VFD class / automotive / help with regen braking

leitmotif

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Join Date
Nov 2004
Location
Seattle Wa. USA
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Am teaching a class Oct 18 and 25 Basic VFD and 3 phase motor applied to electric vehicle.

Have VFD and motor setup made by Fuji for GE field demos (thank you Steve Bailey).

VFD is GE AF 300 B rated 1/2 HP 230 VAC
Motor is Fuji 1/4 HP 200 VAC 3 phase 1800 RPM

Built model car to demonstrate VFD and motor operation start stop accel decel etc etc. Works fine on level smooth floor. Motor drives car via 28:1 gearbox (not a worm) and then a 20 tooth driver and 30 tooth driven gears.

Factory VFD and motor setup works just fine and braking works. Set on bench and wrapped line on gearbox output shaft (5/8) and lifted and lowered 100 lb weight. Braking was tested with 50 lb weight. Started in downward at 60 Hz and dialed freq back to 30 or 40. Slowed and held load fine. Further dial down to say 5 or 10 resulted in loss of braking and load runaway. Unfortunatelly I did not measure braking voltage in this testing.

Want to demonstrate regen braking and use lites to show when regen is active. Problems at this stage:
1. Model car lacks traction (with heavy load drive wheel breaks loos and car skids down ramp). Working on that.
2. Only value I know for existing braking resistor (factory stock "default" is 180 ohm. Do not know voltage produced under braking, do not know wattage value for brake resistor.
3. Based on ignorance in #2 I have no idea of how many 60W 120 VAC lites 17.7 (cold) ohms I need. Picked these as an arbitrary starting point - can change lite bulb parameters as / if needed.
4. Have enquiry into GE local rep have not heard back yet.
5. Tried forum search "braking resistor values" "braking resistor" no luck.

Using car to demonstrate regen may not work (may not overcome traction problem and may have to have very heavy load - toting 400 lbs to class is not attractive). Motor gearbox as capstan may be only option. While mechanically this is no different than a car students may not easily recognize it is the same - trying to make class as "student friendly" as possible.

Still want to demonstrate the regen action using lite bulbs. Open to ideas suggestions or other illuminating thoughts.

Dan Bentler
 
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leitmotif said:
1. Model car lacks traction (with heavy load drive wheel breaks loos and car skids down ramp). Working on that.
2. Only value I know for existing braking resistor (factory stock "default" is 180 ohm. Do not know voltage produced under braking, do not know wattage value for brake resistor.
3. Based on ignorance in #2 I have no idea of how many 60W 120 VAC lites 17.7 (cold) ohms I need. Picked these as an arbitrary starting point - can change lite bulb parameters as / if needed.

Are those Lights or (Miller) lites?

Seriously though, it sounds like you have a pretty cool project. Under resistive braking, the drive connects the braking resistor across the drive's DC bus. You probably should check it with a meter, but the DC bus should be about 324VDC (230V * 1.41)
 
"Loss of braking and load runaway" ?

Sounds like you need either a better motor model in the VFD, or an encoder for feedback.

Typical Vector drives should be able to hold full torque at zero speed without losing control.

For light bulbs, I'd start with enough in series to meet the voltage rating of the DC Buss on the drive * 1.20 (or thereabouts).
 
jimbo3123 said:
Are those Lights or (Miller) lites?

Seriously though, it sounds like you have a pretty cool project. Under resistive braking, the drive connects the braking resistor across the drive's DC bus. You probably should check it with a meter, but the DC bus should be about 324VDC (230V * 1.41)

Gave up on any kind of beer around electrical a long LONG time ago. Hydraulic lunch is not a good idea for young & dumb Navy electricians.

One other drawback I am dealing with - I do not have a drawing of internal wiring of the VFD. GE has great connection diagrams to hook it up for almost any configuration BuT no detailed info on VFD "guts" nor on the braking resistor. I think you just answered my question of why there is no voltage on the braking resistor terminal P and DB while the motor was running -- there is a switch - aha!!!!!!

Tried using 5 130 volt 6 ohm (cold) lites in series. Did not get a glimmer - couse with no (limited) traction it was hard to get dynamic braking. Think I may have solution to traction problem.
After that is fixed we go back to electrical problems.

