3phase motor question

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Smash

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Philadelphia suburbs, PA USA
Having trouble keeping a 3 phase 208V 5hp motor running. This is for and exhaust fan use for auto exhaust. Voltage reads low between legs 1 & 3 at around 198V all other readings are consistent at 208. This is a car dealership with a large service area. Also has had its share of bad contractors over the years. Things are not labeled correctly and some overall bad work. I believe the voltage problem I'm having is from unbalanced load across the 3 phases. Single phase loads & even 120V loads share this panel. Contractors would just twin up anything to get there job done. I know this unbalance is well past the 2% acceptable limit. My question is how do I balance the load on this panel if the loads are distributed around the large 13 bay service area. There will never be a consistent load when loads are being used at different times. My motor is not real happy with this voltage and trips the 3 phase breaker within minutes. The place is at compacity power wise and is always an adventure looking for a place to pull power for anything. So finding any available clean power is futile. I even pulled the motor had it serviced and there is no problem. Has to be unbalanced load on this 3 phase 4 wire service. What options do I have ? Spending 2K on a special wound 200V motor probably isn't an option. Ty
 
Voltage imbalance is rough on motors but did you take current readings and how do they compare to the name plate?

Did you check the voltage at the panel? At the service?
 
Smash said:
Having trouble keeping a 3 phase 208V 5hp motor running. This is for and exhaust fan use for auto exhaust. Voltage reads low between legs 1 & 3 at around 198V all other readings are consistent at 208. This is a car dealership with a large service area. Also has had its share of bad contractors over the years. Things are not labeled correctly and some overall bad work. I believe the voltage problem I'm having is from unbalanced load across the 3 phases. Single phase loads & even 120V loads share this panel. Contractors would just twin up anything to get there job done. I know this unbalance is well past the 2% acceptable limit. My question is how do I balance the load on this panel if the loads are distributed around the large 13 bay service area. There will never be a consistent load when loads are being used at different times. My motor is not real happy with this voltage and trips the 3 phase breaker within minutes. The place is at compacity power wise and is always an adventure looking for a place to pull power for anything. So finding any available clean power is futile. I even pulled the motor had it serviced and there is no problem. Has to be unbalanced load on this 3 phase 4 wire service. What options do I have ? Spending 2K on a special wound 200V motor probably isn't an option. Ty
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Voltage imbalance will not in and of itself cause a breaker to trip. Voltage imbalance causes the motor to develop negative sequence currents that then create negative torque, so the motor has to "fight itself" to maintain the speed to drive the load, which causes it to pull more current to do the same work. That heats up the motor disproportionately compared to the average current being drawn, so it shortens the motor insulation life. But unless that current is extreme, the breaker would not care. If the current IS that high, the motor overload protection should be taking it off line, not the circuit breaker. Unless of course there is no OL relay, or the one that is there tripped so often that someone jumped around it or installed severely over sized heaters (or turned the dial up). 5HP 208V 3 phase is generally too large to have integral motor thermal protection, but it's remotely possible. If so, then it's likely not working. There is more to this story somewhere.

If the OL relay is there and correct, yet the breaker is tripping after 5 minutes, I'd suspect something else. You say the motor has been serviced? By who and how? This sounds suspiciously like a situation where the motor is heating up and something is going to ground or becoming a phase-to-phase short, which WOULD trip the breaker. Might be the motor, but it might be the motor leads too. Could there be water in the conduit?
 
Are there motor overloads on this circuit. I would think voltage unbalance would trip overloads before it would throw a breaker.

What's your breaker size? What's the FLA on the motor? What does the motor pull?

Are all terminations in the panel and at the motor tight?
 
Bad connection somewhere in the circuit, bad bearing in the motor, bad winding, in that order is what I would look for.

I know the op said he had the motor checked out, but I don't know what that means and sometimes problems like this don't show up till there is a good load on the motor.
 
iwire said:
Voltage imbalance is rough on motors but did you take current readings and how do they compare to the name plate?

Did you check the voltage at the panel? At the service?
Click to expand...
One of first things that needs checked. If supply voltage isn't correct it isn't the motor that has a problem. If this voltage is still unbalanced in similar manner with no load - you need to look further upstream for the problem.
 
Saturn_Europa said:
Are there motor overloads on this circuit. I would think voltage unbalance would trip overloads before it would throw a breaker.

