An induction motor has two basic electrical parts: a rotor and a stator. The stator is the stationary electrical component composed of wire coils that create an electromagnet when energized.

The rotor is the rotating component, also capable of being an electromagnet. The rotor is located inside the stator. When AC current is supplied to the stator, it produces a rotating magnetic field, which then induces a separate magnetic field in the rotor. The induced magnetic field of the rotor is attracted to and follows the rotation of the magnetic field of the stator. Because the field of the rotor is induced, there is always some lag time between the speed of the rotor and the magnetic field speed of the stator. This is referred to as “slip”, with the rotor’s speed always lagging a bit behind the stator’s magnetic field.

In a permanent magnet motor, permanent magnets are on the rotor. When current is supplied to the stator, it does not induce a magnetic field on the rotor; rather, the permanent magnetic field of the rotor is synchronous with that of the stator.

Inotherwords,therotorspinsatthesame speed as the stator’s magnetic field.

One of the consequences of the rotor speed matching the magnetic field of the stator, is that there is no energy loss due to “rotor resistance”. In an induction motor, the rotor’s magnetic field resists induction by the stator, to a certain extent. The permanent magnet motor does not suffer this loss of energy, and this largely contributes to the higher efficiency of permanent magnet motor driven variable-speed pumps compared to induction motor driven single speed pumps. Energy efficient single speed pumps might have an induction motor efficiency of 75% while variable-speed pump’s permanent magnet motors are as high as 92% efficient.

Permanent magnet vs induction motor Photo credit: newenergyandfuel.com