Question

For an electric motor as a closed system, the rate of energy transfer in by electric current must equal the rate of power out through the rotating shaft plus the rate of energy transfer by heat from the motor to its surroundings. true or false

Answer 1

Answer:

False. Apart from heat loss, there are some other energy loss factors in electric motors.

They are:

(1) Core losses

(2) Windage losses

(3) Eddy Current

(4) Stray Load

Explanation:

STATOR RESISTANCE/HEAT LOSS(Heat loss I^2*R), which is the product of the square of the current multiplied by the resistance of the stator winding. The rotor also experiences I^2*R losses in the squirrel-cage rotor bars, called rotor resistance loss (rotor I^2*R).

CORE LOSSES also occur, originating in the lamination steel. Core losses include hysteresis losses, which result from reorientation of the magnetic field within the motor’s lamination steel, and eddy current losses resulting from electrical currents produced between laminations due to the presence of a changing magnetic field.

These electrical currents occur in both stator and rotor cores, but primarily in the stator, as these losses are proportional to the frequency of the current. The frequency of current in the rotor bars is only a small fraction of the line frequency, as the rotor current frequency is proportional to slip (the difference between operating speed and synchronous speed). Both the stator and rotor laminations have an insulated coating to reduce shorting losses (eddy current) from adjacent laminations.

Friction losses are from the motor bearings and lubrication.

WINDAGE LOSSES combine losses from the rotor spinning in air that creates drag and those from cooling fans used on the motor, along with friction losses in the bearings.

STRAY LOAD losses also are present.