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:

1. Earth continuity: This ensures safety by verifying that there is a low-resistance path to earth, preventing electric shock.
2. Continuity between the ends of each coil: This ensures that the coils are properly connected, allowing the motor to function effectively.

The insulation resistance test ensures that there is no electrical connection between:

1. Each of the three coils.
2. The coils and earth. This test is crucial to prevent short circuits and ensure the reliability and safety of the motor operation.

The squirrel-cage induction motor operates on the principle of electromagnetic induction. When a three-phase supply is connected to the stator, a rotating magnetic field is produced. This field induces an electromotive force (e.m.f) in the rotor, which is typically constructed of aluminum or copper bars (the 'squirrel cage'). The induced current in the rotor creates its own magnetic field.

The interaction between the magnetic fields of the stator and rotor produces a force that causes the rotor to rotate in the direction of the rotating magnetic field. The motor continues to run as long as there is a supply of electricity, efficiently converting electrical energy into mechanical energy.

The interlocking contacts are identified as MC3 (N/C) and MC2 (N/C). These contacts ensure that both the star and delta configurations of the motor cannot be energized at the same time, thus preventing damage to the motor.

MC1 (N/O1) is connected in parallel with the start button so that when the start button is pressed and released, MC1 will remain energized. This creates a latch effect which keeps the circuit closed, allowing the motor to continue running even after the start button is released.

The star-delta control circuit is used to reduce the starting current of a motor. When the start button is pressed, MC1 is energized, closing contacts MC1 (N/O1) and MC1 (N/O2).

Simultaneously, MC2 (N/C) will open, preventing MC3 from being energized, and thus the motor will start in the star configuration. After a predetermined time, the timer will energize T (N/O), which will close T (N/O) and de-energize MC2, allowing the motor to transition to the delta configuration. MC3 opens and the motor runs in delta, thus ensuring smooth operation and reduced inrush current.

To calculate the synchronous speed ($N_s$): Given:

• 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} Therefore, the synchronous speed is approximately 333.33 r/min.

To calculate the percentage slip,

• Rotor speed ($N_r$) = 955 r/min
• Synchronous speed ($N_s$) = 333.33 r/min

The formula for percentage slip is: ext{Slip} = rac{N_s - N_r}{N_s} imes 100 Substituting the values: ext{Slip} = rac{333.33 - 955}{333.33} imes 100 = 4.5 ext{\\%} Thus, the percentage slip is 4.5%.