Physics • Year 12 • Module 6: Electromagnetism • Lesson 16
AC Induction Motors and Generators
Lock in the key vocabulary, the three-stage induction motor mechanism, and the synchronous speed formula before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: squirrel cage rotor, stator, slip speed, synchronous speed, three-phase AC, slip, turbine generator, Faraday’s Law, Lenz’s Law, rotating magnetic field. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The stationary part of an AC motor containing the electromagnet windings that are connected to the AC supply. | |
| 1.2 | A rotor made of conducting bars connected at both ends by rings, with no external electrical connections, resembling a cage. | |
| 1.3 | The speed of the rotating magnetic field produced by the stator; calculated using ns = 120f/p. | |
| 1.4 | The difference between the synchronous speed and the actual rotor speed. | |
| 1.5 | The percentage by which the rotor speed falls below synchronous speed; typically 2–5% in induction motors. | |
| 1.6 | Three alternating currents separated by 120° in phase, used to create a smooth rotating magnetic field in the stator. | |
| 1.7 | A magnetic field pattern that appears to rotate around the inside of a motor stator when three-phase AC flows through the windings. | |
| 1.8 | The law stating that an emf is induced in a conductor when the magnetic flux through it changes. | |
| 1.9 | The law stating that an induced current opposes the change in flux that caused it. | |
| 1.10 | A generator driven by a steam, gas, or water turbine to produce large-scale electrical power for the grid. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 In an AC induction motor, the rotor is connected to the power supply by brushes and slip rings. T / F
2.2 The rotor of an induction motor turns due to currents induced by the rotating stator field. T / F
2.3 If the rotor reached synchronous speed, torque production would increase to its maximum value. T / F
2.4 A 4-pole motor running on 50 Hz mains supply has a synchronous speed of 1500 RPM. T / F
2.5 In a power station generator, the stator (field winding) rotates while the rotor remains stationary to produce the output current. T / F
2.6 An induction motor requires no brushes or commutator, which makes it more reliable for continuous industrial operation than a DC motor. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
Faraday · flux · induced · Lenz · rotating · slip · stator · synchronous
When three-phase AC flows through the ___________ windings of an induction motor, a ___________ magnetic field is produced inside the motor casing. According to ___________’s Law, this changing magnetic ___________ induces an emf in the conducting bars of the squirrel cage rotor. By ___________’s Law, the resulting ___________ currents in the rotor create a magnetic field that opposes the relative motion, pushing the rotor in the direction of the stator field. The rotor always turns slightly slower than the ___________ speed of the stator field — this difference is called ___________.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 Why is slip necessary for an induction motor to produce torque?
4.2 State the formula for synchronous speed and identify what each symbol represents.
4.3 In a power station generator, what is the function of the rotor and what energy conversion takes place?
4.4 State one advantage of an AC induction motor over a DC motor for long-running industrial use.
5. Label the induction motor stages
The four boxes below describe the stages of induction motor operation out of order. Number them 1–4 in the correct sequence and write the physics law (Faraday or Lenz) that applies to each stage where relevant. 6 marks
| Order (1–4) | Stage description | Law applied |
|---|---|---|
| The interaction between the rotor magnetic field and the stator field produces a net torque, turning the rotor. | ||
| Three-phase AC flows through the stator windings, creating a rotating magnetic field. | ||
| The induced currents in the rotor bars create their own magnetic field that opposes the relative motion. | ||
| The rotating stator field sweeps past the rotor bars, inducing an emf and driving currents through the squirrel cage. |
Q1 — Term–definition match
1.1 stator • 1.2 squirrel cage rotor • 1.3 synchronous speed • 1.4 slip speed • 1.5 slip • 1.6 three-phase AC • 1.7 rotating magnetic field • 1.8 Faraday’s Law • 1.9 Lenz’s Law • 1.10 turbine generator.
Q2 — True / false with correction
2.1 False. In an AC induction motor there are no brushes or slip rings; the rotor has no electrical connection to the supply. Currents are induced electromagnetically.
2.2 True.
2.3 False. At synchronous speed, there is no relative motion between the rotor and the stator field, so no flux change occurs in the rotor, no currents are induced, and torque drops to zero.
2.4 True. ns = 120 × 50 / 4 = 1500 RPM.
2.5 False. In a power station generator, the rotor (the electromagnet, excited by DC) rotates inside the stationary stator, whose windings carry the induced output current.
2.6 True.
Q3 — Cloze paragraph
In order: stator / rotating / Faraday / flux / Lenz / induced / synchronous / slip.
Q4.1 — Why slip is necessary
Slip is necessary because the rotor must turn slower than the stator field to maintain a relative motion between the two. Without relative motion, there is no changing magnetic flux in the rotor bars, so no emf is induced (Faraday’s Law), no currents flow, and no torque is produced.
Q4.2 — Synchronous speed formula
ns = 120f / p, where ns is synchronous speed (RPM), f is supply frequency (Hz), and p is the number of magnetic poles in the stator.
Q4.3 — Generator rotor function and energy conversion
In a power station generator, the rotor is an electromagnet energised by a DC excitation current. As the turbine spins the rotor inside the stator windings, the changing magnetic flux through the stator induces a large alternating emf. The energy conversion is from mechanical (kinetic) energy to electrical energy.
Q4.4 — Advantage of induction motor over DC motor
An AC induction motor has no brushes or commutator. This eliminates a major source of mechanical wear and electrical arcing, making the motor more reliable, lower in maintenance requirements, and suitable for continuous long-running industrial applications such as pumps, fans, and conveyor drives.
Q5 — Induction motor stage sequence
Correct order (top to bottom of original table):
- 4 — Torque produced. No specific single law label (result of the two-field interaction).
- 1 — Three-phase AC creates rotating field. No induction law (this is the input stage).
- 3 — Rotor field opposes relative motion. Lenz’s Law.
- 2 — Rotating field induces emf and current in rotor bars. Faraday’s Law.
Marking note: Award 1 mark per row for a correct order number AND a correct law where applicable (or blank/“N/A” for the stages where no induction law applies).