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A DC Motor whose poles are made of Permanent Magnets is known as Permanent Magnet DC (PMDC) Motor. The magnets are radially magnetized and are mounted on the inner periphery of the cylindrical steel stator. The stator of the motor serves as a return path for the magnetic flux. The rotor has a DC armature, with commutator segments and brushes.
The speed control of the PMDC motor cannot be controlled by using flux control method as the flux remains constant in this type of motor. Both speed and torque can be controlled by armature voltage control, armature rheostat control, and chopper control methods. These motors are used where the motor speed below the base speed is required as they cannot be operated above the base speed.
There are three types of Permanent Magnet Materials used in PMDC Motor. The detailed information is given below.
Alnicos
Alnicos has a low coercive magnetizing intensity and high residual flux density. Hence, it is used where low current and high voltage is required.
Ferrites
They are used in cost sensitive applications such as Air conditioners, compressors, and refrigerators.
Rare earths
Rare earth magnets are made of Samarium cobalt, neodymium-iron-boron. They have a high residual flux and high coercive magnetizing intensity. The rare earth magnets are exempted from demagnetizing problems due to armature reaction. It is an expensive material.
The Neodymium iron boron is cheaper as compared to Samarium cobalt. But it can withstand higher temperature. Rare earth magnets are used for size-sensitive applications. They are used in automobiles, servo industrial drives and in large industrial motors.
The PMDC motors are used in various applications ranging from fractions to several horsepower. They are developed up to about 200 kW for use in various industries. The following applications are given below.
· PMDC motors are mainly used in automobiles to operate windshield wipers and washers, to raise the lower windows, to drive blowers for heaters and air conditioners etc.
· They are also used in computer drives.
· These types of motors are also used in toy industries.
· PMDC motors are used in electric toothbrushes, portable vacuum cleaners, food mixers.
· Used in a portable electric tool such as drilling machines, hedge trimmers etc.
Following are the advantages of the PMDC Motor.
· They are smaller in size.
· For smaller rating Permanent Magnet reduces the manufacturing cost and thus PMDC motor are cheaper.
· As these motors do not require field windings, they do not have field circuit copper losses. This increases their efficiency.
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