4.1 Refer to FIGURE 4.1 and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2016 - Paper 1

A three-phase voltage supply is connected across the stator windings. This setup generates three-phase currents that establish a rotating magnetic field in the stator. This rotating magnetic field induces currents in the rotor conductors, which create a magnetic field around the rotor. The interaction between these fields results in the rotor turning.

If one phase of the stator winding is an open circuit, the motor will still rotate, but it will not develop the correct torque necessary for full operation. The reduced phase results in an imbalance that diminishes the overall performance of the motor.

1. Three-phase induction motors require less maintenance as they have fewer moving parts compared to single-phase motors.
2. For the same size, three-phase motors deliver a higher torque than single-phase motors.

Checking the insulation resistance is crucial to ensure that the insulation is intact and without faults. This check helps prevent any electrical faults or short circuits when the motor is energised.

Check for cracked frames or ensure the motor can rotate smoothly without obstruction.

To calculate the synchronous speed, use the formula: n_s = rac{60 imes f}{p} Given that the frequency (f) is 50 Hz and the number of poles (p) is 18: n_s = rac{60 imes 50}{18} = 1000 ext{ r/min}

Using the calculated synchronous speed and accounting for slip: $n_r = n_s (1 - slip)$ where slip = 0.04: $n_r = 1000 (1 - 0.04) = 960 ext{ r/min}$

To find the input kVA, apply the formula: S = rac{ ext{√3} imes V_L imes I_L}{1000} Substituting the values: S = rac{ ext{√3} imes 380 imes 8.5}{1000} = 5.59 ext{ kVA}

The active power output can be calculated using: $P = ext{√3} imes V_L imes I_L imes ext{cos} θ imes η$ Given that the power factor (cos θ) is 0.8 and efficiency (η) is 0.95: $P = ext{√3} imes 380 imes 8.5 imes 0.8 imes 0.95 = 4.25 ext{ kW}$

The overload unit protects the motor and operator from overload conditions. When triggered, it disconnects the motor from the power supply, thereby preventing damage from overheating.

A practical application is for conveyor systems, where it starts the first drive motor and after a predetermined time interval, it starts the second drive motor.

If the N/O MC1 HOLD IN contact is faulty and does not close, pressing the Start button will energise motor 1 (MC1), but once the Start button is released, motor 1 will not remain energised due to the lack of a holding circuit.

The starter controls the operation of the motor, ensuring it starts smoothly and safely. It includes features such as overload protection and control circuits to manage multiple motors in sequence.