"Braking" with GE VFD

Just an FYI, I know Fuji makes some of the drives but SIEI (formerly Amicon) also manufactures drives brand labeled by GE. I think the GV300 is one of those.

Personally I dont care much for SIEI.

I just got done with a meeting with our regional GE Drives Specialist. Here is an update:

1) GE/Fuji builds VFDs for Saftronics. The labelling I mentioned is probably a lead time issue with the contractor.

2) According to the GE manual, decelleration torque can be set from 20% to 150% of name plate. This is without external braking resistors or regenerative braking. The duration of the torque without tripping varies, of course, with the torque level.

3) As was mentioned above, Function 41 can be set to 0, which will modulate the decelleration torque to prevent over voltage trips, In this mode the accelleration time will be varied as required to eliminate the over voltage.

I'll be back on site in a few weeks, and at that time I'll modify Parameter 41 and do some testing on decelleration time. I'll let you guys know what I find out.

Any comments?

i don`t believe it

I just got done with a meeting with our regional GE Drives Specialist. Here is an update:
2) According to the GE manual, decelleration torque can be set from 20% to 150% of name plate. This is without external braking resistors or regenerative braking. The duration of the torque without tripping varies, of course, with the torque level.

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It`s not going to happen!
Believe this!

Tom, basically it comes down to this. An inverter with no snubber braking and no flux braking will have essential NO braking capacity---maybe 1% of motor torque. Your braking rate will be entirely determined by rotating inertia and friction losses in the load.

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Unless these drive are set up in such away that if one is stopping and you can divert the over voltage from the one that`s stopping into one thats still running. That juice has got to go some where and if you don`t get it out of the drive it`s going to show up as smoke! You can pusle it through a resistor, put into an autotransformer or with a four quadrant drive use an external resistor. 2 cents

Originally posted by Guest1:

Quote:
I just got done with a meeting with our regional GE Drives Specialist. Here is an update:
2) According to the GE manual, decelleration torque can be set from 20% to 150% of name plate. This is without external braking resistors or regenerative braking. The duration of the torque without tripping varies, of course, with the torque level.

It`s not going to happen!
Believe this!

Click to expand...

I disagree. The drives ability to produce torque is not fundamentally coupled to it's ability to get rid of energy. Yes, they do affect each other. But they are not the same thing. Think of the disk brakes on your car. If you drive down the road with the brake applied you melt the brakes down. But the brakes will happily produce torque at a consistent level (more or less) until they are completely cooked. A drive will do the same thing. The thing that prevents the drive from blowing up on overvoltage is bus voltage monitoring. The monitoring will either disable the drive or artificially limit the torque to control bus voltage. But in the absence of monitoring the drive will continue to produce decel torque until components start to blow up from voltage breakdown. Like Tom said, the duration of the torque without tripping varies with the torque level. Its actually a torque-speed product, but that's semantics.

I guess the result ends up being the same no matter how you look at it: the drive/motor combination can't decelerate the load. If the drive is shutting down any time you have a decel torque limit above 10% does that really mean the drive can't put out more than 10% decel torque? I don't believe that is true. If you don't correctly account for the cause of the effect, finding a solution can be a bit difficult.

Keith

See Attached:

think about it

I disagree. The drives ability to produce torque is not fundamentally coupled to it's ability to get rid of energy. Yes, they do affect each other.

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The motor can use the energy sent to it, but what happens when you send energy back? I agree if the drive had disk brakes or eqivalent this would be great because disk breaks is a good way to stop, but this is not the case. Unless the drive has something to go along with it to handle regen and you try to stop to fast its got to trip or smoke. Now for GE to claim to have 150% braking torque on a drive the size Tom`s talking about they have to have something in this drive to handle alot of power. I would be willing to bet the only way the drive is going to have 150% breaking torque is after you add the breaking resistors. Not that the drive is not capable of 150% breaking torque, but it`s not going to have any or very little breaking torque without the resistors. I hope Tom lets us know. another 2 cents

Good questions, Guest1. I asked the GE Drives Specialist where the energy went. He couldn't give a firm quantitative answer without some checking, of course. (This is a fairly esoteric problem, apparently.) In essence, though, he indicated that the energy is dissipated as heat - some in the motor, some in the IGBTs, some in the DC bus. I've emailed him this page, and perhaps he will respond here. If I get any more info, I'll certainly pass it on. This has got me bugged.

One thing to keep in mind, is that power and energy are different. At a given speed the power is a function of torque.

hp = T lb-ft x rpm / 5252

The deceleration torque is a function of current.

Power is the rate of energy used over a period of time. The total energy turned to heat is a constant for my blower at a given speed. If it is done quickly you need a lot of torque, and the energy has to be dissipated as heat quickly and requires high torque. The high reverse torque will show up as high current.

So, it appears to me that there are two sets of factors that dictate the answer to my original question:

1) How high a current can the VFD produce to decellerate the motor, and how high a current can the VFD absorb from the motor as it essentially turns into a generator? The answer to this questions dictates the decelleration torque available.

