5.1 List THREE electrical inspections that need to be carried out on a three-phase motor before commissioning it - NSC Electrical Technology Power Systems - Question 5 - 2019 - Paper 1

Protective devices are essential to safeguard equipment and personnel. They help prevent damage during fault conditions such as overloads and short circuits. By cutting the power during such events, they protect the motor from overheating and potential failure, thus ensuring operational safety.

The motor starter in FIGURE 5.3 is an automatic sequence starter, specifically designed for controlling the starting and stopping of the motor.

If contactor MC1 N/O is faulty and remains permanently closed, it would keep the circuit energized, causing the motor to run continuously. This condition can lead to overheating and potential motor damage as the motor would not stop even when the start button is released.

When the Start button is pressed, the coil of contactor MC1 energizes, keeping the circuit closed. The contact MC1 N/O will maintain power to the main circuit. Upon releasing the Start button, the circuit remains energized through the closed contact MC1 N/O. At the same time, MC1 N/C opens, de-energizing the timer. After a predetermined time, the timer allows contactor MC2 to energize, thus controlling the transition of the motor.

The main function of contactor MC1 is to control the power supply to the motor safely, ensuring it can be started and stopped as intended based on the pushbutton controls.

The starting current is reduced in a star-delta motor starter to prevent a high inrush current when the motor starts. By initially connecting the motor winding in a star configuration, the phase voltage is reduced, thereby lowering the starting current. This helps in preventing damage to the motor and overloads in the electrical system.

To calculate the apparent power, we use the formula:

S_{ap} = rac{√3 imes V_L imes I_{PH}}{1000}

Substituting the values:

$S_{ap} = √3 × 380 × 29 = 19.09 ext{ kVA}$

To find the synchronous speed, we use the formula:

n_s = rac{60 imes f}{p/2}

Given that Number of poles = 12,

n_s = rac{60 imes 50}{12/2} = 1500 ext{ rpm}

The slip can be calculated using the formula:

S = rac{n_s - n_r}{n_s} imes 100

Where:

• $n_s$ = Synchronous speed = 1500 rpm
• $n_r$ = Rotor speed = 1400 rpm

Substituting the values:

S = rac{1500 - 1400}{1500} imes 100 = 6.67 ext{%}