4.1 After a motor has been started and is running, what other functions does the starter perform? 4.2 Name THREE parts of a three-phase induction motor - NSC Electrical Technology Power Systems - Question 4 - 2017 - Paper 1

1. Stator
2. Rotor
3. Bearings

A star-delta starter reduces the voltage across the motor during starting. This leads to a lower starting current, minimizing the inrush current that might damage the motor and prevent nuisance tripping of the supply.

To calculate the current drawn from the supply, use the formula:

$I_L = \frac{P}{\sqrt{3} \times V \times cos \theta}$

Substituting the given values:

$I_L = \frac{15000}{\sqrt{3} \times 380 \times 0.9} \approx 25,32 A$

The apparent power (S) can be calculated using:

$S_{apparent} = \frac{P}{cos \theta}$

So, substituting the values:

$S_{apparent} = \frac{15000}{0.9} \approx 16667 kVA$

The phase current can be found using:

$I_{phase} = \frac{I_L}{\sqrt{3}}$

Thus,

$I_{phase} = \frac{25.32}{\sqrt{3}} \approx 14.43 A$

Testing the insulation resistance is crucial to ensure safety and to prevent faults that could lead to electric shock, equipment damage, or motor failure. It ensures that the insulation is intact and can withstand operational conditions.

The expected insulation resistance reading should typically be high, indicating good insulation integrity, often in the range of megaohms. A reading significantly below this indicates potential insulation breakdown.

The direction of rotation can be changed by reversing any two of the three supply lines connected to the motor.

Protective devices are essential to safeguard the motor against overloads and short circuits, ensuring the longevity of the motor and maintenance of safe operational conditions.

If one phase fails, the motor will continue to run, but with reduced performance. The remaining phases will handle the output power, but this can lead to overheating and eventual motor damage if not addressed.

The function of a star-delta starter is to reduce the starting current of the motor, limiting it to 3 to 4 times the full-load current.

The star-delta starter initially connects the motor in a star configuration, which reduces the voltage and current. After a brief period, it switches to the delta configuration, providing full voltage to allow the motor to run at full power.

The overload unit monitors the current flowing to the motor. If the current exceeds a preset limit for a specified time, the unit opens the circuit, disconnecting power to protect the motor.

A control circuit for a direct-on-line starter typically includes a start button, stop button, contactor, and overload relay. The primary components control power to the motor and ensure safe operation.