2) How much energy can the VFD absorb as heat, where within the VFD/motor system is the energy converted to heat, and how fast can that energy be transferred to the ambient air? The answer to this dictates how long a given torque can be maintained, and how often the decelleration at a given torque can be performed.

To expand on the auto analogy above, the gripping force and diameter of the brake determines the stopping torque on the wheel. The ability of the brake to dissipate the energy determines how many stops you can make without fade or cooking the pads. Disk brakes aren't better than drum brakes (remember those?) because they provide more torque, but rather because they get rid of heat faster and so don't fade as much in hard service.

Tom, the F41 parameter in your pdf file attachment only applies when there is braking capacity added to the drive. A basic drive will have this parameter available but it is unusable because, with no place to disipate the braking energy, the DC bus Overvoltage Fault will occur before any significant braking occurs.

What it comes down to is that, as soon as the motor begins acting as a brake, the braking energy flows back to the drive and accumulates on the DC bus. The drive, in its basic form, has no facility for disipating this energy--that's NO FACILITY!! So, NO BRAKING!!

In fact, the drive heatsink on modern drives is sized only for motoring energy losses which amount to only about 2% of motor KW. There is no additional capacity for getting rid of braking energy.

Some smaller drives are advertised to have a braking chopper included in the standard furnish. This is simply the "seventh" transistor and functions as a voltage sensitive switch to diverge energy to the braking terminals if the DC bus goes over a specified voltage. This still doesn't get you any braking because there is no place to disipate energy, simply a switch to divert it off the DC bus. Now, if you add a brake resistor to the braking terminals, then you get braking because the excess energy is not only diverted off the DC bus but is converted to heat in the braking resistor. That gets it out of the system instead of accumulating on the DC bus and causing a fault.

You can, of course, set the decel on a basic drive as short as you wish, but if the drive has the extra intelligence to extend the decel time when the DC bus pumps up too high, you will still only get the natural coast down time since no additional braking has been provided for. If it doesn't have this additional intelligence, then you simply get a High DC Bus Fault and you coast to stop anyway.

Bottom line, if you want braking you need to spend extra, either for a braking resistor, a complete snubber brake package, or a regen drive.

Sorry!

I'm currently writing a functional description for a machine we are designing and building. I an noticing there, too, that i don't get my point across well. Something else to work on, I guess.

Tom correctly broke the issue down into it's two component parts. Even if the drive has no way to get rid of the regenerated energy, it doesn't change the fact that it's power section is capable of delivering 150% deceleration torque. It just may not be able to use it because the drive immediately goes overvoltage when it tries to do it.
What's the difference, you ask. It can be a big difference. Earlier in this thread there was a blanket statement made that drives cannot produce as high a braking torque as they can motoring torque. No clarification was made there. So if this were true, adding braking resistors wouldn't help since, according to the blanket statement, the drive can't produce the required braking torque anyway. It's important to correctly understand the source of any problem. This is NOT a torque problem in the strictest sense. It is an energy dissipation problem. It just looks like a torque problem since you can 'get rid' of the problem by decreasing the torque requirement.

My comment about the torque-speed product affecting the duration without tripping was a backward way of saying this is a power or energy issue as opposed to a torque issue. If you found a speed-torque combination that allows you to stop without a bus overvoltage and you cut your speed in half, you should be able to significantly increase your torque since the peak power would drop by half and the total energy would drop by a factor of 4 when using the original torque value.

Keith

DickDV said:

Tom, the F41 parameter in your pdf file attachment only applies when there is braking capacity added to the drive. A basic drive will have this parameter available but it is unusable because, with no place to disipate the braking energy, the DC bus Overvoltage Fault will occur before any significant braking occurs.

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I am not real sure that this statement is correct. As I understood it, the F41 parameter does not provide braking, it simply extends the Decel time as necessary to prevent an overvoltage fault from occuring. I am probably in no way an expert on drives, and have learned a ton from this site, but I can tell you that I had a near identical size GE drive as is being discussed here. During startup, I had decel time set at 45 sec. This was fine under most situations, but during certain process conditions, if there was a shutdown, I would occassionally get an overvoltage fault. I could only stop this from occuring by making an extremely long decel time, like 90-100 secs. I didn't want this decel time all of the time, as 45 secs seemed to work fine most of the time. I changed the F41 parameter to "0", changed my decel time back to 45 secs, and it has worked ever since. No faults. I had assumed that this was because during the few times it needs a longer decel time, it was automatically adjusting for it. Am I thinking wrong?

Thanks,
Marty

Obviously with the drive on regen , then the excess energy will result in a rise in bus voltage , we will assume that the drive is fitted with a small internal braking resistor , that under normal conditions will clip the excess bus voltage .
I have seen a situation with MM420's and an MM440 connected to a common DC bus where the braking resistor was faulty , and instead of the MM440 doing what it was supposed to and dumping excess bus voltage , the bus was causing one of the other drives to trip on overvoltage .
Stick a meter on the DC bus and measure it during a fast stop .

DickDV said:

Bottom line, if you want braking you need to spend extra, either for a braking resistor, a complete snubber brake package, or a regen drive.

