What is Permanent Magnet DC (PMDC) Motor? What are the Advantages, Disadvantages and Applications of Permanent Magnet DC Motor?

Permanent Magnet DC (PMDC) Motor:

A permanent-magnet DC (PMDC) motor is similar to an ordinary DC shunt motor except that its field is provided by permanent magnets instead of salient-pole wound-field structure. Figure (a) shows 2-pole PMDC motor whereas Figure (b) shows a 4-pole wound-field d.c. motor for comparison purposes.

Advantages of PMDC Motor:

1. In very small ratings, use of permanent-magnet excitation results in lower manufacturing cost.
2. In many cases a PMDC motor is smaller in size than a wound-field d.c. motor of equal power rating.
3. Since field excitation current is not required, the efficiency of these motors is generally higher than that of the wound-field motors.
4. Low-voltage PMDC motors produce less air noise.
5. When designed for low-voltage (12 V or less) these motors produced very little radio and TV interference.

Disadvantages of PMDC Motor:

1. Since their magnetic field is active at all times even when motor is not being used, these motors are made totally enclosed to prevent their magnets from collecting magnetic junk from neighbourhood. Hence, as compared to wound-field motors, their temperature tends to be higher. However, it may not be much of a disadvantage in situations where motor is used for short intervals.
2. A more serious disadvantage is that the permanent magnets can be demagnetized by armature reaction mmf causing the motor to become inoperative. Demagnetization can result from (a) improper design (b) excessive armature current caused by a fault or transient or improper connection in the armature circuit (c) improper brush shift and (d) temperature effects.

(c) Permanent Magnet DC (PMDC) Motor

Applications:

1. Small, 12-V PMDC motors are used for driving automobile heater and air conditioner blowers, windshield wipers, windows, fans and radio antennas etc. They are also used for electric fuel pumps, marine engine starters, wheelchairs and cordless power tools.
2. Toy industry uses millions of such motors which are also used in other appliances such as the toothbrush, food mixer, ice crusher, portable vacuum cleaner and shoe polisher and also in portable electric tools such as drills, saber saws and hedge trimmers etc.

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