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Permanent Magnet DC Motor [PMDC Motor]

July 12, 2020

Permanent magnet DC motor or PMDC motor is a separately excited motor. This motor is different from all other types of motors. From the name permanent magnet, we use a permanent magnet to create the required magnetic field.

And I should tell you that PMDC motor generally operates on 6V,12V, or 24V Dc supply obtained from batteries or rectifiers.

Applications of Permanent Magnet DC motor

This motor is very simple in construction.

But unfortunately, field control of this motor isn’t possible so we use only small DC motors. It applies to stuff as follows;

  • Windshield wipers.
  • Washer.
  •  Automobiles as a starter motor.
  •  Blowers in heaters and air conditioners.
  •  Personal computer disc drives.
  •  Wheelchairs.
  •  Toys.
  •  And also in small fractional and sub-fractional KW motors.

Types of Permanent Magnet Materials

We usually use three types of permanent magnet materials in this motor:

  • Alnicos

We usually use alnicos where we require low current and a high voltage.

Because it has the advantage of low coercive magnetizing intensity and also high residual flux density.

  • Ferrites

They are evident in cost-sensitive applications. Such as air conditioners, compressors, and refrigerators because they are very stable and difficult to demagnetize.

  • Rare earth

The focus of rare earth is for size-sensitive applications like automobiles, servo industrial drives, and large industrial motors. Additionally, they have high residual flux and high coercive magnetizing intensity. Since, they comprise of samarium cobalt, neodymium, iron, and boron.

Construction of Permanent Magnet DC motor

PMDC motor has the same construction of any motor there is.

  • The stator

This is a steel cylinder mounted on the inner periphery of the magnets. These magnets are usually made from rare earth materials or neodymium. The magnets are mounted that the N-pole and S-pole faced towards the armature.

The stator also serves as a low reluctance return path for the magnetic flux. In the permanent magnet, we don’t need field coils. However, they are used along with the permanent magnet to compensate for the magnetic strength lost by permanent magnets.

  • The rotor

The PMDC motor rotor is similar to other DC motors. There is a core made of a number of varnishes insulated, and slotted circular laminations of steel sheets used to reduce eddy loss in the armature. Also, carbon or graphite brushes are placed on the commutator segments to supply current to the armature.

Working Principle of Permanent Magnet DC Motor

It is the same working principle of DC motor when a carrying conductor comes inside a magnetic field. A mechanical force is experienced by the conductor and we define the direction of this force by Fleming’s left-hand rule.

Similarly, it is the same in permanent magnet; we place the armature inside the magnetic field of the permanent magnet. This armature rotates in the direction of the generated force and the compilation of force produced by each conductor produces a torque that tends to rotate the armature.

The equivalent circuit of this motor

The field in PMDC motor produced by permanent magnets.

So, we don’t need to draw field coils in the equivalent circuit. The equivalent circuit is very simple that is just a voltage supply connected to the armature by the armature resistance.

So, the voltage equation will be:


I: armature current.
Eb: the back emf.

Advantages of Magnet DC motor

PMDC motor has great advantages over other types of the motor as:

  • It’s very simple in construction.
  • There is no need for a field excitation arrangement.
  • Input power doesn’t affect the efficiency of the PMDC motor.
  • We don’t use field coils so there is more space and the overall size of the motor reduces.
  • It’s cheaper and economical for fractional KW applications.

Disadvantages of Magnet DC motor

Here are some disadvantages:

  • The strength of the field may get weak due to the demagnetizing effect of armature reaction. Because we cannot compensate for the armature reaction.
  • There is a chance of getting demagnetized poles due to excessive armature current during starting.
  • We have a fixed and limited field in the air gap that cannot be controlled externally and it makes the control of the motor speed difficult.

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