Changing frequency
An induction motor, as mentioned earlier, is a constant-speed device. Its speed depends on the number of pules provided in the stator. This assumes that the voltage and frequency of the supply to the motor remain constant.
Several methods can be used to vary the speed of an AC motor. The stator or primary winding can be connected to change the number of poles. For exam ple, reconnecting a 4-pole winding so that it becomes a 2-pole windi ng will double the speed. This method can give specific alternate speeds but not gradual “peed changes. Or, the slip can be changed for a given load by varying the line voltage. However, torque is pro portional to the square of the voltage, so reducing the line voltage rapidly reduces the available torque and will soon cause the motor to “tall.
An excellent way to vary the speed of a squirrel-cage induction motor is to vary the frequency of the applied volt age. To maintain a constant torque, the ratio of voltage to frequency must be kept constant, so the voltage must be varied simultaneously with the fre quency. Modern adjustable frequency controls perform this function. At con stant torque, the horsepower output increases directly as the speed Ill creases.
For a 60-Hz motor, increasing the supply frequency above 60 Hz will cause the motor to be loaded in excess of its rating, which must not be done except for brief periods. For a supply frequency of less than 60 Hz, the speed will be less than the design speed of the motor. As the frequency is reduced, the voltage should also be reduced, to maintain a constant torque. Sometimes it is desirable to have a high starting torque or to have a constant horsepow er output over a given speed range. These and other modifications can be obtained by varying the ratio of voltage to frequency as required. Some controllers are designed to provide constant torque up to 60 Hz and constant hp above 60 Hz to permit higher speeds without overloading the motor.
The speed of an A C induction motor can be changed over a very wide range, from perhaps 10% to 20 % of 60-Hz rated speed up to several times rated speed. However, several cautions must be observed. At higher speeds, care must be taken not to exceed the hp rating of the motor. At speeds more than 10% above rating, the manufac turer must be consulted as to the abili ty of the motor to withstand the mechanical forces involved. At low speeds, roughly 20% of rated speed or less, especially if the motor is fan cooled, care must be taken not to exceed the permitted motor tempera ture rise. If speed gets too low, the motor may “cog”-the rotor jumping from one position to the next instead of rotating smoothly-or it may stall completely.
Capability, versatility and flexibility of the Af’ induction motor is a matter of fact. To obtain maximum suitability and effectiveness when selecting and applying AC induction motors, many other factors must be considered. These include type of application, enclosure, mountings, coupling, bear ings, insulation, temperature ratings, initial costs, operating cost, energy rat ing, and starting and control require ments.