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Physics Topics can be both theoretical and experimental, with scientists using a range of tools and techniques to understand the phenomena they investigate.
What is an Electric Motor? How it Works?
A motor is a device which converts electrical energy into mechanical energy. Every motor has a shaft or spindle which rotates continuously when current is passed into it. The rotation of its shaft is used to drive the various types of machines in homes and industry.
Electric motor is used in electric fans, washing machines, refrigerators, mixer and grinder, electric cars and many, many other appliances (see Figure). A common electric motor works on direct current. So, it is also called D.C. motor, which means a “Direct Current motor”. The electric motor which we are going to discuss now is actually a D.C. motor.
Principle of a Motor
An electric motor utilises the magnetic effect of current. 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. When the coil rotates, the shaft attached to it also rotates. In this way the electrical energy supplied to the motor is converted into the mechanical energy of rotation.
Construction of a Motor
An electric motor consists of a rectangular coil_ABCD of insulated copper wire, which is mounted between the curved poles of a horseshoe-type permanent magnet M in such a way that it can rotate freely between the poles N and S on a shaft (The shaft is a long cylindrical rotating rod at the centre of the coil which has not been shown in Figure to keep the diagram simple). The sides AB and CD of the coil are kept perpendicular to the direction of magnetic field between the poles of the magnet.
This is done so that the maximum magnetic force is exerted on the current-carrying sides AB and CD of the coil. A device which reverses the direction of current through a circuit is called a commutator (or split ring). The two ends of the coil are soldered (or welded) permanently to the two half rings X and Y of a commutator. A commutator is a copper ring split into two parts X and Y, these two parts are insulated from one another and mounted on the shaft of the motor. End A of the coil is welded to part X of the commutator and end D of the coil is welded to part Y of the commutator.
The commutator rings are mounted on the shaft of the coil and they also rotate when the coil rotates. As we will see later on, the function of commutator rings is to reverse the direction of current flowing through the coil every time the coil just passes the vertical position during a revolution. In other words, commutator rings reverse the direction of current flowing through the coil after every half rotation of the coil.
We cannot join the battery wires directly to the two commutator half rings to pass current into the coil because if we do so, then the connecting wires will get twisted when the coil rotates. So, to pass in electric current to the coil, we use two carbon strips P and Q known as brushes. The carbon brushes P and Q are fixed to the base of the motor and they press lightly against the two half rings of the commutator. The battery to supply current to the coil is connected to the two carbon brushes P and Q through a switch.
The function of carbon brushes is to make contact with the rotating rings of the commutator and through them to supply current to the coil. It should be noted that any one brush touches only one ring at a time, so that when the coil rotates, the two brushes will touch both the rings one by one.
Working of a Motor
When an electric current is passed into the rectangular coil, this current produces a magnetic field around the coil. The magnetic field of the horseshoe-type magnet then interacts with the magnetic field of the current-carrying coil and causes the coil to rotate (or spin) continuously. The working of a motor will become more clear from the following discussion.
Suppose that initially the coil ABCD is in the horizontal position as shown in Figure. On pressing the switch, current from battery enters the coil through,carbon brush P and commutator half ring X. The current flows in the direction ABCD and leaves via ring Y and brush Q.
(i) In the side AB of the rectangular coil ABCD, the direction of current is from A to B (see Figure). And in the side CD of the coil, the direction of current is from C to D (which is opposite to the direction of current in side AB). The direction of magnetic field is from N pole of the magnet to its S pole.
By applying Fleming’s left-hand rule to sides AB and CD of the coil we find that the force on side AB of the coil is in the downward direction whereas the force on side CD of the coil is in the upward direction. Due to this the side AB of the coil is pushed down and side CD of the coil is pushed up. This makes the coil ABCD rotate in the anticlockwise direction (see Figure).
(ii) While rotating, when the coil reaches vertical position, then the brushes P and Q will touch the gap between the two commutator rings and current to the coil is cut off. Though the current to the coil is cut off when it is in the exact vertical position, the coil does not stop rotating because it has already gained momentum due to which it goes beyond the vertical position.
(iii) After half rotation, when the coil goes beyond vertical position, the side CD of the coil comes on the left side whereas side AB of the coil comes to the right side, and the two commutator half rings automatically change contact from one brush to the other. So, after half rotation of the coil, the commutator half ring Y makes contact with brush P whereas the commutator half ring X makes contact with brush Q (see Figure).
This reverses the direction of current in the coil. The reversal of direction of current reverses the direction of forces acting on the sides AB and CD of the coil. The side CD of the coil is now on the left side with a downward force on it whereas the side AB is now on the right side with an upward force on it. Due to this the side CD of the coil is pushed down and the side AB of coil is pushed up. This makes the coil rotate anticlockwise by another half rotation.
(iv) The reversing of current in the coil is repeated after every half rotation due to which the coil (and its shaft) continue to rotate as long as current from the battery is passed through it. The rotating shaft of electric motor can drive a large number of machines which are connected to it.
It should be noted that the current flowing in the other two sides, AD and BC of the rectangular coil is parallel to the direction of magnetic field, so no force acts on the sides AD and BC of the coil.
We have just described the construction and working of a simple electric motor.
In commercial motors :
(a) the coil is wound on a soft iron core. The soft iron core becomes magnetised and increases the strength of magnetic field. This makes the motor more powerful. The assembly of soft iron core and coil is called an armature.
(b) the coil contains a large number of turns of the insulated copper wire.
(c) a powerful electromagnet is used in place of permanent magnet.
All these features together help in increasing the power of commercial electric motors.