Machine Rotary Phase Converter Problem

[L98Fiero]

I have a 15 hp RPC that has run well for most of 20 years but has started acting up.

Initially I had a start cap blow up about three months ago while the RPC was running, then about two weeks ago, it blew again, this time on start up. I replaced it and thinking I had done something wrong, started it again and after it ran for 15 or 20 minutes there was a rattle of contacts an the 'on' light flickered.

I checked all the capacitors and even replaced the potential relay and it started but after a short time the rattle returned. With that in mind, I took the complete unit to the distributor who checked the system, installed a physically larger start cap of the same values and gave it a good bill of health so I brought it home, connected it and had the same results.

I then left the cover off so I could check the voltages while running and see what was happening. When the RPC was running with nothing connected to it, when the problems started again, I could see the rattle was from the potential relay but also, there were sparks at the terminals of the start cap and one of the electrolytic run caps. There was also evidence of the drain resistor on the start cap getting hot enough to mark one of the leads that was resting on it.

Does anyone have an idea where to go with this next? BTW the run caps all checked out within about 3% of nominal.

 

[BaitMaster]

1. Check the stakon terminals that connect to the caps. They have a tendency to loosen up and cause issues. Could be the sparks your seeing.

2. Check the start cap contactor contacts. This should be turned on and off by the potential relay. Those contacts, if welded together, or sketchy in any way, could cause this. Sometimes the pads fall off the bus bar and get loose, then intermittently make contact.

3. Check the motor connections if they are in the peckerhead of the motor. A loose connection in there could cause this.

4. Check your 1phase breaker. Sometimes the contacts get crappy in there and won’t trip it, but will lose contact intermittently under load.

5. Check your main contactor contacts for the same.

6. After replacing the main breaker, and checking all of the above, check the potential relay by replacing it with a timer if readily available and a potential relay is hard to find. Use an on delay timer and a normally closed contact to disengage the start capacitor contactor about 0.3 seconds after the main contactor engages. The RPC should run as normal in this configuration and if it does, it’s the potential relay. You could leave it like this or replace with the relay.

Hope this helps, keep us updated.

 

[L98Fiero]

1. Check the stakon terminals that connect to the caps. They have a tendency to loosen up and cause issues. Could be the sparks your seeing.

2. Check the start cap contactor contacts. This should be turned on and off by the potential relay. Those contacts, if welded together, or sketchy in any way, could cause this. Sometimes the pads fall off the bud bar and get loose, then intermittently make contact.

3. Check the motor connections if they are in the peckerhead of the motor. A loose connection in there could cause this.

4. Check your 1phase breaker. Sometimes the contacts get crappy in there and won’t trip it, but will lose contact intermittently under load.

5. Check your main contactor contacts for the same.

6. After replacing the main breaker, and checking all of the above, check the potential relay by replacing it with a timer if readily available and a potential relay is add to find. Use an on delay timer and a normally closed contact to disengage the start capacitor contractor about 0.3 seconds after the main contactor engages. The RPC should run as normal in this configuration and if it does, it’s the potential relay. You could leave it like this or replace with the relay.

Hope this helps, keep us updated.

Not being an electrician, I assume the 'peckerhead of the motor' is the junction box on the motor, and yes, they are tight, that was actually one of the first things I checked, so simple I never even thought of mentioning it, one would think the distributor would have checked it too.

The contactors are not welded, that also was checked though not pulled apart to physically check if the contacts were loose. I'll change the 1 phase breaker, hopefully that will help.

 

[BaitMaster]

I would pull the contactors apart if you can. If they are a NEMA type, it’s easy.

They didn’t fix it, so I wouldn’t put too much weight into what they did. Everything is suspect. They likely did not run it with a load. Just bumped it without a load and said yup it’s good.

Peckerhead is indeed the motor junction box.

90% of the time with electrical issues it’s something dead simple. I’ve seen “experts” miss the simplest problems and send bills for it.

I drove 6.5 hours there and back to diagnose and repair a packaging machine. It was a loose wire. One loose wire. Half an hour of work, 13 hours travel. The client had 3 separate local contractors come and spend 6+ hours a piece, fail, and send bills.

There is no problem too simple for someone to miss.

 

[L98Fiero]

I would pull them apart if you can. If they are a NEMA type, it’s easy.

They didn’t fix it, so I wouldn’t put too much weight into what they did. Everything is suspect. They likely did not run it with a load. Just bumped it without a load and said yup it’s good.

Peckerhead is indeed the motor junction box.

When I ran it this afternoon, it was without a load and there was still a problem.

 

[BaitMaster]

Then do the breaker. If that doesn’t work, bypass potential relay with a timer. Should be one of the two if contactors and connections are all good.

 

[Susquatch]

@BaitMaster has good advice.

The first chapter of any decent electrical repair guide should say: Connections, connections, connections. Don't assume it's complicated. Check the connections.

The last chapter of any decent electrical repair guide should say: Connections, connections, connections. Don't assume it's complicated. Check the connections.

 

[L98Fiero]

@BaitMaster has good advice.

The first chapter of any decent electrical repair guide should say: Connections, connections, connections. Don't assume it's complicated. Check the connections.

The last chapter of any decent electrical repair guide should say: Connections, connections, connections. Don't assume it's complicated. Check the connections.

