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. Less maintenance due to the absence of slip rings and brushes.
2. Explosion-proof construction because of 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.

Rotor speed is the speed at which the rotor rotates in an attempt to reach the synchronous speed.

The synchronous speed ($n_s$) is calculated as:

n_s = rac{60 imes f}{p}

Substituting the given values: n_s = rac{60 imes 50}{3} = 1000 ext{ r/min}

The percentage slip is calculated using the formula:

ext{Percentage Slip} = rac{n_s - n_r}{n_s} imes 100

Substituting the values:

ext{Percentage Slip} = rac{1000 - 950}{1000} imes 100 = 5 ext{ \\%}

1. Iron losses.
2. Mechanical losses.

The efficiency ($η$) can be calculated as:

η = rac{P_{in} - ext{losses}}{P_{in}} imes 100

Substituting the values:

η = rac{5000 - 600}{5000} imes 100 = 88 ext{ \\%}

The output power ($P_{out}$) can be calculated as:

$P_{out} = P_{in} imes η$

Substituting the values:

$P_{out} = 5000 imes 0.88 = 4400W = 4.4kW$

Timer.

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

The MC2 N/O, 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 the MC1 N/O2 contact, disconnecting the parallel section of the control circuit, disabling timer T1, thus MC2 will not be energized.