Permanent magnet brushless DC motor MCQ Question – 2021

Permanent magnet brushless DC motor MCQ Question -2021


Ques.1. In a Brushless DC (BLDC) motor, the construction of the motor is similar to

  1. Stepper
  2. Universal
  3. DC Motor
  4. Synchronous Motor

Answer.4. Synchronous Motor


BLDC motors are one type of synchronous motor. This means the magnetic field generated by the stator and the magnetic field generated by the rotor, rotate at the same frequency. BLDC motors do not produce the “slip” which is normally seen in induction motors. BLDC motors come in single-phase, 2-phase, and 3-phase configurations. Corresponding to its type, the stator has the same number of windings. Out of these, 3-phase motors are the most popular and widely used. A brushless motor is constructed with a permanent magnet rotor and wire-wound stator poles. Electric energy is converted to mechanical energy by the magnetic attractive forces between the permanent magnet rotor and the rotating magnetic field induced in the wound stator poles.


Ques.2.  In BLDC motor field winding is kept on

  1. Stator
  2. Rotor
  3. Can be rotor and stator
  4. No field winding on the rotor or stator

Answer.2. Rotor


Brushless motors are similar to AC motors in that a moving magnetic field causes rotor movement or rotation. Both motor types use stator windings and have no brushes. Brush-less motors are also similar to permanent magnet (PMDC) motors since they have linear characteristics. Also, both motor types use permanent magnets to generate one field, The brushless motor is, in essence, a hybrid, which combines the best attributes of both the AC and DC motors.

Brushless DC motor

The configuration of the brushless motor most commonly used in contemporary systems is shown in Fig. In this motor, the rotor consists of permanent magnet field winding and the stator consists of windings. These windings are termed “commutation” windings. By passing a current through a winding, a magnetic field is set up with which permanent magnets on the rotor interact. This results in the rotation of the rotor.


Ques.3.  Which of the following is not an advantage of BLDC motor over conventional DC motor.

  1. Less maintenance
  2. Long life
  3. No risk of explosion or possibility of RF radiation
  4. Low cost

Answer.4. Low cost


Disadvantage of BLDC motor

Cost: Rare-earth magnets are much more expensive than other magnets and result in an increased motor cost. Electronic equipment such as Hall sensor embedded in stator can also increase the cost.

Limited constant power range: A large constant power range is crucial to achieving high vehicle efficiencies. The permanent magnet BLDC motor is incapable of achieving a maximum speed greater than twice the base speed.

Magnet demagnetization: Magnets can be demagnetized by large opposing magnetomotive forces and high temperatures. The critical demagnetization force is different for each magnetic material. Great care must be brought to cooling the motor, especially if it is compactly built.

High-speed capability: The surface-mounted permanent magnet motors cannot reach high speeds because of the limited mechanical strength of the assembly between the rotor yoke and the permanent magnets.


Ques.4. In BLDC motor driver module operation, we may not require ____

  1. SCRs
  2. Power transistors
  3. FETs
  4. Transistors

Answer.4. Transistors


The schematic diagram of a brushless dc motor driver module is shown in Fig. The stator (armature) and rotor are connected in three-phase. The stator is connected to a variable voltage current source through an inductor and an inverter comprising six SCRs (Si to S6). In place of SCRs, power transistors or FETs could be used according to the power rating of the motor.

BLDC operation

Diodes are connected across SCRs to protect these from the L(di/dt) voltage induced in the armature coil undergoing commutation. Position sensors placed on the rotor provide the signal to the sensor decoders and gate drivers which cause the SCRs to be fired in sequence so as to be in synchronism with the rotor’s mechanical position. The stator and rotor fields thus get locked into each other and remain in synchronism at any rotor speed.


Ques.5. Typical brushless motor doesn’t have __________

  1. Commutator
  2. Permanent magnet
  3. Electronic controller
  4. Fixed armature

Answer.1. Commutator


The brushless DC (BLDC) motor operates in a similar way to a brushed DC motor and has the same torque and speed characteristic curve. As the name implies, BLDC motors do not have mechanical brushes for commutation as traditional DC motors do, but they must be electronically commutated in order to produce rotational torque. Since there are no brushes to wear out and replace, BLDC motors are highly reliable and almost maintenance-free.



