DC Motor or Direct Current Motor: What is it? (Diagram Included)

What is a DC Motor?

Principle of DC Motor

When a current-carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move.

In other words, when a magnetic field and an electric field interact, a mechanical force is produced. The DC motor or direct current motor works on that principle. This is known as motoring action.

Fleming’s left hand rule determines the direction of rotation in a DC motor. Extend the thumb, index finger, and middle finger of your left hand perpendicular to each other. The index finger points in the direction of the magnetic field, the middle finger in the direction of the current, and the thumb shows the direction of the force on the motor’s shaft.

Structurally and construction wise a direct current motor is exactly similar to a DC generator, but electrically it is just the opposite.

Unlike a generator, we supply electrical energy to the input port of a DC motor and obtain mechanical energy from the output port. The block diagram shows this process, where the supply voltage (E) and current (I) go to the electrical port, and the mechanical output (torque T and speed ω) comes from the mechanical port.

Parameter K relates the input and output port variables of the direct current motor.

So from the picture above, we can well understand that the motor is just the opposite phenomena of a DC generator, and we can derive both motoring and generating operation from the same machine by simply reversing the ports.

Detailed Description of a DC Motor

To understand the DC motor in details lets consider the diagram below,

The circle in the center represents the direct current motor. On the circle, we draw the brushes. On the brushes, we connect the external terminals, through which we give the supply voltage.

On the mechanical terminal, we have a shaft coming out from the center of the armature, and the shaft couples to the mechanical load. On the supply terminals, we represent the armature resistance R_a in series.

Now, let the input voltage E, is applied across the brushes. Electric current which flows through the rotor armature via brushes, in presence of the magnetic field, produces a torque T_g. Due to this torque T_g the dc motor armature rotates.

As the armature conductors are carrying currents and the armature rotates inside the stator magnetic field, it also produces an emf E_b in a manner very similar to that of a generator.

The generated Emf E_b is directed opposite to the supplied voltage and is known as the back Emf, as it counters the forward voltage.
The back emf like in case of a generator is represented by

Where, P = no of poles
φ = flux per pole
Z= No. of conductors
A = No. of parallel paths
and N is the speed of the DC Motor.

So, from the above equation, we can see E_b is proportional to speed ‘N.’ That is whenever a direct current motor rotates; it results in the generation of back Emf. Now let’s represent the rotor speed by ω in rad/sec. So E_b is proportional to ω.

So, when the application of load reduces the speed of the motor, E_b decreases. Thus the voltage difference between the supply voltage and back emf increases that means E − E_b increases.

Due to this increased voltage difference, the armature current will increase and therefore torque, and hence speed increases. Thus a DC Motor is capable of maintaining the same speed under variable load.

Now armature current I_a is represented by

Now at starting,speed ω = 0 so at starting E_b = 0.

Because the armature winding electrical resistance (Ra) is small, a DC motor has a very high starting current without back Emf. Therefore, a starter is needed to start a DC motor. As the motor rotates, back Emf is generated, gradually reducing the current as the motor speeds up.

Types of DC Motors

Direct motors are classified according to the connection of the field winding to the armature.

There are 3 main types of DC Motors:

1. Shunt wound DC motor
2. Series wound DC motor
3. Compound wound DC motor