Another unfortunate reality is the idea that is starting in my head that I am trying to cover too much ground in an introductory basic VFD class. Maybe the regen braking is a topic better covered in an advanced class. I am in process of putting it together for the first session and I may (??) be overlooking the keep it simple rule.

Dan
 
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Dan, I wonder if it might be easier to loop the wire to the brake resistor through a current transducer and have the transducer drive a meter. That way you wouldn't have to worry about matching the resistance and wattage of the lamps to the existing resistor. It might even turn out to be easier to observe the deflection of the meter than to detect changes in the intensity of the lamps.

I don't think you should ignore the regen aspect with the class. When you're applying electric motors to vehicles, regen is basic. What's the point of putting an electric motor on a vehicle if you don't use the vehicle's kinetic energy to recharge the batteries.
 
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Steve Bailey said:
Dan, I wonder if it might be easier to loop the wire to the brake resistor through a current transducer and have the transducer drive a meter. That way you wouldn't have to worry about matching the resistance and wattage of the lamps to the existing resistor. It might even turn out to be easier to observe the deflection of the meter than to detect changes in the intensity of the lamps.

I don't think you should ignore the regen aspect with the class. When you're applying electric motors to vehicles, regen is basic. What's the point of putting an electric motor on a vehicle if you don't use the vehicle's kinetic energy to recharge the batteries.
Steve

BRILLIANT idea. (pun as poor as it was not intended)
I really like that - still visual and a lot simpler and a lot more compact.
Suggestion for current transducer P/N etc? The trouble is I do not know yet if the voltage at terminals P and DB is AC or DC. If you know pretty much off top of head then great -- otherwise let me get it to do regen again and take a measurement.

I just knew I was going to the right place for inspiration.
All goes to show one guy does not have all the good ideas.

Thanks for feedback on the regen. At least I am not going willy nilly and biting off more than I can chew.
Dan Bentler
 
I'm not sure about the voltage either. When the load drives the motor, the motor is an AC generator, but the bus is DC. With no brake, an overhauling load generates a bus overvoltage fault, so you would think that the generated AC current has to get rectified before getting dumped onto the DC bus. The question is, where is the braking resistor in the circuit?
 
The braking resistor will be across the DC bus through a chopper transistor. I don't think you really want to use the light bulbs, though. Their resistance changes significantly between cold and operating temp. For example, the hot resistance of the 60W lightbulb you talk about is 240 Ohms. You may have a hard time getting a high enough resistance when cold to stay below the chopper current flow but low enough when hot to allow you you burn off bus energy. I would find a power resistor for this app.


Keith
 
OK let us think about this for a minute.
Power generated by motor when being driven with overhauling load
1. NO BRAKE RESISTOR
would have to be rectified and then connected to DC buss. Voltage generated by motor would have to be greater than DC buss voltage or DC bus supply turned off?

2. WITH BRAKE RESISTOR
power from motor would go directly to resistor. There is no connection to DC bus. There would be no need to rectify. There would have to be a switch in the braking circuit. If there werent then the braking circuit would drain power from the motor supply leads.

Sure would be nice to have a schematic of how GE AF300B is wired internally. Would not have to be pestering you guys.

Dan Bentler
 
With no brake resistor, energy from the overhauling load gets stored in the bus capacitors. When they get fully charged the bus voltage goes high and the drive shuts down on a bus overvoltage fault.

With the brake resistor connected, the energy from the overhauling load gets directed across the resistor and dissipated as heat. If the resistor is not properly sized to the load, it will overheat. Generally there is a thermistor included in the braking resistor which you wire in series with the motor thermal leads.

Yes, there is a switching unit that directs the current flow across the resistors when the motor is driven by the load. If you were to apply the VFD to a vehicle, you would replace the braking resistor with a battery charging unit.

Thinking about it, the braking resistor circuit must be DC. There is only one resistor circuit. If if was AC, there would have to be three resistors, one for each phase.
 
Steve

Found a drawing in a textbook for dynamic braking. It says exactly what you said. Amazing what you can get from a drawing. Thank you for your time.

OK now that I know it is DC I know how to measure.
Dan
 
leitmotif said:
OK let us think about this for a minute.
Power generated by motor when being driven with overhauling load
1. NO BRAKE RESISTOR
would have to be rectified and then connected to DC buss. Voltage generated by motor would have to be greater than DC buss voltage or DC bus supply turned off?

