A machine which converts electrical energy into mechanical energy for driving something is known as motor. A motor which converts direct current (dc) electrical energy into mechanical energy is called dc motor.
Operation Principle of DC Motor:
Operation of dc motor is based on the principle of Lorentz force law. When current flows through the conductor (coil), it generates magnetic field. The magnetic field produced by the current interact with another surrounding magnetic field. Due to the interaction of these two magnetic fields, current carrying conductor experiences mechanical force which causes it to rotate. In order to understand the working of dc motor, we must know the two most common terms used to describe the windings of machine.
Field winding applies to that winding which produces main magnetic field in a machine. In case of dc motor, field winding is applied on the stator (stationary part of motor).
Armature winding applies to that winding where main voltage is induced in the machine. In case of dc motor, armature winding is applied on the rotor (rotating part of motor).
Working of DC Motor:
A simple dc motor has stationary set of field magnets in the stator. Stator provides a constant main magnetic field (alternate north and south poles). Armature (rotating part) is simple coil placed between the constant magnetic field (field winding). Armature coil is connected to dc power source through a pair of commutator rings. The above discussion can be understand by the following diagram.
When armature winding is excited by dc power source, current starts to flow through the armature coil due to which magnetic field is produced. Armature conductors are also under the effect of magnetic field produced by the stator. Due to the interaction of these two (both stator and armature current) magnetic fields, armature conductors experience a mechanical force (torque) which tends to rotate the armature. This is how dc motor works (starts to rotate).
Direction of Force:
The direction of mechanical force acting on the armature conductor is perpendicular to both the wire and main magnetic field. Moreover, the direction of mechanical force can be found by Fleming left hand rule.
Fleming left hand rule:
Fleming left hand rule states that open the left hand in such a way that forefinger, middle finger and thumb will be perpendicular to each other. Let the forefinger points in the direction of magnetic field and middle finger in the direction of current flowing through the armature conductors. In that case, thumb will point the direction of mechanical force acting on the armature conductors.
Basically, Fleming left hand rule helps us to determine the direction of rotation of dc motor. Now, lets try to understand the working of dc motor in detail.
Explanation of DC Motor:
The below given figure is of simple dc motor in which a current carrying armature coil attached with a commutator segments is placed in a magnetic field. A coil is connected between each adjacent commutator segments having different polarities. By applying Fleming left hand rule, it is clear that armature coil will move in the clockwise direction according to the direction of acting mechanical force .
When coil rotates and move from one side to the other, left side of the coil which contains current in the inward direction will come to the right and corresponding force will be upward at the right-hand side. Similarly, the direction of acting mechanical force on the left side will now be downward. At the moment this happens, coil will start to move in the counterclockwise direction (since now torque is acting in counterclockwise) which is not what we required.
Actually, as the armature rotates and move from one side to other, sides of the armature gets reversed due to which the direction of acting mechanical force is also changed. Reversal of mechanical force will make the coil to move in the counterclockwise direction which is not needed. To continue the rotation of dc motor, direction of acting mechanical force(torque) need to be remains same.
Here the question arises??
How to get Unidirectional torque in DC Motor?
In order to rotate the motor in one direction, direction of mechanical force acting on the armature coil needs to be remains same when armature coil reversed after a half rotation. In simple words, the direction of mechanical force acting on the right side must always be downward and left side force must be upward so that the armature coil continues to rotate in one (clockwise in this case) direction.
The direction of mechanical force acting on the armature coil can be remains same by the following methods. One way is to change the direction of main magnetic field after every half rotation or you will have to reverse the direction of armature current. Since it is difficult to change the direction of main magnetic field (changing north south position after every half rotation), hence only way is to change the direction of armature current.
In order to reverse the direction of armature current, commutator is used. The main purpose of commutator in dc motor is to reverse the direction of armature current every time when the coil moves perpendicular (after half rotation) to the magnetic field. This phenomenon helps to keep the direction of magnetic force acting on the armature conductor and current to be always remains same. In this way, motor will continue to rotate in one direction (unidirectional torque).
Smooth rotation of DC Motor:
One problem during the rotation of motor is that when armature coil is perpendicular to the main magnetic field, torque induces in the coil becomes zero. No induce torque means that the motor stops rotating. To overcome this problem, a large number of armature coils are used in the actual dc motor with having a separate commutator pair for each coil. By this arrangement, when one coil is perpendicular to the field, the other coils induces the torque. More the number of armature coils, smoother will be the rotation of dc motor.