Question 1

What is the synchronous speed of a 4-pole motor operating on a 60 Hz supply?

Accepted Answer

1800 RPM. Synchronous speed = (120 × frequency) / number of poles = (120 × 60) / 4 = 1800 RPM.

Question 2

What is 'slip' in an AC induction motor?

Accepted Answer

Slip is the difference between synchronous speed and actual rotor speed, expressed as a percentage. It is necessary for torque production in induction motors.

Question 3

What does the service factor (SF) on a motor nameplate indicate?

Accepted Answer

Service factor indicates the permissible overload multiplier a motor can handle continuously without damage under specified conditions. For example, an SF of 1.15 means the motor can run at 115% of its rated load.

Question 4

What is the main difference between a squirrel-cage induction motor and a wound-rotor induction motor?

Accepted Answer

A squirrel-cage rotor has cast aluminum or copper bars shorted by end rings, while a wound-rotor has actual windings brought out through slip rings, allowing external resistance to be added for speed and torque control.

Question 5

How does reversing a three-phase motor's direction of rotation differ from reversing a single-phase motor?

Accepted Answer

A three-phase motor is reversed by swapping any two of the three supply leads. A single-phase motor typically requires reversing the leads of either the start or run winding.

Motors & Controls – Electrician Journeyman Exam

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