Another Question about Single Phase motors?

I have a 208-230 volt 1 PH 3/4 hp motor. General Electric.

It's a used motor. I had a 'bug' in it when I first got it. I literal bug. When you turned the shaft it make a horrible scraping sound. I took the motor apart and discovered that something like a mud wasp must have been building a mud nest in this motor. That's what was causing the horrible scraping sound. Once all that was cleaned out and the motor reassembled it rolls over smooth and quiet. So I'm guessing it might be good to do now. The internal parts looked to be in good shape other than the mud wasp nest.

Some things to note? This motor is single phase. 208-230 volt only according to the label on it.

Here's the label:



It has no internal switches that I could see. It's a very simple motor. It also has no start capacitor attached to the motor.

But as you can see on the label they include a wiring schematic which I redrew here for clarity:



Now I'm a bit confused. First it doesn't say what size cap to use.

Secondly according to this schematic the capacitor is wired in directly and permanently. Is that right? Is this a start-up cap or something else?

I never wired up a motor like this before to 220. So I'm a little leery about it. I guess I could just try this and see what happens?

Although I have no clue what size cap to use? 100 MFD? That would be my first guess.

I can't believe they didn't include that info on the schematic when they printed the label.

I'd like to get this motor up and running. Should I just wire in a 100 MFD cap directly and hope that works? I'm going to put that cap in a box in case it blows up. I don't want it blowing up in my face.

This is a simple capacitor run motor. You undoubtedly have one on the blower for your heating/air conditioning system. You may be able to get information from GE, if not, Look for other similar motors and prorate the capacitor size by the horsepower. This motor is reversible by switching the capacitor to the other incoming power lead.

Remember that a replacement capacitor must be rated for continuous running. A starting capacitor will almost certainly blow up.

Bill

9100 said:

Remember that a replacement capacitor must be rated for continuous running. A starting capacitor will almost certainly blow up.

Bill

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How could I remember that? I didn't even know that! Thanks for the info Bill. Now I'll have to look for the correct capacitor. I did try looking this GE motor up by the GE part number, but I haven't been able to find it yet. That exact part number doesn't produce any results.

One clue......

A black bakelite case and screw or solder terminals , with one end closed and the other having a rubber seal, means an electrolytic starting type, that you do not want.

A run type will typically be in a metal case, either "obround" (racetrack shaped) or possibly cylindrical, with terminals out one end, usually "faston" type, and often labeled "oil filled".

Can also be a similar case, or a plastic case, labeled "film type" or the equivalent.

There are exceptions, so watch out.

You do NOT want anything that looks like this:

I'm learning.

Ok, I just found this in a PDF about the difference between run and start capacitors:

The simplest way to explain the mechanics of a capacitor would be to compare it to a battery; both store and release electricity. Capacitors are charged with electricity then release its stored energy at a rate of sixty times per second in a 60 cycle alternating current system. The sizing is critical to motor efficiency just as sizing of batteries is critical to a radio. A radio that requires a 9V battery will not work with a 1.5V size battery. Thus, as the battery becomes weaker the radio will not play properly. A motor that requires a 7.5 uF capacitor will not work with a 4.0 uF capacitor. Much the same way, a motor will not run properly with a weak capacitor. This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings. Motor manufacturers spend many hours testing motor and capacitor combinations to arrive at the most efficient combination. There is a maximum of +10% tolerances in microfarad rating on replacement start capacitors, but exact run capacitors must be replaced. Voltage rating must always be the same or greater than originalcapacitor whether it is a start or run capacitor. Always consult manufacturers to verify correct capacitor size for the particular application

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So now I know I need a run capacitor, and that the microfarads are critical. But I still have no clue how many microfarads I need. I can't believe they didn't include that information on the motor itself. It just says "separate capacitor". It doesn't say anything about the size.

I think I know how this is all going to end. Unfortunately.

I've been watching YouTube videos tonight on how to choose a run capacitor for a motor. The overwhelming consensus appears to be that if you don't have the original cap or can't read it, then call the manufacturer and ask them what capacitor is uses.

Sounds good on the surface.

The problem is that I bought this motor used. I have no clue what it came out of, or what the original run capacitor was. All I know is that it's a GE with the part number on the motor label.