Yes...a simple difference in potential and control board switching. The VFD control board and the exisitng IGBTs perform the rectification quite naturally:

When the motor is regenerating, it is capable of feeding AC variable frequency power back through the IGBT modules to the DC bus with help from the VFD brain. The drive is monitoring and controlling the current flow but the cause of the flow is the fact that the peaks coming back "out the top" of the IGBTs is greater than the DC voltage of the capactor banks making up the DC bus. The drive is also watching the DC voltage level of the Bus. When it exceeds a safe limit, the controller quits firing the IGBT gates, lets the motor coast, and possibly sets a fault (sometimes fault levels and resposes are configurable).

leitmotif said:
2. WITH BRAKE RESISTOR
power from motor would go directly to resistor. There is no connection to DC bus. There would be no need to rectify. There would have to be a switch in the braking circuit. If there werent then the braking circuit would drain power from the motor supply leads.

I think this is incorrect for modern drives. If the potential difference at the motor is greater than what is present at the IGBT terminals, then current will flow onto the VFD DC bus.

My understanding of braking resistors is that there is another set of solid state devices between the resistor and the drive DC Bus. When the voltage level is detected to be above the level which indicates that regeneration is occurring, the single swithing device will switch on and burn off some of that voltage as heat in a single resistor.

I have seen some older drives which use external contactors to appy regeneration directly to three resistors. In those cases, your statement ... "power from motor would go directly to resistor. There is no connection to DC bus" ... would apply correctly. Those systems were higher maintenance and sometimes required specialized overlapping contactors which actually would momentarily soak up power from the VFD output when engaged. One of the worst panel fires I ever had to help repair was when one of those overlapping contactors auxilliary contacts got stuck and failed to shut down a VFD forcing the resistors to handle regenration from an extremely high inertia load in addition to the drive output. I would think those types of regeneration circuits are not the stuff of electric cars, when DC bus and line regeneration are so readily available now, and much more efficient and reliable.

Most of my recent experiences with resistor braking (aka dynamic braking) have the Solid State Brake Ccontroller wired directly to the DC Bus and a single resistor. Some drives have an extra IGBT for this built in with two external terminals and you simply choose a resistor, hook it up, and configure the drives braking properites. Other, more generic systems have a package which includes a separate solid state switcher and resistor connected through the DC bus terminals through fuses or overload contacts for over current protection.

The Ohms are usually spec'd as a range, and the wattage too. The more current the braking IGBT or SCR can handle, the more current can be burned off by choosing the biggest baddest toaster element you can afford.

There are also drives which can actually use the input circuit (converter section) of the VFD in reverse to put AC back onto the incoming line. See Line Regeneration.

I hope DickDV fixes any errors I may be making in my redneck understanding of this...

leitmotif said:
Sure would be nice to have a schematic of how GE AF300B is wired internally. Would not have to be pestering you guys.

I have a GE-Fuji drive with a simliar part number at work which is no longer in service. I also have some pdfs for it at work...if things are slow enough tomorrow, I will see if it has any helpful info.

I would expect, in a vehicle, you would not care about line generation, and be more concerned with having a big fat DC bus and some sort of charge controller that could more gradually feed that excess current back into batteries rather than wasting it as heat through resistors, but that is an advanced subject for the old baby blue Fleetside to maximize mileage, not necessarily applicable to your class.

Paul
 
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QUOTE I would expect, in a vehicle, you would not care about line generation, and be more concerned with having a big fat DC bus and some sort of charge controller that could more gradually feed that excess current back into batteries rather than wasting it as heat through resistors, but that is an advanced subject for the old baby blue Fleetside to maximize mileage, not necessarily applicable to your class.
UNQUOTE

Paul first and foremost thank you for confirming my confusion and confirming what I learned when I finally found the drawing for dynamic braking in the textbook.

I agree -- with VFD in vehicle the way to go is use dynamic braking to charge battery. That is why I am going to the effort to be able to show visually when the motor is being dynamically braked. My thinking was "when these lights come on the motor is acting like a generator ie dynamic braked and this power can be used to charge battery" Seeing is believing ??

IF you have the time and can find a drawing that would be great.

We're gonna get fully independent 4WED (E for electric) on that Jeep yet.

Dan
 

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