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Quizzes > Electronics & Gadgets

The rotor turns because of magnetic fields: motor quiz

Quick, free quiz to test your motor physics, from stator pole attraction to torque. Instant results.

Editorial: Review CompletedCreated By: Brett HallenUpdated Aug 27, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art motor control quiz showing rotor stator magnetic field and capacitor icons on golden yellow background

This quiz helps you check your understanding of why the rotor turns because of magnetic fields and core motor control points. For a quick refresher on how machines convert energy, see our generator energy transformation quiz. You can also review forces and motion quiz or explore field line directions to visualize stator and rotor interaction.

Which force causes the rotor to turn in an electric motor?
Gravitational force
Lorentz force
Frictional force
Centrifugal force
The Lorentz force arises from the interaction of current and magnetic field, producing torque on the rotor. This is the fundamental mechanism in most electric motors. It is described by F = I × B × l in conductor segments.
What law describes how a current-carrying conductor in a magnetic field experiences a force?
Ohm's Law
Coulomb's Law
Lorentz Law
Faraday's Law
The Lorentz law (often called Lorentz force law) explains the force on a current-carrying conductor in a magnetic field. It quantifies how moving charges are deflected by magnetic fields. This principle underpins torque production in motors.
In a simple DC motor, the commutator serves to:
Supply field excitation
Reduce friction
Reverse current direction in the rotor windings
Increase voltage
The commutator periodically reverses current in the rotor coils, ensuring continuous torque in one direction. Without reversal, the torque would alternate and stall the rotor. It is essential to DC motor operation.
What creates the rotating magnetic field in an AC induction motor stator?
Two-phase winding
Three-phase winding
Permanent magnets
A single-phase winding
A three-phase stator winding produces a rotating magnetic field when supplied by balanced three-phase currents. This rotating field induces currents in the rotor, generating torque. Three phases are standard for smooth rotation.
The term 'slip' in an induction motor refers to:
Stator winding resistance
Difference between input and output current
Difference between synchronous and rotor speed
Rotor frictional loss
Slip is defined as (Ns - N) / Ns, where Ns is synchronous speed and N is rotor speed. It measures how much the rotor lags behind the rotating field. Slip is essential for torque production.
Which component is used in a capacitor-start single-phase motor to provide phase shift?
Diode
Resistor
Inductor
Capacitor
A start capacitor creates a phase-shifted current in the auxiliary winding to produce a rotating field. This delivers starting torque in single-phase motors. The capacitor is switched out once running.
In a DC motor, reversing the armature current reverses the:
Direction of rotation
Magnetic field strength
Stator winding configuration
Voltage applied
Reversing armature current changes the direction of the Lorentz force, flipping the torque direction. This reverses the rotor's rotation. It is the basic method for DC motor control.
What is the synchronous speed of a 4-pole motor on 60 Hz power?
3600 rpm
900 rpm
1200 rpm
1800 rpm
Synchronous speed Ns is given by 120 × f / P. For f=60 Hz and P=4, Ns = (120×60)/4 = 1800 rpm. This is the speed of the rotating field.
Induced currents in an induction motor rotor are caused by:
Capacitor phase shift
Direct DC excitation
Rotating air-gap magnetic field
Mechanical gearing
The stator's rotating magnetic field cuts the rotor conductors, inducing currents by Faraday's law. These currents interact with the field to produce torque. No brushes or DC exciters are needed.
Which law explains the direction of induced EMF opposing the change in flux?
Coulomb's Law
Faraday's Law
Lenz's Law
Ampère's Law
Lenz's Law states that induced EMF always opposes the change in magnetic flux that produces it. This opposition is fundamental to induction in motor rotors. It ensures energy conservation.
The split-phase motor derives its starting torque from:
DC shunt winding
Shaded pole winding
Permanent magnets
Two stator windings with phase shift
A split-phase motor uses a start winding and main winding displaced by about 90° electrical. The phase-shifted currents produce a weak rotating field. This yields starting torque.
What determines the direction of rotation in a three-phase induction motor?
Rotor bar shape
Phase sequence of supply
Capacitor size
Stator core material
Changing the order of the three-phase supply sequence reverses the rotating magnetic field. This, in turn, reverses rotor rotation. Phase sequence is the simplest method.
The main purpose of a centrifugal switch in capacitor-start motors is to:
Engage the start winding at all speeds
Control rotor slip
Regulate line voltage
Disconnect the start winding above a set speed
A centrifugal switch disconnects the start winding (and start capacitor) when the rotor reaches about 70 - 80% of full speed. This prevents overheating and reduces losses. It ensures efficient running.
Torque in an electric motor is proportional to the product of:
Current and resistance
Speed and power
Magnetic flux and current
Voltage and frequency
Motor torque T ? ? × I, where ? is magnetic flux and I is armature current (or rotor current). Stronger field or higher current yields more torque. This relation holds for many motor types.
What is the role of the stator in an induction motor?
Supply DC to the rotor
Reduce bearing friction
Create the rotating magnetic field
Provide mechanical support only
The stator winding, when energized by AC, produces a rotating magnetic field in the air gap. This field induces currents in the rotor, creating torque. The stator also houses the core and frame.
