When manufacturing plant owners and facility managers look for ways to reduce energy consumption, they often see lighting as the main opportunity, so it’s the first upgrade they make. In actuality, the percentage of total energy used in a plant for lighting is much smaller than the percentage used to power equipment and machinery.
In industrial settings, motor-driven systems use 60 to 90 percent of the total power. The purchase price of an electric motor is only about 2 percent of the life cost . Nearly 98 percent of life cost comes from the cost of the electricity the motor uses. To put that in perspective, the purchase price of an electric motor is equal to the cost of electricity to operate that motor continuously for about one month.
Energy efficiency is based on the losses inside the motor during power conversion from electrical to mechanical energy. The major loss is stator resistance loss (stator I2R), which is the product of the square of the current multiplied by the resistance of the stator winding. The rotor also experiences I2R losses in the squirrel-cage rotor bars, called rotor resistance loss (rotor I2R). 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. While these losses are not easily calculated, IEEE 112 method B provides a procedure for determining them based on test data.
