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 Electronics - Question 4 - 2017 - Paper 1

1. Stator
2. Rotor
3. Bearings

A star-delta starter is used to reduce the voltage applied to the motor at startup. This lower voltage helps in minimizing the starting current, thus reducing stress on the motor and electrical supply. This method also decreases the chances of causing a burnout by limiting the inrush current.

To calculate the current drawn from the supply, use the formula: $I_L = \frac{P}{\sqrt{3} \times V \times cos θ}$ Substituting in the values: $I_L = \frac{15000}{\sqrt{3} \times 380 \times 0.9} \approx 25.32 A$

The apparent power can be calculated using the formula: $S = \frac{P}{cos θ}$ Substituting the given values: $S = \frac{15000}{0.9} \approx 16667 VA \text{ or } 16.67 kVA$

To find the phase current, use: $I_{phase} = \frac{I_L}{\sqrt{3}}$ Therefore: $I_{phase} = \frac{25.32}{\sqrt{3}} \approx 14.43 A$

Testing insulation resistance is crucial to ensure safety and prevent electrical shock. It safeguards the motor from insulation failures that can lead to short circuits or equipment damage.

Typically, a good insulation resistance reading between motor windings should be in the megohms range, indicating effective insulation without leakage current.

The direction of rotation of a three-phase motor can be changed by reversing the connections of any two of the three supply lines to the motor.

Protective devices are essential to prevent damage during faults, overloads, and unusual operating conditions. They help in maintaining safe operation and protecting both the motor and the connected equipment.

If one phase fails, the motor will continue to operate but will experience overheating and possible damage. The imbalance may lead to reduced efficiency and could cause electrical faults if continued.

The function of the starter is to safely control the voltage and current supplied to the motor during startup to prevent excessive starting current.

It achieves this function by initially connecting the motor in a star configuration to reduce startup voltage and then switching to delta for normal operation.

An overload relay operates by monitoring the current flowing through the motor. If the current exceeds a set limit, it trips, disconnecting the power to prevent overheating and damage.

The control circuit typically includes a contactor to switch the motor on and off, an overload relay for protection, and a start/stop pushbutton arrangement. (A diagram should be drawn as part of the response.)