7. Define power factor and explain its significance in electrical installations. 1. Define an electrical machine and provide two examples. (4 marks) 2 What is the principle of "uperation of a DC motor? (4 marks) 3.List three types of AC motors and provide a brief description of each (6 marks) 4. Calculate the speed of a 4-pole AC motor operating on a 50 Hz supply (6 marks) 5.Explain the function of a transformer in electrical systems. (4 marks) 6.What is the difference between star and delta connections in three-phase systems? (4 marks) (4 marks) 8. Calculate the power consumed by a 3-phase motor drawing a current of 10 A at a voltage of 400 V with a power factor of 0.8. (6 marks) 9 What are the different earthing methods used in electrical installations? (4 marks) 10 Explain the importance of using an auto-transformer starter for AC motors (4 marks) Section B 60 marks Answer any three questions in this section 11. a Explain the construction of a DC generator. (7 marks) b. Describe the working principle of a DC motor. (7 marks) C. Calculate the efficiency of DC motor that delivers 5 kWoutput power and has an input power of 6 KW. (6 marks) (8 marks) Explain the DOL, starting method (8 marks) b. Compare Shar-Della and Auty-Transformer starting methods. C. Cidolate the starting current for a motor rated at 10 A using DOL starting. (4 marks) 13. a. Englain the different between step-up and step-down transformers. (8 marks) b. Discas the importance of transformer ratings. (5 marks) C. Caloulare the scientisty voltage of a step-down transformer with a primary voltage of and a more's ration a (7 marks)

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7. Define power factor and explain its significance in electrical installations. 1. Define an electrical machine and provide two examples. (4 marks) 2 What is the principle of "uperation of a DC motor? (4 marks) 3.List three types of AC motors and provide a brief description of each (6 marks) 4. Calculate the speed of a 4-pole AC motor operating on a 50 Hz supply (6 marks) 5.Explain the function of a transformer in electrical systems. (4 marks) 6.What is the difference between star and delta connections in three-phase systems? (4 marks) (4 marks) 8. Calculate the power consumed by a 3-phase motor drawing a current of 10 A at a voltage of 400 V with a power factor of 0.8. (6 marks) 9 What are the different earthing methods used in electrical installations? (4 marks) 10 Explain the importance of using an auto-transformer starter for AC motors (4 marks) Section B 60 marks Answer any three questions in this section 11. a Explain the construction of a DC generator. (7 marks) b. Describe the working principle of a DC motor. (7 marks) C. Calculate the efficiency of DC motor that delivers 5 kWoutput power and has an input power of 6 KW. (6 marks) (8 marks) Explain the DOL, starting method (8 marks) b. Compare Shar-Della and Auty-Transformer starting methods. C. Cidolate the starting current for a motor rated at 10 A using DOL starting. (4 marks) 13. a. Englain the different between step-up and step-down transformers. (8 marks) b. Discas the importance of transformer ratings. (5 marks) C. Caloulare the scientisty voltage of a step-down transformer with a primary voltage of and a more's ration a (7 marks)

Answer 1

1. **Define an electrical machine and provide two examples.** - An electrical machine is a device that converts electrical energy into mechanical energy or vice versa. Examples include motors and generators.2. **What is the principle of operation of a DC motor?** - A DC motor operates on the principle that a current-carrying conductor experiences a force in a magnetic field. This force generates rotational motion, allowing the motor to convert electrical energy into mechanical energy.3. **List three types of AC motors and provide a brief description of each.** - a. Induction Motors: These motors operate on the principle of electromagnetic induction. They are self-starting and require no external starting mechanism. - b. Synchronous Motors: These motors rotate at a speed proportional to the frequency of the AC supply. They are used in applications requiring constant speed. - c. Universal Motors: These motors can operate on both AC and DC supplies. They are versatile and used in applications like refrigerators and air conditioners.4. **Calculate the speed of a 4-pole AC motor operating on a 50 Hz supply.** - Speed = (120 * Frequency) / Number of Poles - Speed = (120 * 50) / 4 - Speed = 1500 RPM5. **Explain the function of a transformer in electrical systems.** - A transformer functions to transfer electrical energy between two or more circuits through electromagnetic induction. It can step-up or step-down voltage levels, allowing for efficient transmission and utilization of electrical power.6. **What is the difference between star and delta connections in three-phase systems?** - Star Connection: In a star connection, the three-phase loads are connected in a star configuration. Each load is connected between a neutral point and one of the phases. This configuration provides a neutral point, allowing for unbalanced loads and easy fault protection. - Delta Connection: In a delta connection, the three-phase loads are connected in a delta configuration. Each load is connected between two adjacent phases, forming a closed loop. This configuration does not have a neutral point, making it suitable for balanced loads and requiring more careful fault protection.8. **Calculate the power consumed by a 3-phase motor drawing a current of 10 A at a voltage of 400 V with a power factor of 0.8.** - Power = √3 * Voltage * Current * Power Factor - Power = √3 * 400 * 10 * 0.8 - Power = 8.89 kVA9. **What are the different earthing methods used in electrical installations?** - a. Solidly Grounded: In this method, the neutral conductor is connected to the earth at a single point, providing a low impedance path for fault current. - b. Solidly Isolated: In this method, the neutral conductor is not connected to the earth, and fault current is limited by the impedance of the system. - c. Bonded Neutral: In this method, the neutral conductor is connected to the earth at multiple points, creating a path for fault current but with higher impedance than solidly grounded systems.10. **Explain the importance of using an auto-transformer starter for AC motors.** - Auto-transformer starters are used to reduce the starting current of AC motors. By reducing the voltage applied to the motor during startup, they limit the inrush current, protecting the motor and the electrical system from high initial currents.11. **a. Explain the construction of a DC generator.** - A DC generator consists of a stator (stationary part) and a rotor (rotating part). The stator includes a magnetic field structure, while the rotor houses the armature windings. When the rotor spins, it induces a current in the armature windings, generating DC electricity.11. **b. Describe the working principle of a DC motor.** - A DC motor operates on the principle that a current-carrying conductor experiences a force in a magnetic field. This force generates rotational motion, allowing the motor to convert electrical energy into mechanical energy.11. **c. Calculate the efficiency of a DC motor that delivers 5 kW output power and has an input power of 6 kW.** - Efficiency = (Output Power / Input Power) * 100 - Efficiency = (5000 / 6000) * 100 - Efficiency = 83.33%12. **Explain the DOL starting method.** - DOL (Direct On-Line) starting method involves applying full line voltage directly to the motor during startup. This method provides high starting torque and is suitable for motors with high starting currents.12. **b. Compare Shad-Della and Auty-Transformer starting methods.** - Shad-Delta starting method involves initially connecting the motor in a star configuration and then switching to a delta configuration after the motor reaches a certain speed. This method reduces the starting current and torque. - Auty-Transformer starting method uses an autotransformer to reduce

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