# Single phase motor wont start



## murph7489 (May 8, 2016)

I have a 10 hp single phase 240v motor that runs an air compressor. The motor buzzes really loud then trips the 50 amp breaker after about 5 seconds.

It has 4 capacitors all of them are 370-440v 50 mf run capacitors wired together and they are all fairly new. 


the motor nameplate says
10 hp 240v 46.3 amps
cap 500/50mf 440-440v 

So anyways the motor spins freely when I spin it by hand. I am wondering if it requires a larger start capacitor or if it is something else like perhaps a centrifugal switch inside the motor itself is bad or stuck..I have also eliminated the unloader valve and emptied the tank it still did not start...

There is a reset button that has three wires coming from the; (thermal overload?)
magnetic starter
capacitor 
motor
I am wondering if this could also be the problem, but it seems like if it were bad there would be no humming at all but not sure about that one.

anyways when I flip the breaker on it buzzes really loud and 5 seconds later the breaker trips...

thanks for any advice in advance, I don't know why they even make a single phase motor so big....


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## Ty the electric guy (Feb 16, 2014)

There should be a starting capacitor. Single phase motors need some phase shift to start. 

It seems like someone changed the caps and rewired them. There should be 500 mF caps in there too.

Depending on how many times you tried starting it like this the motor might be fried.


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## murph7489 (May 8, 2016)

thats what I thought...so I should get two 500 mF caps and wire them in with the two run caps..


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## sbrn33 (Mar 15, 2007)

That breaker is not even close to big enough. Did it run before? If so I would say the cetrifical switch is stuck in the run winding mode.


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## varmit (Apr 19, 2009)

A 50 amp breaker will never hold a 46 amp motor. The minimum breaker size would be 90 to 100 amp and this could be questionable depending on the starting characteristics. 

Your description sounds like either severe voltage drop, centrifical switch or bad caps.


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## Ty the electric guy (Feb 16, 2014)

They don't just go in parallel with the run caps. There should be a wiring diagram on the motor. The centrifugal switch needs to take the starting caps out of the cct. 


Sent from my iPhone using Tapatalk


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## murph7489 (May 8, 2016)

It did run before and I was told that it buzzed for a couple of seconds and then start...the circuit is about 100 yards away from the main panel too..which is another issue I was thinking, how in the hell would a 50 amp breaker withstand a 46 amp load on startup


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## Ty the electric guy (Feb 16, 2014)

sbrn33 said:


> That breaker is not even close to big enough. Did it run before? If so I would say the cetrifical switch is stuck in the run winding mode.




Breaker isn't the issue of its sitting there buzzing. Could be the centrifugal switch though. Caps are easier to check first.


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## JRaef (Mar 23, 2009)

My suspicion is that because the breaker is too small, it means the conductors are too small as well, and possibly the supply transformer too. So EVERY time that motor attempted to start, it caused a severe voltage drop. That may have been what prevented the breaker from tripping, but would have made the motor take longer to accelerate, meaning it overloaded the centrifugal switch and burned it out. Centrifugsl switches are only meant to be in the circuit for a second or two, they are not supposed to be carrying current for very long or very often. Someone may or may not have tried to "fix" it by changing the caps (what people sometimes refer to as a "hard start kit"), but that would have just served to accelerate the demise. 

10HP is at the upper end of acceptability for single phase motors, partly because it consumes a large portion of the capacity of a typical single phase transformer to try to start it.


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## Nom Deplume (Jul 21, 2013)

I will bet the centrifugal switch is stuck. I would take the end bell off and clean the gunk out of the weights and clean the contacts of the switch.

This is the most common problem with capacitor start motors.


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## murph7489 (May 8, 2016)

Looks like I will have to take the fan off in order to get to the centrifugal switch correct ?


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## murph7489 (May 8, 2016)

Here is the motor, looks like I will have to take the fan off to get to the centrifugal switch...


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## murph7489 (May 8, 2016)

sorry I had to figure out how to add a pic..its big I know...


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## Nom Deplume (Jul 21, 2013)

No photo.
The centrifugal switch is usually on the end opposite the shaft.
Be careful and don't lose any bearing shims.

Are you sure that motor is capacitor start?
It looks like TEFC.


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## murph7489 (May 8, 2016)

It is tefc....but it has 4 capacitors too...thought it might need a start capacitor...


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## Nom Deplume (Jul 21, 2013)

murph7489 said:


> It is tefc....but it has 4 capacitors too...thought it might need a start capacitor...


Since it is TEFC, the switch may be ok, but it may be worthwhile to check it.


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## murph7489 (May 8, 2016)

so if I have someone flip on the breaker and spin the motor by hand and it starts up, that would in effect rule out the centri-switch and point me in the direction of it needing a start capacitor..but then the question is, what size would that be...

all the nameplate says is cap 500/50 400-440 v... but there are 4 new less than two months old run caps rated at 440 v 50mF hooked up now


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## frenchelectrican (Mar 15, 2007)

I was hoping that was CSCR motor and that is pretty big one for single phase verison...

However.. If you can start the motor by hand .

