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 that there is an uninterrupted electrical path in the circuit. It verifies that:

• There is continuity between the ends of each coil.
• There is continuity between the three coils and earth.

The insulation resistance test aims to ensure there is no electrical connection (short circuit) between:

• Each of the three coils.
• The three coils and earth. This test is critical for safe motor operation.

In a squirrel-cage induction motor, a three-phase supply is connected to the stator windings, creating a rotating magnetic field. This field induces an electromotive force (e.m.f.) in the rotor, producing a current that generates its own magnetic field. The interaction between the stator and rotor magnetic fields causes rotational movement. The rotor aims to catch up with the rotating field, resulting in a speed difference known as slip.

The interlocking contacts are MC3 (N/C) and MC2 (N/C) which prevent simultaneous operation of both contactors, ensuring safe operation of the motor.

MC1 (N/O1) is connected in parallel with the start button to keep the circuit closed after the start button is released. This latches the circuit, ensuring the motor remains energized even after the initial start signal.

In the star-delta control circuit, when the start button is pressed, MC1 is energized, closing the N/O contacts and opening the N/C contacts, allowing the motor to operate in star configuration. After a preset time, the timer activates, closing T (N/O) and subsequently switching to delta configuration by energizing MC3 and opening MC2. This allows the motor to operate efficiently once it reaches a certain speed.

To calculate the synchronous speed: Given:

• Total number of poles ($p$) = 18
• Frequency ($f$) = 50 Hz Using the formula: N_s = rac{120 imes f}{p} = rac{120 imes 50}{18} = 333.33 ext{ r/min}

The percentage slip can be calculated using: ext{Slip} = rac{N_s - N_r}{N_s} imes 100 Where:

• $N_r$ = rotor speed = 955 r/min. Thus, ext{Slip} = rac{333.33 - 955}{333.33} imes 100 = 4.5 ext{ %}