Wednesday, 30 November 2016

Permanent Magnet DC (PMDC) Motor – Construction, Working and Application

There are two types of winding in a Conventional DC Motor, namely Filed Winding and Armature Winding. The purpose of Field Winding is to produce the working magnetic flux in the air gap and wound on the stator of the motor whereas armature winding is wound on the rotor. But in Permanent Magnet DC Motor or PMDC Motor, we do not use field winding rather permanent magnet is used to have the working flux in the air gap. The construction of rotor of PMDC is same as that of the Conventional DC Motor i.e. rotor of PMDC Motor consists of armature core, armature winding and commutator. Stationary Carbon Brushes are kept pressed on the commutator as in conventional DC motor.

PMDC Motors are extensively used in automobiles for windshield wipers and washers, for blowers used in air conditioner and heaters, to raise and lower windows, in personnel computers disc drives etc. As millions of automobiles are manufactured per year, PMDC motors are also produced in millions. Maximum power rating for PMDC motor used in industry has been around 150 kW.

The major advantage of PMDC motor is that they require no field winding and hence no field current rather permanent magnet is used for having working flux in the air gap. Because of this the energy required in producing the field flux is saved. Not only has this, because of absence of field winding the size of PMDC motor reduced which is a great advantage.
The major disadvantage of PMDC motor is that it has limited capacity of producing working flux in the air gap. However, due to development of some new magnetic material like Somarium Cobalt and Neodymium Iron Boron, this problem has been resolved to some extent.

The equivalent circuit of an PMDC motor is shown in figure below. Mind that, in this equivalent circuit filed circuit is shown as is the case with conventional DC Motor.

In a PMDC Motor, flux Ø is constant and therefore we can write as

Ea = Km ωm

Te = KmIa

where Ea = Back emf generated
Te = Electromagnetic Torque
Km = Ea Ø is called speed-volt constant or torque constant. The value of Km depends upon the number of filed poles and the armature conductors etc.

Thus for the equivalent circuit sown above, we can write

Vt = IaRa + Ea

Ra = Armature winding resistance

Vt = IaRa +Km ωm

ωm = (Vt - IaRa) / Km

From the above equations, it can be seen that performance of PMDC motor is same as that of DC Shunt Motor having constant field. Therefore the speed of PMDC motor can be controlled by Armature Voltage Control, Armature Rheostat Control and Chopper Control.

You may like to read Speed Control of DC Motor

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