Ques.6. The advantages of BLDC motors are

  1. High Efficiency
  2. Compactness
  3. Low Maintenance
  4. All of the above

Answer.4. All of the above


Advantages of BLDC motors are:

High efficiency: BLDC motors are the most efficient of all-electric motors. This is due to the use of permanent magnets for the excitation, which consumes no power. The absence of a mechanical commutator and brushes means low mechanical friction losses and, therefore, higher efficiency.

Compactness: The recent introduction of high-energy-density magnets (rare-earth magnets) has allowed achieving very high flux densities in the BLDC motor. This allows achieving accordingly high torques, which in turn allows making the motor small and light.

Ease of control: The BLDC motor can be controlled as easily as a DC motor because the control variables are easily accessible and constant throughout the operation of the motor.

Ease of cooling: There is no current circulation in the rotor. Therefore, the rotor of a BLDC motor does not heat up. The only heat production is on the stator, which is easier to cool than the rotor because it is static and on the periphery of the motor.

Low maintenance, great longevity, and reliability: The absence of brushes and mechanical commutator suppresses the need for associated regular maintenance and suppresses the risk of failure associated with these elements. The longevity is therefore only a function of the winding insulation, bearings, and magnet life-length.

Low noise emissions: There is no noise associated with the commutation because it is electronic and not mechanical. The driving converter switching frequency is high enough so that the harmonics are not audible.


Ques.7. BLDC can be used in place of

  1. Synchronous motor
  2. Normal brushed DC motor
  3. Induction motor
  4. Air motor

Answer.2. Normal Brushed DC Motor


The brushless DC (BLDC) motor operates in a similar way to a brushed DC motor and has the same torque and speed characteristic curve. As the name hints, BLDC motors do not have mechanical brushes for commutation as traditional DC motors do, but they must be electronically commutated in order to produce rotational torque.


Ques.8. The speed-torque characteristics of the BLDC motor are similar to that of

  1. DC Shunt Motor
  2. DC Series Motor
  3. Induction Motor
  4. Compound Motor

Answser.1. DC shunt Motor

The speed-torque characteristic of the BLDC motor is similar to DC shunt Motor as shown in the figure

Speed torque Bldc

in a BLDC motor, the torque remains fairly constant for a speed range up to the rated speed. Once the motor speed is beyond the rated speed, the output torque starts dropping and is limited by the rated power. The motor can be run-up to the maximum speed, which can be up to 150% of the rated speed


Ques.9. The starting characteristic of the BLDC motor is similar to

  1. DC Shunt Motor
  2. DC Series Motor
  3. Induction Motor
  4. Compound Motor

Answer.2. DC Series Motor


The starting characteristics of the BLDC motor are similar to DC series Motor.

During starting of the BLDC motor both the Speed and Back EMF is zero.

Starting characteristic BLDC

Since the armature winding resistance is small, the starting current will be large in a short period of time. It may reach several times or more than ten times the normal operating current. Within the allowable range, the large starting current is helpful to the acceleration of the rotor so that the motor can quickly start even under full load.

Moreover, the armature current increases rapidly in the instant of starting. Thus, the electromagnetic torque is much larger than the load torque so that the speed increases rapidly. Consequently, the back-EMF will increase so that the growth of the armature current becomes slower until it reaches the maximum. Then, the armature current begins to decrease. The decreased current will lead to a decrease of the electromagnetic torque, so the rising acceleration of the speed becomes smaller. When the electromagnetic torque and load torque achieve the dynamic balance, the speed will stay in the rated value, i.e. the BLDC motor will maintain steady-state operation.


Ques.10.  The sensor in BLDC motor is Placed in

  1. Stator side
  2. Rotor Side
  3. Either stator or Rotor
  4. None of these

Answer.3. Either Stator or Rotor


BLDC motors typically have three Hall effect sensors mounted either to the stator or to the rotor side which is known as six-step commutation.

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