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I understand what you are saying, Dick, but to some extent this flies in the face of my experience. We have never used a drive with regen or braking resistors. The coast down time on these machines when the power is simply removed is around five minutes. But if a VFD is used to provide a controlled stop I have been observed the blowers going from operating speed to full stop in 30 to 45 seconds. Obviously the energy is going somewhere!

Another consideration is deceleration during speed changes while the blower is tracking a flow setpoint change. A speed decrease of five to ten Hz takes place within three of four seconds. Of course the torque required to move the air helps in decelleration, but this is also true in a coast down if power is removed. This leads me to think that at least some decelleration is being provided by the drive.

I think Marty's explanation of how the F41 parameter works when set to zero is correct.

Believe me, I'm not arguing for the sake of argument. This phenomenon has me puzzled, and I'd like to understand what is going on.

martyh, I don't think we disagree on F41. I was simply referring to the fact that F41 doesn't give in the software any braking that is not provided for in the hardware.

You can set a zero there, as you did, but it doesn't add braking either. It just prevents a DC Bus Voltage Fault by extending the decel ramp.

Tom, if natural coast to stop on your fan is five minutes, and using the drive decel ramp reduces it to 30-40 seconds, then, without a doubt you have braking capacity in the system somehow. I don't know how since I'm not that familiar with the brands you are using.

There must be some braking capacity added to the drive in its standard form. I'd be interested to what is added to make braking happen.

I believe you said you use C-H and A-B. A careful reading of the specs or a call to Tech Service might get an answer. I've never encountered an Allen-Bradley AC drive with even 1% braking capacity standard but, who knows, maybe that's changed.

If you find out or if someone close to these brands is on here, I sure like to know what's going on.

This is, indeed, an energy dissipation problem. The motor has kinetic energy. Newton says that without some opposing force being applied, the motor will spin forever.

I've already mentioned some of the forces that are contributing to the motor stopping: bearing friction, windage (different source of friction), PN losses, transmission losses, etc. If the motor has more energy than these losses can account for, then its energy will be converted into something else. In this case, that "something else" is DC Link voltage due to the inverse-voltage protection diodes across the IGBT's that act as a rectifier when the motor regenerates electricity.

This energy is going to be fed into the capacitors on the DC Link bus. There is nowhere else for it to go. There is no way for the drive to stop or even otherwise mitigate the accumulation of this energy as voltage on the DC bus, except by giving it an alternate path, such as a braking resistor.

Let's go back to basic induction: When a magnetic field has relative motion (perpendicular) to a conductor, an electro-motive force (EMF) is produced in the conductor. The spinning motor has a magnetized rotor, and its stator windings are perpendicular to this rotating magnetic field, by design. This is going to produce an EMF in the Stator windings.

The Drive's input rectifier, assuming it is a diode converter front end, cannot allow this force to be passed to the incoming line. Therefore, the only place for the energy to go is the DC link. However, the DC link is being supplied by the incoming AC (rectified). Assuming a 460V drive, then you'll have an absolute maximum of about 650 VDC on the DC bus, assuming peak charging of the DC link capacitors.

The Motor's EMF is either going to be voltage or current, or a combination of the two. Since the motor is (for the first few milliseconds) not generating enough EMF to overcome the incomer line, the EMF becomes a voltage on the motor terminals and the drive output terminals. Once this voltage exceeds the level necessary to forward bias the IGBT-protection diodes, then current will flow from the motor onto the DC link. This regenerated motor voltage MUST exceed 460 VAC in order to forward bias the inverter's diodes. This increased voltage will "pump up" the DC link voltage unless it is dissipated. That's what the chopper and braking resistor do; they dissipate the motor's regenerated energy in the form of heat.

If the drive is providing a "deceleration torque," then it is providing power to the motor. Torque times speed is power. Speed is changing (due to the deceleration), so you have torque and (delta) speed, which is power. This power is opposing the energy from the motor. Where does the motor energy go?

The only places for the energy from the motor to go are:

1. Back onto the line, to be used elsewhere. This is not the case, as the front end converter is not bi-directional.

2. Be converted into heat in the motor. This can be the case, as the drive can provide a constant DC to the stator windings. This DC will provide a high flux level, which provides only a braking torque to the rotor. The energy in the motor will be converted to heat in the rotor windings. (See Lens' Law or Flux braking on an internet search for more information.) Any "switching" of the inverter devices to accomplish the negative torque will, likewise, cause some rotor heating.

3. Be converted into heat elsewhere, such as a braking resistor as already discussed.

4. Have the current path for the EMF interrupted. This will allow the rotor's magnetic field to dissipate, causing no more regenerated EMF, and therefore no more braking. The motor will coast to a stop with only friction losses causing it to slow. Any drive power supplied to the motor in this case will serve to re-magnetize the rotor, and give us the other three scenarios once again.

I see no other place for the motor's energy to go. Any other method of braking must account for the rotational energy's conversion. The friciton brakes that have been discussed convert the rotational energy into heat, either in the drum or the disk.