Off Topic - 575V VFD Needed

[Steve Etter]

Pierre,

Are you saying that the multi-speed inputs have no affect when in reverse? If so, at what speed do you go in reverse assuming you can go in reverse at all?

Steve

 

[DickDV]

Plain line reactors are simply series inductors placed in all three three-phase legs. They can be placed in the supply leads to the drive or in the motor leads coming out of the drive. Their effect on drive-motor system performance varies significantly depending on which place you put them. In every case, however, being simply inductors, they display a low impedance to low frequencies and higher impedance to higher frequencies, just as you would expect. Since they are inductors, you can expect them also to drop a small amount of AC voltage even at 60Hz.

Reactors are occasionally spec'ed in microhenries but more often are spec'ed in percent. This is highly confusing and leads to no end of trouble and, of course, more snake oil and false claims by the manufacturers. Just for the record, when a reactor is rated in percent, it means that, at its rated voltage and maximum continuous current at its rated frequency (60Hz, of course, for US reactors), it will drop its rated percent of the supply voltage across its terminals. So, by example, if you have a 5% reactor rated for 460V and 5 continuous amps, it will drop 460V times 5% or 23V when it is conducting 5 amps.

When reactors are placed in the supply leads to an inverter, the usual purpose is to prevent high frequency harmonics from the drive getting back into the AC supply grid. This is usually a no-problem deal unless you've got low voltage to start with. In that case, the extra voltage dropped across the reactor at heavy loads can get you into trouble since the drive cannot output more voltage than it gets in. Some other harmonic reduction scheme may be needed for this situation.

When reactors are placed in the motor leads, the usual purpose is to block high frequency components in the drive output pulses from traveling down the motor leads and causing trouble in the motor insulation. This is not usually a problem unless the motor leads get long enough to cause the high frequency components to reflect back from the motor towards the drive where they encounter new pulses coming from the drive. When that happens, the pulses can become additive and the motor insulation sees much more voltage than it should. Of course, the reactors also drop voltage as current passes thru them and you can again get low voltage conditions at the motor altho this is not usually a big deal. The bigger problem is on precision speed and torque requlating sensorless vector drives where the drive cannot properly identify motor operating parameters because it can't see the motor accurately thru the reactors. The drive is left to include the reactors in the motor model even tho they don't belong in the model. On open loop or closed loop (shaft encoder feedback) systems this is not a problem.

There are additional complexities with motor lead reactors when you are dealing with motor overspeeds and output frequencies over 60Hz but I think the above will suffice for most users.

Hope I've helped clear up some questions on this.

 

[Eric Nelson]

Another Question For Our 'In-House' Drive Guru...

...at heavy loads can get you into trouble since the drive cannot output more voltage than it gets in...

This brings up a question I have regarding motor nameplate voltage vs. incoming line voltage. Early this year, I had 2 A-D GS1 200V class 2HP drives fail within the first few days in the field from what I suspect was low incoming voltage. Both customers have 208V power, but I'll bet it only actually REACHES that 208V on a 'good' day...

Both machines were used with 230V Baldor 2HP motors, so I had shipped them with the "Motor Nameplate Voltage" parameter set for 230V. According to Baldor, these motors are 'Usable at 208V', so when the customers replaced the drives, I had them set that parameter to 208V and have had no more problems since (Eric crosses fingers).

Since I was A.) Using these drives at their maximum 2HP rating, B.) Asking for 240V to the motor, and C.) Only giving them (at most) 208V, was I just asking for trouble? Also, does my changing the nameplate volts from 230 to 208V really help, or am I just pushing my luck?...

Since then, I have switched to the new Telmecanique Altivar 11 drives since my Schneider distributor beat A-D's (already low) price for the same size drive. These Tele 2HP drives have a M-U-C-H bigger heatsink than a 2HP GS1, so maybe they will better at withstanding any low voltage torture?...

In the future, might I be better off switching to a larger (say 3HP) drive? Or would that just be a waste of money?...

beerchug

-Eric

P.S. Here's a photo of the bridge rectifier in one of the dead drives. It doesn't smell too good either...
[attachment]

 

[DickDV]

I am not familiar with all of the design details of Baldor drives but, assuming that they are similar to most other drives out there, I don't think operation on low voltage will cause damage. In most cases, if you loose an input bridge, you have a drive with inadequate input surge protection designed in(a common cost saving move).

As far as operating a motor with a higher nameplate voltage rating than the supply, it is important to enter motor nameplate data exactly as it is presented on the nameplate. The entered frequency and voltage usually determine the slope of the output Volts/Hertz characteristic curve. This in turn defines the available torque from the motor.

If you have a 230V motor and enter 208V for nameplate voltage, you cause the Volts/Hertz curve to lay a little flatter effectively reducing the voltage to the motor at all speeds by the proportion of 208/230. On the other hand, if you set the nameplate voltage for 230V and only have 208 to deal with, the drive will provide the proper voltage to the motor at all speeds up to the point where it runs out of voltage (at 208). This way, only at speeds above the 208V point will the motor be a little short of voltage.

However, none of this should result in a drive failure. The motor will require more current at a given load when receiving low voltage so the inverter will have to conduct that higher current but, within the design limits of the drive, that should not result in drive failure.

I my view, based on what you have told us here, you have plain old drive component failure, not a misapplication.

 

[Eric Nelson]

Thanks for the great info Dick!...

Now that it seems they were, in fact, defective, I guess I should have returned the drives to A-D. Oh well, they're so cheap we just replaced them for the customers at no charge, but if ANOTHER one blows, you can be sure I'll be on the horn to A-D!... :nodi:

beerchug

-Eric

 

[CaseyK]

Eric

Being extremely conservative, I would probably go with the 3 hp myself.

Most of the inverter bypass panels I did at GE were for Baldor. This was several years ago. The drives were okay, and at the time, the motors wee fantastic. Lately, reports I've received are the drives are fair, motors are fair at best. As Dick said, you should have been okay.

When I requested info / PM's about A-D's drives and motors, I will say that I was not flooded with favorable responses. On a scale of 3, they rated a 2.0

I have seen 230 on 208 several times without problems. I did projects in one plant were some areas had 208. I never knew of any problems.

When I can't explain a failure like that, I blame it on a transient voltage pike, cuased by a power company switching operation. Has about 90% acceptance rate. Feel free to use it.

best regards....casey