How to test air compressor electric motor

[CalsXS2]

I'm electronically challenged,,lol. My 5hp 220v electric motor won't do anything. I've had all the caps tested. Two were good, one bad. I replaced it. Still nothing.

Pulling the cover off of the motor I have 125v on both hot legs coming in. I also have 125v on the neutral wire. Is this normal or is this telling me something.

Pleas advise. Thanks

 

[wyliesdiesels]

You have 125v from hot to what?

And a 240v motor doesn't need a neutral.

125v from neutral to what?

Did this motor previously work?

Does it have a starter?

How are you testing the motor?

Gonna need more info.

Pictures would help as well.

 

[CalsXS2]

I'm a dummy. The white wire is not nuetral. It's a hot wire..

The compressor has been running fine for several years. Then one day when it tried to come on it just made a couple of spins then died.

I'm touching my hot wires coming into the motor to ground.

 

[Bert_]

Testing to ground tells you nothing, the motor does not use the ground to run. Test from one hot line to the other. You need to see 240 in order for the motor to work.

 

[CalsXS2]

Testing to ground tells you nothing, the motor does not use the ground to run. Test from one hot line to the other. You need to see 240 in order for the motor to work.

So I put my test leads in all different combinations on the terminals. I have no voltage anywhere.

 

[Bert_]

Then start going up stream, test for voltage at whatever device is before the motor.

 

[The Cobbler]

is the the switch calling for the compressor to run? if not there won't be voltage at the motor .

 

[Lelandwelds]

I am a Mr Magoo with electricity. I am extremely cautious.

If you took the belt to the pump off, will the motor run? If you touch the frame, do you get a shock? With the belt off, can you turn the motor?

I don't understand what you touched to measure a voltage. L1 and L2 together should measure 240 volts. L1 to a neutral or a ground should read 120 v. Same for L2. Ground to neutral should read zero.

You have 4 wires coming from panel? Compressor was working and you made no changes, right? It just died with no warning? Do you have power at the breaker? Any burnt smells? Notice any unusual over heating before early death?

 

[Moosefire66]

Subscribed. I have similar problems with my compressor

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[CalsXS2]

Then start going up stream, test for voltage at whatever device is before the motor.

I think it's the pressure switch. Looking at the pic below, I get 125v on both legs coming into the switch. But I only 125v on one side coming out going to the motor.

What's funny is I see a little burns spot on top of the plastic connector on the lower black wire. But that side has power. It's the other (white wire) side that doesn't.

So will only getting 125v on one side to the motor cause this "Do Nothing". Or could I have more problems. Thanks

 

[LS6 Tommy]

Sounds like you have a bad switch.

Tommy

 

[The Cobbler]

process of elimination, to rule in the switch as culprit, remove the wires on the line side and connect them to the load side . the motor should run , assuming you don't have other issues . just don't leave it run and overpressure, oh, and disconnect the power supply while working on it

 

[CalsXS2]

Definitely the pressure switch. I by-passed it with some jumpers and I'm back in business,,,lol. Well. As soon as I get a new switch that is.

Thanks for all the help.

 

[The Cobbler]

is there a magnetic starter hooked up to that switch? the contacts are not rated for 5 HP on those switches , which probably why it burned up

 

[CalsXS2]

is there a magnetic starter hooked up to that switch? the contacts are not rated for 5 HP on those switches , which probably why it burned up

Forgive my ignorance, but I don't know what a starter is. There's no key on it. LOL. Just kidding. I'm not THAT stupid,,,lol.

But I assume you're talking about something to do with a start capacitor?? It has 3 capacitors. Can you tell anything but the pics below.

 

[CalsXS2]

Also. You might be onto something about the switch. According to the specs I just found the switch I'm using is only rated for 3hp.

What ***** is I bought it from an industrial compressor sales place that is huge in the USA. Can't recall the name right now. It's what they recommended. Charged me big bucks for it. Then I found the exact one at rural king for $15.

Can you recommend me a switch?

Here's what I have now.

https://www.condor-usa.com/air-compressor-switches-mdr11-usa

 

[The Cobbler]

you need a switch like this https://www.ebay.com/itm/MAGNETIC-M...480038&hash=item4ac51de286:g:EWgAAOSwPc9Wvf1-

so the lesser rated pressure switch now controls the mag starter which is rated for the 5hp motor . the magnetic starter switches on and off the motor, the pressure switch controls the mag starter

 

[Bert_]

Don't waste money throwing parts at it. Test for voltage upstream of the motor until you find where you loose power. That will tell you which part is defective. It sounds like the problem is BEFORE the motor.

 

[CalsXS2]

Don't waste money throwing parts at it. Test for voltage upstream of the motor until you find where you loose power. That will tell you which part is defective. It sounds like the problem is BEFORE the motor.

Read post #13 above. I found the problem. It's the pressure switch.

 

[Lelandwelds]

I don't know what a starter is.

But I assume you're talking about something to do with a start capacitor?? It has 3 capacitors. Can you tell anything but the pics below.

That's the hold over name for a contactor or relay. It's a low amperage switch that operates a higher current circuit. Five horsepower (23 amps) is where the optional/ required equipment crossover point is.

Start capacitors dump a chunk of current to get the motor started.

 

[wyliesdiesels]

A motor starter is a contactor/relay AND an overload relay for wire and motor protection.

 

[Lelandwelds]

A motor starter is a contactor/relay AND an overload relay for wire and motor protection.