A shaded-pole motor achieves starting torque by using:
Electronic inverter
Shaded copper rings on poles
Permanent magnets
Auxiliary capacitor
Shaded-pole motors have copper bands (shades) around part of each pole face. These create a delayed magnetic flux in the shaded portion, producing a weak rotating field. This yields starting torque.
What is the effect of increasing rotor resistance in a wound-rotor induction motor?
Increases synchronous speed
Shifts maximum torque toward lower speeds
Lowers starting torque
Reduces slip
Adding external resistance to the rotor circuit increases the slip at which maximum torque occurs (pull-up torque). This moves the torque-speed curve so the motor can start under heavier load. It also reduces starting current.
In a capacitor-run induction motor, the run capacitor:
Replaces the main winding
Is only in circuit during startup
Remains in circuit during normal operation
Acts only to reduce harmonics
Run capacitors are connected in series with the auxiliary winding continuously. They improve phase displacement, torque, and power factor across the speed range. They are sized smaller than start capacitors for continuous duty.
Which phenomenon causes a cage rotor to act as a single-turn secondary winding?
Electrostatic coupling
Eddy-current damping
Skin effect
Faraday induction
The squirrel-cage rotor consists of bars shorted at ends, forming a single-turn conductor. The rotating stator field induces currents in these bars by Faraday's law. This induction produces torque.
The Kloss equation is used to estimate:
Synchronous motor efficiency
Capacitor start current
Induction motor torque-slip characteristic
DC motor armature current
Kloss's equation approximates the torque-slip curve of an induction motor. It expresses normalized torque as a function of slip, showing a peak at pull-out torque. It is useful for quick performance estimates.
In vector control of AC motors, the d-q axes represent:
Dynamic and quasi-static currents
Differential and quasi currents
Direct and quadrature flux components
Delayed and quick flux terms
The d (direct) axis aligns with the rotor flux, while the q (quadrature) axis is orthogonal. Vector control decouples torque and flux control by separately regulating Id and Iq. This method achieves fast dynamic response.
What is 'cogging torque' in permanent magnet machines?
Frictional torque in bearings
Torque ripple due to current harmonics
Load torque fluctuations
Detent torque from magnet - tooth attraction
Cogging torque arises from the interaction between permanent magnets and stator slots when no current flows. It causes torque pulsations at low speed. Designers mitigate it via skewing or slot/pole combinations.
In a permanent magnet synchronous motor (PMSM), field weakening is used to:
Increase low-speed torque
Balance stator currents
Extend maximum speed beyond base speed
Eliminate rotor currents
Field weakening reduces the net air-gap flux at high speeds by injecting opposing current. This allows operation above base speed at the cost of reduced torque. It extends the constant power region.
The pull-out torque of an induction motor is the:
Torque at standstill
Maximum torque before stalling
Torque at synchronous speed
Average torque during start
Pull-out torque (or breakdown torque) is the peak torque the motor can develop before losing synchronism. Beyond this, the rotor cannot follow the rotating field. It defines the motor's load capacity.
Which parameter primarily affects the starting current of an induction motor?
Rotor inertia
Rotor impedance
Stator resistance
Air-gap flux density
High rotor impedance (especially resistance) reduces starting current because it limits induced current. Low impedance allows large currents at standstill. Designers sometimes add resistance for better start performance.
A brushless DC (BLDC) motor uses electronic commutation to:
Replace mechanical brushes and commutator
Increase rotor inertia
Suppress back-EMF
Enhance frictional forces
Electronic controllers switch the stator phases in synchronism with rotor position sensors. This replaces brushes and commutators for wear-free operation. BLDC motors achieve high reliability and efficiency.
The Thevenin equivalent of an induction motor's stator can help calculate:
Rotor temperature rise
Torque - slip curve
Stator current harmonic distortion
Starting torque
Replacing the stator circuit with its Thevenin equivalent simplifies rotor circuit analysis. It allows calculation of starting torque and currents using reduced parameters. This method is standard in induction motor theory.
Which quantity remains nearly constant in a shunt-wound DC motor under varying load?
Armature current
Field flux
Speed
Torque
In a shunt DC motor, the field winding is connected in parallel with the armature. The field current and thus flux remain almost constant under load variations. This gives good speed regulation.
Core losses in an AC motor are due to:
Air resistance
Mechanical friction
Eddy currents and hysteresis
Copper resistance heating
Core (iron) losses include hysteresis loss from magnetizing cycles and eddy-current loss induced in laminations. They depend on flux density and frequency. Laminated cores reduce eddy losses.
The Kloss equation for induction motors is derived under the assumption of:
Harmonic-free supply
Saturated rotor flux
Permanent magnet rotor
Linear magnetic circuit
Kloss's equation assumes linearity in the magnetic circuit and neglects saturation. It also presumes constant air-gap flux and neglects stator resistance. These simplifications enable a closed-form torque-slip relation.
In deep-bar rotors, the skin effect during start:
Reduces effective resistance
Reverses torque direction
Increases effective resistance
Eliminates rotor currents
At high frequencies (as seen at startup), induced currents concentrate near the bar surface (skin effect). This effectively increases rotor resistance, boosting starting torque. At running speed slip and frequency drop, reducing resistance.
