6.1 Name THREE parts of a cage rotor - NSC Electrical Technology Power Systems - Question 6 - 2023 - Paper 1

Rotor slip is defined as the difference between the synchronous speed and the rotor speed, expressed as a percentage of the synchronous speed. It is given by the formula: ext{Slip} = rac{n_s - n_r}{n_s} imes 100 where $n_s$ is the synchronous speed and $n_r$ is the rotor speed.

The synchronous speed ($n_s$) of a motor can be calculated using the formula: n_s = rac{60 imes f}{p} Substituting the values: n_s = rac{60 imes 50}{4} = 750 ext{ r/min}

To calculate the rotor speed ($n_r$) with a slip of 5%, we can use the formula: n_r = n_s - rac{ ext{Slip} imes n_s}{100} Calculating: n_r = 750 - rac{5 imes 750}{100} = 712.5 ext{ r/min}

The combined true power ($P_t$) is given by: $P_t = P_1 + P_2 = 75 ext{ kW} + 50 ext{ kW} = 125 ext{ kW}$

The combined reactive power ($Q_t$) is obtained by adding the reactive powers: $Q_t = Q_1 + Q_2 = 45 ext{ kVAr} + 21.79 ext{ kVAr} = 66.79 ext{ kVAr}$

The apparent power ($S$) can be calculated using: S = rac{(P_t)^2 + (Q_t)^2}{ ext{VA}} Calculating: S = rac{(125,000)^2 + (66,790)^2}{ ext{VA}} = 141.72 ext{ kVA}

The power factor ($ext{pf}$) can be calculated as: ext{pf} = rac{P_t}{S} = rac{125,000}{141.72 imes 1,000} = 0.88

The contacts that create the interlocking function are MC1 N/C and MC2 N/C.

A normally closed contact allows current to flow under normal operating conditions. If an overload condition occurs, the contact will open, cutting off the current and protecting the motor from damage.

MC2 cannot be energised at the same time as MC1 because the interlocking contact (MC1 N/C) is wired in series with the control circuit for MC2. When MC1 is engaged, the N/C contact for MC1 will open, preventing the energisation of MC2.

If MC1 N/O is faulty and permanently open, it will prevent the energisation of the motor. As a result, pressing the START 1 button will not initiate motor operation, effectively disabling the motor control circuit.