Starting Torque & Unbalance

Question:

Unbalanced phase voltages make a three-phase motor run hotter, and the numbers have been published in many places. What about starting torque? Does Unbalance affect that, and if so how much?

Answer:

Although little has been published on this subject, some tests indicate that accelerating torque (including breakdown torque, which governs a motor’s capability to ride through voltage disturbances without stalling) is reduced by a percentage about 2/3 of the percentage voltage unbalance.

Another source suggests a greater effect, with a 6% unbalance producing 10% less torque. Our conclusion is that while the influence of unbalance on torque is much less than the effect on current and temperature, the relationship is highly variable with both motor design and amount of unbalance.