Q51. The main difference between an open-loop and a closed-loop servo system is the presence of
A) Error amplifier
B) Feedback path
C) Controller
D) Motor
Answer: B
Explanation: Closed-loop systems include feedback to continuously correct the output; open-loop systems do not.
Q52. Which of the following devices converts shaft position into an electrical signal?
A) Potentiometer
B) Amplifier
C) Transistor
D) Converter
Answer: A
Explanation: Potentiometers are position transducers used for feedback in servo systems.
Q53. The torque-speed characteristic of a DC servo motor is
A) Linear
B) Exponential
C) Constant
D) Parabolic
Answer: A
Explanation: Torque is directly proportional to armature current, giving a linear torque-speed characteristic.
Q54. The rotor of a variable reluctance stepper motor is made of
A) Permanent magnets
B) Soft iron
C) Aluminium
D) Copper
Answer: B
Explanation: Rotor is made of soft iron to allow magnetic reluctance variation with position.
Q55. In a hybrid stepper motor, the rotor has
A) Permanent magnets only
B) Variable reluctance structure only
C) Both permanent magnets and toothed rotor
D) None of the above
Answer: C
Explanation: Hybrid stepper motors combine PM rotor with toothed structure for finer steps.
Q56. If a stepper motor has a step angle of 0.9°, how many steps per revolution does it have?
A) 200
B) 400
C) 100
D) 800
Answer: B
Explanation: Steps per revolution = 360 / Step angle = 360 / 0.9 = 400.
Q57. The response of a servo system to a unit step input depends on
A) Damping ratio and natural frequency
B) Motor inertia only
C) Power rating
D) None of the above
Answer: A
Explanation: System response parameters are governed by damping ratio (ζ) and natural frequency (ωₙ).
Q58. The electrical time constant of a DC servo motor is given by
A) L/R
B) R/L
C) 1/(L×R)
D) L×R
Answer: A
Explanation: Electrical time constant τₑ = L / R where L = armature inductance and R = armature resistance.
Q59. A two-phase servo motor’s stator has
A) Two windings displaced by 90° electrically
B) Two windings in series
C) One winding
D) Three windings displaced by 120°
Answer: A
Explanation: Two-phase AC servo motors have stator windings displaced by 90° electrical for control and reference inputs.
Q60. The damping factor in servo systems affects
A) Steady-state error
B) Overshoot and transient response
C) Gain margin
D) Pulse frequency
Answer: B
Explanation: Damping controls oscillations and overshoot during transient response.
Q61. Which motor type provides highest torque per ampere?
A) DC shunt motor
B) AC servo motor
C) Permanent magnet DC servo motor
D) Universal motor
Answer: C
Explanation: PM DC servo motors have high torque-to-current ratio due to strong permanent magnets.
Q62. The pull-in torque of a stepper motor is
A) Maximum torque at which motor starts and synchronizes
B) Torque when running at full speed
C) Same as detent torque
D) Torque when power is off
Answer: A
Explanation: Pull-in torque defines the torque with which the motor can start, stop, or reverse synchronously.
Q63. Servo motors operate on which control principle?
A) On–off control
B) Error signal control
C) Voltage–current feedback
D) Power feedback control
Answer: B
Explanation: Servo motors move in proportion to the error signal generated between reference and feedback signals.
Q64. Stepper motor rotor movement is
A) Continuous
B) Discrete
C) Random
D) None of these
Answer: B
Explanation: Stepper motors rotate in discrete steps corresponding to each input pulse.
Q65. The main cause of loss of synchronism in a stepper motor is
A) Overloading
B) Low voltage
C) Excessive stepping rate
D) All of these
Answer: D
Explanation: All factors—overload, low supply voltage, or high stepping rate—can cause step loss (loss of synchronism).
Q66. The main feedback element in modern servo drives is
A) Optical encoder
B) Potentiometer
C) Tachogenerator
D) Rheostat
Answer: A
Explanation: Encoders provide precise digital position and speed feedback in modern servo systems.
