5.3. Motors
Motors
A motor is an electrical machine, that has opposite conversion than that of a generator – it converts electrical energy into mechanical rotary movement.
Motors can be divided into AC and DC according to their power supply, can vary from step motors used in control circuits to huge installations in the MW range.
The most common type of motors used on board are three-phase alternating current asynchronous motors with a squirrel cage rotor.
Motor construction and operation
A motor consists of a rotor and stator. The stator, as the name suggests, stays stationary and is made of the frame with slots for insulated copper coils, called windings.
The rotor can be also called the armature and can be wound (like the stator, with coils of copper, especially in DC motors) or made of bars that are connected by rings, like a squirrel cage rotor. The rotor rotates due to the interaction of magnetic fields and its movement is transferred to the shaft.
An armature or rotor is mounted on bearings inside this magnetic field. An armature contains another series of windings on an iron frame that also becomes magnetised when a current is passed through the windings. A rotor can have either fixed metal bars but sometimes it has windings, too. When there is an interaction of the opposing magnetic fields between the motor casing (stator) and the armature, or magnetic field drives the rotor, or rotor spins around in its bearings, we have a motor.
Motors have a common operation principle. A magnetic field is established inside the motors case, either with fixed permanent magnets or by using electromagnets (coils of wire, wrapped around iron shoes that become magnetized when current is passed through the windings). The windings create excitation field and are called field windings.
Depending on the motor construction and its details of operation, an electric motor can be series wound, shunt wound, permanent magnet and induction ones.