High Service Voltage vs. Motors
High Service Voltage vs. Motors
(OP)
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%.





RE: High Service Voltage vs. Motors
Muthu
www.edison.co.in
RE: High Service Voltage vs. Motors
RE: High Service Voltage vs. Motors
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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RE: High Service Voltage vs. Motors
What is the rating of three-phase motor here!
RE: High Service Voltage vs. Motors
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
RE: High Service Voltage vs. Motors
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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RE: High Service Voltage vs. Motors
RE: High Service Voltage vs. Motors
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
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"Why not the best?"
Jimmy Carter
RE: High Service Voltage vs. Motors
Rafiq Bulsara
http://www.srengineersct.com