Q67. Stepper motor performance deteriorates at high speed mainly because of
A) Back EMF and inductance
B) Resistance drop
C) Magnetic saturation
D) Shaft misalignment
Answer: A
Explanation: Back EMF and inductance limit current rise at high frequency, reducing torque.
Q68. The speed of a DC servo motor can be increased by
A) Increasing field current
B) Increasing armature voltage
C) Increasing armature resistance
D) Decreasing supply voltage
Answer: B
Explanation: Speed ∝ Armature voltage (in armature-controlled DC servo motors).
Q69. The bandwidth of a servo system represents
A) Power handling capacity
B) Speed range
C) Frequency range for accurate response
D) Torque output range
Answer: C
Explanation: Bandwidth indicates the frequency range within which the system can follow input changes accurately.
Q70. A 1.8° step angle stepper motor will take how many steps to complete 45° rotation?
A) 20
B) 25
C) 50
D) 100
Answer: C
Explanation: Steps = 45° / 1.8° = 25 steps.
Q71. The phase current in a stepper motor lags voltage because of
A) High inductance
B) High capacitance
C) Resistance
D) Load torque
Answer: A
Explanation: High inductance in stepper windings causes current to lag voltage.
Q72. The error signal in a servo system is obtained by
A) Differentiating output
B) Comparing input and feedback
C) Multiplying feedback
D) Integrating output
Answer: B
Explanation: Error = Desired output – Actual (feedback) output.
Q73. The main advantage of AC servo motors over DC servo motors is
A) Simpler construction and no brushes
B) Higher inertia
C) More maintenance
D) Poor speed control
Answer: A
Explanation: AC servo motors are brushless, compact, and require less maintenance than DC types.
Q74. A 4-phase, 50 Hz stepper motor has 100 steps/rev. The pulse frequency for 600 rpm is
A) 1000 Hz
B) 2000 Hz
C) 4000 Hz
D) 6000 Hz
Answer: B
Explanation:
Speed = (Pulse frequency × 60) / Steps per revolution →
Pulse frequency = (600 × 100) / 60 = 1000 Hz.
Q75. The dynamic characteristics of a servo system are determined by
A) Damping ratio and natural frequency
B) Gain only
C) Input voltage
D) Load torque only
Answer: A
Explanation: Dynamic behavior depends on ζ (damping ratio) and ωₙ (natural frequency).
Q76. Stepper motor resonance occurs when
A) Pulse frequency matches mechanical natural frequency
B) Motor operates at high voltage
C) Load is zero
D) Frequency is random
Answer: A
Explanation: Resonance happens when step pulse frequency equals mechanical resonance frequency, causing vibration or loss of steps.
Q77. The torque in a servo motor is proportional to
A) Error signal amplitude
B) Input frequency
C) Feedback voltage
D) Rotor inertia
Answer: A
Explanation: The control signal (error amplitude) determines torque output in servo systems.
Q78. The speed of a stepper motor can be controlled by varying
A) Pulse frequency
B) Step angle
C) Supply current
D) Stator voltage
Answer: A
Explanation: Speed is directly proportional to the pulse rate of the input signals.
Q79. The term “slew rate” in stepper motor refers to
A) Rate of voltage change
B) Maximum stepping speed without missing steps
C) Acceleration limit
D) Torque decay rate
Answer: B
Explanation: Slew rate defines maximum step frequency for stable operation.
Q80. In a servo drive, PID controller is used for
A) Power amplification
B) Error correction and stability
C) Speed reduction
D) Voltage rectification
Answer: B
Explanation: PID controller (Proportional–Integral–Derivative) minimizes error and improves stability.
Q81. A servo system becomes unstable when
A) Gain is too low
B) Phase lag exceeds 180°
C) Damping ratio > 1
D) Load torque is zero
Answer: B
Explanation: Phase lag ≥180° with unity gain causes oscillations (instability).
Q82. The holding torque of a stepper motor is measured in
A) Newton
B) Newton-meter
C) Ampere
D) Watt
Answer: B
Explanation: Torque is measured in N·m (Newton-meter).
Q83. The feedback element in a digital servo motor system is usually
A) Digital encoder
B) Analog potentiometer
C) Tachogenerator
D) LVDT
Answer: A
Explanation: Digital encoders provide high-accuracy position feedback in digital servo drives.
