1. Fill in the blank: The rotor of a DC motor consists of an armature core, winding, commutator, rotating shaft, and fan blade, which is a key part for energy conversion.
2. The excitation mode of a DC motor can be categorized into separately excited, shunt excited, series excited, and compound excited.
3. When a DC motor operates under rated load, its # grade should not exceed 1.5.
4. The rated capacity of a three-phase transformer refers to the maximum apparent power that can be delivered without exceeding the temperature rise limits when the primary winding is supplied with rated voltage. It equals √3 times the product of the secondary winding’s rated voltage and current.
5. In a transformer, the loss caused by the thermal effect of current in the windings is known as copper loss, while the loss due to alternating magnetic fields in the core includes hysteresis and eddy current losses, collectively called iron loss.
6. During a short-circuit test, the low-voltage side is usually shorted, and the high-voltage side is pressurized because the current is large but the applied voltage is low.
7. The part of the AC motor winding embedded in the slot is called the effective part, and the part outside the slot is referred to as the end part.
8. The direction of the rotating magnetic field in a three-phase asynchronous motor depends on the phase sequence of the power supply. If the direction of the magnetic field changes during operation, the rotor's direction will also change.
9. The insulation class of Y-series AC motors is Class B.
10. The rotor of a synchronous motor can be either salient pole or hidden pole. Typically, the rotors of diesel generators and factory synchronous motors are salient poles.
11. The speed of a three-phase asynchronous motor depends on the number of pole pairs (P), slip ratio (S), and supply frequency (f).
12. Winding drying typically uses three methods: oven drying, bulb drying, and current drying.
13. An arc is formed when gas in the contact gap ionizes, creating a conductive path. The electrical energy passing through this region converts into heat and light, causing the emission of light and heat.
14. RL1 series spiral fuses contain quartz sand inside the fuse element. When the current is interrupted, the quartz sand helps extinguish the arc quickly.
15. The contact opening distance of a contactor is the shortest distance between the stationary and moving contacts when they are fully separated.
16. The arc generated in a DC circuit is more difficult to extinguish compared to an AC arc, so arc suppression devices are commonly used.
17. The purpose of zero-position protection in a bridge crane is to prevent the motor from starting if the controller is not at the zero position after power is restored.
18. The fixed part of a magnetic-electric instrument mainly includes the magnet and core, while the rotating part is primarily composed of the coil and pointer.
19. When an RLC series circuit reaches resonance, the current in the circuit reaches its maximum value. This phenomenon is known as resonance.
20. The principle for connecting a three-phase load to a three-phase power supply is: if the load's rated voltage matches the line voltage, it should be connected in a delta (Δ) configuration; if it matches the phase voltage, it should be connected in a star (Y) configuration.
21. The most basic logical operations are AND, OR, and NOT.
22. Transformer cooling methods include oil-immersed self-cooled, oil-immersed air-cooled, and forced oil-air cooled.
23. When a no-load transformer is energized, a magnetizing inrush current occurs, which can reach 6–8 times the rated current.
24. For parallel operation of transformers, the conditions must include equal voltage ratios, same connection groups, and similar short-circuit voltages.
25. Factors affecting the accuracy of insulation resistance measurements include temperature, humidity, and contamination of the insulation surface.
26. The absorption ratio of an insulating medium is the ratio of the insulation resistance measured at 60 seconds to that at 15 seconds.
27. The DC system in a substation supplies power to control circuits, relay protection, automatic devices, and accident lighting, ensuring reliable operation of the station.
28. The charge level of a cadmium-nickel battery is determined by its terminal voltage, not by specific gravity.
29. The operating current for overcurrent protection is set to avoid the maximum load current.
30. Gas protection is installed based on the fact that internal faults in a transformer produce and decompose gases.
31. When the operating voltage exceeds 1.1 times the rated voltage of a capacitor, the running capacitor should be disconnected. The rated voltage of capacitors in factories is specified accordingly.
32. Safety measures in electrical work include work ticket systems, operation ticket systems, site inventory, work permit systems, work supervision, work interruption and transfer, and work termination and power-on procedures.
33. Substation equipment defects are classified into general, serious, and critical defects.
34. During a ground fault in high-voltage equipment, indoor personnel should stay at least 4 meters away from the fault point, and outdoor personnel should stay at least 8 meters away.