FWIW, I completely disconnected the contactor, I could get the coil off but couldn't find a non-destructive way to get to the contacts though when it was out, I checked the resistance on the contact and they all read zero ohms and this will be the second time I took it out and put uit back in so the connections in that regard should be at least as good as they were or better. The connectors on the caps were all tight, the odd thing though, is that they showed marks from arcing on the outside of the push on connector, I didn't spend much time with it running while it was arcing so I didn't see where they were arcing to, the marks were at the top where the wire is crimped and it was on both the run and start caps. The potential relay has a thin white body and you could see major arc flash from the contacts which I though was a bit concerning as well. I haven't run the RPC since reassembly.

 

[BaitMaster]

Solder the connections between the wire and the crimp. The wire is stranded correct? Not solid?

 

[mbond]

The list of thinks that can go wrong with a machine like this (rotary phase converter) isn't long. Bad connections and short circuits. Ruining capacitors seems like just a symptom - a symptom that is consistent with bad connections or short circuits

 

[garageguy]

From my years as an auto tech, seems to me you need to find out why you are getting such a high voltage at the connectors that it is causing arching. Consult an electrician or someone very familiar with RPC's. I have an American Rotary running my grinder so I have some basic understanding. Have you checked your motor? Is there a possibility of a defect there that is causing a high voltage feed back to the RPC ? Just throwing that out there. Good luck. Also check your wiring. Disconnect your terminals and pull on the connectors. if the wire "stretches" thats bad.

 

[jcdammeyer]

My two cents here. Crimped connectors should never be soldered to stranded wire. The reason is the solder wicks up along the wire under the insulation. So now there's a hard spot a short distance from the crimped connector. Vibration over time can break the wire at that hard spot but you can't see that it's broken. Worse is it can be very intermittent or have a notable resistance. Yet it looks good from the outside and when you measure with a meter it appears fine.

So I agree with @garageguy that if the wire stretches that's bad.

 

[RobinHood]

Speaking of connections: here is what I just pulled out of my RPC…

Heard an arcing noise during start-up the last few times; it kept getting worse; so was time to investigate.

Those start caps are well over 20 years old - so no complaints. Off to the motor supplier in the AM to get new ones.

 

[BaitMaster]

My two cents here. Crimped connectors should never be soldered to stranded wire. The reason is the solder wicks up along the wire under the insulation. So now there's a hard spot a short distance from the crimped connector. Vibration over time can break the wire at that hard spot but you can't see that it's broken. Worse is it can be very intermittent or have a notable resistance. Yet it looks good from the outside and when you measure with a meter it appears fine.

So I agree with @garageguy that if the wire stretches that's bad.

I’ve never seen this happen.

However, it is a possibility, obviously.

Our CSA panel shop has never had a single complaint/warranty claim on a soldered connection failure in a like manner.

There specifically are RPC OEM’s that solder conductors to the spades on the capacitors instead of using crimp on connectors to deal with failures in crimps. I have never seen one of those connections fail, but have seen many spade connectors fail.

That being said, if the conductor is heated by an inadequate heat source and it takes a long time to get up to soldering temperature, the solder does creep up the wire, as you mentioned. If an adequate heat source is used the solder hardly creeps, as the area to be soldered gets hot fast, the solder wicks to the desired area, and the heat source is removed.

Soldered connections in a high vibration environment ( aircraft, robot, heavy machinery ) will fatigue.
A stationary control panel typically does not qualify for that designation, considering that off the shelf contactors/relays/timers/switches are used.

In a stationary environment you are way more likely to have humidity corrosion build up a layer of oxides, leading to a small but ever growing carbon buildup causing massive I2R issues leading to a catastrophic crimp failure.

Solder eliminates bad connections like that.

 

[jcdammeyer]

I’ve never seen this happen.

However, it is a possibility, obviously.

Our CSA panel shop has never had a single complaint/warranty claim on a soldered connection failure in a like manner.

There specifically are RPC OEM’s that solder conductors to the spades on the capacitors instead of using crimp on connectors to deal with failures in crimps. I have never seen one of those connections fail, but have seen many spade connectors fail.

That being said, if the conductor is heated by an inadequate heat source and it takes a long time to get up to soldering temperature, the solder does creep up the wire, as you mentioned. If an adequate heat source is used the solder hardly creeps, as the area to be soldered gets hot fast, the solder wicks to the desired area, and the heat source is removed.

Soldered connections in a high vibration environment ( aircraft, robot, heavy machinery ) will fatigue.
A stationary control panel typically does not qualify for that designation, considering that off the shelf contactors/relays/timers/switches are used.

In a stationary environment you are way more likely to have humidity corrosion build up a layer of oxides, leading to a small but ever growing carbon buildup causing massive I2R issues leading to a catastrophic crimp failure.

Solder eliminates bad connections like that.

I believe the rule for not soldering is in boats. Couldn't quote you rules. Might be a "Trumpism"? Hey is that going to be a new term?

 

[BaitMaster]

I believe the rule for not soldering is in boats. Couldn't quote you rules. Might be a "Trumpism"? Hey is that going to be a new term?

Solder does not create an adequate mechanical connection. It is brittle and break prone in mobile environments like boats.

In marine applications you aren’t allowed to solely use solder as a means of connection, it can be supplementary to stop corrosion induced conductivity degradation etc.

The electrical connection solder makes is top notch.

The BEST connections are crimps though. Not from Napa. Not from the hardware store.

We are talking specialty mil spec level goodness, used with approved 1000$ connector specific crimping pliers to boot, then heat shrunk with approved shrink. They use those on aircraft, etc. They actually crimp so good and right they “cold weld” the conductor….

But for us normies….. and even as a pro technician….. properly used solder is an ace in the deck for connection reliability. Specifically the electrical portion of the connection.

Absolutely it does embrittle the conductor of used in a flexy/vibratey environment though. It’s not a good mechanical connection for conductors. Your right on that.