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22/12/2021

Construction & Working of PMDC Motor

 

Principle

‘Interaction between current carrying conductor and rotating magnetic field in the rotating machine produces torque. ’ The construction of Permanent Magnet DC Motor is similar to that of DC Motor except the stator of PMDC motor is made of permanent magnet in place of field winding.

Construction

  • The construction of Permanent Magnet DC Motor is shown in the Figure, in which permanent magnet is used in place of field winding.
  • This permanent magnet is connected to cylindrically stator such that the opposite polarity face to each other. 
  • It provides complete path for stator flux. The construction of rotor is similar to that armature winding, brush and commutator.


construction-of-pmdc-motor.png



Permanent Magnet

  • The Permanent magnet does not require any supply current for N – poles and S – poles. 
  • It has constant magnetic field and the direction of field from N – pole to S – pole. 
  • The magnetic field strength is maximum at pole magnet ends.
  • The ferrite magnet, alnico magnet or rate earth magnet is used as permanent magnet.

( 1 ) Ferrite magnet

  • The ferrite magnet is made of barium or strontium. 
  • This type of magnet is used in the fractional kW motor due to low cost and high resistance against de-magnetization.

( 2 ) Alnico magnet

  • It is used for motor rating of 1 kW to 150 kW.

( 3 ) Rate earth magnet

  • It is used for high rating motor. 
  • The rare earth magnet is made of samarium, cobalt and neodymium iron boron. 
  • It provides high resistance against demagnetization & low susceptibility.
  •  The cost of rare earth magnet is very high.

Working

  • The DC supply of 6 V, 12 V or 24 V is given to armature through rectifier unit. 
  • The torque is produced due to interaction between permanent magnet flux and current flows through armature winding.

Torque – Speed Characteristics

  • The cross – magnetizing and de – magnetizing effect is produced due to interaction of armature flux and field flux in the DC Shunt Motor. 
  • The field flux decreases due to low reluctance ( high permeability ) of stator iron core when load current increases. 
  • This will result in decrease in torque – speed characteristics.
  • The flux in the permanent magnet is constant due to high reluctance of it.
  •  The torque – speed characteristic becomes linear due to unaffected by armature current interaction.  
  • As the speed decreases, the torque increases when the armature voltage is kept constant. 
  • The torque – speed characteristics shift to upward when the armature voltage increases.

torque-speed-characteristic-of-pmdc-motor.png



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Speed control

  • As the permanent magnet flux remains constant in this motor, the speed control is not achieved by the field flux therefore the speed control of motor is done through armature voltage control. 
  • This type of motor is used where speed control below normal ( rated ) speed requires.

Features

  • The size of PMDC motor is small as compared to field wound DC Motor for same rating.
  • The cost of low rating PMDC motor is low due to excitation is provided by permanent magnet
  • Low air noise in the low voltage PMDC motor
  • Effect of radio interference is significant when motor is designed for low voltage

 


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