6.1 Name the TWO types of rotor construction used in induction motors - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1

Rated speed is defined as the maximum speed at which the motor effectively operates under no load conditions. Full-load speed, on the other hand, is the speed at which the motor operates when maximum load is applied.

The synchronous speed ( s) of a motor can be calculated using the formula: ns = rac{60 imes f}{p} Where: f = frequency (50 Hz) p = number of poles = 2 × pole pairs = 8 (since there are 4 pole pairs) Thus: ns = rac{60 imes 50}{8} = 375 ext{ rpm}

Percentage slip can be calculated using the formula: ext{Slip} = rac{n_s - n_r}{n_s} imes 100 Where: n_s = synchronous speed = 750 rpm (from 6.2.2) n_r = rotor speed = 725 rpm Substituting the values: ext{Slip} = rac{750 - 725}{750} imes 100 = 3.33 ext{ \\%}

1. Check if there is any play on the axis of the rotor.
2. Inspect if the bearings rotate smoothly when turned by hand.

The input power (P) can be calculated using the formula: P = rac{\sqrt{3} imes I_L imes V_L imes \cos \theta}{1000}
Substituting the given values: $P = \frac{\sqrt{3} \times 5 \times 380 \times \cos(20°)}{1000} = 3.09 kW$

The reactive power (Q) can be calculated using the formula: $Q = \frac{\sqrt{3} \times I_L \times V_L \times \sin \theta}{1000}$ Substituting the values: $Q = \frac{\sqrt{3} \times 5 \times 380 \times \sin(20°)}{1000} = 1.13 kVAR$

The output power (P_o) can be calculated as follows: $P_o = P_{input} \times \frac{Efficiency}{100}$ Where: Efficiency = 90% Substituting the input power calculated from 6.4.1: $P_o = 3.09 kW \times \frac{90}{100} = 2.78 kW$

To connect the motor in delta, connect:

• U1 to V2
• W1 to U2
• V1 to W2 This creates a triangular connection that allows the motor to operate effectively under delta configuration.

Component A is identified as an Overload Normally Closed contact.

MC1/N.O. serves as a hold-out contact ensuring that the timer relay (MC1) and the star contactor (MC2) do not energise until MC1 is energized.

Interlocking ensures that MC2 and MC3 do not energise simultaneously, which prevents any electrical faults and unsafe operating conditions.

When the timer has completed its timing, the Normally Closed contact opens, de-energising MC2. This also closes MC2/N.C. which opens MC3/N.C., ensuring that MC2 remains de-energised and the motor transitions to delta operation.