Stepper Motor Interview Question Answer - 2

State the main disadvantage of the VR stepper motor.

Disadvantage of VR stepper motor

• The main disadvantage of the VR stepper motor is that it hasn’t any detent torque ( which is necessary to retain the rotor at main position in the case of power failure)

Explain the term : Multi stack VR stepper motor

Multi stack VR stepper motor

• It consists of number of axially isolated sections or stacks along its axial length. 
• Each stack is excited by a separate winding.

Give reason : The variable reluctance stepper motor is capable at high stepping rate.

• Due to high torque to weight ratio

State the other name of permanent magnet step motor.

• Stamped construction or sheet metal step motor

Give reason : The PM stepper motor is called as stamped step motor.

PM stepper motor

• The rotor of the PM consists of smooth cylindrical permanent magnet radially magnetized with alternate N and S poles therefore it is also called as stamped step motor.

Compare drive circuit : Bipolar and Unipolar

Bipolar ( Monofilar )

• Single winding per bobbin
• Drive circuit requires in order to reverse the direction of current in the winding
• Positive and negative supply or bridge inverter requires.

Unipolar ( Bifilar )

• Two windings per bobbin
• Drive circuit energized only one winding at a time
• Only one power supply and two switches per bobbin requires to operate a motor

Which type of stator and rotor does the Permanent magnet stepper motor consist of?

PM stepper motor

• Stator : It consists of wound projecting poles.
• Rotor : It is made of cylindrical permanent magnet.

State the salient features of the PM stepper motor.

Features of PM stepper motor

• High detent torque as compared to VR stepper motor
• It requires low power as compared to VR stepper motor.
• High inertia
• Low acceleration
• It produces more torque / stator current as compared to VR stepper motor.

Which type of rotor construction the hybrid stepper motor consists of?

Hybrid stepper motor - Rotor

• The rotor of the stepper motor is made of three sections.
• The two outer sections are similar to variable reluctance rotor and in between a magnet is placed.
• The two outer sections are offset circumferentially from each other by one half tooth pitch.

What is reason behind the name ‘Hybrid stepper motor’?

Hybrid stepper motor

• The rotor of the hybrid stepper motor is made of both combined variable reluctance and permanent magnet therefore it is termed as ‘Hybrid stepper motor’.

Describe the function of permanent magnet rotor in the Hybrid stepper motor.

• The permanent magnet rotor provides good damping characteristics in the hybrid stepper motor.

Which type of shape the holding torque curve of the stepper motor consists of?

• The shape of the holding torque curve is inverted sinusoidal. 
• It is graphically representation of holding torque versus rotor displacement.

Which type of data the specification sheet of a stepper motor consists of?

Specification data sheet

• Voltage
• Number of phases
• Maximum Speed
• Maximum holding torque
• Step angle
• Step accuracy
• Types of drive : Unipolar and Bipolar

Explain the term : Cogging torque or Detent torque

Cogging torque

• The residual torque in the permanent magnet or hybrid stepper motor is called as cogging torque.

At which condition the torque versus rotor displacement curve is sinusoidal?

• The torque versus rotor displacement curve is sinusoidal below holding torque.

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