Motor Rating

Nameplate Ratings-Nameplate rating are exactly what they sound like. These are the rated values for a given motor as shown on the motors nameplate. Rated values can be thought of as nominal design values for the motor. For example a very common rated voltage is 460 volts. The voltage coming into the motor may likely vary by as much as 10% (this is per common NEMA standards), therefore the motor may run on an actual voltage range of about 410 to 510 volts but its design or rated voltage is 460.

Rated voltage-Voltage is rated per the NEMA (National Electrical Manufacturers Association) Common rated voltages include 115 and 230 for single phase motors and 460 volts for three phase motors. These are rated voltages and actual design limits are +/- 10% per NEMA therefore actual operating ranges (voltage coming into the motor) could be 104-126 volts for a 115V rated motor, 207-253 for a 230V rated motor and 414-506 for a 460V rated motor.

Rated Amperage-Rated amperage is typically the full load amperage. This is a current the motor will draw as full design load.

Rated Frequency-rated frequencies will either be shown as 50 or 60 Hz. In North America, 60 Hz is the standard frequency. 50 Hz is used everywhere else.

Rated Horsepower-Rated horsepower is based on the rated/full load torque and speed also given on the nameplate. Horsepower is calculated per the following equation:

Note: see free equations sheet on the homepage for more horsepower equations

PE exam strategy: Motor sizes are available in “frames” or steps. Sizing should be done by calculating required motor load including any given efficiencies and then choosing the next available size UP.

Heat Output

Motors, aside from creating power in the form of horsepower will always give off a certain amount of heat. This heat dissipation is sometimes called copper losses or winding losses. Since real motors are never 100% efficient, it can be assumed that a certain percentage of motor power will be dissipated in the form of heat. Other forms of energy dissipation in motors can be vibration and noise but these typically contribute much less than that of heat and for most PE exam problems can most likely be ignored unless they are specifically mentioned in the problem statement.
In some cases, you can think of a motors efficiency as a measure of heat dissipation. Efficiency is the ratio of power output measured at the motor shaft (otherwise known as brake horsepower) and the power input into the motor which is a function of voltage and current (usually shown as kVA) consumed to the motor. In order to compare these two values (Hp and kVA), we must get them into equivalent units. kVA is calculated from the measured volts and amps input to the motor per:

To convers kVA to Hp use the following equation:

Where pf is the power factor for the motor (usually 0.8). Motor efficiency can be shown per the following:

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