For almost a year now, the electro-mechanical circuit that many folks helped me design has been working perfectly (reference: Seeking Basic Non-Electronic Gate Opener Circuit.) A problem arose for the first time today. When the gate didn't work I found the motor running with the belt slipping. It was quite hot but had not yet tripped the thermal overload switch. Upon diagnosis, I found that one of the SPDT relays, the one that controls the motor rotation direction, had failed. I was surprised because I had chosen very robust ratings- 20a relay rating- for the gate opener motor, which draws 5a! This was replaced and all seemed well until the 1/2 hp motor wouldn't start. This motor has a capacitor with 4 terminals: there is line voltage across two of the terminals, with the other two connected to the aforementioned relay to reverse the rotation. The motor would sit and buzz, but manually spinning the motor shaft to give the motor a start would cause the motor to run properly in either direction, and otherwise the gate would work properly. While I would think the capacitor would be highly suspect there is a complication. Under load the voltage to the motor drops from 115 to 109 volts. There has been an intermittent problem, both at the breaker box and the pole, supposedly caused by salt corrosion. (The gate is near the ocean.) Although the motor is rated at 120V, my understanding is that +/- 10% is permissible. This would mean that 109V was OK, but before calling the electric company I wanted your opinion. So I'm thinking there are 3 potential issues causing the motor not starting by itself: bad capacitor, low line voltage, and possible damage to the motor caused by overheating earlier (even though the overload didn't trip.) Please help me sort this out. Anything else you can think of? Can anything be ruled out?
Many thanks for your help last year, and now.
Frank

 

Sounds like the capacitor is the problem. Buzzing but not running until given a spin is fairly diagnostic.

 

Agree on the capacitor, ensure it, or any replacement is AC motor rated run capacitor, I would stay away from Chinese origin versions.
Max.

 

I am thinking that it is not a coincidence that the cap failed when the relay failed. The motor running hot for an extended period may have done in the cap.

 

It has occurred to me that it is just possible that the motor overheating burnt out the winding which the capacitor feeds, which would give the same symptoms. Can you check the windings for continuity?

 

It has occurred to me that it is just possible that the motor overheating burnt out the winding which the capacitor feeds, which would give the same symptoms. Can you check the windings for continuity?

Thanks to all (hello again to Max who helped me greatly a year ago! Unfortunately the new Grainger cap- same mfd and vac rating- which I haven't yet tried has Chinese characters on it!)
I should note that the starting problem didn't occur until several tests after the new relay was installed- all was well for awhile.
As to the possible winding damage I should be receiving a used replacement motor early this week. Will try the cap first, then replace the motor if n.g.
No one seems to have implicate the line drop. I'll assume 109v should not affect the motor start.
Will have more info tomorrow. Again thank you all!
Frank

 

Yes, I will check the windings if the cap doesn't fix things.

 

Yes, I will check the windings if the cap doesn't fix things.

Update: It was just the capacitor! Glad I have the new motor for backup tho as they have been discontinued for many years. Now I just have to worry about Max' China comment.
Now a final question: Why would a Dayton relay rated at 30a fail after a year?
The motor draws 5, more at startup obviously, but still.
The relay was a dpdt and was only using one side, so switched to the other side. Still I will get a new one, same kind as not much room and fits nicely.
This time I will put the sets of poles in parallel. That should give me a 60a capacity, or am I missing something?
Looking forward to hearing back. You guys are so sharp.
Frank

 

Please note corrections from my original post: relay is DP and rating is 30a.

 

The capacitor, btw, was not identified as an a/c motor capacitor but has ratings identical to the original.

 

Paralleling the relay contacts will get you a 60A carrying capacity but as the contacts are unlikely to make or break at exactly the same time it doesn't double the making/breaking current. It is likely the arcing, instigated by the inductive load, which reduces the life of the contacts. Using a varistor (TVS or MOV) across the contacts should help.

 

The capacitor, btw, was not identified as an a/c motor capacitor but has ratings identical to the original.

A few years back there was a importer of Chinese origin machines, he used to change the capacitors out before resale.
Also you can pick up cheap Telemecanique motor contactors on ebay.
Max.

 

I usually these to replace the Chinese versions, Cornell Dubilier, (CDE), http://www.cde.com/solutions/motor-run.
The former are false economy if a motor burns out as a result.
Max.

 

I usually these to replace the Chinese versions, Cornell Dubilier, (CDE), http://www.cde.com/solutions/motor-run.
The former are false economy if a motor burns out as a result.
Max.

The link is to motor run capacitors. Except for size, I guess run and start are the same? I need 50-60 mfd, 250vac which I thought was classified as a start.

 

Start are often bipolar electrolytic due to them only being in circuit for a few seconds, if longer they fail fast.
Max.

 

Paralleling the relay contacts will get you a 60A carrying capacity but as the contacts are unlikely to make or break at exactly the same time it doesn't double the making/breaking current. It is likely the arcing, instigated by the inductive load, which reduces the life of the contacts. Using a varistor (TVS or MOV) across the contacts should help.

 

So I would have two varistors, one for each throw, correct?

 

So I would have two varistors, one for each throw, correct?

I don't know just how the relay is wired but yes, you probably would, one across each pole of the switch.

 

I don't know just how the relay is wired but yes, you probably would, one across each pole of the switch.

Nothing complicated about the relay. So the two varistors would have a common connection to the "wiper", the other end of each going to each throw terminal, correct? (SPDT)
Could you please suggest specific varistors and their ratings? Thanks.

 

Here is a guide.
Max.