Synchronous electric motors have a rotational output speed that is exactly synchronized with the frequency of the AC line powering them. In the case of the 120 volt, 60 cycle AC power standard in the United States, the motor's rotational velocity in RPM will always be a whole number multiple of 60. Synchronous motors therefore provide a frequency lock between the AC power source, usually a regulated utility company, and whatever device uses this type of motor. These motors are infinitely useful wherever accurate timing is required--clocks, timers and many machines.
Larger synchronous motors used in industrial machines have active rotor windings excited by a DC current through solid commutator rings, in addition to the AC current powering the stator windings. This provides constant synchronous rotational speed and high output torque under varying load conditions. Small synchronous motors such as those used in small clocks and timers are usually non-excited in that the rotor has no rotor windings excited by DC current. They are used solely for their synchronous speed characteristics because they have low output torque.
Excited Synchronous Motors
Because the output speed of these motors is purely based on the frequency of the AC power driving them, they have found widespread use in manufacturing industries where many processes must be coordinated. Variable frequency power systems can regulate the speed of a limitless number of synchronous motors from the same control system, greatly simplifying automation strategies.
Non-Excited Hysteresis Motors
The rotors of hysteresis motors are made from a material such as soft iron that is capable of being repeatedly magnetically charged and discharged based on the intensity and frequency of the stator winding magnetic poles surrounding it. They are called hysteresis motors since they rely on the hysteresis or lag between the start of the momentary magnetization process and the achievement of the full magnetization of the material. These motors are far simpler and less costly to manufacture than excited types and lend themselves well to low-cost mass-produced electrical products.
Constant Torque Motors
A unique characteristic of hysteresis-type synchronous motors is that their output torque remains constant as they accelerate from fully off to operating speed. This makes them infinitely useful in constant torque applications. First, they can be used as continuous springs to set tension on belt drives or to remove mechanical free play from sensitive mechanisms. Or they can be used to uniformly accelerate or decelerate wheels and mechanisms from stop to full speed without surging the power supply. Finally, they are used for continuous regulatory speed control.
AC Power Stabilization
Electrical utilities may use huge synchronous motors with flywheels to stabilize the sine-wave frequency characteristics of the grid.
Synchronous motors are found in every type of appliance that utilizes a clock or timer such as washers, dryers, refrigerators, water softeners and air-conditioning controls. In days of old, they were found in record players and reel-to-reel tape decks too.
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