A change in efficiency as a function of load is an inherent characteristic of motors. Operation of the motor at loads substantially different from rated load may result in a change in motor efficiency (see Fig. 3).
Generally, the full-load efficiency of motors increases with physical size and rated output of motors.
For the same power rating, motors with higher speeds generally, but not always, have a higher efficiency at rated load than motors with lower rated speeds. This does not imply, however, that all apparatus should be driven by high-speed motors. Where speed- changing mechanisms, such as pulleys or gears, are required to obtain the necessary lower speed, the additional power losses could reduce the efficiency of the system to a value lower than that provided by a direct-drive lower-speed motor.
A definite relationship exists between the rated speed (1/min) and the efficiency of a polyphase induction motor, i.e., the lower the rated speed, the lower is the efficiency, for slip is a measure of the losses in the rotor winding.(Slip of an induction motor is the difference between synchronous speed and operating speed). Slip, expressed in percent, is the difference in speeds divided by the synchronous speed and multiplied by 100. Therefore, Design N cage-induction motors having a slip at full-load of less than 5 percent are more efficient than motors having a higher slip and should be used when permitted by the application.
For loads such as pumps, fans and air compressors, it may be possible to make a significant saving in energy by utilizing a multispeed motor or by using a variable frequency drive (VFD). However, it should be noted that the efficiency of a multispeed motor at each operating speed is somewhat lower than that of a single-speed motor having a comparable rating. Single- winding (Dahlander), multispeed motors are generally more efficient than two-winding, multispeed motors.
Motors which operate continuously or for long periods of time provide a significant opportunity for reducing energy consumption. Examples of such applications are processing machinery, air moving equipment, pumps, and many types of industrial equipment.
While many motors are operated continuously, some motors are used for very short periods of time and for a very low total number of hours per year. Examples of such applications are valve motors, dam gate operators, industrial door openers, fire pumps and sewage pumps. In these instances, a change in motor efficiency would not substantially change the total energy cost since very little total energy is involved and may decrease the required performance.
A modest increase of a few percentage points in motor efficiency can represent a rather significant decrease in percentage of motor losses. For example, for the same output, an increase in efficiency from 75 to 78.9 percent, from 85 to 87.6 percent, or from 90 to 91.8 percent represents a 20 percent decrease in losses in each case.
As efficiency typically increases with the size of the motor, high-voltage machines with output powers well exceeding 1 MW usually have an efficiency above 95%.
NOTE While an electric motor’s output power increases with the square of it’s size the permissible heat dissipation increases almost linearly. Therefore, a higher efficiency is an inevitable precondition for the design of larger motors .
Motor losses electric motor converts electrical energy into mechanical energy and in so doing incurs
losses which are generally described as follows:
Electrical (stator and rotor) losses (vary with load) – Current flowing through the motor windings produces losses which are proportional to the current squared times the winding resistance (I²R). Rotor losses also increase with slip.
Iron (core) losses (essentially independent of load) – These losses are confined mainly to the laminated core of the stator and to a lesser degree the rotor. The magnetic field, essential to the production of torque in the motor, causes hysteresis and eddy current losses.