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Question on a single phase motor repair

Sweet Dreamer

Aluminum
Joined
May 26, 2013
Location
Pennsylvania
I have a 1 HP single phase 115 volt motor. The start-up points are burned out and I can't find replacement points for this motor. I would like to repair it using an external relay to replace the start-up points. I was told that I can do this by using a timer to control how long the relay will remain closed. I plan on using a 555 timer circuit to control the delay on the relay.

My question is how do I decide how long to have the relay delay before opening?

The motor will be used to power a small metal lathe.

Thank you.
 
I have a 1 HP single phase 115 volt motor. The start-up points are burned out and I can't find replacement points for this motor. I would like to repair it using an external relay to replace the start-up points. I was told that I can do this by using a timer to control how long the relay will remain closed. I plan on using a 555 timer circuit to control the delay on the relay.

My question is how do I decide how long to have the relay delay before opening?

The motor will be used to power a small metal lathe.

Thank you.

Just make new contact points. It isn't hard.

Look at them.

Big f*****g deal, yah?

New capacitors can be had. "Snubber" as well. This is not the first motor to need such things.

If no joy, just go and get another motor. Pennsyslvania is not the Sahara desert. No shortage.
 
If you are set on using an external circuit to turn the relay on and off, you might want to sense the motor speed and use that to determine timing, instead of a fixed delay.
 
For the cat's meow, use a current relay to cut off the start. When the current draw drops the start cuts out. Current will only drop when the motor comes up to speed. That can also be done with a triac and a sense resistor providing gate voltage.

But, for a timer, I would set 0.3 second and adjust from there. Or, just put a push-button, "hold until started". Simple and no problem.

If the motor is easily replaced, not buried under everything, just replace it. Small motors are cheap at tag sales, etc. If they spin, and do not smell burnt, odds are they are OK.
 
For the cat's meow, use a current relay to cut off the start. When the current draw drops the start cuts out. Current will only drop when the motor comes up to speed. That can also be done with a triac and a sense resistor providing gate voltage.

That's a good idea. I wouldn't have thought of that.


But, for a timer, I would set 0.3 second and adjust from there. Or, just put a push-button, "hold until started". Simple and no problem.

I actually thought about the push to start button idea. I might go with that. I'll be the only one who uses this machine so I'll know how to start it.


If the motor is easily replaced, not buried under everything, just replace it. Small motors are cheap at tag sales, etc. If they spin, and do not smell burnt, odds are they are OK.

Nothing's as cheap as free. I already have this motor and tons of relays, etc. So repairing it is pretty much zero cost. To even go out and try to find a cheap motor would cost me time and money just in running around looking for one. But yeah, if I new where there was one and it wasn't too far to go I'd probably go pick it up. Assuming it was indeed cheap.

I might go with the push-button method. That way if I do decide to start the motor under load I can probably just hold the start button down a tad longer. That might end up being more versatile than a fixed timer.
 
What is soooo special about keeping this motor alive ?

Answer: $$$$$$$

If I can fix this motor with an external relay that fix will basically be FREE for me.

Where else am I going to find a 1 HP single phase motor for FREE?

So I guess what makes this motor sooooo special is that I already have it.

If you know where I can pick up another one for FREE, please do let me know.

In fact, if I knew where I could pick up another 1 HP single phase motor for even as low as $25 locally, I'd go get it. But I don't even know where I could find one that cheap.

If we start getting up to much over 50 bucks, this FREE motor becomes quite special again. :)
 
I believe you are looking for a Steeveco potential relay. These are often used to start rotary phase converter idler motors. Sometimes they need to have a helper relay alongside them to handle larger motor inrush currents, for a one hp application it would probably work by itself. If you poke around a bit on this sub-forum you could probably find the exact model number.

Many modern single phase motors use a relay like this, instead of a centrifugal switch.
 
Answer: $$$$$$$

If I can fix this motor with an external relay that fix will basically be FREE for me.

Where else am I going to find a 1 HP single phase motor for FREE?

So I guess what makes this motor sooooo special is that I already have it.

If you know where I can pick up another one for FREE, please do let me know.

In fact, if I knew where I could pick up another 1 HP single phase motor for even as low as $25 locally, I'd go get it. But I don't even know where I could find one that cheap.

If we start getting up to much over 50 bucks, this FREE motor becomes quite special again. :)

I THINK I finally sent them off for scrap after 40 years of tripping over them.. , but... used to save the motors out of wringer-washer, clothes-dryer, and small sheet-offset litho presses, clean-up plain bearings, put in new thrust washers, and make contacts (they are simple rivets) for the burnt ones on the centrifugal switches.

