1. Fill in the blank: The rotor of a DC motor consists of the armature core, winding, commutator, rotating shaft, and fan blade, which is an essential component for energy conversion.
2. The excitation modes of a DC motor include 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 when the primary winding is supplied with rated voltage, ensuring the temperature rise of the core does not exceed the rated value. It equals √3 times the product of the secondary winding’s rated voltage and rated current.
5. In operation, the loss caused by the thermal effect of current in the winding is called copper loss, while the loss due to alternating magnetic fields in the core is known as iron loss, which includes hysteresis and eddy current losses.
6. During a short-circuit test, the high-voltage side is usually energized to short-circuit the low-voltage side 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, while 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 is determined by the phase sequence of the supply. If the direction changes during operation, the rotor will also change direction.
9. The insulation class of Y-series AC motors is Class B.
10. The rotor of a synchronous motor has two types: salient pole and hidden pole. Most diesel generators and factory synchronous motors use 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. Common methods for drying windings include oven drying, bulb drying, and current drying.
13. An arc forms when gas in the contact gap ionizes, creating a conductive path. The energy from the current passing through this region is converted into heat and light.
14. The RL1 series spiral fuse contains quartz sand inside the fuse element, which helps extinguish arcs quickly when the circuit is broken.
15. The opening distance of a contactor’s contacts is the shortest distance between the static and dynamic contacts when fully separated.
16. Arcs in DC circuits are harder to extinguish than in AC circuits, so arc suppression devices are commonly used.
17. Zero-position protection on a bridge crane prevents accidental startup if the controller is not at zero after power is cut off.
18. The fixed parts of a magnetic-electric instrument mainly consist of the frame and coil, while the rotor is composed of the moving coil and other components.
19. When an RLC series circuit reaches resonance, the current in the circuit is maximized. This phenomenon is known as resonance.
20. For a three-phase load connected to a three-phase power supply, if the load's rated voltage matches the line voltage, it should be connected in delta. If it matches the phase voltage, it should be connected in star.
21. The most basic logical operations are AND, OR, and NOT.
22. Transformer cooling methods include oil-immersed natural cooling, oil-immersed air cooling, and forced oil-air cooling.
23. When a no-load transformer is energized, a magnetizing inrush current may occur, reaching up to 6–8 times the rated current.
24. Transformers operating in parallel must meet conditions such as equal voltage ratios, same connection groups, and similar short-circuit voltages.
25. Factors affecting insulation resistance measurement include temperature, humidity, and surface contamination.
26. The absorption ratio of an insulating medium is the ratio of the insulation resistance at 60 seconds to that at 15 seconds.
27. The DC system in a substation supplies power to control signals, relay protection, automatic devices, and emergency lighting.
28. The charge level of a cadmium-nickel battery is determined by terminal voltage, not specific gravity.
29. Overcurrent protection setting is based on the maximum load current.
30. Gas protection is triggered when internal faults in a transformer produce or decompose gas.
31. If the operating voltage exceeds 1.1 times the capacitor’s rated voltage, it should be removed from service.
32. Safety measures in electrical work include work ticket systems, operation tickets, site inspections, permits, supervision, and termination procedures.
33. Substation equipment defects are categorized as general, serious, and critical.
34. During a ground fault in high-voltage equipment, indoor personnel should stay at least 4 meters away, and outdoor personnel at least 8 meters away.
