Current Limiting Device for 690v Circuit

If I understand you correctly you are trying to power up 60 variable speed drives (or similar) that have been (say) sitting around for a while and so need to reform the electrolytic capacitors that are "leaky" due to excessive leakage current.

If it were me (and I test and service variable speed drives) I would do the following

1) For your test/service setup, switch to a fuse or circuit breaker with a motor rated curve to handle the inrush current. Standard fuses and circuit breakers are not designed for these high inrush currents and will likely operate.

2) Limit the inrush current and power (protecting both the capacitors and the fuse) by either dropping the supply voltage to the drive using a variac (ideal) or inserting a resistor in series (not ideal or recommended be me unless you have no access to a variac). I would suggest using the variac option as this allows you to ramp the voltage up which greatly reduces the chances of stressing and blowing out the drive's input protection current limiting resistor (or similar).

3) Test one drive at a time.

Chances are just performing 1) and 3) should fix your problem. I could go on about opening up the drive and disconnecting the capacitors and charging them up from 0V using a ramping charge voltage but this is only really applicable for large (or possibly very old) drives where disconnection is possible and where the person doing the testing really knows what they are doing (it is rather dangerous).

Additionally drive manufacturers suggest leaving the drive (and hence capacitors) powered for 8 hours prior to connecting the drive to a motor (load), I guess this is to help capacitor reforming but I never do it I just run the drive under light load.

Correct only going to power one invertor at a time. Use of a variac will be a good idea. Thankyou

Please excuse a reply from a mechanical engineer, but I was under the impression that one of the uses for inductors is to limit inrush current in situations like yours.

As far as I understood the main problem is in-rush current in an emty capacitor. So the sollution can be to charge the capacitor with a limited current. Normaly in large UPS and frequency inverters a resistor in parralel with DC bus contactor is used at first to equal the voltage potential on the rectifier side and on the capacitor (or battery). Only then the contactor is closed.

I have used one of these devices when the active-circuit current limiting scheme went up in smoke on a large rack-mount audio amplifier I was repairing.

Ametherm NTC surge thermistors

It looks like they have a wide variety to choose from, and they were willing to send me samples at my request. Perhaps one of these would work for you?

Tom D.

This can be best done with an off the shelf DC power supply with adjustable current limit for one drive at a time, if the DC terminals of the drive are accessible. Set the current limit to 250mA...800mA depending on the size of the drive. Connect the DC supply to DC terminals of the drive and ramp up the DC voltage to 1.35*Rated AC Input voltage of the drive. Supply the drive for atleast an hour. If you are doing this simultaenously for 60 drives, you will have to know the total capacitance of all the drives connected and size the charging resistor correctly.

The problem with using an NTC Thermistor or any resistor and shorting contactor for THIS application is the cumulative effect of all of those VFDs. It would work if you ALWAYS had EXACTLY the same number and sizes of VFDs connected where you can calculate the proper amount of series resistance to limit the current. But too much and you don't do the job before shunting it (or the thermistor fails to heat properly) and too little means you don't have the proper effect.

As Jack OAT said, the proper way to do this is with a variac. A motorized one would give you the ability to ramp the voltage slowly over time.

Thanks JRaef for that info, I didn't really know much about the application, and probably should have said so. Those thermistors are good and simple fit when applicable.

Tom D.

ntc or ptc resistors seems the easiest to me. you might be able to pull of an active current limit by using a shunt regulator and an high voltage fet. look at page 31. http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=tl431&fileType=pdf

dont forget to calculate the power in the resistors.