The majority of electric motors are made to operate between 50% and 100% of their rated load.
The maximum efficiency is typically in the range of 75% of the rated load. Therefore, the allowable load range for a 10-horsepower (hp) motor is between 5 and 10 hp; its peak efficiency is at 7.5 hp. Below roughly 50% load, a motor’s efficiency tends to drop off sharply.
It is feasible to determine motor loads by measuring actual motor speed with a tachometer. A battery-powered stroboscopic tachometer is the safest, most practical, and frequently most accurate type of tachometer.
Avoid using mechanical tachometers, plug-in tachometers, and tachometers that demand that the motor be stopped in order to apply paint or reflective tape.
Slip Measurements in Electric Machines/Motors
If the engineer is familiar with the technique, slip measurements can be used to determine the motor load.
Due to its ease of use and safety benefits, the speed/slip method of measuring motor part-load is frequently preferred. The majority of motors are designed so that a tachometer or strobe light can see the shaft.
The slip method’s accuracy, however, is not very good. The biggest question mark is the manufacturer-reported nameplate full-load speed tolerance of 20% that NEMA permits.
Manufacturers typically round their claimed full-load speed values to a multiple of 5 rpm due to this wide tolerance.
The slip method depends on the discrepancy between full-load nameplate and synchronous speeds, which may seem minor given that 5 rpm is a tiny percentage of the full-load speed. A seemingly insignificant 5 rpm discrepancy results in a 12 percent variation in the computed load when there is a 40 rpm “correct” slip.
Slip also fluctuates inversely with respect to the square of the voltage at the motor terminals, which is subject to a separate NEMA tolerance of 10%. Of course, the slip load equation can be modified to include a voltage adjustment factor.