Science & STEM

The rotor turns because of magnetic fields: motor quiz

Moderate2-5mins

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.

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1Which force causes the rotor to turn in an electric motor?
2What law describes how a current-carrying conductor in a magnetic field experiences a force?
3In a simple DC motor, the commutator serves to:
4What creates the rotating magnetic field in an AC induction motor stator?
5The term 'slip' in an induction motor refers to:
6Which component is used in a capacitor-start single-phase motor to provide phase shift?
7In a DC motor, reversing the armature current reverses the:
8What is the synchronous speed of a 4-pole motor on 60 Hz power?
9Induced currents in an induction motor rotor are caused by:
10Which law explains the direction of induced EMF opposing the change in flux?
11The split-phase motor derives its starting torque from:
12What determines the direction of rotation in a three-phase induction motor?
13The main purpose of a centrifugal switch in capacitor-start motors is to:
14Torque in an electric motor is proportional to the product of:
15What is the role of the stator in an induction motor?
16A shaded-pole motor achieves starting torque by using:
17What is the effect of increasing rotor resistance in a wound-rotor induction motor?
18In a capacitor-run induction motor, the run capacitor:
19Which phenomenon causes a cage rotor to act as a single-turn secondary winding?
20The Kloss equation is used to estimate:
21In vector control of AC motors, the d-q axes represent:
22What is 'cogging torque' in permanent magnet machines?
23In a permanent magnet synchronous motor (PMSM), field weakening is used to:
24The pull-out torque of an induction motor is the:
25Which parameter primarily affects the starting current of an induction motor?
26A brushless DC (BLDC) motor uses electronic commutation to:
27The Thevenin equivalent of an induction motor's stator can help calculate:
28Which quantity remains nearly constant in a shunt-wound DC motor under varying load?
29Core losses in an AC motor are due to:
30The Kloss equation for induction motors is derived under the assumption of:
31In deep-bar rotors, the skin effect during start:
32Space vector PWM is advantageous because it:
33In reluctance motors, torque is produced by:
34Field-weakening control in a PMSM requires controlling which current component?
35The air-gap torque in an induction motor can be calculated from:
36In an induction generator, to self-excite, capacitors must supply:
37Magnetic saturation of the stator core leads to:
38Harmonic currents in the stator winding produce:
39The negative-sequence component in a three-phase supply causes:
40Finite element analysis (FEA) in motor design helps to:
41d-q axis theory transforms three-phase currents into:
42Maximum torque per ampere control in PMSM aims to:
43Homopolar (zero-sequence) flux in an AC machine arises when:
44Pull-in capability of an AC motor indicates:
45The Maximum Torque Coefficient of an induction machine is:
46In permanent magnet machines, high air-gap MMF harmonics cause:
Learning Goals

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.

Study Guide

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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Updated Feb 21, 2026