Rocks & mirror
Encyclopedia of Marine and Energy Technology

Electric motors


The electric motor converts electrical energy to mechanical energy. The most frequently used type of electric motor is the motor powered by an alternating current supply: the AC-motor. AC motors are not limited in the power they can produce and can be considered as two main types; asynchronous and synchronous. The synchronous AC-motor is less popular, however it has higher efficiency and is frequently used as the propulsion motor in an electric drive. DC-electric motors are powered by a direct current power supply and are not frequently used. They require commutator brushes (this limits their power output), they also require more maintenance than AC-motors.

Asynchronous motor, induction motor – The asynchronous motor has the simplest design of all electric motors. It is brushless by nature and the cage rotor is very rigid, which makes relatively high speeds possible. These qualities are the reason, that the PWM frequency converter  is designed for this type of motor. Although high-speed motors make a gearbox necessary, the weight (and price-) reduction of a high-speed motor, compared to a low-speed one is generally worthwhile. Asynchronous motors can be manufactured with power levels beyond 10MW.

Synchronous motor  – The disadvantage of the synchronous motor is that it uses sliprings to power the rotor windings with DC-supply. Its rotational speed matches the frequency of the AC-supply and it runs synchronously. In case the load torque exceeds the maximum allowable torque, synchronisation will be lost and no torque will be transferred at all.