Q451. Servo motors are preferred in CNC machines because they offer?
A) Open-loop control
B) High torque at low speed and precise feedback
C) High inertia
D) Constant speed only
Ans: B
Explanation: CNC machines need accurate position and torque control — features of servo motors.
Q452. The driver circuit for a stepper motor mainly converts?
A) DC voltage to AC voltage
B) DC voltage to pulse sequence
C) AC to DC
D) DC to sine wave
Ans: B
Explanation: Stepper drives generate pulse sequences to energize motor windings in order.
Q453. Servo amplifiers use feedback from?
A) Load
B) Motor shaft sensor
C) Controller output
D) External signal
Ans: B
Explanation: Feedback from encoders/resolvers on motor shaft controls position/speed precisely.
Q454. The primary advantage of a hybrid stepper motor is?
A) High power factor
B) High precision and torque
C) Low current
D) High frequency
Ans: B
Explanation: Hybrid steppers combine PM and variable reluctance for high precision and torque.
Q455. Stepper motors are inherently?
A) Closed-loop
B) Open-loop
C) AC synchronous
D) Induction-type
Ans: B
Explanation: Steppers operate open-loop, responding to input pulses without feedback.
Q456. The “hold-in range” of a servo motor defines?
A) Range of frequencies for stable operation
B) Range of input voltage
C) Range of position it can maintain under load
D) Torque limit
Ans: C
Explanation: Hold-in range = the position range where servo maintains control without oscillation.
Q457. The driver for stepper motors typically includes?
A) H-bridge circuits
B) Transformers
C) Rectifiers
D) Relays
Ans: A
Explanation: Stepper drivers use H-bridge MOSFETs or transistors to switch phase currents.
Q458. Servo motors require which type of supply?
A) Fixed DC
B) Controlled variable supply
C) Constant AC
D) Pulsed AC
Ans: B
Explanation: Servos use controlled PWM supply for torque/speed regulation.
Q459. The “pull-out torque” decreases as?
A) Speed increases
B) Temperature decreases
C) Voltage increases
D) Load decreases
Ans: A
Explanation: At higher speed, inductive effects reduce torque capability.
Q460. Stepper motor position is determined by?
A) Number of pulses
B) Pulse width
C) Voltage magnitude
D) Frequency
Ans: A
Explanation: Each pulse = one step, so position depends on pulse count.
Q461. The main drawback of open-loop stepper control is?
A) Missed steps under overload
B) High cost
C) Low torque
D) Excessive feedback
Ans: A
Explanation: Without feedback, missed steps cause position errors.
Q462. The encoder in a servo motor acts as a?
A) Speed and position sensor
B) Current sensor
C) Load sensor
D) Power monitor
Ans: A
Explanation: Encoders give speed and position feedback to maintain accuracy.
Q463. In a servo motor, “stiction” refers to?
A) Static friction before movement starts
B) Dynamic torque loss
C) Electrical noise
D) Step loss
Ans: A
Explanation: Stiction = static friction, affecting precise low-speed control.
Q464. Stepper motor acceleration control prevents?
A) Resonance
B) Overshoot
C) Loss of steps
D) All of these
Ans: D
Explanation: Controlled acceleration avoids resonance, overshoot, and missed steps.
Q465. The control input for a DC servo amplifier is typically?
A) Error voltage
B) Speed command
C) Pulse signal
D) Torque signal
Ans: A
Explanation: Error voltage represents difference between desired and actual value.
Q466. The stability of a servo system is analyzed using?
A) Bode plot
B) Torque-speed curve
C) Step response only
D) Pulse test
Ans: A
Explanation: Bode plot helps evaluate gain margin and phase margin for stability.
Q467. The damping factor of a servo system should ideally be?
A) 0
B) 1
C) Between 0.6–0.8
D) Above 2
Ans: C
Explanation: Optimum ζ = 0.6–0.8 for fast response with minimal overshoot.
Q468. Stepper motor resonance occurs at?
A) Certain step rates
B) Any speed
C) High torque
D) Low current
Ans: A
Explanation: Mechanical resonance occurs at specific step frequencies.
Q469. The electrical time constant of a stepper motor is determined by?
A) L/R of the winding
B) Voltage × current
C) Speed/torque ratio
D) Phase difference
Ans: A
Explanation: Time constant = L/R, affects current rise and speed response.
Q470. Servo drive “tuning” adjusts?
