Permanent magnet Dc motor
Permanent magnet Dc motor or PMDC motor is a separately excited motor, this motor has different from all other types of motors, and 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.
This motor is very simple in construction.
But unfortunately, field control of this motor isn’t possible so we use only at small Dc motors.
So, we don’t need to control speed, so we use it in simple applications like:
• Windshield wipers.
• Automobiles as a starter motor.
• Blowers in heaters and air conditioners.
• Personal computer disc drives.
• 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:
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.
We usually use ferrites in cost-sensitive applications like air conditioners, compressors, and refrigerators because they are very stable and difficult to demagnetize.
We usually use the rare earth for size_sensitive applications like automobiles, servo industrial drives and large industrial motors because they have a high residual flux and high coercive magnetizing intensity as they made of samarium cobalt, neodymium_iron_boron.
Construction of permanent magnet Dc motor:
PMDC motor has the same construction of any motor there is ( you can see the construction of any DC motor here >> Construction of DC motor ):
• The stator:
It’s a steel cylinder mounted on its inner periphery the magnets, these magnets are usually made from rare earth materials or neodymium, and 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, but we also use them along with the permanent magnet to compensate the magnetic strength lost by permanent magnets.
• The rotor:
The PMDC motor rotor is similar to other Dc motor, there is a core made of a number of varnishes insulated, slotted circular laminations of steel sheets used to reduce eddy loss in the armature, also a carbon or graphite brushes 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 experienced by the conductor and we define the direction of this force by Fleming’s left-hand rule.
It is the same in permanent magnet; we place the armature inside the magnetic field of the permanent magnet and this armature rotates in the direction of generated force and the compilation of force produced by each conductor produces a torque which tends to rotate the armature.
The equivalent circuit of this motor:
The field in PMDC motor produced by permanent magnets as we say.
So, we don’t need to draw field coils in the equivalent circuit, the equivalent circuit is very simple 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:
From the last introduction and the applications of PMDC motor, we deduce that PMDC motor has great advantages over other types of the motor as:
- It’s very simple in construction.
- There is no need of field excitation arrangement.
- input power doesn’t effect on the efficiency of PMDC motor.
- We don’t use field coils so there is more space so the overall size of the motor reduces.
- It’s cheaper and economical for fractional KW applications.
Disadvantages of PMDC motor:
Unfortunately look like all other types we can find some disadvantages as:
• The strength of the field may get weak due to demagnetizing effect armature reaction because we cannot compensate the armature reaction.
• We also have a chance of getting demagnetized poles due to excessive armature current during starting.
• We have a fixed and limited field in the air gap which cannot be controlled externally and it makes the control in the speed of this motor difficult.
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