Q101. In a servo system, the error signal becomes zero when
A) The system is at steady-state
B) Output equals input
C) Output is maximum
D) Load is minimum
Answer: B
Explanation: When feedback signal equals the reference signal, error = 0, indicating perfect tracking.
Q102. The torque-speed curve of a stepper motor is
A) Hyperbolic
B) Linear
C) Non-linear
D) Exponential
Answer: C
Explanation: Stepper torque decreases non-linearly with increasing speed due to inductance and back emf.
Q103. The transient response of a servo system is improved by
A) Increasing damping ratio
B) Increasing gain
C) Increasing inertia
D) Increasing load
Answer: A
Explanation: Higher damping reduces overshoot and improves transient behavior.
Q104. The frequency response of a servo system shows
A) System gain variation with frequency
B) Torque variation
C) Speed variation with load
D) Current variation
Answer: A
Explanation: Frequency response indicates system sensitivity to input frequency changes.
Q105. The term “slew rate” in a servo drive refers to
A) Maximum speed change rate
B) Torque per current ratio
C) Armature voltage rate
D) Error voltage
Answer: A
Explanation: Slew rate defines maximum rate of change of speed or position command in servo control.
Q106. In stepper motor drives, micro-stepping improves
A) Speed only
B) Torque only
C) Resolution and smoothness
D) Efficiency only
Answer: C
Explanation: Micro-stepping divides each step into smaller increments for smoother and finer motion.
Q107. The field control in DC servo motor gives
A) Wide speed range
B) High torque
C) Low armature current
D) Reduced speed
Answer: A
Explanation: Weakening the field increases speed beyond base speed (field control method).
Q108. The steady-state error of a type-1 servo system for a step input is
A) Zero
B) Finite
C) Infinite
D) Depends on gain
Answer: A
Explanation: Type-1 systems have one integrator → zero steady-state error for step input.
Q109. If a stepper motor receives 5000 pulses, each pulse of 1.8°, total rotation will be
A) 5 rev
B) 10 rev
C) 25 rev
D) 15 rev
Answer: B
Explanation: Each revolution = 200 steps → 5000/200 = 25 rev × 1.8° = 9000° = 25 revolutions.
Q110. The torque-to-inertia ratio of a servo motor determines
A) Speed of response
B) Power factor
C) Efficiency
D) Accuracy
Answer: A
Explanation: Higher torque-to-inertia ratio gives faster acceleration and response.
Q111. In a stepper motor, “pull-out torque” is
A) Torque at which synchronism is lost
B) Starting torque
C) Holding torque
D) Average torque
Answer: A
Explanation: Pull-out torque defines the maximum load torque before losing step synchronization.
Q112. A 3.6° step angle motor completes one revolution with how many pulses?
A) 100
B) 200
C) 360
D) 180
Answer: D
Explanation: Steps per revolution = 360 / 3.6 = 100.
Q113. The position error constant (Kp) of a servo system determines
A) Steady-state error for step input
B) Transient response
C) Frequency range
D) Damping ratio
Answer: A
Explanation: Position error constant defines steady-state accuracy for step input.
Q114. The major disadvantage of DC servo motor is
A) Brush wear and maintenance
B) Low torque
C) High cost
D) Heavy rotor
Answer: A
Explanation: Brushes require periodic replacement and cause electrical noise.
Q115. Stepper motor step angle can be reduced using
A) Gear reduction
B) Increased current
C) Lower voltage
D) Reduced frequency
Answer: A
Explanation: Gear reduction provides smaller output step angles (higher precision).
Q116. An AC servo motor operates on
A) Two-phase supply
B) Three-phase supply
C) Single-phase supply only
D) DC supply
Answer: A
Explanation: AC servo motors use two-phase stator windings for control and reference.
Q117. The transfer function of a DC servo motor (armature control) is
A) K / (s(Ts + 1))
B) K / (s² + 2ζωₙs + ωₙ²)
C) K / s
D) K / (1 + sT)
Answer: A
Explanation: Armature control DC servo has first-order transfer function = K / [s(Ts + 1)].
Q118. The resolution of a 200-step/rev stepper motor with 16 micro-steps per step is
A) 200
B) 3200
C) 400
D) 1600
Answer: B
Explanation: Total steps = 200 × 16 = 3200 steps/rev.
Q119. The servo amplifier output controls
A) Motor torque
B) Supply voltage
C) Load power
D) Feedback current
Answer: A
Explanation: Amplifier provides current proportional to error signal, controlling torque.
Q120. Stepper motor’s holding torque is proportional to
A) Stator current
B) Rotor speed
C) Pulse frequency
D) Voltage only
Answer: A
Explanation: Holding torque depends directly on current in energized windings.
Q121. Servo system gain is increased to
A) Increase steady-state error
B) Improve accuracy and response
C) Reduce feedback
D) Decrease damping
Answer: B
Explanation: Higher gain improves accuracy and speed but may reduce stability if too high.
Q122. Stepper motor loses steps mainly due to
A) Inertia mismatch
B) Insufficient torque
C) High stepping rate
D) All of the above
Answer: D
Explanation: All these factors lead to loss of synchronization.
Q123. The response speed of a DC servo motor is inversely proportional to
A) Inertia
B) Voltage
C) Torque
D) Feedback
Answer: A
Explanation: Lower inertia allows faster acceleration and response.
Q124. The feedback signal in a servo system is derived from
A) Motor shaft motion
B) Control input
C) Error detector
D) Amplifier
Answer: A
Explanation: Shaft encoders or sensors provide feedback proportional to actual motion.