2. True/False Questions:
1. The rated power of a transformer refers to the maximum power allowed by the secondary side when the primary is supplied with rated voltage and the temperature rise does not exceed the allowable limit. (×)
2. The iron core of a transformer gradually oxidizes and rusts, causing the no-load current to decrease. (×)
3. The speed of a three-phase asynchronous motor depends on the power frequency and pole pairs, not the slip rate. (×)
4. The lower the rotor speed, the higher the slip and rotor frequency. (√)
5. When checking for short circuits in a three-phase motor, each branch must be disconnected. (√)
6. As the load current increases, the output voltage of a transformer inevitably decreases. (×)
7. A contactor with an arc suppressor must be energized before starting the motor. (×)
8. In series magnetic blowout mode, the direction of the magnetic force is independent of current direction. (√) Parallel mode: same. (×)
9. The short-circuit ring on the AC contactor core will not release the moving iron core. (×)
10. Changing a parameter in a circuit affects the voltage between two points. (×)
11. Self-induced EMF is proportional to the rate of current change, not the magnitude. (√)
12. Increasing the frequency of 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 not measure signal size. (×)
15. A purely resistive circuit has a power factor of 1, but a power factor of 1 doesn’t mean only resistors are present. (×)
16. As load increases, copper loss and temperature rise increase, along with combined flux. (×)
17. No-load transformers have low power factor due to high reactive current. (√)
18. Lower voltage leads to lower current in a motor with rated torque. (×)
19. Cracked explosion-proof pipe requires immediate power-off and repair. (×)
20. Single-phase motors are larger and less efficient than three-phase ones of the same capacity. (√)
21. Portable voltmeters and ammeters are considered part of the power distribution device. (×)
22. “Jumping†of a circuit breaker means it closes and then opens. (√)
23. High-voltage isolators with two blades per pole increase conduction area. (×)
24. A low-voltage circuit breaker with shunt and voltage loss trip is called a double-trip device. (×)
25. Reactors are used to increase impedance and limit short-circuit current. (√)
26. AC and DC contactors use short-circuit rings to reduce noise. (×)
27. Working in flammable areas is safe if precautions are taken. (×)
28. The anti-rust coating on explosion-proof equipment should not be removed. (×)
29. Simpler relays with fewer contacts are more reliable. (√)
30. Gas relay responds to all transformer faults. (×)
3. Short Answer Questions:
1. What happens if a diode is shorted, open, or reversed in a single-phase bridge rectifier?
Answer: A shorted diode causes half-wave rectification and short-circuits the other half. An open diode results in half-wave rectification. A reversed diode prevents rectification and no current flows through the load.
2. What is the main role of a transformer in the power system?
Answer: To step up voltage for efficient transmission, reducing losses and enabling long-distance power delivery.
3. What are the main parameters of a transformer?
Answer: Rated voltage, current, 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 protect Class A insulation and ensure longevity, as losses generate heat that could damage the transformer.
5. What are the hazards of improper circuit breaker speed?
Answer: Too high speed may damage components; too low speed may fail to clear faults quickly or cause contact burnout.
6. What are the wiring methods for a voltage transformer?
Answer: YY, YY, and VV connections.
7. What should be considered when deactivating a voltage transformer?
Answer: Deactivate related protections, remove the secondary fuse to 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, manually check contacts, install within 5° tilt, and avoid oiling contacts.
10. What is reverse braking?
Answer: Reversing the power supply to stop the motor quickly, using the opposing torque to brake, but consumes significant energy.
11. What factors should be considered when choosing a power system?
Answer: Cable voltage rating, long-term load, laying method, and environmental conditions.
12. What is lightning protection grounding and what parts does it include?
Answer: Grounding to safely direct lightning current to earth, including the lightning rod, down conductor, and grounding electrode.
13. What is the safety voltage and its values?
Answer: Safe voltage values are 65V, 36V, and 12V depending on the environment.
14. Why do AC contactors make noise and how to fix it?
Answer: Noise is due to poor suction, caused by dust, damaged short-circuit ring, low voltage, or stiff springs. Cleaning, repairing, or replacing parts can help.
15. How to choose a fuse for a lighting circuit?
Answer: Main line fuse should be slightly larger than branch fuses, and each branch should be sized to the load current.
16. How to select a thermal relay for a motor?
Answer: Based on motor rated current, typically 1.2 times the rated current. Can be directly or indirectly connected.
17. What are the wire threading requirements?
Answer: Total wire area should not exceed 40% of conduit cross-section, no joints or twists, and wires of different circuits cannot be in the same conduit.
18. What is a secondary circuit and its classification?
Answer: Secondary circuits monitor, control, and protect equipment. They include current, voltage, control, and signal loops.
19. What is the purpose of working grounding?
Answer: Reduce touch voltage, quickly isolate faulty equipment, and reduce insulation requirements.
20. Is the no-load current of an asynchronous motor appropriate as a percentage of rated current?
Answer: Large motors: 20–35%, small motors: 35–50%.
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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