Three-Phase Motors & Starters is one of 9 topics in the Grade 12 Electrical Technology CAPS syllabus, covered across 3 lessons on Study Vault. Below are real practice questions from this topic with full worked answers.
Sample practice questions
Easy
1. What produces the rotating magnetic field in a three-phase induction motor?
The three-phase stator windings, spaced 120 degrees apart
The squirrel cage rotor bars alone
An external DC supply to the rotor
Slip rings and brushes on the rotor
Answer: The three-phase stator windings, spaced 120 degrees apart — The three stator windings, spaced 120 degrees apart and supplied with three-phase AC, produce the rotating magnetic field that sweeps around the inside of the stator.
Easy
2. Does the squirrel cage rotor have any external electrical connection?
No, the rotor has no external electrical connection at all
Yes, it connects via brushes and slip rings
Yes, it connects directly to the three-phase supply
Yes, it connects via a separate DC excitation supply
Answer: No, the rotor has no external electrical connection at all — A squirrel cage rotor has no external electrical connection; its bars are short-circuited by end rings, and current is induced in them purely by electromagnetic induction from the stator field.
Medium
3. Name the principle by which the rotating stator field induces a voltage (and therefore current) in the short-circuited rotor bars.
Answer: electromagnetic induction — Electromagnetic induction is the principle by which the rotating stator field induces a voltage, and therefore a current, in the short-circuited squirrel cage rotor bars.