What's your breaker size? What's the FLA on the motor? What does the motor pull?

Are all terminations in the panel and at the motor tight?
Click to expand...

and how far from the panel is it and what size wire is it wired with?

Does the motor drive a pulley to the fan? If so, is that in good shape?

Is the motor 208 or 230V? You're really gonna have problems running a 230V motor on 208/208/198V.
 
GoldDigger said:
Worst case if remediating the supply voltage is not practical: install a VFD.

Sent from my XT1585 using Tapatalk
Click to expand...

I have to agree with gold digger. Either a VFD or set a dedicated 3 phase xfmr just for the motor. I saw where someone mention what your line voltage was feeding the transformer and since it is in direct ratio it is a huge question as well.


Sent from my iPhone using Tapatalk
 
Excess current in the 198 volt leg might be the reason it draws voltage in that leg down. Cause for this could be ground fault in that winding of the motor, but the fault is near the neutral point in the motor instead of near the supply point so the impedance of the winding limits current enough you don't get instant trip of the breaker.
 
Jraef said:
This sounds suspiciously like a situation where the motor is heating up and something is going to ground or becoming a phase-to-phase short, which WOULD trip the breaker. Might be the motor, but it might be the motor leads too. Could there be water in the conduit?
Click to expand...

Put the clamp around all three wires at the same time as close as you can to the motor without including the conduit. You will see more than a flick or two on the lowest read out if something's going to the ground in an amount that matters. If you don't see anything there, you rotate the leads forward. 1->2, 2->3 and 3->1. If the issues are from up above the problem stays where it was. If it's the motor, it will follow the rotation around.
 
iwire said:
Voltage imbalance is rough on motors but did you take current readings and how do they compare to the name plate?
Click to expand...
I agree. Checking the currents would/should be the first step. Before concluding that voltage imbalance is the problem.
 
Electric-Light said:
Put the clamp around all three wires at the same time as close as you can to the motor without including the conduit. You will see more than a flick or two on the lowest read out if something's going to the ground in an amount that matters. If you don't see anything there, you rotate the leads forward. 1->2, 2->3 and 3->1. If the issues are from up above the problem stays where it was. If it's the motor, it will follow the rotation around.
Click to expand...
As you do this note which combination has the best results as far as current imbalance on the individual leads. Use that connection. I've used that method on Open Delta with a Hi leg. Not sure if it does you any good on a Wye.
 
Smash said:
Having trouble keeping a 3 phase 208V 5hp motor running. This is for and exhaust fan use for auto exhaust. Voltage reads low between legs 1 & 3 at around 198V all other readings are consistent at 208. This is a car dealership with a large service area. Also has had its share of bad contractors over the years. Things are not labeled correctly and some overall bad work. I believe the voltage problem I'm having is from unbalanced load across the 3 phases. Single phase loads & even 120V loads share this panel. Contractors would just twin up anything to get there job done. I know this unbalance is well past the 2% acceptable limit. My question is how do I balance the load on this panel if the loads are distributed around the large 13 bay service area. There will never be a consistent load when loads are being used at different times. My motor is not real happy with this voltage and trips the 3 phase breaker within minutes. The place is at compacity power wise and is always an adventure looking for a place to pull power for anything. So finding any available clean power is futile. I even pulled the motor had it serviced and there is no problem. Has to be unbalanced load on this 3 phase 4 wire service. What options do I have ? Spending 2K on a special wound 200V motor probably isn't an option. Ty
Click to expand...
Stick a cheap VFD on it - will balance volts. If that is the issue, prob solved.

Sent from my SM-G900V using Tapatalk
 
mike_kilroy said:
Stick a cheap VFD on it - will balance volts. If that is the issue, prob solved.

Sent from my SM-G900V using Tapatalk
Click to expand...
Unless that is not the problem. If it's a short in the motor or a loose connection, the drive will likely trip to protect itself, making it a fault detector. If it's a short in the motor leads, the transistors might blow, making it an EXPENSIVE fault detector.

Better to find and solve the real problem before throwing more hardware at it.
 
Jraef said:
Unless that is not the problem. If it's a short in the motor or a loose connection, the drive will likely trip to protect itself, making it a fault detector. If it's a short in the motor leads, the transistors might blow, making it an EXPENSIVE fault detector.

