Dc motor working principle is that it works on the principle of Lorentz law which states that “the current-carrying conductor experiences a force when placed in a magnetic and electric field.”

The basic working principle of the DC motor is that whenever a current carrying conductor is placed in the magnetic field, it experiences a mechanical force. The force experienced is called Lorentz force.

Directions of force experienced is Determined by Fleming’s left hand rule and it’s magnitude is given by Force F =. BIL

Where B = Magnetic Flux,I =Magnitude of Current and L = Length of the conductor in the magnetic field.

Fleming’s Left Hand Rule

Fleming’s left-hand rule states that if we stretch the thumb, the forefinger and the middle finger of our left hand such that they are mutually perpendicular to each other then the forefinger gives the direction of current and middle finger points in the direction of magnetic field and the thumb points towards the direction of the force or motion of the conductor.

Thumb :-Direction of Thrust or direction of motion of conductor.

Index :-Direction of field

Middle :-Direction of current

What is a DC Motor ?

DC motor is an electrical machine which converts electrical energy into mechanical energy. Tools, toys, and appliances all employ small DC motors. The universal motor is a lightweight brushed motor that can run on direct current and is used in portable power tools and appliances. Larger DC motors are being employed in electric vehicle propulsion, elevator and hoist drives, and steel rolling mill drives.

DC Motor Working

As we know that dc motor works on lenz’s law. So, it requires magnetic flux and current carrying conductor.

Working principle of DC Motor

The above animations will help you to understand the working principle of dc motor

How DC motor works ?

Let us consider a Consider a coil which carry DC current through brushes and commutator. These commutator segments rotate freely about its axis.

DC Motor Working Principle

The commutator segment that comes in contact with the left brush get positive polarity, while the one that comes in contact with the right brush get negative polarity. This causes current to flow through the coil.

Using Fleming’s Left-Hand Rule, the conductor on the left side always feels an upward force, while the conductor on the right side feels a downward force. As a result, dc motors produce unidirectional torque.

Commutator which make possible to achieve unidirectional torque. Otherwise,When the direction of movement of the conductor in the magnetic field is reversed, the direction of force is reversed.
This is how a DC motor works.

EMF used in DC Motor

Fundamental laws of nature state that no energy conversion is possible unless there is something to oppose the conversion. This opposition is given by magnetic drag in generators, but back emf is present in dc motors.

When the armature of a motor rotates, the conductors cut the magnetic flux lines, causing an emf to induce in the armature conductors, according to Faraday’s law of electromagnetic induction. The induced emf is directed in the opposite direction as the armature current (Ia).

Significance of Back EMF

1. Magnitude of back end is directly proportional to speed of the motor.

The magnitude of the back emf is proportional to the speed of the motor. Consider the case where the load on a dc motor is suddenly reduced. In this case, the required torque is small in comparison to the current torque. Because of the excess torque, the motor’s speed will begin to increase.

As a result of being proportional to speed, the magnitude of the back EMF will also increase. The armature current will begin to decrease as the back EMF increases. Because torque is proportional to armature current, it will decrease until it is sufficient for the load. As a result, the motor’s speed will be regulated.

2. A DC motor is made self-regulating because of back EMF.

If a dc motor is suddenly loaded, the load will cause the speed to decrease. Back emf will decrease as speed decreases, allowing more armature current. Increased armature current will result in increased torque to meet the load requirement. As a result of the presence of the back emf, a dc motor becomes self-regulating.

This is all about working principle of DC Motor


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