# 3 phase motor help



## rcar (Apr 28, 2013)

First off I am a resi electrician. My brother in law has a air compressor at his body shop that is going through starters left and right. After the 3rd starter he bought a new motor thinking that was it. Since then 2 more starters have burnt up. I went and looked at it today and I'm not real sure what I'm supposed to have. I checked all the connections and everything seemed to be clean and secure. 

Voltage incoming from poco. and at starter while off.

A: 120
B: 120
C: 208

When clamped motor running.

A: 39a
B: 0a
C: 39a
Obviously something wrong here, I just don't know how.









Best pic I could get.

Nameplate reads:
15 hp
230/460
40/20a


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## Pete m. (Nov 19, 2011)

Looks like you have a 120/240 delta service based on those voltage readings.

Since the coil for the contactor is 120 volt has anyone made sure it wasn't on the high phase?

Also, have you checked the voltage with the motor running? Looks like it's single phasing from the ampacity readings.


Pete


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## JRaef (Mar 23, 2009)

That's were I would start; why are you reading 0A on B phase? You might have a broken wire between the starter and the motor somewhere. The coil voltage source is important too, so if you made a meter reading error (i.e. the clamp wasn't closed all the way), check that next. If that's OK, then start looking at the pressure switch. There is supposed to be a cut-in and cut-out pressure setting. People who don't know what that means will sometimes try to set those too close to each other, which makes the compressor bang on and off too often. That destroys the starter first, then the motor.


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## 3DDesign (Oct 25, 2014)

Is this fed from fuses?


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## McClary’s Electrical (Feb 21, 2009)

Stop being iec rated starters and by a nema starter.


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## rcar (Apr 28, 2013)

Pete m. said:


> Looks like you have a 120/240 delta service based on those voltage readings.
> 
> Since the coil for the contactor is 120 volt has anyone made sure it wasn't on the high phase?
> 
> ...


Thanks, I'll check those out.



JRaef said:


> That's were I would start; why are you reading 0A on B phase? You might have a broken wire between the starter and the motor somewhere. The coil voltage source is important too, so if you made a meter reading error (i.e. the clamp wasn't closed all the way), check that next. If that's OK, then start looking at the pressure switch. There is supposed to be a cut-in and cut-out pressure setting. People who don't know what that means will sometimes try to set those too close to each other, which makes the compressor bang on and off too often. That destroys the starter first, then the motor.


Kind of like on a well pump? 20/40 30/50 type thing? 



3DDesign said:


> Is this fed from fuses?


No, the breaker is in good condition and tests o.k.



mcclary's electrical said:


> Stop being iec rated starters and by a nema starter.


I'll look in to this also.


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## varmit (Apr 19, 2009)

If you really have no current on the "B" phase, something is open and the motor is single phasing. If this is an IEC starter, check the overload relay. These are a common failure item. Other possibilities: wire loose/broken, fuse blown, faulty breaker, panel buss bar loose/burnt, motor winding open, bad contact in contactor, loose utility connection at service.

Is there any other three phase equipment in this shop? If so, does it function properly?

To narrow the search, check to voltage at all point of the circuit, on all phases, with the motor on. 

An air compressor is a brutal application for an IEC starter. If the FLA of the motor is 40 amps, use at minimum a 63 amp contactor.


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## micromind (Aug 11, 2007)

I'd bet more on a bad current reading than anything else. 

If a 40 amp motor is indeed single-phasing, the current on the two hot phases would be a LOT more than 39. Also, being connected to an air compressor, it wouldn't even be running if it were single phasing. 

The only way that a 40 amp 3Ø motor can read 39 amps is if all 3 phases are good. 

If it runs ok and doesn't trip its O/Ls, I'd leave it alone. 


Also, as noted, if a clamp meter isn't fully closed around the conductor, the reading will be less than actual.


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## varmit (Apr 19, 2009)

I was assuming that the amp readings were inaccurate.

The compressor may not be relieving head pressure. This would cause a high starting current that would shorten the starter life.


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## rcar (Apr 28, 2013)

I triple checked the amp readings. Twice with just me and then a third time with my bro in law. I'll go through this list of things you guys gave me and report back. 

