High Service Voltage vs. Motors

[hbendillo]

I have a hospital client who wants to replace a 480-volt, three-phase vacuum pump that they have had a lot of trouble with burning out motors and contactors. We discovered that the voltage a the motor is about 503 volts. Interesting enough the voltage at the service equipment is 500 volts. The feeder is a long run of conduit, probably 300 feet. Do motors generally have the tolerance for this kind of overvoltage? If the motor is rated 480-volt it is less than 5% which seems reasonable. But, I have seen motors rated at 460-volt which would put us about 9.3%.

 

[edison123]

Have you checked the load current ? May be it's the overload that is killing the motor than the overvoltage. +- 5% should not burn the motor.

Muthu

http://www.edison.co.in

 

[hbendillo]

Load current is on the money. I didn't think the overvoltage was a big problem but I don't often find the service voltage this high on a commercial building.

 

[electricpete]

I am used to seeing 460vac as nameplate, 480vac as nominal system / distribution voltage.

NEMA MG-1 allows for 10% voltage variation.

Higher voltage at the motor than at the service entrance, even though there is 300’ run of cable to the motor? That would be strange. Maybe conditions changed between measurements. Did they actually measure at the motor location ? (That would normally be a challenge).


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[RRaghunath]

If the motor is really far away from the source, could it be that the motor is failing during starting!
What is the rating of three-phase motor here!

 

[rbulsara]

Causes could be different if multiple motors are failing. or is this just related to one motor?

Need a lot more info about the system and frequency of failures.
What size (HP) motors are burning out?
Is single phasing investigated?
Are there automatic PF correction capacitors installed? Ferroresonance could play a role if combined with long cables.

Has any analysis by a good repair shop made on root cause of the motor failures?

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

Good point.

If power factor correction capacitors were installed, it would increase voltage at motor terminals, but should not be above source voltage if properly undercorrected. If voltage really is higher at motor terminals, it could indicate the motor is overcorrected either by caps or combination caps plus cables. IF those capacitors are switched with the motor, in that case overvoltage would be expected during coastdown.

Just a thought based on very limited info, not a diagnosis.


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[hbendillo]

This is the only piece for equipment that has given them these kind of problems. At one point we found that is was running backwards. This is a twin 10 HP pump, alternating operation, vacuum pump. There is no power factor correction employed by the hospital. I don't know if the power company is using any on the incoming primary but I can check. Single phasing has been investigated and not a problem. This machine has burned probably a half dozen motors and contactors but has been running well for the last year with little problems. The client is going to completely replace it now and was curious about the voltage condition before he did so.

 

[waross]

Standard motor voltages in North America are multiples of 115 Volts.
With chronically high voltage, you may be having higher voltages during off peak hours. If your 503 Volts is going any higher at other times of the day it won't take much to saturate the motor. But the higher currents should activate the motor overload devices. Does the motor have proper overload protection? Has the overload protection been set higher to avoid nuisance trips?
A possible scenario is high voltage causing nuisance trips. The overloads are reset to avoid nuisance trips. Nuisance trips are avoided but nuisance burnouts occur.
But you say that contactors are also burning. That indicates excessive voltage drop on starting. With a 300 foot run, the starting current must be considered rather than the running current. If the conductors are sized for 5% voltage drop while running, as per NEC and CEC minimum requirements, you may expect 25% or 30% voltage drops when starting. That may easily burn contactors and/or motors. The 500+ Volts incoming may have worked in your favor.
Concerning a higher voltage at the motor, my first reaction has to be meter error.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Hire some experienced engineer to look at the issue. Cannot cover all bases from remote and limited info. A good motor repair shop can assist in narrowing the probable cause of motor failures.

Rafiq Bulsara

http://www.srengineersct.com