SECTION 7: MOTOR HEATING AND POWER SUPPLY VOLTAGE
There are two major causes of motor heating: copper losses and iron losses. Copper losses are the easiest to understand; this is the heat generated by current passing through a resistance, as in the current passing through the motor’s winding resistance. Often this is referred to as “I2R” dissipation.
This cause of motor heating is at a maximum when the motor is stopped and rapidly diminishes as the motor speeds up since the inductive current is inversely proportional to speed.
Eddy current and hysteresis heating are collectively called iron losses. The former induces currents in the iron of the motor while the latter is caused by the re-alignment of the magnetic domains in the iron. You can think of this as “friction heating” as the magnetic dipoles in the iron switch back and forth. Either way, both cause the bulk heating of the motor. Iron losses are a function of AC current and therefore the power supply voltage.
As shown earlier, motor output power is proportional to power supply voltage, doubling the voltage doubles the output power. However, iron losses outpace motor power by increasing non-linearly with increasing power supply voltage. Eventually the point is reached where the iron losses are so great that the motor cannot dissipate the heat generated. In a way this is nature’s way of keeping someone from getting 500HP from a NEMA 23 motor by using a 10kV power supply.
At this point it is important to introduce the concept of overdrive ration. This is the ration between the power supply voltage and the motor’s rated voltage. An empirically derived maximum is 25:1, meaning the power supply voltage should never exceed 25 times the motor’s rated voltage or 32 times the square root of motor inductance. Below is a graph of measured iron losses for a 4A, 3V motor. Notice in Figure 16 how the iron losses range from insignificant to being the major cause of heating in the motor compared to a constant 12W copper loss (4A times 3V).