Thanks


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## micromind (Aug 11, 2007)

varmit said:


> I was assuming that the amp readings were inaccurate.
> 
> The compressor may not be relieving head pressure. This would cause a high starting current that would shorten the starter life.


It sure will!!!

Especially IEC stuff. It'll barely handle a close-coupled pump and the long start time of a compressor is often more than it can handle. 

A 3Ø motor can usually start a fully loaded compressor, but it'll take longer to get up to speed. A NEMA starter can usually handle this, but it puts a real strain on an IEC one.


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## bobelectric (Feb 24, 2007)

A 3 phase circuit with a neutral for the 120 volt coil?


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## Bootss (Dec 30, 2011)

varmit said:


> I was assuming that the amp readings were inaccurate.
> 
> The compressor may not be relieving head pressure. This would cause a high starting current that would shorten the starter life.


does the compressor have an unloader for starting?


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## wendon (Sep 27, 2010)

The first thing I'd do is replace the pressure switch. I presume the contacts are burning up. Very possible a bad pressure switch is chattering the points before it shuts off. Another thing would be to try feeding the control circuit with a separate source.


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## pjg (Nov 11, 2008)

Are the voltages in post #1 phase to phase or phase ground? If phase to phase ,check A to C as well


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## JRaef (Mar 23, 2009)

Micromind is right, it can't be that the motor is single phasing, a 3 phase motor will almost never start with single phase power. Without the third phase there is no relative rotation, so it just shudders back and forth until the fuses blow or the breaker trips. A 3 phase motor will CONTINUE to run if it is ALEADY spinning when the phase is lost, that's why phase loss protection is important. But if there is no current flowing in one phase, it will not start unless maybe there there is zero load on it, in which case it may randomly run in either direction, it becomes a 50-50 chance. Then the current will increase by about 50% for a given load. The one single exception I have seen, just once, what's that current flowed in all 3 phases to get it to start, then the jerk from starting the motor caused one terminal to lose the connection and the motor continued to run. But in this case, the current in the other two legs is too low for that to be the case, unless that motor is maybe twice as big as it needs to be in the first place. That RARELY happens with compressors, it's usually the other way around. It almost has to be a measurement error.

So it might be a bad choice on the motor starter selection, but it's more common on a situation like this that the compressor is being run at too high of a duty cycle. That means either you are demanding more air than it can deliver, which usually just means the motor never shuts down, or the differential is too narrow on the pressure switch. If the motor never shuts down, it doesn't damage the contacts, leaving the pressure switch settings, or the control circuit is allowing the contactor coil to chatter.


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## rcar (Apr 28, 2013)

OK so, I'm a moron to start. The reason I was getting 0a is I had my meter on the 40a setting and it is pulling 41a. So I moved to the 400a setting and this is what I was getting. 

A: 32
B: 36
C: 41

These numbers are odd especially given what I was reading yesterday. I made sure I was closed clamp and had the correct phase.

Voltage while motor running at breaker and line/load of starter. 

A: 119
B: 119
C: 207

The pressure switch is at 100/150. 

On the line side of the starter, going from left to right are phases B, C, then A. Am I wrong in thinking they should be ABC left to right? Where does the control get its power?

Seeing as though pressure switches are cheap I will have him change that out and see what happens.


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## 51360 (Jun 9, 2014)

rcar said:


> OK so, I'm a moron to start. The reason I was getting 0a is I had my meter on the 40a setting and it is pulling 41a. So I moved to the 400a setting and this is what I was getting.
> 
> A: 32
> B: 36
> ...


Interesting thread. Much better readings. 

One of those cases where it would be nice to be there with you, no disrespect. Listening to the compressor start, and visual checks can help immensely.

Borgi


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## rcar (Apr 28, 2013)

Borgi said:


> Interesting thread. Much better readings.
> 
> One of those cases where it would be nice to be there with you, no disrespect. Listening to the compressor start, and visual checks can help immensely.
> 
> Borgi


No doubt, I have no delusions of me knowing wtf I'm doing here. If it wasn't family I wouldn't touch it. You know how this **** goes though. They think since I'm an electrician I can fix anything with a wire to it.