Q84. The servo amplifier amplifies
A) Reference voltage
B) Error signal
C) Output signal
D) Feedback signal
Answer: B
Explanation: Servo amplifier boosts the error signal to drive the servo motor.
Q85. The step angle of a stepper motor can be reduced by
A) Increasing rotor teeth
B) Reducing stator poles
C) Reducing rotor teeth
D) Decreasing supply voltage
Answer: A
Explanation: More rotor teeth result in smaller step angles (higher resolution).
Q86. The main advantage of hybrid stepper motor is
A) High torque and small step angle
B) Low torque and large step angle
C) Cheap construction
D) Simple control
Answer: A
Explanation: Hybrid type provides high torque with small step angle and excellent resolution.
Q87. The transfer function of a simple DC servo motor is
A) Type 0
B) Type 1
C) Type 2
D) Type 3
Answer: B
Explanation: DC servo motor has one integrator (1/s), making it a type-1 system.
Q88. In a position control system using servo motor, the output is
A) Speed
B) Acceleration
C) Angular displacement
D) Voltage
Answer: C
Explanation: Servo position control outputs precise angular displacement.
Q89. Stepper motor’s incremental movement depends on
A) Frequency and number of pulses
B) Voltage only
C) Torque only
D) Current only
Answer: A
Explanation: Each pulse gives a step; pulse frequency decides speed; pulse count decides displacement.
Q90. The time constant of a servo system affects
A) Response speed
B) Load torque
C) Output power
D) Step angle
Answer: A
Explanation: Smaller time constant → faster response.
Q91. A DC servo motor with constant field control varies speed by changing
A) Armature voltage
B) Field current
C) Load torque
D) Resistance
Answer: A
Explanation: Armature-controlled DC servo varies speed by changing armature voltage.
Q92. The output of a tachogenerator is proportional to
A) Speed
B) Position
C) Torque
D) Power
Answer: A
Explanation: Tachogenerator generates voltage directly proportional to rotational speed.
Q93. When the damping ratio (ζ) = 1, the servo system is
A) Underdamped
B) Critically damped
C) Overdamped
D) Oscillatory
Answer: B
Explanation: ζ = 1 → critically damped, providing fastest non-oscillatory response.
Q94. Which motor is best suited for CNC feed drives?
A) DC shunt motor
B) Stepper motor
C) Synchronous motor
D) Induction motor
Answer: B
Explanation: Stepper motors provide precise incremental movement ideal for CNC positioning.
Q95. The transfer function of a servo motor determines
A) Dynamic behavior
B) Step angle
C) Supply voltage
D) Shaft design
Answer: A
Explanation: Transfer function expresses system’s dynamic response to input.
Q96. The efficiency of stepper motor decreases with
A) Speed
B) Torque
C) Voltage
D) Step angle
Answer: A
Explanation: Efficiency drops at higher speeds due to current lag and core losses.
Q97. Servo motors are preferred over stepper motors when
A) Feedback and smooth motion are required
B) Open-loop control is sufficient
C) Cost must be low
D) Torque is unimportant
Answer: A
Explanation: Servo motors provide continuous, feedback-controlled motion with higher torque and speed.
Q98. The load torque in a servo system acts as
A) Negative feedback
B) Disturbance input
C) Reference signal
D) Error amplifier
Answer: B
Explanation: Load torque is an external disturbance the system must overcome to maintain position.
Q99. For precise micro-positioning, which motor is best suited?
A) DC motor
B) Induction motor
C) Stepper motor
D) Synchronous motor
Answer: C
Explanation: Stepper motors provide accurate micro-stepping control for fine positioning.
Q100. A hybrid stepper motor has rotor teeth = 50 and stator teeth = 200. The step angle is
A) 1.8°
B) 0.9°
C) 3.6°
D) 7.2°
Answer: A
Explanation: Step angle = 360 × (Nr – Ns) / (Nr × Ns) ≈ 1.8° for typical hybrid configurations.
No comments:
Post a Comment