Space vector PWM is advantageous because it:
Eliminates need for current sensors
Requires fewer gate pulses
Reduces switching harmonics and optimizes DC bus utilization
Simplifies hardware implementation
Space vector PWM uses the inverter's switching states to approximate the reference voltage vector. It yields lower harmonic distortion and better DC bus usage. It is widely used in high-performance drives.
In reluctance motors, torque is produced by:
Alignment tendency to minimum reluctance path
Eddy-current damping
Permanent magnet attraction
Interaction of currents and magnets
Reluctance motors produce torque as the rotor tries to align with the stator's rotating field at positions of minimum magnetic reluctance. No windings or magnets are on the rotor. They rely on anisotropic rotor geometry.
Field-weakening control in a PMSM requires controlling which current component?
Direct-axis current (Id)
Negative-sequence current
Quadrature-axis current (Iq)
Zero-sequence current
Injecting negative Id current opposes the rotor flux, weakening the field. This allows operation above base speed at constant power. Iq controls torque independently.
The air-gap torque in an induction motor can be calculated from:
Rotor copper losses and slip
Stator core losses
Mechanical output power only
Frictional losses
Air-gap torque equals the air-gap power (input minus stator losses) divided by synchronous speed. It can also be derived from rotor copper losses divided by slip. This gives electromagnetic torque before mechanical losses.
In an induction generator, to self-excite, capacitors must supply:
High-frequency signals
Reactive power to magnetize the machine
DC excitation to the rotor
Real power to the stator
To operate as a self-excited induction generator, capacitors provide reactive power to establish the air-gap flux. Without an external grid, this reactive supply is critical to build voltage. Proper sizing ensures voltage regulation.
Magnetic saturation of the stator core leads to:
Increase in power factor
Linear increase in flux with current
Lower core losses
Reduced incremental inductance and higher magnetizing current
When the core saturates, additional magnetizing current yields less flux increase, reducing incremental inductance. This causes a rise in no-load current and losses. Saturation affects performance and heating.
Harmonic currents in the stator winding produce:
Higher efficiency
Reduced core losses
Uniform torque
Torque pulsations and additional losses
Harmonic currents generate rotating fields of different speeds, causing torque ripple (pulsations). They also increase iron and copper losses. Designers use winding distribution and filters to minimize harmonics.
The negative-sequence component in a three-phase supply causes:
Higher torque
Improved power factor
Reverse rotating field and heating
Forward rotating field
Negative-sequence currents produce a rotating field opposite to the main field. This backward field induces currents in the rotor, causing additional losses and heating. It is a key concern in unbalanced supplies.
Finite element analysis (FEA) in motor design helps to:
Accurately predict flux distribution and losses
Quickly estimate efficiency without modeling
Eliminate need for prototyping
Reduce magnetic saturation
FEA solves Maxwell's equations over the motor geometry, giving precise flux, torque, and loss predictions. It helps optimize core shapes, slot designs, and cooling. While it reduces prototypes, it does not eliminate them entirely.
d-q axis theory transforms three-phase currents into:
Three harmonic components
Rotor flux vectors
Single-phase equivalent only
Two orthogonal DC quantities
The d-q transform converts three-phase AC currents into two steady DC components in the rotor reference frame. This simplifies control of AC machines as in DC motors. It is the foundation of field-oriented control.
Maximum torque per ampere control in PMSM aims to:
Operate at base speed only
Maximize flux
Eliminate field-weakening
Minimize copper losses for given torque
MTPA control sets Id and Iq to achieve a given torque with minimum current magnitude, reducing copper losses. It operates below base speed in PMSM drives. It improves efficiency under light loads.
Homopolar (zero-sequence) flux in an AC machine arises when:
Phase currents are balanced
Rotor is saturated
Field current is zero
Sum of phase currents is non-zero
Zero-sequence flux appears when the algebraic sum of three-phase currents is not zero. It cannot cross the air-gap in a symmetrical machine and typically circulates in the stator or winding return. It causes unwanted losses.
Pull-in capability of an AC motor indicates:
Ability to start on load
Maximum torque at synchronous speed
Power factor at no load
Minimum slip under load
Pull-in capability is the motor's ability to accelerate from standstill to synchronous speed under load. It depends on the torque-speed characteristic and load inertia. It is critical for synchronous and reluctance machines.
The Maximum Torque Coefficient of an induction machine is:
Equal to unity at pull-out
Directly proportional to supply frequency
Independent of slip
Inverse function of rotor resistance ratio
The maximum torque coefficient kTmax = 2/(?s/Lr*Rr/Rth)^0.5, showing inverse dependence on rotor resistance ratio. It is derived from the torque equation. Designers use it to predict pull-out torque.
In permanent magnet machines, high air-gap MMF harmonics cause:
Better torque smoothness
Improved power factor
Additional torque ripple and losses
Lower magnet demagnetization risk
Harmonic MMF components interact with stator currents to produce undesirable torque pulsations. They also induce eddy currents in rotor magnets and core, increasing losses. Designers try to minimize harmonics via winding and magnetization shaping.
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Study Outcomes