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## 51360 (Jun 9, 2014)

rcar said:


> No doubt, I have no delusions of me knowing wtf I'm doing here. If it wasn't family I wouldn't touch it. You know how this **** goes though. They think since I'm an electrician I can fix anything with a wire to it.


I know exactly what you speak of!  My sisters call me often and somehow expect me to solve their problems over the phone. Sometimes you can, but other times it's best to be there.

Your getting great advice from the previous posters. 

Borgi


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## micromind (Aug 11, 2007)

To a motor, voltage from phase to neutral/ground doesn't matter, it sees only phase to phase voltage. 

Very often with 240 ∆ systems, there's a lot of load on the two 120V legs and very little on the high leg. This results in imbalanced phase to phase voltages. Even a small voltage imbalance will cause a fairly large current imbalance. 

There are several causes for voltage imbalance in addition to the one I mentioned, the most common is voltage drop across things like breakers, starters, bad connections, any of which will cause current imbalance to go up. 

First, I'd measure voltage phase to phase as close to the motor as you can safely get. This is usually at the load side of the starter. If it's more than 5 volts difference, you'll need to look further. 

Measure phase to phase voltage as close to the service as you can. Usually this is the line side of the main breaker. If there's more than 5 volts difference, I'd get the POCO involved as it is likely their problem. 

If the service voltage is good and the voltage at the starter is bad, you have voltage drop across something in the system. 

Begin with a 'fall of potential' test at various parts of the system. This test involves measuring voltage across a device. At a starter, for instance, measure voltage from phase A on the line side to phase A on the load side. The reading should be low; usually less than 0.5 volt. If it's more than a volt or so, you've found a bad component. 

In your case, I'd begin with the starter, next would be the disconnect (if there is one), then the breaker in the panel. 

When doing fall of potential on a breaker, go from the load side of the breaker to the line side of the bus. Get as much of the component in the test as you can.

If this is the only 3Ø load, I'd do the main breaker as well.


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## FF301 (Jan 12, 2014)

what are the voltage readings going from phase to phase ? 
you are going from phase to ground or neutral by the high leg indication.
compare voltage reading on the line and load sides phase to phase with the motor running, this will indicate voltage drop if the contacts are bad, contacts arc every time they open/close and if not big enough will develop high resistance do to "burnt contacts".
it might pay to just over size the contactor and as mentioned use mena rated they are stronger.
if voltages on line side of contactor are low during run time then go back and tighten all connections in the service a poor connection will result in voltage drop also. this could go all they way back to meter and even the service entrance and utilities. what is the main service rating, you really should not see any voltage drop on a larger service for just one motor.

do a visual inspection on main service looking for signs of over heating or burnt wires, just tighten everything to be sure of good connections. this includes were breakers are bolted into the buss. and neutrals can cause many problems people think are on the line side.


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## 51360 (Jun 9, 2014)

FF301 said:


> what are the voltage readings going from phase to phase ?
> you are going from phase to ground or neutral by the high leg indication.
> compare voltage reading on the line and load sides phase to phase with the motor running, this will indicate voltage drop if the contacts are bad, contacts arc every time they open/close and if not big enough will develop high resistance do to "burnt contacts".
> it might pay to just over size the contactor and as mentioned *use mena rated they are stronger.*
> ...


Am I missing something, the picture I seen, the starter was a size 2 NEMA starter.

Correct me if I'm wrong.

Borgi


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## hardworkingstiff (Jan 22, 2007)

micromind said:


> Begin with a 'fall of potential' test at various parts of the system. This test involves measuring voltage across a device. At a starter, for instance, measure voltage from phase A on the line side to phase A on the load side. The reading should be low; usually less than 0.5 volt. If it's more than a volt or so, you've found a bad component.


This is a great starting point, excellent advice.


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## hardworkingstiff (Jan 22, 2007)

rcar said:


> On the line side of the starter, going from left to right are phases B, C, then A. Am I wrong in thinking they should be ABC left to right?


BCA is the same rotation as ABC, and that's what is important to the motor.

I'm guessing that C phase is landed in the middle of that starter because it's the high leg.


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## glen1971 (Oct 10, 2012)

rcar said:


> OK so, I'm a moron to start. The reason I was getting 0a is I had my meter on the 40a setting and it is pulling 41a. So I moved to the 400a setting and this is what I was getting.
> 
> A: 32
> B: 36
> ...