Then the starting system is allready comperised...

Inproper start cap connection. 

Bad starting swich 

Burnted or damaged starting winding..

Those are one of most common signs for hard starting..

If you did have 240 volts at the breaker panel and someone throw the switch . you will see voltage drop but how much it drop that will varies depend on supply conductor size , distance , transformer size.. 

But please check the wiring dagram for cap connection..


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## murph7489 (May 8, 2016)

Yeah actually I already did that sir, actually I disconnected the motor at the magnetic starter and found 238v...doing construction elec I don't see a whole lot of this type of stuff and I've never seen a single phase motor this big on an air compressor. Troubleshooting is all about experience and knowledge. The more you have of each, the faster and better you are at it...


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## telsa (May 22, 2015)

Air compressor...

Good grief.

It needs an "unloader"

I strongly suspect THAT"S your problem.


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## Nom Deplume (Jul 21, 2013)

telsa said:


> Air compressor...
> 
> Good grief.
> 
> ...


Good idea. Check this too.
I have found that the problem is always electrical until we prove it otherwise.


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## murph7489 (May 8, 2016)

well actually that was the first thing I checked..if there is a failed unloader the pistons would not be able to move with the air trapped in the heads...and that is clearly not the case


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## frenchelectrican (Mar 15, 2007)

murph7489 said:


> Yeah actually I already did that sir, actually I disconnected the motor at the magnetic starter and found 238v...doing construction elec I don't see a whole lot of this type of stuff and I've never seen a single phase motor this big on an air compressor. Troubleshooting is all about experience and knowledge. The more you have of each, the faster and better you are at it...


Yuh.. That fine as you stated 238 volts .. Ok what size conuctor it have in there i know you mention 100 yards that is pretty good distance there...

I have seen much larger single phase motors the biggest one i know is 20 hp ( yuh i ran into larger one but that in med / high voltage motor ) 

That big motor i did work on was hard on POCO pole pig ...

I am supect the distance and conductor size did cause the hard start ..


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## Bootss (Dec 30, 2011)

Be careful if you take it apart not to break the points and Mount insulator of starting switch when you pull it apart.
its internal near one of the bearings


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## sbrn33 (Mar 15, 2007)

I hope he comes back and lets us know what happens here. I would be willing to bet it is the centrifugal switch. It could be a start cap but I doubt it. I do know that it needs at least a 90 amp breaker. 
Still would like to know if this thing ever ran....


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## sbrn33 (Mar 15, 2007)

If it was an unloader it would start after sitting for awhile.


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## murph7489 (May 8, 2016)

I STRONGLY suspect I have found the issue..I haven't been out to the site yet, but I did some research and found a wiring diagram for the motor which makes a LOT more sense now... 

Who ever worked on this thing before installed 4 capacitors on it..however they must have just assumed that it needed 4 run capacitors, ie; he took one out ran down to the supply house and said give me 4 of these.... Unfortunately he didn't notice that two of the capacitors need 500 mF anyways that is one issue that I know is wrong...

Here is the wiring diagram I got and looking at this, it all makes sense..So far...I will know more when I get these installed


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## JRaef (Mar 23, 2009)

murph7489 said:


> so if I have someone flip on the breaker and spin the motor by hand and it starts up, that would in effect rule out the centri-switch ...


I think you are misinterpreting how the centrifugal switch works here. When at rest, the switch is closed and the start cap is in the circuit. That's what allows the motor to start because it provides the phase shift that gives it a rotation direction. Once started, it keeps spinning in that direction and the start cap is no longer needed, so the centrifugal switch opens. 

If the switch is stuck open or the contacts have burned away, the start cap is not in the circuit and the motor sits there and hums or even just shakes back and forth because there is no relative rotation. So if you spin it by hand, you are giving it that relative rotation and it will run normally, until you turn it off. So this confirms even more that it is likely the centrifugal switch.


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## murph7489 (May 8, 2016)

You are Correct I completely worded that wrong..thanks


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## murph7489 (May 8, 2016)

Oh yeah and it has #6 conductors and If I remember correctly I am allowed to install a larger breaker to handle the inrush load, and keep the #6 conductors because I have an overload protection device inline, while the breaker protects against short circuit and ground faults.


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## Nom Deplume (Jul 21, 2013)

murph7489 said:


> Oh yeah and it has #6 conductors and If I remember correctly I am allowed to install a larger breaker to handle the inrush load, and keep the #6 conductors because I have an overload protection device inline, while the breaker protects against short circuit and ground faults.


10hp single phase motor at 230v is rated at 50amps.
50 amps x 125% = 62.5 amps
#6 @ 75°c = 65 amps

The breaker can be 250% of the 50 amp rating.
The max size breaker can be 125 amp on the #6 wire.

A 50 amp breaker may trip on startup, but what you describe still points to the centrifugal switch.

Has this motor ever run on the 50 amp breaker? If so the problem may be in the motor and upsizing the breaker may burn up the motor.


