Refer to Figure 2.2 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2017 - Paper 1
Question 4
Refer to Figure 2.2 below and answer the questions that follow.
---
**Figure 2.2**: Three-Phase Motors and Starters
---
Worked Solution & Example Answer:Refer to Figure 2.2 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2017 - Paper 1
Step 1
4.1 What advantages do three-phase motors have over single-phase motors?
96%
114 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
Three-phase motors require less maintenance compared to single-phase motors because they have fewer parts. Additionally, for motors of the same size frame, three-phase motors deliver a higher torque, which enhances their performance in various applications.
Step 2
4.2 What are the consequences of starting a three-phase motor if the rotor is not rotating freely?
99%
104 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
If the rotor is not rotating freely and is switched on, it may cause extensive damage to both the motor and the operator. This is due to the high starting currents that can lead to overheating and potential mechanical failure.
Step 3
4.3 What checks should be performed on electrical connections before operating the motor?
96%
101 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
It is crucial to ensure that all electrical connections are secure. Moreover, the insulation resistance between the windings and the earth should be tested to prevent any short circuits, which could pose a safety hazard.
Step 4
4.4 What does an electrical fault between the rotor and stator indicate?
98%
120 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
An electrical connection indicating a short circuit between the rotor and stator signifies a fault condition. This can potentially cause damage to the motor if not addressed immediately.
Step 5
4.5 Explain the types of losses in a three-phase motor.
97%
117 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
In a three-phase motor, there are mechanical losses, which include friction and windage losses, and iron losses, which are associated with the hysteresis and eddy currents in the motor core. These losses can impact the efficiency and performance of the motor.
Step 6
4.6.1 Calculate the output power when the input power is 15,000 W and the power factor is 0.8.
97%
121 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
The output power can be calculated using the formula: Pout=5imesextcosheta
Substituting the values, we get: Pout=15,000Wimes0.8=12,000W
Thus, the output power is 12 kW.
Step 7
4.7.1 How does improving the power factor affect motor performance?
96%
114 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
Improving the power factor will allow more power to be available to do the work required of the motor, enhancing its operational efficiency.