What are your phase to phase voltages?

Where is the control power coming from? What voltage is the coil rated for? If the control is coming from the first and second lines or if there is a control transformer and it is being fed from the first and second phases and you have a high leg terminated to the second terminal, that may be some of your issues..


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## micromind (Aug 11, 2007)

Borgi said:


> Am I missing something, the picture I seen, the starter was a size 2 NEMA starter.
> 
> Correct me if I'm wrong.
> 
> Borgi


I see a Square D NEMA size 2 starter as well. Square D is one of the better brands, and this one looks to be in good shape, but the green wire on the 120 coil bothers me a bit........


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## McClary’s Electrical (Feb 21, 2009)

micromind said:


> I see a Square D NEMA size 2 starter as well. Square D is one of the better brands, and this one looks to be in good shape, but the green wire on the 120 coil bothers me a bit........


I made an assumption. I couldn't see the pic


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## JRaef (Mar 23, 2009)

rcar said:


> ...
> On the line side of the starter, going from left to right are phases B, C, then A. Am I wrong in thinking they should be ABC left to right? Where does the control get its power? ...


OK, now we are getting somewhere. The current imbalance isn't really that bad for a 120/240 Delta system, it's always going to be unbalanced, that's the cost of using that kind of connection. Besides, that would not cause damage to the starter. A NEMA Size 2 starter like that is going to be rated for a minimum of 45A, you are only drawing 41 on the highest pole, shouldn't be a problem at all.

But the phasing information, combined with your photo and the Sq. D Wiring book (click to download), is making it apparent that someone may have messed up here. In your system, C is the High Leg, it is supposed to be B phase (but it does occasionally vary with legacy utilities). However, the standard Sq. D wiring diagram (page 48 of the pdf in the link) taps off of A and B for control power. In your case with 120V available from A or B to neutral, your 120V coil hot would have to be tapped off of A or B, which would have been fine, but instead, because someone likely THOUGHT that a High Leg system would have 208V on B to Neutral, they probably changed to to be C and neutral. Then because your system is NOT a B phase high leg, it actually double-psyched them...

Part of why I think this may have happened is because of the fact that they ALSO ran one side of the coil directly to ground, as evidenced by your photo showing the green wire as micromind noticed. That's evidence of a hack who did not know that you are not supposed to do that. So the person who hacked this in the first place was likely not savvy enough to notice the double-psych on the stinger leg, and ended up doing exactly what should not have been done, put 208V on the 120V rated coil. It would work for a while, but the coil would get very hot, eventually swell and jam the armature, either open or closed. Open is a safer fail in that the motor does not run when you want it to, but it could also fail closed, which means the motor does not STOP when it is supposed to, just like having welded contacts.

So before you go any further, use your meter to read the ACTUAL voltage to that coil, assume nothing. Also, get rid of that green wire and make that connection to the real neutral (white) wire. Even if it is reading 120V right now, if he cheated and just went 120V hot to a ground wire, and the starter box is bonded locally but not to the service ground, you could have a ground loop setup because of differences in ground resistance between the locations that could be giving you fits as well.


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## rcar (Apr 28, 2013)

Sorry for going MIA. We had a death in the family last weekend that took me out of work. The more I thought about the situation and the hack jobs mentioned I decided to tell him to change out the pressure switch and see what happens. If I make any changes and it fails again, even though it wouldn't be my fault, they might try and hold me liable. Before this thread I didn't have the slightest idea how far the rabbit hole would go. I need to learn hands on from someone more experienced before I go changing things. Thanks for all the replies. I definitely know a sh*t ton more now than when this started. You guys are great.


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## ELECTRICK2 (Feb 21, 2015)

When you take the voltage readings for a 3 phase motor always measure line to line.
What part of the starter dies? Contacts or coil?.
If it's the coil go through the control circuit, same as a light circuit.
What's the voltage at the coil when motor is running .
Measure with both leads on the coil, not with one on ground.
If it's the contacts I would look at the pressure switch.
Post a pic of interior of dead contractor.
Does the pressure switch use a mercury bulb?
I had a motor that kept tripping overloads cuz the mercury switch rocked back and forth causing the o/l to trip.


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