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

The rated speed of an induction motor is the maximum speed at which the motor effectively operates under no load. In contrast, the full-load speed is the speed at which the motor operates when it is connected to its maximum load. This difference arises due to slip, which occurs because the rotor would not reach the synchronous speed.

To calculate the synchronous speed ( s) of the motor, use the formula: n_s = rac{60 imes f}{p} where:

• $f$ is the frequency in Hz,
• $p$ is the number of pole pairs. Given:
• $f = 50$ Hz,
• $p = 4$ (as it has four pole pairs),

Calculating: n_s = rac{60 imes 50}{4} = 750 ext{ rpm}

To calculate the percentage slip, use the formula: ext{Slip (\%)} = rac{n_s - n_r}{n_s} imes 100 where:

• $n_s = 750$ rpm (synchronous speed),
• $n_r = 725$ rpm (rotor speed).

Substituting the values: ext{Slip (\%)} = rac{750 - 725}{750} imes 100 = 3.33\%

Two mechanical inspections that could be performed are:

1. Checking for any play on the rotor axis and ensuring it turns freely.
2. Inspecting the bearings for smooth operation when turned by hand and checking for excessive grease on the bearing bushes.

To calculate input power, use the formula: P = rac{ ext{√3} imes I_L imes V_L imes ext{cos} θ}{1000} Given:

• $I_L = 5$ A
• $V_L = 380$ V
• $θ = 20°$

Calculating: P = rac{ ext{√3} imes 5 imes 380 imes ext{cos} (20°)}{1000} \\ \\ P \approx 3.09243 ext{ kW}

To calculate reactive power, use the formula: Q = rac{ ext{√3} imes I_L imes V_L imes ext{sin} θ}{1000} Given:

• $I_L = 5$ A
• $V_L = 380$ V
• $θ = 20°$

Calculating: Q = rac{ ext{√3} imes 380 imes 5 imes ext{sin} (20°)}{1000} \\ \\ Q \approx 1.12555 ext{ kVAr}

To calculate output power, use: P_{out} = P_{in} imes rac{Efficiency}{100} where:

• $P_{in} \approx 3.09243$ kW (Input Power)
• Efficiency = 90%

Calculating: $P_{out} \approx 3.09243 imes 0.90 = 2.78319 ext{ kW}$

Redraw the terminal box to show the connections for a delta configuration, ensuring that each winding of the motor is connected in such a way that:

• U1 connects to W2,
• W1 connects to U2,
• V1 connects to W3,
• W3 connects to U1,
• V2 connects to W1,
• and the connection closes as per the standard delta connection diagram.

Component A is identified as a Normally Closed (N/C) contact.

MC1/N_O2 serves as a hold-out contact ensuring that the timer relay and the star contactor MC2 do not energise until MC1 is energised.

Interlocking ensures that contactors MC2 and MC3 do not energise simultaneously, preventing short circuits or overloads as well as ensuring safe motor operation.

Once the timer has elapsed, the Normally Closed contact (T N/C) opens and de-energizes MC2. Consequently, this closes MC3/N/C, which opens MC1/N_O2 to ensure that MC2 remains de-energized, allowing the motor to operate in delta mode.