Thread: VFD - Reforming capacitors

Need to hook up a Siemens MicroMaster VFD. Worked fine the last time it was used but it's been a couple of years and the instructions clearly insist on reforming the capacitors before putting it back in action. Simple process of 2 hour holds at 60v, 120v, 180v and 240v.

Simple if I had a variable AC power supply which, of course, I don't. Seems like a number of inexpensive 0V to 120v variacs on eBay. No idea how an AC variac works. Can those be hooked up to 240v and give me 25, 50 and 75 percent of input voltage or will I just let the magic smoke out of the variac by doing this?

Thanks.
Steve

Steve

You can use a 110 Variac without issue. They do go "over voltage" 10% or so
So 50V 110V, and 135V Pretty close ;-) Then right to line.

NOTE I am not saying to hook up a 110 Variac to 220, I'm not sure how that would work, Might be OK, no real load just to energize a VFD.

I have a 240V variac on the bench also, Paired tandem coils on a single shaft. I did use it to bring up a long stored 440V VFD in the same way. It could be there is a good reason for the tandem construction. I have not thought about it.

I could be completely wrong, but getting the initial low voltages in might be enough.

I have a pair of the MicroMaster 420s, Both 5.5 Kw One on the band saw, and the other powering the Weiler lathe. They work well!

Cal

another maybe cheaper way that will work: pull old lamp out of junk bin.... cut cord and use the resulting 2 wires to feed the ac input to ur vfd (prob any 2 of 3 ph inputs if 3 ph shud be ok).... put in small wattage bulb (25w if u got one), then bigger wattage after an hour or so.... then do same but with 2 bulbs in series with 220v in....... altho 120v current limited input from 1 60 watt bulb is 99.9% sure to sufficiently reform it w/o harm... u really dont need to go upto the full 220vac input...

btw, some drives wont supply the ac input to the dc bus caps unless above a certain voltage so u shud measure across the big caps with voltmeter while u r doing this to verify the juice is getting to them.... if not, u will have to figure out why not (input softstart relay or plain old input contactor, etc etc.... if push comes to shove just stick a little like 1n4004 diode in series with the lamp cord and put directly across the cap bank...

Use a 120V variac with a 240:120 V single phase control power transformer.

Feed the output from the variac to the 120V leads on the transformer and connect the drive to the 240V leads from the transformer and start your reforming activities.

Often the control power transformer can be operated from either 240 or 480 volt primaries - using the taps on the transformer you can then reform either 240V or 480V drives using this setup.

MG

Thanks for the tip!

I have all those elements on my bench.....

now I just need a thoroughly "rested" VFD ;-)

Use the 'lamp' trick, and wire in an ordinary dimmer switch in series too. Use a couple of different bulb sizes (a 15w, 25w, 50w, and 100w?) to give you some choices...

wow. if u add up the sum of the parts they equal more than the inital thoughts themselves funny thing is we use ALL these ideas when we reform 480v drives! 120v open style variac mounted in an old discarded servo drive case, with a $ 1.99 harbor freight digital vom mounted on top wired to the output, then thru 100 watt lite bulb in socket, then into small 250va saved from trash 120->230/460 xfmr! dont laugh! its on the shelf and used 3-4 times a year!

Thanks for the ideas. Can't believe that with the number of times I've calculated voltage drop for a circuit it never occurred to me to just add a resistance to the 120v legs to force a voltage drop. Duooh!!

Now if I can just find the box with the dimmer switches!

Steve