So what's going to happen when I call GE? I don't claim to be able to look into the future but I'm willing to bet that their answer will be that the product is discontinued and they advise that I just buy a new motor.

It's a throwaway world we live in today. I'm willing to bet that's how this will end.

Hopefully I'll be surprised and they'll actually be able to tell me what size cap to buy. I LOVE being wrong in these kinds of situations.

There's nothing wrong with this motor as far as I can see. All I need is a proper size run cap and it should spin right up. A new motor this size is going to cost $150 to $200. A financial fate I would prefer to avoid if possible. A motor becomes useless just because we don't know what size cap to use? That's a pretty sad situation isn't it?

Shouldn't there be some way to determine the right size cap? What if I put a 35uF run cap on it and fire it up. Can't I then take some voltage measurements and decide from there wither I need a higher or lower capacitance? I guess I should go to an electronics forum with these kinds of questions huh?

I'll try to call GE tomorrow and see what I can find out. Maybe they'll surprise me and give me a capacitance size. That'd be great! I'll post the results of that phone call.

For a "PSC" motor, which is what this seems to be, you actually can determine the capacitor size.

The motor will draw the least current with the right size capacitor, but this is WHEN LOADED. So if you can load the motor as it will be in use, and try a capacitor, then try a different sized capacitor, the one puling less current is closer to the right size.

Too big, and current goes up again.

As a point of calibration, we had a client who used these motors to run fans. Easy load to test..... They used a 10 uF for a 1 HP motor, as I recall, and 7.5 uF for a half HP motor, at 220VAC. (I'm not totally sure about the size of the smaller motor now)

Another client had a motor driving a pump and a fan at the same time. Only 1/3 HP, but it had, IIRC, a 9 uF motor run capacitor, which sounds big, but that one was at 120VAC. You expect about double size at half voltage, so that is within range of being as expected.

Going by that, you might start around the 7.5 uF, and see how it runs. Compare the current to the motor nameplate rating.

Good advice. I would use an oscilloscope to measure both the voltage and phase angle, but I doubt that you have one so Jerry's advice is the way to go. You can be pretty far off on the first hack and still get meaningful results that will allow you to home in on the best value.

Bill

9100 said:

Good advice. I would use an oscilloscope to measure both the voltage and phase angle, but I doubt that you have one so Jerry's advice is the way to go. You can be pretty far off on the first hack and still get meaningful results that will allow you to home in on the best value.

Bill

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Actually I have several oscilloscopes. I have quite a bit of experience in electronics. I just don't often work with large AC motors. I usually work with small PM motors, servos, or stepper motors. Those I can deal with.

If it's all about matching phase angles I can deal with that. I just need a starting point. I can't measure anything until the motor is up and running.

You want I think a 90 degree shift between the currents in the two windings.

Somewhere around 5-20uF is likely fine for starting unloaded - you can even just spin it. It's only when running continuously at high load (typical for fans) that sizing becomes critical.

The reason for the lowest current is that with the proper cap, the power factor of the motor gets close to 1.

So you can, if you have the means, just check the power factor, but overall current is probably the easiest.

SomeoneSomewhere said:

You want I think a 90 degree shift between the currents in the two windings.

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I'm trying to think of how I would measure this. Here's the schematic again:

View attachment 299841

I'm guessing that measuring between T8 and GND relative to the signal between T5 and GND should always be 90 degrees since that's just the line coming in and would need to be what it is as it comes in.

So that means that I would need to compare the waveform from T8 and T2 relative to the waveform from T5 to T2 and adjust the capacitor to balance those two signals to 90 degrees. Is that right?

I'll need a dual trace scope to compare the phase of these two waves.

Or, without a scope I could just measure the currents in the line-in on T8 and T5 and adjust the cap to make those two currents the same?

You'll need to forgive me. My mind isn't crystal clear anymore. It takes me a while to figure out simple things.

Using a current meter would probably be the easier way to go. I wouldn't need to measure both currents simultaneously. Just measure one, and then the other, and if they are different I know I'm not at 90 degrees.

