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 refers to the maximum speed at which the motor effectively operates under no-load conditions. In contrast, full-load speed is the speed at which the motor runs when it is under maximum load conditions.

The synchronous speed can be calculated using the formula:

n_s = rac{60 imes f}{p}

Where:

• $f$ = frequency in Hz (50 Hz)
• $p$ = number of pole pairs (4)

Substituting the values:

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

The percentage slip can be calculated using the formula:

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

Where:

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

Substituting the values:

ext{Slip} = rac{750 - 725}{750} imes 100 = 3.33 ext{%}

Two mechanical inspections that could be conducted are:

• Check for any play on the axis of the rotor.
• Inspect if the bearings run smoothly and freely when turned by hand.

The input power can be calculated using the formula:

P = rac{ ext{√3} imes V_L imes I_L imes ext{cos} heta}{1000}

Substituting the values:

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

P = rac{ ext{√3} imes 380 imes 5 imes ext{cos}(20°)}{1000} = 3.09 kW

The reactive power can be calculated using the formula:

Q = rac{ ext{√3} imes V_L imes I_L imes ext{sin} heta}{1000}

Substituting the values:

Q = rac{ ext{√3} imes 380 imes 5 imes ext{sin}(20°)}{1000} = 1.13 kVar

The output power can be calculated by first finding the input power ($P_{ ext{input}}$) and then adjusting for efficiency:

P_{ ext{output}} = P_{ ext{input}} imes rac{ ext{Efficiency}}{100}

Assuming the input power was calculated correctly as 3.09 kW:

$P_{ ext{output}} = 3.09 kW imes 0.90 = 2.78 kW$

In the ANSWER BOOK, draw the terminal box with the following configuration for delta connection:

• Connect U1 to V2, U2 to W1, and U3 to W2.
• Ensure all terminals are properly labeled and show the motor connections clearly.

Component A is an overload normally closed (N/C) contact.

MC1/N.O. serves as a hold-out contact that ensures the timer relay and the star contactor MC2 does not energize until MC1 is energized.

Interlocking is crucial as it prevents MC2 and MC3 from energizing simultaneously, ensuring the safe operation of the star-delta starter.

Once the timer has completed its cycle, the normally closed contact (T N/C) opens, de-energizing MC2. This action closes MC2/N/C and turns off MC3, ensuring the motor operates in delta mode.