A) PID parameters
B) Supply voltage
C) Pulse frequency
D) Armature current
Ans: A
Explanation: Tuning = adjusting PID gains for best dynamic response.
Q471. Microstepping improves?
A) Torque accuracy and smoothness
B) Power factor
C) Frequency response
D) Voltage regulation
Ans: A
Explanation: Microstepping creates smoother motion and higher resolution.
Q472. Which feedback device gives absolute position?
A) Incremental encoder
B) Absolute encoder
C) Tachogenerator
D) Potentiometer
Ans: B
Explanation: Absolute encoders output unique position codes per shaft angle.
Q473. Servo system bandwidth is limited by?
A) Mechanical inertia and motor dynamics
B) Voltage rating
C) Current capacity
D) Feedback resistance
Ans: A
Explanation: Inertia and motor response limit system bandwidth.
Q474. The “following error” in a servo drive means?
A) Difference between commanded and actual position
B) Error due to temperature
C) Mechanical misalignment
D) Encoder malfunction
Ans: A
Explanation: Following error = lag between desired and actual motion.
Q475. Stepper motor winding insulation failure mainly causes?
A) Step loss
B) Overheating and phase short
C) Torque increase
D) No effect
Ans: B
Explanation: Insulation breakdown leads to short circuit & overheating.
Q476. The torque constant of a DC servo motor depends on?
A) Magnetic field strength
B) Armature resistance
C) Commutator material
D) Shaft load
Ans: A
Explanation: Torque constant Kt ∝ magnetic flux of field system.
Q477. Stepper motor speed control is achieved by varying?
A) Pulse frequency
B) Pulse amplitude
C) Step angle
D) Phase voltage
Ans: A
Explanation: Speed ∝ Pulse frequency directly.
Q478. Servo drive fault “overcurrent” occurs when?
A) Motor is stalled
B) Power loss
C) Encoder fails
D) Controller reset
Ans: A
Explanation: Stalling or heavy load draws excessive current causing trip.
Q479. Stepper motor phase current is usually limited by?
A) Current chopper
B) Transformer
C) Series resistor
D) Relay
Ans: A
Explanation: Chopper circuit controls winding current for protection.
Q480. Servo motors can be AC or DC depending on?
A) Drive type
B) Load torque
C) Supply frequency
D) Phase shift
Ans: A
Explanation: AC or DC servos depend on drive and control circuit used.
Q481. The torque developed by a stepper motor depends on?
A) Phase current
B) Step angle
C) Number of phases
D) All of these
Ans: D
Explanation: Torque ∝ current, phase count, step angle, and design.
Q482. In servo systems, “deadband” means?
A) Range of no response
B) Maximum overshoot
C) Power loss
D) Feedback delay
Ans: A
Explanation: Deadband = small input change that produces no output movement.
Q483. Overdamped servo response results in?
A) Slow but stable motion
B) Oscillatory motion
C) Unstable control
D) Zero steady-state error
Ans: A
Explanation: Overdamping eliminates oscillations but slows response.
Q484. Stepper motor coils are energized in?
A) Sequence
B) Random order
C) Parallel
D) Continuous all-on
Ans: A
Explanation: Sequential energization causes step-by-step rotation.
Q485. Servo control systems operate on?
A) Feedback principle
B) Feedforward only
C) Open-loop
D) None
Ans: A
Explanation: Servo = closed-loop feedback control mechanism.
Q486. The servo drive “enable” input is used to?
A) Activate motor power
B) Adjust speed
C) Reset error
D) Measure current
Ans: A
Explanation: Enable input allows motor to receive drive power.
Q487. Stepper motor “holding torque” acts when?
A) Motor is powered but not rotating
B) Motor off
C) At high speed
D) During deceleration
Ans: A
Explanation: Holding torque maintains position when energized but stationary.
Q488. Servo drive overvoltage fault occurs during?
A) Rapid deceleration (regen braking)
B) Low load
C) Idle state
D) Encoder failure
Ans: A
Explanation: Regenerative energy during deceleration raises bus voltage.
Q489. Stepper motor “detent torque” exists due to?
A) Permanent magnet rotor
B) Field winding
C) Stator core
D) Shaft friction
Ans: A
Explanation: PM rotor creates magnetic alignment torque without current.
Q490. Servo “lag error” can be reduced by?
A) Increasing proportional gain
B) Increasing derivative gain
C) Both A & B
D) Reducing integral action
Ans: C
Explanation: Higher P and D gain reduce lag, improve tracking.