Q125. The open-loop gain of a servo system affects
A) Stability
B) Torque
C) Efficiency
D) Step angle
Answer: A
Explanation: High open-loop gain may lead to oscillations if not compensated.
Q126. The torque developed by a DC servo motor is proportional to
A) Armature current
B) Armature voltage
C) Armature resistance
D) Flux × voltage
Answer: A
Explanation: , and for constant flux, .
Q127. In a stepper motor, increasing supply voltage
A) Improves high-speed torque
B) Reduces torque
C) Has no effect
D) Increases step angle
Answer: A
Explanation: Higher voltage allows current to build faster, improving high-speed torque.
Q128. The primary feedback device in a closed-loop stepper drive is
A) Encoder
B) Potentiometer
C) Thermistor
D) LVDT
Answer: A
Explanation: Encoders track actual position for feedback correction.
Q129. The accuracy of a stepper motor is typically within
A) ±1% of step angle
B) ±5% of step angle
C) ±10% of step angle
D) ±20% of step angle
Answer: A
Explanation: Stepper motors are highly accurate, generally within ±1% of step size.
Q130. In servo motors, position error occurs due to
A) Load torque
B) Inertia
C) Friction
D) All of the above
Answer: D
Explanation: Mechanical imperfections and load variations cause small steady-state errors.
Q131. The purpose of derivative control in a servo system is to
A) Increase damping
B) Reduce steady-state error
C) Increase speed
D) Improve accuracy
Answer: A
Explanation: Derivative term adds damping to minimize overshoot.
Q132. The time response of a servo motor is measured by
A) Rise time, settling time
B) Frequency
C) Torque
D) Step angle
Answer: A
Explanation: Rise and settling times describe dynamic time response.
Q133. The efficiency of a DC servo motor is typically
A) 50–60%
B) 70–80%
C) 85–90%
D) 95%
Answer: C
Explanation: DC servo motors have high efficiency in precision control.
Q134. Stepper motor can run in open-loop mode because
A) It steps predictably per pulse
B) It has high inertia
C) It’s AC type
D) It uses feedback
Answer: A
Explanation: Known step angle ensures predictable open-loop motion.
Q135. In a servo motor, torque is maximum when
A) Rotor speed is zero
B) Rotor speed equals synchronous speed
C) Load is zero
D) Error is zero
Answer: A
Explanation: At zero speed, maximum current flows → maximum torque.
Q136. The controller in a servo system mainly functions to
A) Generate control signal
B) Regulate speed
C) Amplify voltage
D) Rectify AC
Answer: A
Explanation: Controller computes error and generates control signal to minimize it.
Q137. The torque of a stepper motor depends on
A) Step rate
B) Supply current
C) Magnetic design
D) All of these
Answer: D
Explanation: Torque depends on all — current, step frequency, and magnetic circuit.
Q138. The “pull-in range” of a stepper motor refers to
A) Speed range in which motor starts/stops without losing synchronism
B) Maximum speed
C) Holding torque limit
D) Resonance frequency
Answer: A
Explanation: Pull-in range defines the safe operating step rate zone.
Q139. Stepper motors are not suitable for
A) Continuous speed applications
B) Position control
C) CNC machines
D) Robotics
Answer: A
Explanation: Continuous smooth motion is difficult due to discrete stepping.
Q140. In a servo motor, back emf is proportional to
A) Speed
B) Torque
C) Current
D) Voltage
Answer: A
Explanation: .
Q141. The servo system stability can be increased by
A) Adding derivative control
B) Reducing feedback
C) Increasing inertia
D) Increasing gain only
Answer: A
Explanation: Derivative control adds damping → improves stability.
Q142. The phase margin of a servo system is related to
A) Damping ratio
B) Speed
C) Torque
D) Load
Answer: A
Explanation: Phase margin represents damping; higher margin means more stable.
Q143. The maximum speed of a stepper motor is limited by
A) Inductance
B) Supply voltage
C) Load inertia
D) All of these
Answer: D
Explanation: All contribute to limiting step rate and synchronization.
Q144. The signal amplifier in a servo loop is known as
A) Power amplifier
B) Error amplifier
C) Both A and B
D) None
Answer: C
Explanation: It amplifies the error signal to sufficient level for motor drive.
Q145. The transfer function corresponds to
A) DC servo motor
B) AC servo motor
C) Stepper motor
D) Universal motor
Answer: A
Explanation: Armature-controlled DC servo model.
Q146. The bandwidth of a servo motor system should be
A) High
B) Low
C) Zero
D) None
Answer: A
Explanation: Higher bandwidth → faster response and better tracking.
Q147. The angular resolution of a 400-step motor is
A) 1.8°
B) 0.9°
C) 0.45°
D) 7.2°
Answer: B
Explanation: Step angle = 360/400 = 0.9°.
Q148. Stepper motor’s step accuracy is unaffected by
A) Temperature
B) Load variations within torque limits
C) Voltage
D) Pulse width
Answer: B
Explanation: Within rated torque, step accuracy remains constant.
Q149. The error signal polarity in a servo system determines
A) Direction of correction
B) Speed
C) Gain
D) Phase lag
Answer: A
Explanation: Sign of error defines direction of motor rotation.
Q150. A servo motor rated 24 V, 2 A, develops torque of 0.5 N·m. Its torque constant is
A) 0.25 N·m/A
B) 0.5 N·m/A
C) 0.1 N·m/A
D) 1 N·m/A
Answer: A
Explanation: Torque constant .
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