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. There is an uninterrupted electrical connection between the terminals.
2. There is continuity between the ends of each coil.

The insulation resistance test is conducted to ensure:

1. There is no electrical connection between the coils and earth.
2. There is no path for current to escape that may lead to faults.

When a three-phase supply is connected to the stator winding, a rotating magnetic field is produced. This flux induces an electromotive force (e.m.f) in the rotor, causing current to flow through it and generating a magnetic field. The interaction between the stator's rotating field and the rotor's generated field creates forces that make the rotor turn, allowing the motor to operate.

The interlocking contacts are MC3 (NC) and MC2 (NC). These ensure that MC1 operates correctly without simultaneous energization.

MC1 (N/O1) is connected in parallel with the start button so that when the start button is pressed, the circuit remains closed even after release, thus maintaining the operation of the motor.

When the start button is pressed, MC1 will be energized, closing MC1 (N/O1) and MC1 (N/O2). This energizes MC2 (N/C) and starts the motor in star mode, winding coils together. After a preset time, timer T will open T (N/O), de-energizing MC2 and closing MC3. The motor runs in delta configuration, enhancing its operational capacity while preventing overload.

To calculate the synchronous speed: Given that $p = 18$, we have: $N_s = \frac{60 \cdot 50}{18} = 166.67 \, \text{r/min}$

To calculate the percentage slip: $\text{Slip} = \frac{N_s - N_r}{N_s} \times 100$ Substituting the values: $\text{Slip} = \frac{(166.67 - 955)}{166.67} \times 100 = 4.5\%$