6.1 Name TWO continuity tests to be performed on a three-phase motor. 6.2 Explain the term cogging with reference to induction motors. 6.3 State TWO advantages of cage-type induction motors over wound rotor-type motors with slip rings and brushes. 6.4 Name TWO applications of squirrel-cage induction motors where constant speed and torque is essential. 6.5 Label points A, B and C on the characteristic curve in FIGURE 6.5 below. 6.6 Differentiate between synchronous speed and rotor speed. 6.7 The rotor of a three-phase induction motor with 3 pole pairs per phase rotates at 950 r/min when connected to a 380 V/50 Hz supply. Given: $V_L = 380$ V $f = 50$ Hz $n_r = 950$ r/min $p = 3$ Calculate the following: 6.7.1 Synchronous speed of the motor. 6.7.2 Percentage slip. 6.8 The input power to a three-phase motor is 5 kW. The losses are determined as 600 W. 6.8.1 Name TWO types of losses other than copper losses that influence efficiency of the motor. 6.8.2 Calculate the efficiency of the motor. 6.8.3 Calculate the output power of the motor. 6.9 The circuit in FIGURE 6.9 below is used to control TWO three-phase motors. Answer the questions that follow. 6.9.1 Identify component T. 6.9.2 Explain the purpose of having TWO overload units in the circuit. 6.9.3 Identify the error in the circuit. 6.9.4 Explain how this error affects the operation of the circuit.

NSC Electrical Technology Power Systems PLC Logic and Outputs: 6.1 Name TWO continuity tests to

6.1 Name TWO continuity tests to be performed on a three-phase motor - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1

Question 6

6.1 Name TWO continuity tests to be performed on a three-phase motor. 6.2 Explain the term cogging with reference to induction motors. 6.3 State TWO advantages of ... show full transcript

Worked Solution & Example Answer:6.1 Name TWO continuity tests to be performed on a three-phase motor - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1

Step 1

Name TWO continuity tests to be performed on a three-phase motor.

96%

114 rated

Answer

Continuity test between the ends of each coil.
Continuity test between the frame of the motor and earth.

Step 2

Explain the term cogging with reference to induction motors.

99%

104 rated

Answer

Cogging is the tendency of the rotor rod to remain stuck under a stator tooth due to the direct magnetic attraction between the two.

Step 3

State TWO advantages of cage-type induction motors over wound rotor-type motors with slip rings and brushes.

96%

101 rated

Answer

Less maintenance due to the absence of slip rings and brushes.
Explosion proof because of the absence of brushes and slip rings that cause sparking.

Step 4

Name TWO applications of squirrel-cage induction motors where constant speed and torque is essential.

98%

120 rated

Answer

Cranes.
Conveyor belts.

Step 5

Differentiate between synchronous speed and rotor speed.

97%

117 rated

Answer

Synchronous speed is the speed at which the magnetic field in the stator rotates, while rotor speed is the speed at which the rotor rotates in an attempt to reach the synchronous speed.

Step 6

Calculate the synchronous speed of the motor.

97%

121 rated

Answer

$n_s = \frac{60 \times f}{p} = \frac{60 \times 50}{3} = 1000$ r/min

Step 7

Calculate the percentage slip.

96%

114 rated

Answer

$\text{Percentage slip} = \frac{n_s - n_r}{n_s} \times 100 = \frac{1000 - 950}{1000} \times 100 = 5\%$

Step 8

Name TWO types of losses other than copper losses that influence efficiency of the motor.

99%

104 rated

Answer

Iron losses.
Mechanical losses.

Step 9

Calculate the efficiency of the motor.

96%

101 rated

Answer

$\eta = \frac{P_{in} - \text{losses}}{P_{in}} \times 100 = \frac{5000 - 600}{5000} \times 100 = 88\%$

Step 10

Calculate the output power of the motor.

98%

120 rated

Answer

$P_{out} = P_{in} - \text{losses} = 5000 - 600 = 4400$ W (or 4.4 kW)

Step 11

Identify component T.

97%

117 rated

Answer

Timer.

Step 12

Explain the purpose of having TWO overload units in the circuit.

97%

121 rated

Answer

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

Step 13

Identify the error in the circuit.

96%

114 rated

Answer

The MC1/N/O2 auxiliary contact (Hold contact) connected in parallel with the start button is omitted.

Step 14

Explain how this error affects the operation of the circuit.

99%

104 rated

Answer

The moment the start button is released, MC1 will de-energize, opening MC1/N/O2, disconnecting the parallel section of the control circuit, thereby disabling timer T1, thus MC2 will not be energized.

6.1 Name TWO continuity tests to be performed on a three-phase motor - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1

Worked Solution & Example Answer:6.1 Name TWO continuity tests to be performed on a three-phase motor - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1

Name TWO continuity tests to be performed on a three-phase motor.

Explain the term cogging with reference to induction motors.

State TWO advantages of cage-type induction motors over wound rotor-type motors with slip rings and brushes.

Name TWO applications of squirrel-cage induction motors where constant speed and torque is essential.

Differentiate between synchronous speed and rotor speed.

Calculate the synchronous speed of the motor.

Calculate the percentage slip.

Name TWO types of losses other than copper losses that influence efficiency of the motor.

Calculate the efficiency of the motor.

Calculate the output power of the motor.

Identify component T.

Explain the purpose of having TWO overload units in the circuit.

Identify the error in the circuit.

Explain how this error affects the operation of the circuit.

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