Electrical Motors

Motors convert electrical energy into rotational mechanical energy. Electrical motors are used to power many devices used in everyday life like refrigerators, washing machines, elevators, ventilation fans etc. Industrial applications include bulk material handling pumps, rock crushers, machinery operations, compressors, etc.

Basic construction of an electrical motor

An electrical motor consists of a stationary part or stator, and a rotor, which is the rotating part connected to a shaft that couples the machine to its mechanical load. The shaft and rotor are supported by bearings so that they can rotate freely.

Depending on the type of machine, either the stator or the rotor or both contain current-carrying conductors configured into coils. Slots are cut into the stator and rotor to contain windings and their insulation. Currents in the windings set up magnetic fields and interact with fields to produce torque.

Normally, the stator and the rotor are made of iron to intensify the magnetic field. Also, if the magnetic field alternates in direction through the iron with time, the iron must be laminated to avoid large power losses due to eddy currents (In certain parts of some machines, the field is steady and lamination is not necessary).

Armature and Field windings

In most types of electrical machines, a given winding can be classified as either a field winding or as an armature winding (please note that, for induction motors, these are simply referred to as stator windings and rotor conductors).

The primary purpose of a field winding is to set up the magnetic field in the machine. The current in the field windings is independent of the mechanical load imposed on the motor except in series-connected motors. The armature winding carries current that depends on the mechanical power produced. Typically the armature current amplitude is small when the load is light and larger for heavier loads. If the machine acts as a generator, the electrical output is taken from the armature. In some electrical machines, the field is produced by permanent magnets hence a field winding is not required here.

Related article: Basics of Electric Circuits

Motor Principle

As indicated earlier, an electric motor is a machine that converts electric energy into mechanical energy. Its action is based on the principle that, when a current carrying a conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s left-hand rule and whose magnitude is given by F = BIL Newton

Where,

B = Flux density

I = Current

L = Length

Fleming’s left hand rule states that, when a current-carrying conductor is placed in an external magnetic field, the conductor experiences a force perpendicular to both the field and to the direction of the current flow.

If we consider a part of a multipolar dc motor shown below:

When the field magnets in the above figure are excited, and its armature conductors supplied with current from the supply mains, they experience a force tending to rotate the armature. The armature conductors under N-pole are assumed to carry current downwards (crosses) and those under S-poles, to carry current upwards (dots). By applying Fleming’s left-hand rule, the direction of the force on each conductor can be found. This is shown by small arrows placed above each conductor. Each conductor experiences a force F which tends to rotate the armature in anticlockwise direction. These forces collectively produce a driving torque which sets the armature rotating. The commutator in the motor reverses current in each conductor as it passes from one pole to another hence helps to develop a continuous and unidirectional torque.

Related: Factors to consider when selecting a Motor for a particular application