PMDC Motor Basics

Wednesday greetings from team!

Our technical department is cool as always so  we are here again with our new short post about PMDC Motor.

Don’t procrastinate – read it 😉

We already wrote about Flemming’s left hand rule, but perhaps, there are new readers who didn’t read our previous post, so we’ll shortly explain it again.

Flemming’s left hand rule– this rule is applicable for d.c motors. The rule states that, whenever a current carrying conductor is placed in a magnetic field, a force acts on the conduct in the direction perpendicular to both, the current and the magnetic field.

Here you can see Fleming's Left Hand Rule demonstration
Fleming’s Left Hand Rule demonstration

Since all D.C motors work on Flemming’s left hand rule, it’s essential to establish a magnetic field.

This magnetic field is produced by the means of magnets.

When permanent magnets are used to create the magnetic field, the motor is called a Permanent Magnet motor or a  PMDC motor.

Construction of PMDC Motor:

STATOR – is a steel cylinder with permanent magnets placed at the inner periphery of the steel cylinder.

PMDC motor construction

The magnets are placed so that the alternate poles of the magnets are facing the armature or the rotor (i.e if the north pole of one magnet is facing the rotor, the south pole of the next magnet will be facing the rotor).

ROTOR- is made of varnish insulated, slotted circular laminated steel sheet. The rotor consists of core, winding and commutator.

Carbon or graphite brushes are places with spring pressure on the commutator segments to supply current to the rotor.

Working principle of the PMDC Motor:

As mentioned earlier, the D.C motor works on Flemming’s left-hand rule, when the current carry conductor comes inside a magnetic field, it experiences a mechanical force which is governed by the left-hand rule.

The armature/ rotor is placed in the magnetic field of the permanent magnet(i.e the stator); the armature rotates in the direction of the generated force.

The conductors on the armature experiences the mechanical force of F = B.I.L Newton, where

B =  magnetic field in tesla;

I = current in Amperes;

L = length of the conductor in meters;

Each conductor experiences forces and the compilation of these forces result in torque, because of which the armature rotates.

Advantages of PMDC Motor:

  1. No need for field excitation arrangement
  2. No input power is used for the excitation of the field, thus increasing the efficiency of the motor.
  3. No field coil, thus the size of the motor is small.
  4. Cheap and economical for fractional kw application.

Disadvantages of PMDC Motor:

  1. There is a chance of getting the poles permanently demagnetized (partial) due to excessive during starting, overloading and reverse the condition of the motor.
  2. The magnetic field is present all time, even when the motor is not being used.
  3. A pmdc motor has a lower induced torque per ampere turns of armature current than a shunt motor of the same rating.

Applications of PMDC Motor:

  1. In automobiles for windshield wipers and washers, air conditions, for heaters, to raise the lower windows, etc.
  2. In electric bikes
  3. In toy industries
  4. Used in electric tools like drilling machines, hedge trimmers, etc.

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Here’s to your prosperity, team

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