Electrician Red Seal · Question
A technician is troubleshooting a 480 V, 3-phase, 60 Hz, 15 kW (20 hp) induction motor that fails to start. A voltmeter across the motor terminals while attempting to start shows balanced phase voltages of 480 V. An ammeter shows locked rotor current (LRC) is being drawn, but the motor does not turn. Which of the following is the most likely cause?
If locked rotor current (LRC) is drawn, it indicates that current is flowing through all windings, ruling out an open circuit. If the motor is drawing LRC but n
Start free practice for Electrician Red Seal
150 questions · no signup required · 40 free questions per day
Question: A technician is troubleshooting a 480 V, 3-phase, 60 Hz, 15 kW (20 hp) induction motor that fails to start. A voltmeter across the motor terminals while attempting to start shows balanced phase voltages of 480 V. An ammeter shows locked rotor current (LRC) is being drawn, but the motor does not turn. Which of the following is the most likely cause?
Answer options:
- A winding short circuit within the motor.
- An open circuit in one of the motor phases. ✅ Overload on the motor shaft preventing rotation.
- Incorrect phase sequence of the supply voltage.
Correct answer: Overload on the motor shaft preventing rotation.
Explanation: If locked rotor current (LRC) is drawn, it indicates that current is flowing through all windings, ruling out an open circuit. If the motor is drawing LRC but not turning, it suggests a mechanical issue preventing rotation, such as a seized bearing or an excessive load. A winding short circuit would likely result in even higher, unbalanced currents and rapid tripping.
Start free practice for Electrician Red Seal
150 questions · no signup required · 40 free questions per day
More about Electrician Red Seal
Related Questions
- A residential garage is to be wired with a 15 A, 120 V duplex receptacle. According to the Canadian Electrical
- During a renovation of a commercial kitchen, an electrician needs to replace an old single-phase, 240 V, 5 kW
- A commercial kitchen is being renovated, and several new 4 kW, 208 V, single-phase electric ovens are being in
- What is the minimum working space depth required in front of an electrical panel rated 200 A, 120/240 V, locat
- A Red Seal electrician is setting up temporary power for a construction site. The main temporary service is a
- A new industrial facility requires a 600 V, three-phase, 150 A feeder to supply a motor control centre. The co
More for Electrician Red Seal candidates
Practice tests
Question explanations
- A residential garage is to be wired with a 15 A, 120 V duplex receptacle. According to the Canadian Electrical
- During a renovation of a commercial kitchen, an electrician needs to replace an old single-phase, 240 V, 5 kW
- A commercial kitchen is being renovated, and several new 4 kW, 208 V, single-phase electric ovens are being in
- What is the minimum working space depth required in front of an electrical panel rated 200 A, 120/240 V, locat
Ready to practice?
Free, no signup required. Build a wrong-question list as you go.
Start Free Electrician Red Seal Practice →Related courses
Other Canadian certifications candidates often prepare for alongside this one.