# Control Systems Questions and Answers – Stepper Motors

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This set of Control Systems Multiple Choice Questions & Answers (MCQs) focuses on “Stepper Motors”.

1. A variable reluctance stepper motor is constructed of ______________ material with salient poles.
a) Paramagnetic
b) Ferromagnetic
c) Diamagnetic
d) Non-magnetic

Explanation: The ferromagnetic material has the property of least reluctance. Whenever a ferromagnetic material is placed under the influence of the magnetic field, then the magnetic field lines completely pass through the material. No magnetic flux passes through the air.The reason is air has high reluctance as compared to the reluctance of ferromagnetic material. So salient poles with ferromagnetic material is used for construction of a variable reluctance stepper motor.

2. In a three-stack 12/8-pole VR motor, the rotor pole pitch is
a) 15º
b) 30º
c) 45º
d) 60º

Explanation: Pole pitch is equal to total numbers of armature slots divided by total numbers of poles, we alternatively refer to it as armature slots per pole.
Otherwise, Pole pitch in mechanical degrees = 360 °/ no.of rotor poles
Here no.of rotor poles = 8
Therefore, pole pitch in mechanical degrees = 360°/8 = 45°

3. A stepper motor having a resolution of 300 steps/rev and running at 2400 rpm has a pulse rate of- pps.
a) 4000
b) 8000
c) 6000
d) 10,000

Explanation: Steps per second is mathematically given as SPS = (Rpm/60)*(number of steps per minute) = (2400/60)*300 = 12000
Pulse per second = steps per second /2 = 12000/2 = 6000.
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4. If a hybrid stepper motor has a rotor pitch of 36º and a step angle of 9º, the number of its phases must be
a) 4
b) 2
c) 3
d) 6

Explanation: Step angle is defined as =(Ns-Nr)/(Ns+Nr)*360.

5. The rotor of a stepper motor has no
a) Windings
b) Commutator
c) Brushes
d) All of the mentioned

Explanation: A stepper motor has a cylindrical permanent magnet rotor. Thus it does not contain windings, commutator or brushes mounted on it.

6. A stepping motor is a ____________ device.
a) Mechanical
b) Electrical
c) Analogue
d) Incremental

Explanation: A stepping motor is a motor in which the motion is in the form of steps and is an incremental device in which as the time increases the steps are increased.

7. The rotational speed of a given stepper motor is determined solely by the
b) Step pulse frequency
c) Polarity of stator current
d) Magnitude of stator current.

Explanation: The stator part of a motor is the stationary part of the motor and rotational speed of a given stepper motor is given by the step pulse frequency.

8. Which of the following phase switching sequence represents half-step operation of a VR stepper motor ?
a) A, B, C,A……..
b) A, C, B,A…….
c) AB, BC, CA, AB……..
d) A, AB, B, BC……..

Explanation: In the half step operation of a Variable reluctance motor physical step resolution is multiplied by 2 and control signals appear to be digital rather than analog.

9. A stepper motor may be considered as a ____________ converter.
a) Dc to dc
b) Ac to ac
c) Dc to ac
d) Digital-to-analogue

Explanation: A stepper motor is a motor in which the motion is in steps and it is an increemental device and may be considered as a digital to analog converter.

10. What is the step angle of a permanent-magnet stepper motor having 8 stator poles and 4 rotor poles?
a) 60º
b) 45º
c) 30º
d) 15º

Explanation: Step angle is defined as [(Ns-Nr)/(Ns*Nr)]*360 where Ns is the number of stator poles and Nr is the number of rotor poles.
Step angle = [(Ns-Nr)/(Ns*Nr)]*360
= [(8-4)/(8*4)]*360
Step angle = 45º

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