What is the working principle of an electric motor ?

Electric motor: A motor is a device which converts electrical energy into mechanical energy.

Principle of motor : A motor works on the principle that when a rectangular coil is placed in a magnetic field and current is passed through it. A force acts on the coil which rotates it continuously.

An electric motor uses the attraction and repulsion of magnetic fields to produce motion. The simplest is the permanent magnet motor. A simplified version is shown below.

Placing a coil of wire inside a permanent magnetic field and fixed so it can freely rotate. Pass a current through the coil of wire and it will rotate to the perpendicular position.

Now reverse the current flow and the coil will spin 180 degrees. The brushes and the commutator does the switching directions of the current through the coil at the appropriate moment to keep the coil rotating in one direction. This is the basics of all electric motors.

Fleming’s left hand rule above describes the relationship between the main magnetic field. The current flowing in the coil and the direction of the movement or force

Now we can use electronics to do the switching instead of the commutator and brushes. These motors are called Brushless DC motors.

Larger motors require a stronger magnetic field and more electrical ,power to drive it faster and with more torque. Stronger magnetic fields are created by electromagnets.

AC motors use induction from the stationary windings to create the second magnetic field in the rotor. That induces currents in the rotor and these currents have their own magnetic field which interact ( repel and attract) with the main magnetic field to make the rotor rotate.

How work a electric motor:

1. When electric current is passed into the rectangular coil, this current produces a magnetic field around the coil.
2. The magnetic field of horse shoe-type magnet then interacts with the magnetic field of the coil and causes the coil to rotate continuously.
3. If ABCD is in horizontal position current from battery enters the coil through brush B _{ 1 }, and commutator half ring C:. The current flows in the direction ABCD
   and leaves via ring and brush B _{ 2 }
4. The direction of current is from A to B, the direction of current is from C to D. The force on side AB of the coil is in the downward direction. Whereas the force on the side CD of the coil is in the upward direction. ABCD rotate in anti-clockwise direction.
5. While rotating when the coil reaches vertical position then the brushes B _{ 1 } and B _{ 2 }will touch the gap between the commutator rings and current to the coil is cut off. But the coil does not stop rotating because it has already gained momentum.
6. The coil CD comes on the left side and AB comes to the right side. Again they come in contact with brush B _{ 1 } Current direction is reversed.
7. The reversing of current in the coil is repeated after every half rotation due to which the coil continue to rotate as long as current from the batter is passed through it. The rotating shaft of electric motor can drive a large number of machines which are connected to it.

Passing an electric current through a wire creates a magnetic field. A larger and larger coil of insulated wires will generate a stronger and stronger magnetic field. When this coil of wires is connected to a shaft inside a fixed set of magnets, the like fields will be repelled while at the same time the opposite fields (North & South) will be attracted. This Push - Pull effect will cause these coils (the armature) to begin to rotate the shaft they are attached to. There is a fource known as CEMF, (Counter Electro Motive Force) that opposes the rotation and prevents the motor from rotating at ever increasing speeds.

DC motor working principle


By: Er. Ashok kumar