DC motor or Direct Current Motor converts electrical energy into mechanical energy. The electric motor is used to drive the mechanical loads. The electric motor operated by ac supply is called ac motor and, the electric motor operated by dc supply is called the dc motor.
The DC motor and DC generator are similar in construction, but electrically it is just opposite. The DC generator converts mechanical energy into electrical energy.
Working Principle of DC Motor
The DC motor works on the principle of Lorenz law. When a current carrying conductor is placed in the magnetic field, the conductor experiences a force. Thus, the force is exerted on the conductor when it is placed in a magnetic and electric field. The force cause motor rotation.
The direction of motor rotation can be found by Fleming’s left hand rule.
The Fleming’s left hand rule states that if the index finger,middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of current ,then the direction of thumb shows the direction in which conductor experience the force and the DC motor shaft rotates in that direction.
Working of DC motor or Direct Current Motor
The DC Motor has mainly two parts- Field winding and the armature winding. The armature is the rotating part of the motor and the conductors of the armature experiences the Lorenz force when it is placed in the magnetic field. The magnetic field is produced by the field coils. The field coils are stationary part of the DC motor.
DC supply to armature is given through the carbon brushes. The carbon brushes are required because armature is rotating and its supply part is the stationary. The carbon brushes are softer than the commutator of the DC motor so the brushes get wear after running of the motor for certain period of time. If the brushes are harder than the commutator material the brushes can damage the commutator. The electrographite ( EG grades) are generally used in the DC motor.
The DC motor are named according to the connection of the field winding with the armature winding. There are following types of DC motors.
1. Series Motor
2. Shunt Motor
3. Compound motor
Let us take separately excited DC motor for understanding of working of the DC motor. The armature coil has certain resistance and the voltage drop occurs in the armature when current flows through the armature conductors. Let the resistance of armature is Ra.
When DC voltage(V) is applied across the armature winding, the motor draws high current as back emf(Eb) induced across armature winding is zero. The emf starts developing as soon as the motor starts rotating. The emf developed across the armature winding opposes the main armature dc supply and thus counter balance the supply voltage and , emf developed is called back emf.The back emf(Eb) developed across the armature winding depend on the followings;
1. Field Flux 2. Number of armature conductors 3. Speed of rotation of motor 4. Number of poles 5. Number of parallel path
The back emf(Eb) can be expressed as;
The back emf (Eb) is directly proportional to the speed of the motor. The back emf increase or decrease with an increase or decrease of the motor speed. When the load on the motor is increased the speed of the motor gets decrease. The decreased speed cause decrease in the back emf. The armature current gets increased because the voltage difference between supply voltage and back emf (V-Eb) increases. The torque of the motor is proportional to the armature current, hence the torque gets increased and motor speed again increase. In this way, the motor is capable to maintain the same speed under variable load condition.
The armature current of the DC motor is represented by
At start the DC motor draw large current because the back emf is absent and also the armature resistance of the winding is very low. Therefore, a starter is needed to limit the high starting current of the DC motor. In DC drives, the speed reference is increased in the ramp up manner to limit the starting current.