Working Principle of DC Motor

Hello friends, we are making the blog series on DC motors. In this first blog, we are talking about the working principle of DC motor, how they work? Also, we will talk about the construction of DC motor.

What is DC Motor?

The DC motor is the motor which converts the direct current into the mechanical work. It works on the principle of Lorentz Law, which states that “the current carrying conductor placed in a magnetic and electric field experience a force”. And that force is the Lorentz force.

Types of DC Motor

There are 4 major types of DC motor and they are,

• Series DC Motor
• Permanent Magnet DC Motor
• Shunt/Parallel DC Motor
• Compound DC Motors

MUST READ ON DC MOTORS:

• Different Types of DC Motor 
• Difference Between AC and DC Motor
• Speed Control of DC Motor
• Applications of Dc motor

Construction of DC Motor

Before understanding the working of DC motor first, we have to know about their construction. There are two main parts of the DC motor.

The armature coil consists the commutators and brushes. The commutator converts the AC induces in the armature into DC and brushes transfer the current from rotating part of the motor to the stationary external load. The armature places between the north and south pole of the permanent or electromagnet.

Working Principle of DC Motor

A DC motor is an electrical machine which converts electrical energy into mechanical energy. The basic working principle of the DC motor is that whenever a current carrying conductor places in the magnetic field, it experiences a mechanical force.

Fleming's left-hand rule and its magnitude decide the direction of this force.

Fleming's Left Hand Rule:

If we stretch the first finger, second finger and thumb of our left hand to be perpendicular to each other, and first finger represents the direction of the magnetic field, the second finger represents the direction of the current, then the thumb represents the direction of the force experienced by the current carrying conductor.

F = BIL Newtons

When armature winding is connected to a DC supply, an electric current sets up in the winding.  Permanent magnets or field winding (electromagnetism) provides the magnetic field. In this case, current carrying armature conductors experience a force due to the magnetic field, according to the principle stated above.

The Commutator is made segmented to achieve unidirectional torque. Otherwise, the direction of force would have reversed every time when the direction of movement of the conductor is reversed in the magnetic field. This is how a DC motor works!

Back- EMF of DC motor

According to the fundamental law of nature, no energy conversion is possible until there is something to oppose the conversion. In case of generators, magnetic drag provides this opposition, but in the case of dc motors, there is back emf. Presence of the back emf makes a dc motor ‘self-regulating’.

When the armature of a motor is rotating, the conductors are also cutting the magnetic flux lines and hence according to the Faraday's law of electromagnetic induction, an emf induces in the armature conductors.

The direction of this induced emf is such that it opposes the armature current (I_a). The circuit diagram below illustrates the direction of the back emf and armature current. 

Significance of Back-EMF

Magnitude of back emf is directly proportional to speed of the motor. Consider the load on a dc motor is suddenly reduced. In this case, required torque will be small as compared to the current torque. Speed of the motor will start increasing due to the excess torque. Hence, being proportional to the speed, magnitude of the back emf will also increase. With increasing back emf armature current will start decreasing. Torque being proportional to the armature current, it will also decrease until it becomes sufficient for the load. Thus, speed of the motor will regulate.

On the other hand, if a dc motor is suddenly loaded, the load will cause decrease in the speed. Due to decrease in speed, back emf will also decrease which allows more armature current. Due to increase in armature current  the torque will increase to fulfill the load requirement. 

Hope this article helps you to understand the working principle of DC motor. 

We at Robu.in hope that you found it interesting and that you will come back for more of our educational blogs.