Physics • Year 12 • Module 6: Electromagnetism • Lesson 16
AC Induction Motors and Generators
Build HSC Band 5–6 extended-response technique: analyse experimental data on motor behaviour, evaluate the role of slip, and compare induction and synchronous machines in real-world contexts.
1. Data + scenario: variable-load induction motor experiment (Band 5–6)
8 marks Band 5–6
Scenario. A student tested a 4-pole, 50 Hz induction motor by progressively increasing the mechanical load on the motor shaft. She recorded rotor speed, input current, and shaft torque at each load step. The results are summarised below.
| Load step | Rotor speed (RPM) | Input current (A) | Shaft torque (N m) |
|---|---|---|---|
| No load | 1498 | 2.1 | 0.2 |
| Light load | 1470 | 4.8 | 8.5 |
| Full rated load | 1440 | 9.6 | 18.2 |
| Overload (120%) | 1395 | 14.3 | 22.4 |
Illustrative data. Motor rated: 4-pole, 50 Hz, 230 V. Supply frequency maintained constant at 50 Hz throughout.
Q1. Analyse and evaluate the experimental data to explain how an AC induction motor responds to increasing mechanical load. In your response you must:
- Calculate the percentage slip at each load step and identify the trend.
- Explain, using Faraday’s Law and Lenz’s Law, why slip and induced rotor current increase as load increases.
- Explain why the input current drawn from the supply increases with load, with reference to energy conservation.
- Predict what would happen to the motor if it was continuously operated at 120% load. Use the data to support your prediction.
- Evaluate one limitation of this experimental design and suggest an improvement.
2. Experimental design — determining whether a motor is induction or synchronous (Band 5–6)
7 marks Band 5–6
Research question. An engineer is handed an unmarked AC motor and must determine whether it is an induction motor (with slip) or a synchronous motor (no slip, runs at exactly synchronous speed). Design an investigation using readily available equipment to settle the question.
Constraints: You have access to a variable-load brake, a tachometer (measures RPM), an AC ammeter, a 50 Hz 230 V supply, and a frequency counter. The investigation must be completed in a single laboratory session.
Q2. Design the investigation and present it in the format below.
- State a testable hypothesis that distinguishes the two motor types based on their rotor speed behaviour under load.
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps.
- State the result that would confirm an induction motor and the result that would confirm a synchronous motor.
- State two limitations of the design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Percentage slip calculations: ns = 120 × 50 / 4 = 1500 RPM.
- No load: % slip = (1500 − 1498) / 1500 × 100 = 0.13%
- Light load: % slip = (1500 − 1470) / 1500 × 100 = 2.0%
- Full load: % slip = (1500 − 1440) / 1500 × 100 = 4.0%
- Overload: % slip = (1500 − 1395) / 1500 × 100 = 7.0%
Trend: percentage slip increases with load. [1 — calculations correct with trend stated]
Faraday and Lenz explanation: As load increases, the rotor slows (greater mechanical resistance). The slower rotor falls further behind the stator field, increasing the relative motion between the stator field and the rotor bars. By Faraday’s Law, the rate of change of flux through the rotor bars increases, inducing a larger emf and therefore larger currents. By Lenz’s Law, these larger induced currents create a stronger magnetic force opposing the relative motion — this translates to greater torque on the rotor, partially compensating for the increased load. [1 — Faraday; 1 — Lenz]
Input current and energy conservation: The motor must deliver more mechanical power to overcome the increased load (Power = torque × angular velocity). By conservation of energy, more electrical power must be drawn from the supply. The input current increases because electrical power (P = V × I × cosφ) must increase; as V and frequency are constant, I must increase. The data confirm this: current rises from 2.1 A (no load) to 14.3 A (overload). [1 — links to energy conservation and power equation]
