Permanent Magnet DC ( PMDC ) Motor Interview Question Answer - 1

Describe the different types of Permanent magnet synchronous motor.

The Permanent magnet synchronous motor is classified as below

Permanent magnet synchronous motor (BLAC)(Sinusoidal waveforms)

PM Brushless DC motor ( BLDC )( Trapezoidal waveforms )

Which are the main parts of the BLDC motor?


It consists of winding similar to DC motor armature.


It is made of permanent magnet.

Hall sensors

They are embedded in the stator on the non – driving end of the motor to sense the position of the rotor. When the rotor magnetic pole pass near the hall sensors, they give high or low signal indicates that the N or S pole passes near the sensors.

Give the reason behind the name of ‘BLDC’motor.

The BLDC motor has straight line speed torque characteristics similar to that of permanent magnet direct current motor so it is called as BLDC motor.

Compare the sinusoidal winding and trapezoidal winding in the BLDC motor.

Sinusoidal and trapezoidal winding

The torque output of the sinusoidal winding is smoother than the trapezoidal winding. The sinusoidal winding requires extra winding interconnection which increases the weight and cost of copper stator winding.

Give the examples of rare earth alloys

Rare earth alloy magnet

Samarium cobalt


Alloy of neodymium, ferrite and boron

Compare rare earth alloy magnet and ferrite magnet.

Ferrite magnet

Low flux density per unit volume

Less costly

Rare earth alloy magnet

High flux density per unit volume


High torque as compare to ferrite magnet for same size

Improved size to weight ratio

State the configuration of the BLDC motor.

According of construction of BLDC motor, it is configured as

Outer rotor motor

Inner rotor motor

Special configuration motor

Compare outer rotor and inner rotor BLDC motor.

Outer rotor BLDC motor

High rotor inertia is necessary

The outer rotor motor has much more magnetic material than the inner rotor motor

Application : computer disk, drives, cooling fans

Inner rotor BLDC motor

Lower inertia because of small rotor diameter

Accelerate more rapidly due to small rotor diameter

Most of the inner rotor BLDC motor has multiple phase in order to reduce starting problems

Is there cogging problem in the slotless brushless inner rotor BLDC motor?

Cogging Problem

The stator of the inner rotor BLDC motor has no teeth therefore there is no variation of permeance as the rotor moves resulting no cogging problem.

How the sensing of the rotor position is done in the stepper motor?

Stepper Motor – Rotor Position

The rotor position of the stepper motor is sensed by placing three sensors in the stator on the non – driving end. 

Whenever the rotor magnetic poles pass near the hall sensors, they produce either LOW or HIGH signal which indicates either N or S pole passing near the hall sensors.

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