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## JRaef (Mar 23, 2009)

murph7489 said:


> Oh yeah and it has #6 conductors and If I remember correctly I am allowed to install a larger breaker to handle the inrush load, and keep the #6 conductors because I have an overload protection device inline, while the breaker protects against short circuit and ground faults.


Yes you're right, you can over size the breaker if you have another OL device in the circuit. 

But #6 is not the right conductor size for a 10HP 240V single phase motor. NEC chart lists the FLC as 55A, then you must size the conductors at 125% so 68.5A minimum. #6 is good for 65A, not counting any voltage drop de-rating for distance. I think you need #4 conductors.


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## murph7489 (May 8, 2016)

yeah I agree Jraef it does need #4 and it also needs a 100 amp breaker, and it needs two start capacitors 350v 500mF then I believe it will operate correctly..

I am not discounting the Centrifugal switch theory, non deplume, however that will suss out after I install the proper capacitors. Because the fact is; this compressor was operating for a long time with no problems until someone decided to place 4 run capacitors on a motor that required two start caps and two run caps...I don't know if they had the correct caps installed before then or not, but I cant imagine that motor starting ever without the right caps...


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## murph7489 (May 8, 2016)

lol, well the guy that owns the compressor just told me that he had his friend, a rocket scientist (for real) come look at it yesterday, he says that its the reset switch that p1, p2 and T4 hook up too in the wiring diagram..

I said no, the motor needs two start capacitors and two run capacitors and it now has 4 run capacitors...it's not going to start without the correct start capacitors....then he says well it ran before and it only had the same capacitors that it has now in it and it ran...

oy vey...lolol..


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## Nom Deplume (Jul 21, 2013)

JRaef said:


> Yes you're right, you can over size the breaker if you have another OL device in the circuit.
> 
> But #6 is not the right conductor size for a 10HP 240V single phase motor. NEC chart lists the FLC as 55A, then you must size the conductors at 125% so 68.5A minimum. #6 is good for 65A, not counting any voltage drop de-rating for distance. I think you need #4 conductors.





murph7489 said:


> yeah I agree Jraef it does need #4 and it also needs a 100 amp breaker, and it needs two start capacitors 350v 500mF then I believe it will operate correctly..
> 
> I am not discounting the Centrifugal switch theory, non deplume, however that will suss out after I install the proper capacitors. Because the fact is; this compressor was operating for a long time with no problems until someone decided to place 4 run capacitors on a motor that required two start caps and two run caps...I don't know if they had the correct caps installed before then or not, but I cant imagine that motor starting ever without the right caps...


The OP stated that this was 240v.
Table 430.238 shows a 10 hp motor at 230v is rated at 50 amps.
50 amps x 125% = 62.5 amps
#6 is rated for 65 amps at 75°c

#6 is suitable for this motor at 240 volts. If this were 208 volts then it would need #4.


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## telsa (May 22, 2015)

Nom Deplume said:


> The OP stated that this was 240v.
> Table 430.238 shows a 10 hp motor at 230v is rated at 50 amps.
> 50 amps x 125% = 62.5 amps
> #6 is rated for 65 amps at 75°c
> ...


Because of the distance from the panel, #6 is marginal. 

I don't think it's at issue here, but I would have run #4 if it were my druthers, way back when.

I'm constantly amazed at the choking effect a long branch run has on such induction loads. 

I have a couple of single-phase 10 hp motors out in my shop. 

They REALLY suck down the current during start up.


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## JRaef (Mar 23, 2009)

Nom Deplume said:


> The OP stated that this was 240v.
> Table 430.238 shows a 10 hp motor at 230v is rated at 50 amps.
> 50 amps x 125% = 62.5 amps
> #6 is rated for 65 amps at 75°c
> ...


Oops, you're right, I accidentally read the 208V column.


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## murph7489 (May 8, 2016)

So I just got back from the site..installed two 350v 500mF caps and and it started right up no problem...told the owner his rocket scientist buddy might be able to build a rocket but they don't use single phase motors to start....lol...


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## Nom Deplume (Jul 21, 2013)

telsa said:


> Because of the distance from the panel, #6 is marginal.
> 
> I don't think it's at issue here, but I would have run #4 if it were my druthers, way back when.
> 
> ...


I don't disagree. If I were installing this I would most likely use #4.
I was just pointing out that this was code minimum.


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## Nom Deplume (Jul 21, 2013)

murph7489 said:


> So I just got back from the site..installed two 350v 500mF caps and and it started right up no problem...told the owner his rocket scientist buddy might be able to build a rocket but they don't use single phase motors to start....lol...


That's easier than splitting the end bell off the motor.
Since it was TEFC it should be pretty clean inside unless it was over greased.


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## emtnut (Mar 1, 2015)

JRaef said:


> Oops, you're right, I accidentally read the 208V column.


I was wondering if there was a difference ... CEC table 45 has it at 50 amps too :thumbsup:

Sometimes there _are_ funny differences in the 2 codes !


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## Bootss (Dec 30, 2011)

I wonder if that motor has a voltage/current coil controlling start contacts,instead of centrifugal mechanism?
:thumbup::whistling2::vs_mad::laugh:


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