5.1.1 State the amount of current the motor will draw from the supply at full load - NSC Electrical Technology Power Systems - Question 5 - 2018 - Paper 1

The motor is suitable for South Africa because it operates at a supply voltage of 380 V and a frequency of 50 Hz, which aligns with the standard electrical supply in the region.

The 7.5 kW on the nameplate indicates the rated output power that the motor can deliver to drive the load.

To determine the number of poles, use the formula:

$p = \frac{60 \times f}{n_s}$

Substituting in the values:

1. Frequency (f) = 50 Hz
2. Speed (n_s) = 1500 r/min

Thus,

To calculate efficiency, use the formula:

$\eta = \frac{P_{out}}{P_{out} + P_{loss}} \times 100$

Where:

• $P_{out} = 7500$ W (7.5 kW)
• $P_{loss} = 1200$ W (1.2 kW)

Calculating: $\eta = \frac{7500}{7500 + 1200} \times 100 = \frac{7500}{8700} \times 100 \approx 86.21\%$

The purpose of no-volt protection is to prevent the motor from restarting automatically after a power failure. It ensures that if the supply voltage drops below a prescribed value, the circuit will disconnect the motor to protect the operator and the equipment.

The direction of rotation can be changed by switching any two of the three-phase supply lines. This alters the phase sequence, which effectively reverses the motor's rotation.

1. Check if the end plates are securely fastened.
2. Ensure that the shaft turns freely without any obstruction.