In daily electric machine management, the most commonly used equipment is the electric motor. As the prime mover of machinery, it plays a crucial role in the operation of industrial systems. Any malfunction can lead to significant downtime and operational issues. Therefore, understanding the proper operation and maintenance of electric motors is essential. This article aims to provide key knowledge about hydraulic motor operation and maintenance for everyone involved in the process.
1. The generator stator voltage should not exceed 110% of its rated value, and must not fall below 90% of the rated voltage. It should also meet the plant voltage requirements.
2. The normal operating frequency of the generator should be maintained at 50 Hz, with an allowable variation range of ±0.2 Hz. When the frequency changes, the stator current, excitation current, and temperature of all parts must not exceed their rated values.
3. The stator voltage of the generator is allowed to vary within ±5% of the rated value. When the power factor is at its rated value, the rated capacity remains unchanged. If the stator voltage drops below 95%, the long-term allowable stator current must not exceed 105% of its rated value.
4. During operation, the hydrogen purity of the generator must be at least 96%, and the oxygen content should be less than 2%.
5. The rated power factor of the generator is 0.85. For generators that have not been tested in phase, the power factor may run continuously in the late phase (between 0.95 and 1) when the excitation regulator is in automatic mode. When there is a change in the power factor, the P-Q output curve at the time of hydrogen pressure must not be exceeded.
6. The rate of increase of active load after paralleling generators is determined by the unit, while the rate of increase of reactive load is unrestricted. However, changes in stator voltage should be monitored closely.
7. The insulation resistance of the generator rotor winding should be measured using a 500V megohmmeter, and the value should not be less than 0.5 MΩ.
8. When the three-phase current of the stator is unbalanced, negative sequence current will be generated.
9. During the boosting process, the stator three-phase voltage should rise steadily, and the rotor current should not exceed the no-load value.
10. The insulation resistance of a 6kV motor should be measured using a 2500V megohmmeter, and the result should be greater than 6 MΩ.
11. Under normal conditions, a squirrel-cage motor is allowed to start twice in cold state, with each interval being at least 5 minutes. In hot state, it can start once. Only in emergency situations where the motor does not need to be processed or starts for more than 2–3 seconds, it may start more than once.
12. The insulation resistance of a 6kV high-voltage factory motor should be measured under the same environmental and temperature conditions. If the measurement is lower than 1/3 to 1/5 of the previous value, the cause should be investigated. The absorption ratio R60/R15 should be greater than 1.3.
13. A motor can operate at nominal voltage and within ±1% of the supply frequency, and its rated output remains unchanged.
14. The main transformer can operate at a lower rated load. If the load is small, the upper oil temperature may not reach the specified value, but it can rise to the maximum limit for up to 60 minutes.
15. The three-phase unbalance current of a three-phase AC motor must not exceed 10% of the rated current, and no single phase current should exceed the rated value.
16. For oil-immersed air-cooled transformers, the maximum temperature of the upper oil layer should not exceed 95°C, and generally should not exceed 85°C.
17. When the gas protection secondary circuit is grounded at one point, the heavy gas protection should be switched to the signal position.
18. The upper oil temperature of a forced-oil circulating air-cooled transformer should generally not exceed 75°C, and the maximum should not exceed 85°C.
19. The primary voltage applied to a transformer should not exceed 105% of the tap’s rated value, and the secondary side can carry the rated current.
20. Power plants typically use a double busbar connection. In normal operation, one grounding component should be present on each line. The main transformer is usually grounded via a grounding switch, and the neutral point is directly grounded.
21. 6kV switchgear is equipped with "five-prevention" mechanical locking devices. The locking function includes: when the car switch is closed, it cannot be moved; when the grounding switch is closed, the car switch cannot be pushed into the working position; when the switch is in the working position, the grounding switch cannot be closed; and without the grounding switch, the switch cabinet door cannot be opened.
22. After closing all isolating switches, it is necessary to check that the three-phase contacts are in good contact.
23. Before closing the grounding switch, it must be confirmed that the power switch on all sides is in the disconnected position and that the absence of voltage has been verified.
24. If a load-breaking arc occurs, it should be quickly closed before the arc is broken. If it has already been broken, it must not be closed again. If a load-closing arc occurs, it must not be re-opened.
25. In the absence of a switch in the loop, an isolating switch can be used to perform a loop equalization current with a voltage not exceeding 10 kV and a current below 70 A.
26. The normal operating voltage of a cable line should not exceed 15% of the cable's rated voltage.
27. Electrical equipment should not operate without protection. Partial protection may be deactivated if necessary, but primary protection must not be deactivated simultaneously. It is forbidden to open protective device doors during operation, and wireless communication devices should not be used in the control room.
28. Measure the insulation resistance of low-voltage AC and DC factory motors below 380V using a 500V megohmmeter. The insulation resistance must not be less than 0.5 MΩ.
29. Generator time-limited overload protection reflects the size of the stator current.
30. Stator winding overvoltage protection reflects the magnitude of the terminal voltage.
31. Generator timing negative sequence overcurrent protection reflects the size of the negative sequence current to prevent overheating of the rotor surface.
32. The four limiting factors on the PQ curve of the generator are: heating of the stator windings, heating of the rotor windings, heating of the iron core at the ends of the stator, and stable operating limits.
33. Generator reverse power protection is for turbine protection.
34. Install the grounding wire in the order of ground first, then conductor.
35. Electrical insulating materials are classified based on the maximum temperature they can withstand during normal operation.
36. The current value indicated by an AC meter represents the effective value of the current.
37. The safety distance for equipment without power is 0.7m for 6kV, 1.5m for 110kV, and 5m for 500kV.
38. In power plants, the phase sequence of the three-phase bus is represented by fixed colors: yellow for A, green for B, and red for C.
