Motor contactor failing on one terminal - question and concerns

Hi all,

I discovered today that one my of nearly new motor contactors was not generating 240V on all three of the terminals when closed (240V, 240V, 12V). This resulted in my 3 phase motor simply buzzing. Initially I thought that my motor was toast, but after when I connected a second motor to this contactor and had the same problem I tracked the problem down to the contactor (not the heaters). At the top I had full voltage, when contacted I get the lower voltage. I redid the wiring and it still exhibited the problem, so I put a new contactor in there and it works fine.

My question - what is going on, and is this common? Could it have been something which I did to damage the unit?

I am now concerned that this failure might occur again, when I am not aware/standing next to it. If it were to fail with such different voltages, is it going to end up frying the motor or would the heaters trip? I really wouldn't want a $25 contactor to destroy an expensive motor.

Any thoughts welcome.

Don

Probably your contactor. The contacts do wear out and on name brand types you can buy replacements. But yours is "new", so obviously... make sure the contactor is rated for the motor HP (not just current) and the starting conditions (is it heavily loaded, multiple starts per hour etc , for example?). $25 sure sound cheap for a qualiity contactor..

I had the exact same problem on a conveyor motor last week - replaced the entire contactor instead of just the contacts though (cheaper for the company).

Actually, it's the $34 contactor. The brand is Fuji which I was figuring would be an okay brand (not a drop-ship from China type). It is actually over-rated, it is rated for 7.5HP@240V, but I was starting a 5HP. It probably has fewer than 20 starts, although some of them struggling to turn a compressor pump under pressure.

Thanks for the reply.
Don

You get what you pay for. Buy a good NEMA rated contactor. Forget the IEC crap............

A compressor pump should never be started under load. You need to redo your plumbing. Be glad the contacts did not weld shut from the overload.
Bill D

5HP at 230V is approaching 12Amps continuous and could hit 70-80 amps starting (unloaded) . . . likely close to 100Amps at locked rotor.

$34 sounds like an IEC rated unit running at the top power rating of a given series.

A NEMA contactor w/overloads would run you $200 minimum.

markz528 said:

You get what you pay for. Buy a good NEMA rated contactor. Forget the IEC crap............

Click to expand...

I have a certain reverence for the NEMA contactor, but a properly rated IEC will work just as well. Not knowing the age of your Fuji, but for environmental purposes Japan and parts of Europe, have outlawed cadmium containing contact material. Silver tin oxide is the current favorite. Problem is that tin oxide is too stable to decompose in the break arc and will build up on the contact surface. End result is thermal run away and destruction of the contact.

Tom

1. With contactor unconnected push down on the points and check that the resistance from the source to load side is zero for each contact.
2. Apply coil volts and do the same check.
3. If steps 1 & 2 do not pass the checks then disassemble what you can and see what the contacts look like. The failing contact(s) will probably be blackened.
4. See if you can stretch the contact arm springs to give you more tension.

Describe model number and show any pictures.

Bill D said:

A compressor pump should never be started under load. You need to redo your plumbing.

Click to expand...

Check. The pump wasn't getting up to speed before the loaders loaded up (Qunicy hydraulic unloaders). Not sure if it was my phase converter or what, but it wasn't working great. I have redone the plumbing with a solenoid/timer to keep it unloaded until it is up to speed. This was my attempt to get it back up and running.

rons said:

1. With contactor unconnected push down on the points and check that the resistance from the source to load side is zero for each contact.
2. Apply coil volts and do the same check.
3. If steps 1 & 2 do not pass the checks then disassemble what you can and see what the contacts look like. The failing contact(s) will probably be blackened.
4. See if you can stretch the contact arm springs to give you more tension.

Describe model number and show any pictures.

Click to expand...

Thanks for the suggestions here. I'm embarrassed I didn't do this first but - when I manually points I get zero resistance on all three terminals, but when I energize the coil the third line does not connect. The other two have zero resistance. Not sure why I got 12V above, but on the bench I'm getting no connection.

So I went in, and ugh - looks like the finger of blame points to me:



Looks like the contacts have heated up enough to move, melt and/or fall off:


It is Fuji Duo SC-E05. I'm not seeing replacement contacts.

