What do we mean by voltage unbalance on a three-phase circuit? NEMA Standard MG1-14.36.2 defines it this way:
|Percent voltage unbalance = 100 X||Maximum voltage deviation from average voltage|
Example: Measured phase voltages 450, 460, 467. Average is 459. Maximum deviation from average is 459 minus 450, or 9 volts; % unbalance = 900/459 or 1.96%.
What does voltage unbalance do to a motor? Two things — both bad. First, it causes severely unbalanced line currents, up to 10 times the voltage unbalance. That causes rapid overheating of parts of the motor winding. Temperature rise will increase in proportion to twice the square of the voltage unbalance (4% voltage unbalance means about 30% higher temperature rise). Similar — and often more severe — overheating occurs in the rotor, undetectable by protective devices sensing only stator conditions.
The National Electrical Code rules for properly selecting the motor’s starter overload protection are dependable only for balanced voltages and currents. The selection procedure becomes much more complex, and far less reliable, when significant unbalance exists.
Second, voltage unbalance reduces motor accelerating torque. The motor may therefore be unable to start its load safely, if at all.
What unbalance is allowable? No standard sets a strict limit. Many users rely on a curve in NEMA standards (MG1-14.36) that calls for reducing motor horsepower loading as unbalance increases (to compensate for the extra heating). That curve plots voltage unbalance up to 5%. Hence, a common — but incorrect — assumption is that NEMA “allows” 5% voltage unbalance. On the contrary: in NEMA MG1-12.45, NEMA warns against operating any motor at a voltage unbalance exceeding 1%.
An article in an April 1992 air conditioning trade magazine claimed that “the maximum unbalance is 2%.” Where did that come from? Neither the author nor his manufacturer contacts could produce a factual justification — it’s just “somebody’s practice.” Unbalance of 2% is, again, twice as bad as NEMA recommends.
Electricians, plant operators, or consultants sometimes say “Systems are always unbalanced. The NEMA tolerance isn’t realistic. The utility line isn’t going to be that well-balanced.” Why argue? Measurements on any given circuit are easily made. And whether realistic or not, the NEMA limit is what motor manufacturers have agreed to live with. Applying any motor outside that limit is asking for a voided warranty at best; loss of adequate overheat protection (with possible motor burnout) at worst. Unbalance, whatever its origin, is both measurable and correctable. Ignore it at your own risk.
Richard L. Nailen, P.E.