Q491. Stepper motor open-loop control fails when?
A) Load exceeds pull-out torque
B) Frequency too low
C) Temperature decreases
D) Step angle increases
Ans: A
Explanation: Excess load causes desynchronization → lost steps.
Q492. Servo “notch filter” is used to?
A) Eliminate resonance frequencies
B) Amplify torque
C) Increase damping
D) Shift phase
Ans: A
Explanation: Notch filter suppresses specific resonance frequencies.
Q493. Stepper motor microstepping requires?
A) Sinusoidal current control
B) DC voltage
C) Phase delay
D) Capacitor bank
Ans: A
Explanation: Sine-cosine control creates fine microsteps for smoothness.
Q494. Servo drive feedback loss results in?
A) Fault trip
B) Smooth motion
C) Speed increase
D) Reduced torque
Ans: A
Explanation: Loss of encoder feedback → drive shutdown for safety.
Q495. Stepper motor driver “half-stepping” means?
A) Energizing one or two phases alternately
B) Energizing all phases
C) Reversing polarity
D) Doubling frequency
Ans: A
Explanation: Half-step mode alternates 1-phase and 2-phase excitation for finer steps.
Q496. Servo “velocity loop” primarily controls?
A) Motor speed
B) Torque
C) Position
D) Power factor
Ans: A
Explanation: Velocity loop ensures stable and accurate speed control.
Q497. Stepper driver ICs (like A4988, DRV8825) control current via?
A) PWM chopping
B) Linear resistor
C) Relay switching
D) Transformer
Ans: A
Explanation: PWM chopping regulates phase current precisely.
Q498. Servo tuning “gain too low” causes?
A) Sluggish response
B) Overshoot
C) Oscillation
D) Noise
Ans: A
Explanation: Low gain → slow response and poor tracking.
Q499. Stepper motor maximum torque occurs at?
A) Low speed
B) High speed
C) Mid-speed
D) No load
Ans: A
Explanation: Torque highest at low speed, decreases with frequency.
Q500. Servo “current loop” controls?
A) Motor torque
B) Position
C) Velocity
D) Power
Ans: A
Explanation: Current loop = torque loop, ensures desired torque output.
Q501. Stepper motor resonance can be reduced by?
A) Microstepping and damping
B) Increasing speed abruptly
C) Reducing pulses
D) Decreasing voltage
Ans: A
Explanation: Microstepping + mechanical damping minimizes vibration.
Q502. In servo drives, “feedforward control” improves?
A) Tracking accuracy
B) Stability margin
C) Torque ripple
D) Voltage control
Ans: A
Explanation: Feedforward anticipates motion, reducing lag error.
Q503. Stepper motor maximum speed depends on?
A) Supply voltage and winding inductance
B) Rotor weight
C) Step angle only
D) Encoder feedback
Ans: A
Explanation: Higher voltage and lower inductance increase speed range.
Q504. Servo drive braking resistor is used to?
A) Dissipate regenerative energy
B) Boost voltage
C) Stabilize encoder
D) Control current
Ans: A
Explanation: Dynamic braking resistor absorbs regen energy during deceleration.
Q505. Stepper motors are widely used in?
A) Printers and CNC machines
B) Power transformers
C) Transmission lines
D) Circuit breakers
Ans: A
Explanation: Precise incremental motion makes steppers ideal for printers, CNC, robotics.
Q506. Servo systems avoid “hunting” by?
A) Proper damping and tuning
B) Increasing inertia
C) Decreasing feedback
D) High gain only
Ans: A
Explanation: Optimal damping eliminates oscillatory hunting.
Q507. Stepper motor “full-step” drive energizes?
A) Two phases at once
B) One phase at a time
C) Randomly
D) All together
Ans: A
Explanation: Full-step mode energizes two phases for max torque.
Q508. Servo “position loop” output acts as?
A) Command to velocity loop
B) Torque feedback
C) Encoder pulse
D) Power input
Ans: A
Explanation: Position loop output → velocity command for next inner loop.
Q509. Stepper motor efficiency reduces at high speed due to?
A) Winding inductance
B) Armature reaction
C) Field weakening
D) Core loss
Ans: A
Explanation: High inductance delays current rise, reducing torque and efficiency.
Q510. Servo drives commonly use?
A) PWM inverters
B) Linear amplifiers
C) Transformers
D) Slip rings
Ans: A
Explanation: PWM inverters efficiently control AC servo motors.
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