Better to find and solve the real problem before throwing more hardware at it.
Click to expand...

I Agree! I did not see all the previous posts, and I sent that too quickly. It is only a 'fix' if the issue is indeed a voltage imbalance and the choice is between a $ 500.00 vfd or $ 5000.00 for days of rewiring their power panels to balance their loads better.
 
Smash said:
Having trouble keeping a 3 phase 208V 5hp motor running. This is for and exhaust fan use for auto exhaust. Voltage reads low between legs 1 & 3 at around 198V all other readings are consistent at 208. This is a car dealership with a large service area.
Click to expand...

When you say there are low reading between legs 1&3 where are you measureing the voltage. At the motor, at a sub panel, at the main? Is this low reading constant or only at times. It could be nothing more than a bad connection. Do the light dimm when a big load such as an air compressor or AC unit comes on ?

As Jraef says this may not be the problem with your motor but would still need to be checked out.
 
3 phase motor question

3 phase motor question

Here's the whole story I was called to fix exhaust fan, it always was very loud and rattled a lot. I found a defective burned out 12yr old motor. Staff said worked fine other than the noise for 12yrs. Good enough for me I replaced the motor with a exact replacement. Worked on the housing to level motor, greased bearings, lined up pulleys and installed new belts. Fired it up worked great, staff very happy with how quiet it now was everything seemed fine. Ran for two weeks than stopped. Was called back out motor is free to turn by hand with not much effort belts still tight no wobble no mounting problems. However when turned on motor just has loud hum. That's when I took voltage readings and found the lower voltage between 1 & 3 First thing I think is great the motor is fryed. Brand new motor has to be under warranty. Called Baldor they told me it needs to be sent to one of there shops to be tested for warranty replacement. I go back pull the motor drop it off, get a call a day later motor is fine no problems pulley little out of balance but that's not the problem. Here I am today ready to reinstall motor. Little background who ever built this thing and I do have some pictures did not supply any over current protection, there isn't even a service disconnect outside. I know this company and there a little frugal so telling them they already need to replace the motor $1000 and oh. Y the way another $1000 to protect the new motor even though the old motor (so they say) ran for 12 yrs no problem. Right now they have a 3ph 30amp breaker (plug in mind you next to there main air compressor) feeding a disconnect feeding a contractor controlled by a T101 time clock. Conduit all the way to power room about 100' using # 10 thhn 3 wire with a ground. Will send pics of outdoor motor housing that's all I have right now.
 

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Smash said:
Here's the whole story I was called to fix exhaust fan, it always was very loud and rattled a lot. I found a defective burned out 12yr old motor. Staff said worked fine other than the noise for 12yrs. Good enough for me I replaced the motor with a exact replacement. Worked on the housing to level motor, greased bearings, lined up pulleys and installed new belts. Fired it up worked great, staff very happy with how quiet it now was everything seemed fine. Ran for two weeks than stopped. Was called back out motor is free to turn by hand with not much effort belts still tight no wobble no mounting problems. However when turned on motor just has loud hum. That's when I took voltage readings and found the lower voltage between 1 & 3 First thing I think is great the motor is fryed. Brand new motor has to be under warranty. Called Baldor they told me it needs to be sent to one of there shops to be tested for warranty replacement. I go back pull the motor drop it off, get a call a day later motor is fine no problems pulley little out of balance but that's not the problem. Here I am today ready to reinstall motor. Little background who ever built this thing and I do have some pictures did not supply any over current protection, there isn't even a service disconnect outside. I know this company and there a little frugal so telling them they already need to replace the motor $1000 and oh. Y the way another $1000 to protect the new motor even though the old motor (so they say) ran for 12 yrs no problem. Right now they have a 3ph 30amp breaker (plug in mind you next to there main air compressor) feeding a disconnect feeding a contractor controlled by a T101 time clock. Conduit all the way to power room about 100' using # 10 thhn 3 wire with a ground. Will send pics of outdoor motor housing that's all I have right now.
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First picture was taken after I pulled the new motor. Also was told by Baldo rep that this motor will work fine at 208V even though nameplate does not list 208V. In hindsight I should have gotten a motor which are wound slightly different with a nameplate listing my 208v Also another reason I thought I fried the motor. But they tell me it's ok and the old motor lasted 12 yrs. Baldor does not know of summer brownouts in this area that could lower voltage even more during summer months
 
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