5.1 Define the term synchronous speed of the motor - NSC Electrical Technology Power Systems - Question 5 - 2019 - Paper 1

The continuity test is conducted to ensure:

• There is a complete path for electric current to flow,
• There is continuity between the ends of each coil,
• There is no break in the electrical connections.

The insulation resistance test is conducted to ensure that there is no electrical connection between:

• Each of the windings and earth,
• Each of the windings to ensure they are insulated properly to avoid short circuits.

In a squirrel-cage induction motor, when a three-phase supply is connected to the stator winding, a rotating magnetic field is produced. This rotating field induces an electromotive force (e.m.f) in the rotor.

The induced current in the rotor creates its own magnetic field, which interacts with the stator's rotating field, producing a force that causes the rotor to turn in the same direction as the magnetic field. The difference in speed between the rotating magnetic field and the rotor is known as slip.

The interlocking contacts identified are MC3 (N/C) and MC2 (N/C), which prevent both contactors from being energized simultaneously.

MC1 (N/O1) is connected in parallel with the start button to latch and retain the circuit once the start button is pressed. It ensures that when the start button is released, the circuit remains closed, keeping the motor energized.

When the start button is pressed, MC1 will be energized, closing MC1 (N/O1) and MC1 (N/O2). This action also energizes MC2 (N/C), allowing the motor to run in star configuration, which reduces the starting current. After a preset time, the timer will open T(N/C) and close T(N/O), de-energizing MC2 and energizing MC3, allowing the motor to continue running in delta mode for normal operation.

The synchronous speed can be calculated as follows: N_s = rac{60 imes f}{p} = rac{60 imes 50}{18} = 166.67 ext{ r/min}

The percentage slip can be calculated using the formula: ext{Slip} = rac{N_s - N_{r}}{N_s} imes 100 where:

• $N_{r}$ = rotor speed.

Substituting the values gives: ext{Slip} = rac{166.67 - 955}{166.67} imes 100 = 4.5 ext{ \\%}