QUESTION 6: THREE-PHASE MOTORS AND STARTERS 6.1 Name TWO continuity tests to be performed on a three-phase motor - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1

Cogging is the tendency of the rotor rod to remain stuck under a stator tooth due to the direct magnetic attraction between the two.

1. Cage-type induction motors require less maintenance because of the absence of slip rings and brushes.
2. They are explosion-proof due to the absence of brushes and slip rings that cause sparking.

1. Cranes
2. Conveyor belts

A - Pull-up torque B - Breakdown torque C - Full-load torque

Synchronous speed is the speed at which the magnetic field in the stator rotates, whereas rotor speed is the speed at which the rotor rotates in an attempt to reach the synchronous speed.

To calculate the synchronous speed:

$n_s = \frac{60 \times f}{p}$ Substituting the given values:

$n_s = \frac{60 \times 50}{3} = 1000 \text{ rpm}$

To calculate the percentage slip:

$\text{Percentage slip} = \frac{n_s - n_r}{n_s} \times 100$ Substituting the calculated synchronous speed and the given rotor speed:

$\text{Percentage slip} = \frac{1000 - 950}{1000} \times 100 = 5 \%$

1. Iron losses
2. Mechanical losses

To find the efficiency:

$\eta = \frac{P_{in} - \text{losses}}{P_{in}} \times 100$ Given:

• Input power $P_{in} = 5000 W$
• Losses = 600 W

Substituting these values:

$\eta = \frac{5000 - 600}{5000} \times 100 = 88 \%$

To calculate the output power:

$P_{out} = P_{in} - \text{losses}$ Substituting the values:

$P_{out} = 5000 W - 600 W = 4400 W$ Thus, the output power is 4.4 kW.

T is a timer.

Each overload unit monitors the current drawn by each motor independently.

The error is that the MC1/N/O2 auxiliary contact (Hold contact) connected in parallel with the start button is omitted.

The moment the start button is released, MC1 will de-energize, opening MC1/N/O2, disconnecting the parallel section of the control circuit, disabling timer T1, thus MC2 will not be energized.