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When is a ceramic magnet used as a permanent magnet in a permanent magnet DC motor?

When is a ceramic magnet used as a permanent magnet in a permanent magnet DC motor? Correct Answer in fractional kW motor

Permanent-Magnet DC 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 as shown.

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PMDC motors are much more economical than fractional kilowatt motors.

Working:

  • It usually runs on 6 V, 12 V, or 24 V DC supply obtained either from batteries or rectified alternating current.
  • The torque is produced by the interaction between the axial current-carrying rotor conductors and the magnetic flux produced by the permanent magnets.
     

Speed Control:

  • The only way to control its speed is to vary the armature voltage with the help of an armature rheostat or electronically by using choppers as flux remains constant.
  • Consequently, such motors are found in systems where speed control below base speed only is required.
     

Advantages:

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

Disadvantages:

  • Since their magnetic field is active at all times even when the motor is not being used, these motors are made totally enclosed to prevent their magnets from collecting magnetic junk from the neighborhood.
  • A more serious disadvantage is that the permanent magnets can be demagnetized by armature reaction MMF causing the motor to become inoperative.
     

Applications:

  • 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.
  • The 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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