NEC standrad for voltage unbalance for motors 3

[JatTiw]

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

 

[busbar]

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.

 

[jbartos]

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)

 

[lenne]

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.

 

[DanDel]

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.

 

[busbar]

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

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

 

[busbar]

Short IEEE online article on voltage-unbalance assessment:

http://grouper.ieee.org/groups/1159/1/VUFAss.html

 

[jbartos]

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%.