Polyphase Induction Motors
About two-thirds of the electric power in the U.S. is consumed by motors, with industrial three-phase motors above 5 HP (7 kW) being by far the bulk of that load. They are linear loads and therefore don't contribute to harmonics. They are, however, the major contributor to reduced Displacement Power Factor, which is a measurement of the effective use of system capacity.
Measurements
1. Voltage unbalance
Voltage unbalance should not exceed 1-2% (unless the motor is lightly loaded). Why such a small number? Voltage unbalance has a very large effect on current unbalance, in the neighborhood of 8:1. In other words, a voltage unbalance of 1% can cause current unbalance of 8%. Current unbalance will cause the motor to draw more current than it otherwise would. This in turn causes more heat and heat is the enemy of motor life, since it deteriorates the winding insulation.