ELECTRIC MOTOR:
The basic principles of operation for a motor are the same as that of a generator, except that a motor converts electrical energy into mechanical energy (rotational). Both motors and generators can be explained in terms of a coil that rotates in a magnetic field. In a generator the coil is attached to an external circuit and it is mechanically turned, resulting in a changing flux that induces an emf. In a motor, a current-carrying coil in a magnetic field experiences a force on both sides of the coil, creating a torque which makes it turn. Any coil carrying current can feel a force in a magnetic field; the force is the Lorentz force on the moving charges in the conductor. We know that if the coil is parallel to the magnetic field then the Lorentz force will be zero. The charge of opposite sides of the coil will be in opposite directions because the charges are moving in opposite directions. This means the coil will rotate.
Instead of rotating the loops through a magnetic field to create electricity, a current is sent through the wires, creating electromagnets. The outer magnets will then repel the electromagnets and rotate the shaft as an electric motor. If the current is AC, the two slip rings are required to create an AC motor. An AC motor is shown in right hand side.
If the current is DC, split-ring commutators are required to create a DC motor.
There are many types of motor such as AC, DC, Synchronous, Servo, Stepper, Brushless DC Motor etc. Every motor has a shaft or spindle which rotates continuously when current in passed into it. The rotation of its shafts is used to drive the various types of machines in domestic and industrial purpose. Electric motor is used in electric fans, washing machines, refrigerators, mixer and grinder and many other appliances.
Principle of a DC Motor:
An electric motor utilizes 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 torque acts on the coil which rotates is 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 DC Motor:
The construction of a DC motor is identical to the construction of DC Generator. In other word, A DC generator can be used as a DC motor and vice versa.
Working of a DC Motor:
Suppose that initially the coil ABCD is in the horizontal position as shown in above figure. On pressing the switch, the current enters the coil through carbon brush B2 and commutator half ring R2. The current flows in the direction ABCD and leaves via ring R1 and brush B1.
a) In side AB of the coil, the direction of current is from A to B and the direction of magnetic field is from N to S pole. So, by applying Fleming’s left hand rule to the side AB of the coil we find that it will experience a force in the upward direction.
b) In side DC of the coil, the direction of current is from D to C towards but the direction of magnetic field remains the same from N to S pole as shown in figure. So, by applying Fleming’s lef hand rule to the side DC of the coil, we find that. It will experience a force in the downward direction.
c) We find that the force acting on the side AB of the coil is in the upward direction whereas the force acting on the side DC of the coil is in the downward direction. These two equal, opposite and parallel forces acting on the two sided to the coil form a couple (torque) and rotate the coil in the clockwise direction.
d) While rotating, when the coil reaches the vertical position, then the brushes B1 and B2 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 doesn’t stop rotating because it has already gained momentum due to which it goes beyond the vertical position.
e) When the coil goes beyond the vertical position, the two commutator’s half rings automatically change contact from one brush to the other. This reverses the direction of current through the coil which, in turn, reverses the direction of forces acting on the two sides of the coil. The side AB of the coil now be one the right hand side with a downward force on it, whereas side DC of the coil will come on the left hand side with an upward force on it. In this position also a couple acts on the coil which rotates it in the same direction (clockwise direction). This process is repeated again and again and the coil continues to rotate as long as the current is passing. This is how an electric motors works.
EXERCISE - GENERATORS AND MOTORS
2. Use Faraday’s Law to explain why a current is induced in a coil that is rotated in a magnetic field.
3. Explain the basic principle of an AC generator in which a coil is mechanically rotated in a magnetic field. Draw a diagram to support your answer.
4. Explain how a DC generator works. Draw a diagram to support your answer. Also, describe how a DC generator differs from an AC generator.
5.Explain why a current-carrying coil placed in a magnetic field (but not parallel to the field) will turn. Refer to the force exerted on moving charges by a magnetic field and the torque on the coil.
6. Explain the basic principle of an electric motor. Draw a diagram to support your answer.
7. Give examples of the use of AC and DC generators.
8. Give examples of the use of motors
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