I was hoping to keep the explanation simple. Bringing up heaters and the difference between over load protection and over current protection would complicate things for OP more than necessary. Lots of 5 hp (and almost all smaller) compressors ship without a mag starter.

Maybe you or someone else can very simply explain how both circuit breakers and heaters are needed.

 

[Bert_]

Read post #13 above. I found the problem. It's the pressure switch.

My bad I missed that.

 

[TRWham]

I was hoping to keep the explanation simple. Bringing up heaters and the difference between over load protection and over current protection would complicate things for OP more than necessary. Lots of 5 hp (and almost all smaller) compressors ship without a mag starter.

Maybe you or someone else can very simply explain how both circuit breakers and heaters are needed.

How about:

The circuit breaker protects the wire (mostly from a short circuit), and the overload(s) protect the motor.

Because the breaker will be oversized relative to the motor load to allow for high starting current, then the motor may be overloaded and the CB won't care. The overloads react much more slowly than a breaker, and can be sized right above the expected full load amperage, and will open only if the motor is drawing too much current for a sustained period. The breaker will still react quickly to any short circuit, or any spike in current high enough to trip it (like a mechanical failure that locks up the motor).

Some motors are internally protected and need no overloads, and some breakers (Heinemann for example) can react like an overload and a breaker in one device.

 

[Lelandwelds]

How about:

The circuit breaker protects the wire (mostly from a short circuit), and the overload(s) protect the motor.

Because the breaker will be oversized relative to the motor load to allow for high starting current, then the motor may be overloaded and the CB won't care. The overloads react much more slowly than a breaker, and can be sized right above the expected full load amperage, and will open only if the motor is drawing too much current for a sustained period. The breaker will still react quickly to any short circuit, or any spike in current high enough to trip it (like a mechanical failure that locks up the motor).

Some motors are internally protected and need no overloads, and some breakers (Heinemann for example) can react like an overload and a breaker in one device.

Much better than the gunk I tried to type. Thanks. I had never heard of a hydraulic magnetic breaker. Or one with that can trip at two different rating. Heinemann is some kind of different.

Glad the OP problem was a simple switch. Too bad he dug deeper than needed. Pressure switches fail more often than motors or pumps. I have found some bad check valves that were head scratchers too.

 

[wyliesdiesels]

How about:

The circuit breaker protects the wire (mostly from a short circuit), and the overload(s) protect the motor.

Because the breaker will be oversized relative to the motor load to allow for high starting current, then the motor may be overloaded and the CB won't care. The overloads react much more slowly than a breaker, and can be sized right above the expected full load amperage, and will open only if the motor is drawing too much current for a sustained period. The breaker will still react quickly to any short circuit, or any spike in current high enough to trip it (like a mechanical failure that locks up the motor).

Some motors are internally protected and need no overloads, and some breakers (Heinemann for example) can react like an overload and a breaker in one device.

On a hardwired motor circuit, the breaker ONLY protects against short circuits and ground faults.

The overloads in the motor starter(or internal in the motor) protect BOTH the circuit wire AND motor.

 

[TRWham]

On a hardwired motor circuit, the breaker ONLY protects against short circuits and ground faults.

The overloads in the motor starter(or internal in the motor) protect BOTH the circuit wire AND motor.

I understand your points, however I will stand by my statements as a simplified explanation.

Further, if a compressor fails mechanically and locks up, the breaker can sometimes trip faster than the overloads, depending on how they are sized compared to the circuit breaker. My experience in this case is mostly with inherently protected HVAC compressors. I worked for an HVAC equipment OEM for about 10 years, then a major compressor manufacturer for over a decade following that. We did some cruel things to compressors during UL testing.

If the circuit is sized correctly, a motor can be overloaded at a current well below the ampacity of the wire, so the overloads are not really protecting the wire in that case. My 7.5 hp air compressor is on 50A conductors with a 40A breaker and ~32A overloads. Without overload protection, the motor could run overloaded for quite some time and the wire would still not be damaged.

 

[wyliesdiesels]

Circuit breakers for motor citcuits can be sized @ a maximum of 250% of NEC FLC. So there is some installs out there where the breaker rating is far above the wire ampacity.

 

[TRWham]

Circuit breakers for motor citcuits can be sized @ a maximum of 250% of NEC FLC. So there is some installs out there where the breaker rating is far above the wire ampacity.

Okay, that's a completely fair point, so let's try the following:

For adequate protection, because of the varying current requirements of electric motors across different phases of operation, motor circuits may need the combination of a circuit breaker and additional over-current protection, whether internal or external to the motor.

 

[Lelandwelds]

Okay, that's a completely fair point, so let's try the following:

For adequate protection, because of the varying current requirements of electric motors across different phases of operation, motor circuits may need the combination of a circuit breaker and additional over-current protection, whether internal or external to the motor.

Lol. Still better than my abandoned attempt but what happened to a simple, clear explanation?

 

[TRWham]

Lol. Still better than my abandoned attempt but what happened to a simple, clear explanation?

Beyond my capability, apparently. That last effort does somehow remind me of Al Sleet's ultimate weather forecast.

 

[Bert_]

Breaker protects against short circuits, like a cut wire. There may be 100's-1000's of amps flowing, many times the circuits rating.

Overloads protect against overload (duh), mainly protects the motor but also protects the wire in the process. They will trip with a few amps or 10's of amps over the setting, like an overworked motor.