Prediction at 120% overload: At 14.3 A, the stator windings carry approximately 49% more current than at full rated load (9.6 A). Since resistive heating scales with I2, the heat dissipated is approximately (14.3/9.6)2 ≈ 2.2 times the full-load heat. Continuous operation would cause the winding insulation to overheat and degrade, ultimately causing insulation failure and a short circuit (motor burn-out). The data shows torque at 22.4 N m still increasing, meaning the motor is near its breakdown torque; beyond this, torque falls rapidly and the motor stalls. [1 — heating argument with I2R; 1 — overheating consequence stated]
Limitation and improvement: One limitation is that the experiment does not measure motor winding temperature, so it is impossible to determine how close the motor is to its thermal limit; current alone is an indirect indicator of heating. Improvement: attach a thermocouple to the stator winding and monitor temperature continuously; use a thermal camera to identify hot spots. Alternatively, repeat the test at three load levels to improve reliability of the speed and current measurements. [1 — limitation; 1 — improvement]
Marking criteria (8 marks): 1 = four correct % slip values with trend identified; 1 = Faraday’s Law applied correctly to increasing flux change; 1 = Lenz’s Law applied correctly to torque response; 1 = energy conservation argument linking mechanical output to electrical input and rising current; 1 = I2R heating argument; 1 = specific consequence (insulation failure / motor stall) linked to data; 1 = valid limitation identified; 1 = specific improvement stated.
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If the motor is an induction motor, its rotor speed will decrease measurably (by 2–7%) as mechanical load increases; if it is a synchronous motor, rotor speed will remain constant at 1500 RPM (for a 4-pole, 50 Hz motor) until the motor suddenly stalls when the pull-out torque is exceeded. [1 — testable hypothesis distinguishing both types by speed behaviour]
Variables: Independent variable: mechanical load applied to the motor shaft (varied using the brake). Dependent variable: rotor speed in RPM (measured with the tachometer). Controlled variables: supply voltage (230 V), supply frequency (50 Hz, verified with frequency counter), ambient temperature. [1 — IV, DV and at least two controlled variables]
Procedure: (1) Connect the motor to the 230 V, 50 Hz supply via the ammeter; attach the tachometer to the shaft and set the brake to zero load. (2) Record the no-load rotor speed and input current; compare the rotor speed to the calculated synchronous speed (ns = 120 × 50 / p). (3) Increase the brake load in four equal steps up to the rated load of the motor, recording rotor speed and current at each step. (4) Plot rotor speed vs load force; determine whether speed decreases continuously (induction) or remains flat then drops sharply (synchronous). [1 — four clear steps]
Results that confirm each type: Induction motor: rotor speed decreases progressively with each load step, with slip increasing from ~0% to ~5% at rated load. Synchronous motor: rotor speed remains constant at exactly synchronous speed (1500 RPM for 4-pole, 50 Hz) regardless of load, until the pull-out torque is exceeded and the motor abruptly stalls. [1 — correct distinguishing result for each type]
Limitations: (1) The number of poles of the unknown motor is not stated; without knowing p, the calculated synchronous speed is uncertain, making it harder to determine whether the rotor is running at synchronous speed. Mitigation: use the frequency counter to check supply frequency precisely and measure no-load speed to estimate p. (2) The tachometer may have limited accuracy at the small speed differences involved; a 1% error in reading could mask the slip of an induction motor or mimic it for a synchronous motor. [1 + 1 — two specific limitations]
Improvement: Use a precision optical tachometer with ±1 RPM resolution and repeat each load step three times to obtain a mean speed with uncertainty estimate, improving the reliability of the comparison. [1 — specific improvement]
Marking criteria (7 marks): 1 = testable hypothesis naming the distinguishing feature (speed vs load behaviour); 1 = IV, DV and two controlled variables correctly identified; 1 = four clear procedural steps including measurement of rotor speed at multiple loads; 1 = result confirming induction motor (progressive speed drop); 1 = result confirming synchronous motor (constant speed to stall point); 1 = first valid limitation; 1 = second valid limitation OR improvement.