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NEC standrad for voltage unbalance for motors
3

NEC standrad for voltage unbalance for motors

NEC standrad for voltage unbalance for motors

(OP)
Hi Members
Is their any NEC standrad for acceptable level for unbalance voltage for induction motors

RE: NEC standrad for voltage unbalance for motors


The NEC does not specify, but another ANSI standard—C84.1-1995 [Annex D] discusses derating for unbalanced-voltage operation.  

Use the search function at this site.  It has been discussed at length here.
  

RE: NEC standrad for voltage unbalance for motors

Suggestion: Visit
http://www.nema.org
for MG-1 motor and generator standard
Also, military standard
MIL-STD-1399(NAVY)
SECTION 300A
13 October 1987
SUPERSEDING
DOD-STD-1399(NAVY)
SECTION 300
1 August 1978
(See 6.3)
for the voltage unbalance limit 3% of Type I and II power supplies and 2% for Type III power supply (400Hz)

RE: NEC standrad for voltage unbalance for motors

NEMA definition: voltage unbalance as 100 times the maximum deviation of line voltage from the average voltage on a three-phase system, devided by the average voltage.

NEMA recommends against the use of motors where the unbalance is greater than five percent.

A unbalance of one percent may already void a manufacturer's warranty.

RE: NEC standrad for voltage unbalance for motors

Just a note: during a monitoring survey I conducted to determine why a chiller motor was tripping out on current imbalance, a voltage imbalance of approximately 1.7% was causing a current imbalance of TEN TIMES (17%) that amount. I prefer to see voltage imbalances of less than 1% for induction motors.

RE: NEC standrad for voltage unbalance for motors


According to ABB protective-relay literature, induction-motor current imbalance is equal to

         voltage imbalance x motor locked-rotor current / motor full-load current
  

RE: NEC standrad for voltage unbalance for motors

Suggestion. The mil std above appears to be within the margin of error of the posted link directly above for continuously running load, e.g. pump, compressor, etc., i.e. 3%.

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