I think I can figure this out. I buy a few caps of different sizes and see which size give me the most even current readings.

~~~~~~

About the phone call to GE:

I made the phone call to GE. I got a chat bot that asked me a bunch of useless questions about appliances. It finally directed me to a web site where I can speak with an electric engineer after paying $5 for the privilege of talking with him/her.

I think I'll pass on the GE nonsense and just buy a few run caps and see if I can balance out the motor myself.

When connecting oscilloscopes to line voltages, you should know the scope inputs are referenced to ground and most are wired to GROUND. Using isolation transformers to isolate ground can be dangerous. We use battery operated scopes but still are very cautious with connecting probes and leads. I would not be concerned about a 90 degree phase shift, just make the motor work watching your currents. Also, megger the motor windings to ground before you even start with the cap.

Just Call your local motor shop that rewinds and sells them, even easier. they 100% know

Pathogen said:

I would not be concerned about a 90 degree phase shift, just make the motor work watching your currents.

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Yeah, that's the method I'll use. It's a lot easier.

This is a very common HVAC PSC type motor... used on furnace blowers and outdoor condenser unit fans. The run capacitor will be at least 5 microfarads, probably no higher than 30... and the amount necessary will be based on running load... so wiring it up and testing it with no load is somewhat of a useless challenge. I vote to just wire it up with a 5uf, see what it does, and then switch to a 10, then switch to 5+10 in parallel, etc...

DaveKamp said:

This is a very common HVAC PSC type motor... used on furnace blowers and outdoor condenser unit fans. The run capacitor will be at least 5 microfarads, probably no higher than 30... and the amount necessary will be based on running load... so wiring it up and testing it with no load is somewhat of a useless challenge. I vote to just wire it up with a 5uf, see what it does, and then switch to a 10, then switch to 5+10 in parallel, etc...

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Thanks for taking the time to reply. I'll do as you suggest. I'll need to order some run caps as I don't have any laying around. I'm holding off on that as I am also building an RPC and I'll need capacitors for balancing that out too. So I'm going to wait and order everything at once.

I'm just starting out working with larger AC motors. So I'm just getting my feet wet here. I've worked with a lot of 110 motors before that use regular start up capacitors. But I don't recall ever working with motors that use continuous run capacitors. That's why the schematic on this motor threw me for a loop because I didn't see any switch to kick out the cap.

The RPC I'm building uses both a start-up cap and several "run capacitors" for balancing the 3 phase power.

So this is all new to me. But hopefully pretty soon it will be old hat.

https://acpartsdistributors.com/index.php?main_page=product_info&products_id=26744

I'd call these guys and say you have that motor, and need a cap for it.

Winterfalke said:

https://acpartsdistributors.com/index.php?main_page=product_info&products_id=26744

I'd call these guys and say you have that motor, and need a cap for it.

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Nice work Winterfalke! You found the motor by part number.

They even have the capacitor in the picture with the motor, if the picture was large enough I could have just read the label on the cap.

$530 for this motor. Wow! I only paid $18 for this one. Looks to be in pretty good shape once the mud wasp nest had been cleaned out. So the motor is intended as a condenser fan motor. Interesting. I'm not even sure exactly where I'll use this motor yet. I just bought it to experiment with and learn on. I thought a new one would be $150 to $200. That $530 price tag is pretty wild. After I get mine up and running and test run it for a while maybe I should put it on eBay or Craigslist and see what I can get for it as a used but working motor? If the new price is $530 someone might be happy to grab up on it for $150 or $200. I might be able to make a few bucks in this learning experiment and buy some more $18 motors.

Thanks to Winterfalke I found the capacitor size. Century makes this motor under a different part number, but it appears to be the exact same motor. I found a site that sells these motors and it calls out the cap that they include with it.

Capacitor: 17.5MFD @ 370VAC (Included)

So that's probably a good starting point. I'll buy that size cap and put a load on this motor and see how well the current balances out between the two 110 volt legs. I could probably toss this on my jointer planer just for a crude test to see how well the motor works. It's a little slow for a planer, but just to check to see if the current is balanced it should be okay for a test load.

It's amazing. Keep poking around long enough and you finally get what you want.

We'll have to change the song lyrics: "You can always get what you want".