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 starting voltage applied to the motor, which consequently lowers the starting current. This method minimizes the inrush current and prevents potential damage to the motor, allowing for a smoother start and reducing the chance of burnout.

To find the current drawn from the supply, we can use the formula:

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

Substituting the values:

$I_L = \frac{15000}{\sqrt{3} \cdot 380 \cdot 0.9} \approx 25.32 A$

The apparent power can be calculated using the formula:

$S = \frac{P}{\text{cos } \theta}$

Thus:

$S = \frac{15000}{0.9} \approx 16667 VA = 16.67 kVA$

The phase current can be calculated as:

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

Substituting the previously calculated line current:

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

Testing insulation resistance is crucial to ensure that there is no leakage current that could lead to short circuits or motor failures. It protects the equipment and personnel by preventing electric shock and ensuring reliable operation.

You would expect a high resistance reading, typically in the range of Megaohms, indicating good insulation. A low reading could suggest insulation failure, requiring immediate attention.

The direction of rotation can be changed by reversing any two of the three supply lines to the stator. This alters the magnetic field direction and consequently the motor's rotational direction.

Protective devices such as circuit breakers and overload relays are necessary to safeguard the motor against damage due to overloads, short circuits, or other fault conditions. They help maintain safe operating conditions and prevent equipment malfunction.

If one phase fails, the motor will continue to operate, but it will run inefficiently and may experience a significant drop in torque. Prolonged operation under these conditions can cause overheating and eventual motor failure.

The function of a star-delta starter is to reduce the starting current of a motor at startup, which can be up to three to four times the full-load current.

The starter initially connects the motor in a star configuration, reducing the voltage across the motor windings. After a short time, it switches the motor to a delta connection, allowing full voltage and current for normal operation.

The overload unit is designed to detect excessive current conditions. It will open the circuit and cut power to the motor when the current exceeds a predetermined threshold, protecting the motor from overheating and damage.

The control circuit typically includes components such as a contactor, overload relay, start and stop buttons, as well as auxiliary contacts. A simple diagram can illustrate the relationships between these components and their functions in controlling the motor.