  1. Understand why the rotor turns because of magnetic fields -

    Explain the interaction between stator and rotor magnetic fields to see exactly why the rotor turns because of magnetic forces.

  2. Explain split-phase motor operation -

    Describe how split-phase motor operation uses main and auxiliary windings to create a phase shift for smooth startup torque.

  3. Analyze capacitor motor functions -

    Examine how capacitors are always connected to the start winding to boost starting torque and improve overall motor performance.

  4. Recall voltage requirements in magnetic control systems -

    Identify typical volt levels in magnetic control systems generally operate on ____ volts, and understand their role in relay and contactor circuits.

  5. Differentiate between capacitor-start and permanent-split capacitor motors -

    Compare the operational characteristics, torque profiles, and application scenarios of capacitor-start versus permanent-split capacitor motors.

  6. Apply motor control systems knowledge through a scored quiz -

    Use the motor control systems quiz to assess your grasp of key concepts and pinpoint strengths and areas for further study.

Cheat Sheet

  1. Rotating Magnetic Field Principle -

    The rotor turns because of the rotating magnetic field created by two or more stator windings energized out of phase - typically 90° apart in a capacitor motor. This rotating field induces currents in the squirrel-cage rotor, producing torque via the motor effect, following Fleming's left-hand rule mnemonic: Force = B·I·L.

  2. Split-Phase Motor Operation -

    Split-phase motor operation relies on a start winding with higher resistance and a run winding to create a phase shift, producing the initial rotating field. A centrifugal switch then disconnects the start winding once the motor reaches about 70 - 80% of full speed, ensuring efficient run performance.

  3. Capacitor Motor Functions -

    Capacitor motor functions are all about phase shift and torque boost; a start or run capacitor connected to the start winding shifts current by up to 90°, enhancing starting torque. Remember "capacitors are always connected to the start winding" to recall proper wiring for peak performance.

  4. Control Voltage Standards -

    Magnetic control systems generally operate on 24 volts for safety in commercial controls or 120 volts in industrial settings, with 480 V often used for large motors. Knowing these standard values ensures you select the correct coil and troubleshoot confidently.

  5. Torque Equation & Mnemonic -

    Review the torque equation T = k·Φ·I to link magnetic flux (Φ) and armature current (I) to produced torque, where k is a machine constant. Use the mnemonic "Flux In, Torque Win" to remember that more flux and current equal greater torque - perfect prep for your motor control systems quiz.

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