29 Electric Motors 1.4 Synchronous Motors The synchronous motor is defi ned by the fact that the rotor rotates at the same speed as the magnetic fi eld created by the stator windings. The design of the stator is in many cases similar to that of asynchronous motors, with distributed windings. Some manufacturers use concentric windings (in slot) which enable a more compact motor design and require less copper. The energy savings achieved by the reduced use of copper are however often eaten up by additional losses, which result in harmonics in the air gap fl ux caused by the construction. a)b) Fig. 1.22 Distributed windings.
1.4.1 Permanent Magnet (PM) Motors The simplest way to build a permanent magnet motor (PM motor) is to replace the squirrel-cage rotor of an asynchronous motor with a rotor which is equipped with permanent magnets. When applying a suitable voltage to the stator, a rotating magnetic fi eld will be created in the air gap. The rotor will follow the fi eld at synchronous speed because the magnets are attracted by the rotating fi eld. If the diff erence between rotor speed and the speed of the magnetic fi eld is too big the motor falls out of synchronicity and the motor will stop.
Therefore a suitable controller is required which ensures that speed changes are done by adjusting the feeding frequency continuously and not by switching from one speed to another. In the past PM motors were often used in servo applications with focus on fast and precise operation. These servo motors are typically slim and long in order to have a low inertia for high dynamic applications. To utilise the high-effi ciency characteristic of PM motors in other applications the principle has been transferred to motors in IEC frame sizes. Standard frequency converters can be used in the majority of PM motor systems for operation if suitable control algorithms are implemented in the device.