What phase loss and unbalance do to a motor
When one line opens on a three phase induction motor, the machine keeps turning as a single phase load and the current in the two surviving phases climbs to roughly 1.73 times its previous value to hold the same torque. Because winding loss follows the square of current, heating jumps to about three times normal. If the phase is already open at start, the rotor never breaks away and locked rotor current burns the winding in seconds. Field measurements range from about 1.7 to 2.4 times depending on load and winding connection.
Voltage unbalance is defined by NEMA MG-1 as the maximum deviation from the average of the three line voltages, divided by that average. Current unbalance runs six to ten times the voltage unbalance, and winding temperature rise increases by roughly twice the square of the unbalance percentage. The NEMA derating curve gives 0.98 at 1%, 0.95 at 2%, 0.88 at 3%, 0.82 at 4% and 0.75 at 5%, and continuous operation above 5% is not recommended.
Phase reversal is a direction problem rather than a current problem. Pumps, compressors and conveyors that are damaged by backward rotation need a phase sequence and phase loss relay, since at light load the single phasing current may never reach the overload relay setting. Have a licensed electrician verify supply conditions and protection settings before returning a motor to service.
Frequently Asked Questions
Yes. Below about 50% load the single phasing current may stay under nameplate FLA so the overload relay never trips, yet negative sequence current keeps heating the rotor. Dedicated phase loss protection is the only reliable answer.
Uneven single phase loading across the three phases, a loose or corroded connection on one phase, a blown fuse in a power factor capacitor bank, and mismatched transformer taps are the usual causes. Measure at both no load and full load to narrow it down.