rotary phase converter problem

Anyone know if on a rotary phase converter if anything in the "box" (with the caps and contactors) could cause over heating of the induction motor that is part of this system? I've got one getting hot to the point where I can smell the insulation on the windings burning. A 2 hp motor load is being used and the convertor is rated 5 hp using a Siemens induction motor. The box has 4,

90/5 uf 370 vac caps. A contactor which energizes the system motor, which works, (it pulls in till the motor gets to speed and then drops out), and a time delay relay that I don't know its function. Just started getting hot after 14 years of moderate use. The converter motor comes up to speed and does supply the 3 phase. (Or at least it is running the machines). I am thinking there is a short in the coverter motor windings...? Also are the caps in the box used to start the motor or do they serve some other purpose? Any feed back greatly appreciated. Thanks
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the "box"

the induction



The box has 4,


then drops

The converter

least it is

the coverter

they serve


If they are metal can capacitors they would be in the circuit at all times..if one of those failed the motor would slip and overheat...but they are probably black plastic capacitors in which case they should only be in the start winding circuit on start up as you describe.

bad windings would tend to fail not just run hot. have you checked for bad bearings and rotor contact with the stator? have you checked the amperage draw to be sure the coverter is not being over loaded?

If you have a mix of black plastic and oil filled metal capacitors...remove the metal can capacitor and replace it. Its about due after 15 years. To check it...remove it, use a RESISTOR to short across the leads (300 volt charge so watch out)... then put a standard ohm meter across the terminals, not a digital one. but with a dial...the needle should rise briefly then fall back to zero as the capacitor takes a charge from the meter battery then discharges, If it tests open or shorted with no needle blip..replace it.

The run cap is generally left in the circuit at all times... I have no idea what the time delay relay is is it wired?

have you checked all 3 legs for balanced power...

You can use an ohm meter to check the windings to ground when its depowered...any read at all is bad windings...for a better check buy a megohm meter...follow the directions.

Phil Scott

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Phil Scott

Some of the phase capacitors may be bad. Read your three phase voltages. you should get approximate matches, but it will be slightly off since the phase shift of the third phase is only 90 degrees because it is generated by the capacitors.

Not enough voltage, too much current.

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Does the phase converter start right up or does it take awhile to come up to speed? Bad capacitors or connections in the phase converter junction box will make the converter start slowly. A phase converter can get hot if it's oversized. Has there been a change in the size of the load? My experience doesn't match the other respondent's. It seems like there is a big variance in the voltage on the manufactured leg. The variance seems to be larger with some brands of phase converters than others. This is on pivot irrigation systems with a constantly changing load. Dean

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Dean Hoffman

On Mon, 4 Oct 2004 00:00:18 -0400, put forth the notion that...

I build phase converters, and might be able to help. Your time delay relay is probably being used to switch the start caps out of the circuit. You say you have four 90/5 mfd caps. I'm assuming you mean dual rated caps with three terminals on them instead of two. If that's the case, they're run caps (metal cans instead of plastic). If those are the only caps you have, all or some of them are being used as start caps, but may also be switched over to run mode after the idler motor starts. If they're wired in parallel, 4X90 is a lot of run capacitance for a 5-HP converter. Assuming nothing has changed since you've owned it, my best guess would be that either the time delay relay is no longer timed properly, or you have some bad capacitors. Check your voltages from phase A-B, B-C, and C-A. They should all be within at least 10% or better. Here's a link to one of my converters:

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