39. Equipment with a voltage-to-ground below 250 volts is considered low-voltage equipment. That is why we often refer to the 380V plant power system as low-voltage equipment.
40. During normal generator operation, the three-phase unbalanced current value generally should not exceed 10% of the stator rated value.
41. Phase difference dynamic high frequency protection uses a high frequency signal to compare the current across the protected line.
42. If a disconnector is found to be hot, the load should be reduced until no heat is generated, and ventilation and cooling should be increased. If the heating is severe, the equipment should be taken out of service for processing.
43. When the temperature difference between the highest and lowest temperatures among the stator coils of water-cooled generators reaches 8°C, or the temperature difference between the outlet of the stator bars and the lead pipes reaches 8°C, an alarm should be triggered, the cause identified, and the load reduced accordingly.
44. When the temperature difference between the stator bars of a water-cooled generator reaches 14°C, or the temperature difference between the outlet pipe of the stator reaches 12°C, or the inter-layer temperature measuring element exceeds 90°C, or the outlet water temperature of any stator slot exceeds 85°C, and after confirming the temperature measuring element is correct, immediate action such as reverse flushing and related inspections should be carried out, and the equipment should be stopped to avoid major accidents.
45. The product of the RMS value of the total voltage in a sinusoidal AC circuit and the RMS value of the current includes both active power and reactive power, which is referred to as apparent power.
46. In a circuit, the current flowing into a node equals the current flowing out of that node. This is known as Kirchhoff’s Current Law.
47. Starting from any point in the loop, the sum of potential rises and potential drops around the loop is zero. This is Kirchhoff’s Voltage Law.
48. Among various methods for calculating complex circuits, the branch current method is the most fundamental approach.
49. In inductive circuits, the voltage leads the current. In capacitive circuits, the voltage lags behind the current.
50. In power systems, shunt reactors are commonly used to absorb excess reactive power and reduce system voltage.
51. In a delta-connected three-phase AC power supply or load, the line voltage is equal to the phase voltage.
52. The total power of a symmetrical three-phase AC circuit is three times the power of a single phase.
53. In a symmetrical three-phase AC circuit, the neutral voltage is equal to zero.
54. A short circuit refers to an abnormal connection between two phases or between a phase and ground through an arc or other relatively small impedance.
55. A storage battery is a type of energy storage device that converts electrical energy into chemical energy. When used, it converts chemical energy back into electrical energy and releases it through external circuits.
56. The size of a conductor’s resistance depends not only on its length and cross-sectional area, but also on its material and temperature.
57. In a closed circuit, voltage is the condition that produces current. The size of the current is related to the circuit’s resistance and the terminal voltage.
58. In a series circuit, the voltage distribution across the load is proportional to the resistance of each load. In a parallel circuit, the current distribution in each branch is inversely proportional to the resistance of each branch.
59. When the current in a coil changes, a self-induction electromotive force is generated at both ends of the coil.
60. The direction of the electromagnetic force experienced by a conductor in a magnetic field is determined by the left-hand rule. When a conductor moves in a magnetic field and cuts the magnetic lines of force, the direction of the induced electromotive force is determined by the right-hand rule.
61. The number of periodic changes in alternating current per second is called frequency, denoted by the letter f, with the unit name Hertz and symbol Hz.
62. The maximum instantaneous value of a sinusoidal alternating current in a cycle is called the peak or amplitude of the current.
63. The effective value of AC is obtained by dividing the maximum value by √2.
64. In circuits composed of resistors, inductors, and capacitors, only resistive components consume power, while inductive and capacitive components exchange energy without consuming power.
65. A star-connected power supply without a neutral point is a three-phase, three-wire system. In this system, the line current is equal to the phase current.
66. The stronger the current through a coil, the stronger the magnetic field it generates, and the more magnetic lines pass through the coil.
67. A current-carrying coil can generate a magnetic field, and the strength of the magnetic field is proportional to the current passing through the conductor.
68. The voltage between the three-phase terminals is called line voltage, and the voltage between a terminal and the neutral is called phase voltage. In a star-connected symmetrical circuit, the line voltage is √3 times the phase voltage.
69. The main cause of a short circuit in a power system is the breakdown of the insulation of the current-carrying parts of electrical equipment.
70. The hazards of short circuits to electrical equipment include: (1) thermal effect causing equipment to burn or damage insulation; (2) electrodynamic forces causing deformation and destruction of equipment.
71. Electrical equipment and current-carrying conductors must have sufficient mechanical strength to withstand the effects of electrodynamic forces during a short circuit, and must have adequate thermal stability.
72. Transformers operate based on the principle of electromagnetic induction, converting the voltage and current of an alternating current into the same frequency but different values.
73. The principle of an induction motor is that when a three-phase symmetrical alternating current flows through a winding, it generates a rotating magnetic field. This magnetic field induces current in the rotor, and the interaction between the stator magnetic field and the rotor current generates electromagnetic torque, causing the motor to rotate.
74. Copper and aluminum wires are connected using a transition joint. Direct connection can lead to a potential difference between copper and aluminum, and moisture at the joint can cause ionization and electrochemical corrosion.
75. Near a transmission line, an insulator can induce an inductive charge, a phenomenon known as electrostatic induction of the transmission line.
76. Sulfur hexafluoride (SF6) is a colorless, odorless, non-flammable gas with very stable performance.
77. Batteries are used as control and protection DC power supplies in power plants, offering advantages such as stable voltage and reliable power supply.
78. The active material on the positive plate of a battery is lead dioxide, and on the negative plate is spongy lead.
79. A group of batteries has a capacity of 1200 AH. If discharged at a current of 100 A, the continuous supply time is 12 hours.
----- Editor: Dalan Oil Pump Motor 02 - Procurement Consultant
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