"Tripping over" because the grand plans as GP bench grinder spindles, buffing-wheel and wire brush mounts, and small machine-tool power ran head-on-collision into "cheap ass, open, drip, proof" and proved it wasn't worth five minutes of my time - let alone 20 minutes - to save the fool things!

"Your mileage may vary", but given a DECENT motor can easily last fifty years, I am not so rich as to be able to afford the HIGH PRICE of messing with so-called "free" AKA "SAD junque" disposable Bic grade 1-P motors.

3-Phase.. or top-grade DC - is a whole 'nuther zoo of far worthier animals.
 
I believe you are looking for a Steeveco potential relay. These are often used to start rotary phase converter idler motors. Sometimes they need to have a helper relay alongside them to handle larger motor inrush currents, for a one hp application it would probably work by itself. If you poke around a bit on this sub-forum you could probably find the exact model number.

Many modern single phase motors use a relay like this, instead of a centrifugal switch.
I should have thought of that. I sent Sweet Dreamer here thinking the timer would be the best option. I even did a video covering the potential relay a while back. I guess I need to clean out the cobwebs...
 
I would like to thank everyone for all the replies in this thread. I've only just now had time to do a more detailed inspection of the motor. It turns out that the wingdings are burnt and shorted in places. So this motor is shot. Repairing the start-up points isn't going to save it.

In the meantime I'll be picking up a 7.5 HP 3PH motor to use as a pony motor in a home-built rotary phase converter. I'll be using that phase converter to run a 5 HP lathe. I know I should probably use a 10 HP pony motor, but I found a really nice 7.5 HP motor cheap so I'm going to go with that.

I wonder what size start-up capacitor I should use to get the 7.5 HP motor up and running on 220 single phase? I remember reading somewhere that a good rule of thumb is 75 MFD per HP. Is that about right? That comes out to about 560 MFD or so. And I'm guessing it should probably be rated at about 400 volts just to be safe?
 
Answer: $$$$$$$

If I can fix this motor with an external relay that fix will basically be FREE for me.

Where else am I going to find a 1 HP single phase motor for FREE?

So I guess what makes this motor sooooo special is that I already have it.

If you know where I can pick up another one for FREE, please do let me know.

In fact, if I knew where I could pick up another 1 HP single phase motor for even as low as $25 locally, I'd go get it. But I don't even know where I could find one that cheap.

If we start getting up to much over 50 bucks, this FREE motor becomes quite special again. :)

Your time (unless your doing this at work and using the lab's materials) isn't "Free".

This is "Practical Machinist". if a timer was the most practical way to doo it, the manuf would have done it that way.
 
I would like to thank everyone for all the replies in this thread. I've only just now had time to do a more detailed inspection of the motor. It turns out that the wingdings are burnt and shorted in places. So this motor is shot. Repairing the start-up points isn't going to save it.

In the meantime I'll be picking up a 7.5 HP 3PH motor to use as a pony motor in a home-built rotary phase converter. I'll be using that phase converter to run a 5 HP lathe. I know I should probably use a 10 HP pony motor, but I found a really nice 7.5 HP motor cheap so I'm going to go with that.

I wonder what size start-up capacitor I should use to get the 7.5 HP motor up and running on 220 single phase? I remember reading somewhere that a good rule of thumb is 75 MFD per HP. Is that about right? That comes out to about 560 MFD or so. And I'm guessing it should probably be rated at about 400 volts just to be safe?

Are these motors, caps, and relays "FREE" ??????
 
I just built a 7.5 hp converter. I used 650 MFD for starting, worked fine. I don't think you are going to find 400v starting caps. 250V is fine for starting.
 
This is "Practical Machinist". if a timer was the most practical way to doo it, the manuf would have done it that way.

Where did you get the idea that manufacturers do things the most practical way?

They tend to do things in a way that makes them the most money. Installing centrifugal points is CHEAPER than installing a relay with a timer. So they take the cheap way out to make the most money.

And to make things far worse, after the points burn out they charge you $47.99 for replacement points. A part that shouldn't cost more than $5 max.

I wouldn't look to manufactures for lessons on practicality.

And besides, if you are trying to repair the product and you don't have access to replacement points, but you do have access to relays and timers, what would be the most practical repair for you?

What's practical for a manufacturer may not be practical for you.
 








 
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