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Permanent Magnet DC ( PMDC ) Motor Interview Question Answer - 2

Describe the construction difference between DC motor and BLDC motor.

DC Motor – BLDC Motor

  • The rotor winding must be eliminated from the DC motor in order to make it brush less motor.
  • This can be done by turning the DC motor inside out, it means that the winding placed on the stator and permanent magnet on the rotor.
  • The current reversal in the BLDC motor is automatically possible by using electronic switch instead of commutator and brushes.

Whether the BLDC motor is operated on DC supply or AC supply?

BLDC Motor

  • The BLDC motor cannot be driven directly by the direct current.
  • The current in the external circuit must be reversed at certain rotor position. 
  • Therefore the BLDC motor is actually driven by the three phase bridge inverter or AC supply.

Give reason : The number of phases in the BLDC motor is limited.

Number of Phases

  • The BLDC motor is available in the single phase, two phase and three phase configuration.
  • There are large numbers of coils distributed around the stator but each coil requires its own drive circuit.
  • Therefore the number of phases in the BLDC motor is limited.

State the condition of maximum torque in the BLDC motor. How is it possible?

BLDC Motor – Maximum Torque

  • The torque in the BLDC motor becomes maximum when the stator and rotor magnetic field is 900 with respect to each other.
  • As the stator winding consists of three phase winding, the stator magnetic field advances only by 600.
  • The stator magnetic field must be 900 ahead of rotor magnetic field in order to generate constant torque.
  • It is not possible to maintain 900 torque angle but it is possible an average of 900 torque angle by working between stator magnetic field 600 and 1200.

State the advantages of Permanent magnet synchronous motor.


  • No brushes or slip – rings
  • No sparking
  • No brush maintenance

How many phases are energized during commutation in the three phase BLDC motor?

Commutation Sequence

  • Each commutation sequence has one winding is energized to positive, the other is energized to negative and third is in non – energized or floating.

State the advantages of BLDC motor over DC motor and Induction motor.

Advantages of BLDC motor

  • Fast dynamic response
  • High efficiency
  • Quiet operations
  • High speed range
  • Better torque – speed characteristics

State the applications of the BLDC motor.

Applications of BLDC motor

  • Constant load : Fan, Blower, Pump
  • Variable load : Compressors, Dryers, Washers, Fuel pump control, Electronic steering control, Robotic arm control, Gyroscope control
  • Positioning control : Torque control loop, Position loop control, Computer numeric control machine, Process control, Conveyor control

Give reason : The PMBLDC motor is called as Electronic commutated motor.

Electronic Commutated Motor

  • There is no mechanical commutator in the PMBLDC motor.
  • Therefore the function of current reversal is done by semiconductor switches with rotor position sensor.
  • The semiconductor switch may be IGBT or MOSFET. Therefore the PM BLDC motor is called as electronic commutated motor.

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