Thanks for the help.
Don

The contacts look like they're for a bathroom light switch. Throw this junk away (I'd send it to the manufacturer with my warmest comments) and replace it with a real contactor.

Yum, fried contactor. Add some bacon and coffee and we have breakfast!

Check your RPC voltages as you start the generator. Good chance your generated leg is sagging causing high starting currents. Nothing wrong with correctly specified IEC contactors and overloads - most of the world uses them.

dschad said:

View attachment 276030

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..........


Search for what is called a Contact Kit.
I remember a company in Los Angeles that install new points on whatever you sent them.

Search for what is called a "solid state contactor".

Not worth rebuilding. What happened is that the current was heavy enough and/or there was high contact resistance sufficient to melt the backing. The over heating probably has damaged the insulation systems, most likely the moving contact carrier.

Did you by chance have this wired to the wild leg? Can't tell for sure but the 3thd photo looks like there was a lot bounce as in chatter on one contact, the one on the top. The other interesting observation is that the heating is only on the two outer legs, the center doesn't appear to be affected. Again my guess is the center leg is the wild leg.

From what you have said about the compressor not coming up to speed along with the contactor leads me to believe you connected the contactor coil to the wild leg. When the tank called for air, the contactor closed but as soon as it did, the wild leg voltage dropped below the coil holding voltage, the contacts opened then reclosed and the cycle repeated until the motor got up to speed, if it ever did. The constant make/break will generate a tremendous amount of heat, enough to melt the backings.

Tom

Lakeside53 said:

Check your RPC voltages as you start the generator. Good chance your generated leg is sagging causing high starting currents. Nothing wrong with correctly specified IEC contactors and overloads - most of the world uses them.

Click to expand...

TDegenhart said:

Not worth rebuilding. What happened is that the current was heavy enough and/or there was high contact resistance sufficient to melt the backing. The over heating probably has damaged the insulation systems, most likely the moving contact carrier.

Did you by chance have this wired to the wild leg? Can't tell for sure but the 3thd photo looks like there was a lot bounce as in chatter on one contact, the one on the top. The other interesting observation is that the heating is only on the two outer legs, the center doesn't appear to be affected. Again my guess is the center leg is the wild leg.

From what you have said about the compressor not coming up to speed along with the contactor leads me to believe you connected the contactor coil to the wild leg. When the tank called for air, the contactor closed but as soon as it did, the wild leg voltage dropped below the coil holding voltage, the contacts opened then reclosed and the cycle repeated until the motor got up to speed, if it ever did. The constant make/break will generate a tremendous amount of heat, enough to melt the backings.

Tom

Click to expand...

This is very interesting, esp. about the wild leg. Unfortunately I can't say how it was wired since I have changed many things since the problem first happened. I had done some temporary wiring to test it and other equipment out, and I must admit that I didn't pay very close attention. But what is true is that the contactor coil (120V) was fed from a 240V to 120V transformer using two of the 3 phase lines coming into the compressor electrical box, so there is a good chance (like 66%) that I was not feeding that transformer with the non-wild leg, assuming I just randomly grabbed the wires.

With that in mind - one thing that I believe (although I can't prove it) is that I'm pretty sure that the day the contactor last worked, the compressor switch was ON when I switched in my second idler, and unfortunately the compressor was not isolated from that idler by another switch - so both the idler motor (7.5hp) and the loaded compressor (5hp) were starting at the same time via a running 3hp phase converter.

I was in the converter room blinking, wondering why my idler didn't snap up to speed as it usually did, when I realized what I had done and I hit the power. I don't know if the compressor started at all or not, but I suspect it did not. I don't recall running the pump after this incident, and shortly thereafter I was thinking that I had killed the motor since it started to buzz.


It sounds like my actions/experience comports well with your theory of the bouncing contactor.

Thanks very much for the response. Now that I have been working on my proper installation, I will make sure that I pay attention to which lines are feeding the transformers in my power box.

rons said:

Search for what is called a Contact Kit.
I remember a company in Los Angeles that install new points on whatever you sent them.

Click to expand...

Thanks, but it is probably not cost effective for this one. The new unit was $33.


All the info is very much appreciated!