step..


I'm not sure what you are asking me here. I'll try to answer.. The igbts will be triggered by a microcontroller that has all the pwm and foc coding in it. A throttle and a brake pedal will be plugged Into the microcontroller. The throttle will contribute to calculating the proper frequency and sequence of the igbt triggering. The voltage and current used to trigger the igbts will be figured out once I decide on the model of igbt to buy. The brake pedal will start regen mode and the further it is pressed the more negative torque is produced. At the bottom end of the brake pedal the mechanical brakes will cut in as an emergency measure to ensure the car comes to a complete stop In all ssituations.

The motors I bought came with brakes attached to them. I will most likely remove these as they aren't needed and just add extra drag and weight.

I'll try to clarify what I meant about voltage in regen mode...
So.. for example.. if the rotor is spinning at 900 rpm, the stator will be supplied something like 120v at something like 890rpm to start regen. This will induce a current in the rotor bars, creating a magnetic field that produces a negative torque on the rotor. This will produce a voltage that is fed back to the battery bank as excess power. So If +120v is going in to the stator, then let's say +20v is being produced because of the negative slip of the rotor. *Therefore the net voltage at the battery bank will be instantaneously 140v?

I have a feeling that this is incorrect.. if anyone would like to fill me in on what actually happens with the voltages and their polarity during regen mode, please do.


----------