True or False Questions:
1. The rated power of a transformer refers to the maximum power allowed on the secondary side when the temperature rise does not exceed the allowable limit when the primary side is supplied with rated voltage. (×)
2. As the iron core oxidizes, the vacuum load current decreases. (×)
3. The speed of a three-phase asynchronous motor depends only on the power frequency and number of pole pairs, not on the slip rate. (×)
4. The lower the rotor speed of a three-phase asynchronous motor, the higher the slip and the higher the rotor electromotive force frequency. (√)
5. When using a short-circuit tester to check for a single-phase short circuit in a multi-branch winding, each branch must be disconnected first. (√)
6. No matter the load, increasing the load current always reduces the output voltage. (×)
7. A contactor with an arc suppression hood must be energized before the motor starts. (×)
8. In series magnetic blowout mode, the magnetic blow force direction is independent of current direction. (√) In parallel mode, it is also independent. (×)
9. The short-circuit ring on the core of an AC contactor does not release the moving core. (×)
10. Changing a parameter in a circuit affects the voltage between two points. (×)
11. The magnitude of self-induced EMF is proportional to the rate of current change, not the current itself. (√)
12. Increasing the frequency in an inductive circuit reduces the total current. (√)
13. A tube voltmeter can measure DC, AC, sine, and non-sinusoidal signals. (×)
14. An oscilloscope can display waveforms but cannot measure signal size. (×)
15. A purely resistive circuit has a power factor of 1, but a power factor of 1 does not necessarily mean the circuit contains only resistors. (×)
16. As the load increases, both copper loss and combined flux increase. (×)
17. A no-load transformer has a low power factor due to small active current and large reactive current. (√)
18. If the voltage is below rated, the current will be lower than rated. (×)
19. If the explosion-proof pipe of an oil-filled transformer is damaged, the transformer must be powered off. (×)
20. A single-phase motor is larger but has lower power factor and efficiency than a three-phase motor of the same capacity. (√)
21. Measuring instruments are part of the power distribution device. Portable voltmeters and ammeters are considered such. (×)
22. "Jumping" of a high-voltage circuit breaker means it closes and then opens again. (√)
23. High-capacity isolating switches have two blades per pole to increase cross-sectional area and capacity. (×)
24. A low-voltage circuit breaker with shunt and voltage loss trip is called a double-trip device. (×)
25. Reactors are installed to increase short-circuit impedance and limit current. (√)
26. AC and DC contactors use short-circuit rings to reduce vibration. (×)
27. Even with safety precautions, working in flammable or explosive areas is still dangerous. (×)
28. The grease on explosion-proof equipment prevents rust and should not be removed. (×)
29. Better relays with fewer contacts are more reliable. (√)
30. The gas relay responds to all transformer faults. (×)
Short Answer Questions:
1. What happens if a diode in a single-phase bridge rectifier is shorted, open, or reversed? Answer: A shorted diode causes half-wave rectification, an open diode results in half-wave rectification, and a reversed diode stops current flow entirely.
2. What is the main role of a transformer in the power system? Answer: Transformers step up or step down voltage to facilitate efficient power transmission and meet user voltage requirements.
3. What are the main parameters of a transformer? Answer: Rated voltage, rated current, rated capacity, no-load current, no-load loss, short-circuit loss, impedance voltage, and connection group.
4. Why is the winding temperature rise limited to 65°C? Answer: To ensure the insulation class (Class A) lasts longer and avoids overheating damage.
5. What are the hazards of improper circuit breaker speed? Answer: Too fast or too slow speeds may cause mechanical stress, component damage, or failure to clear faults quickly.
6. What are the wiring configurations for a voltage transformer? Answer: YY, VV, and other standard configurations.
7. What should you note when deactivating a voltage transformer? Answer: Deactivate protection devices, remove secondary fuses, and prevent reverse charging.
8. What is the role of the DC system in a substation? Answer: Provides reliable power for control, protection, and emergency lighting, ensuring safe operation.
9. What are the installation requirements for a contactor? Answer: Check coil voltage, clean the core, inspect contacts, and ensure proper alignment.
10. What is dynamic braking? Answer: Reversing the power supply to create a braking torque, stopping the motor quickly but consuming more energy.
11. What factors should be considered when choosing a power system? Answer: Cable voltage rating, laying method, environmental conditions, and load capacity.
12. What is lightning protection grounding? Answer: Grounding to safely dissipate lightning current into the earth, including lightning rods, leads, and grounding electrodes.
13. What is a safety voltage and its values? Answer: Safe voltage values are 65V, 36V, and 12V depending on the environment's risk level.
14. Why do AC contactors sometimes make noise? Answer: Poor suction due to dust, broken short-circuit rings, or low voltage. Clean, repair, or adjust the voltage.
15. How to choose a fuse for a lighting circuit? Answer: Main line fuses should match the sum of branch fuses, and branch fuses should match the lamp current.
16. How to select a thermal relay for a motor? Answer: Based on motor rated current, with thermal elements around 1.2x rated current. Can be directly or indirectly connected.
17. What are the wire threading requirements? Answer: Total wire area ≤ 40% of conduit, no joints or twists, and separate circuits should not share conduits.
18. What is a secondary circuit and its classification? Answer: Secondary circuits monitor, control, and protect, including current, voltage, operating, and signal loops.
19. What is the function of working grounding? Answer: Reduce body contact voltage, quickly isolate faulty equipment, and lower insulation requirements.
20. Is the no-load current of an asynchronous motor appropriate? Answer: Large motors: 20%-35%, small motors: 35%-50% of rated current.
21. What causes excessive no-load current in an asynchronous motor? Answer: High voltage, large air gap, insufficient turns, incorrect wiring, or aging insulation.
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