50 hp motor low rpm dynamo price in pakistan
12. the regulations stipulate that the operating voltage of the motor can deviate from the rated value by - 5% or + 10% without changing its rated output. Why is the allowable range of high voltage larger?
Answer: as for the impact of voltage deviation from rated value on motor operation, here we only focus on why the specified range of high voltage is different from that of low voltage. To sum up, there are two reasons.
(1) Higher voltage operation is better for the motor than lower voltage operation, causing less adverse effects.
When the voltage is low, the stator and rotor currents increase, resulting in increased losses. At the same time, the cooling conditions become worse when the speed decreases, which will increase the temperature rise of the motor. In addition, the starting and self starting conditions become worse due to the reduction of torque.
It is true that when the voltage increases, the iron loss increases due to the increase of magnetic flux. A little higher temperature has an impact on the stator winding temperature. However, due to the decrease of stator current, the stator winding temperature drops a little. According to the analysis, the influence of the increase of iron core temperature on the increase of stator winding temperature is smaller than that caused by the decrease of stator current. Therefore, the general trend is to reduce the temperature. As for the temperature rise of the iron core itself, it doesn't matter, and there is no harm to the motor. The increase of torque caused by the increase of voltage greatly improves the starting and self starting conditions. As far as insulation is concerned, there will be no danger if the voltage is increased by 10%, because the electrical strength of insulation has a certain margin.
(2) The provisions with large voltage deviation range are easy to meet the requirements for operation, so it may be possible to avoid the use of on load voltage regulating auxiliary transformer. Otherwise, the scope is specified to be small. Even if the on load voltage regulating auxiliary transformer is not used in the design, the operator must frequently adjust the generator voltage or the tap of the main transformer.
13. what should be paid attention to when measuring insulation resistance with a megger?
Answer: (1) generally, there are 500, 1000 and 2500 V megohmmeters. Which one should be selected according to the voltage level of the equipment.
(2) When measuring the insulation resistance of equipment, the power supply must be cut off first, and the equipment with large capacitance (such as capacitors, transformers, motors and cable lines) must be discharged first.
(3) The megger shall be placed at the horizontal position. Before wiring, shake the megger to see if the pointer is at "∞", then short-circuit the "L" and "e" terminals, slowly shake the megger to see if the pointer is at "zero". It is not suitable to use short circuit calibration for semiconductor type megger.
(4) Multi strand flexible wire shall be used for the reference wire of the megger and shall have good insulation.
(5) For overhead lines and electrical equipment connected to overhead lines, in case of thunderstorm, or double circuit overhead lines and buses that cannot be completely powered off, it is prohibited to measure when the induced voltage of the measured circuit exceeds 12V.
50 hp motor low rpm dynamo price in pakistan
(6) When measuring capacitors, cables, large capacity transformers and motors, there must be a certain charging time. The larger the capacitance, the longer the charging time should be. Generally, the reading after the megger rotates for one minute shall prevail.
(7) When telemetering the insulation resistance, the megger shall be kept at the rated speed, generally 120 rpm. When the capacitance of the measured object is large, in order to avoid the pointer swing, the rotating speed can be appropriately increased (such as 130 RPM).
(8) The surface of the object to be measured shall be wiped clean and free of dirt, so as not to leak and affect the measurement accuracy.
(9) Do not touch the measuring part and the terminal of the megger with your hands before the megger stops rotating and the equipment is discharged, so as to avoid electric shock.
14. why is it stipulated that the shaking time is 1 minute when measuring the insulation resistance with a megger?
Answer: megger is used to measure insulation. Generally, it is required that the reading after shaking for one minute shall prevail. Because after DC voltage is applied to the insulator, the current flowing through the insulator (absorption current) will gradually decrease with time. The DC resistivity of insulators is determined according to the steady-state conduction current, and the attenuation time of insulation absorption current of insulators of different materials is different. However, the test shows that the absorption current of most insulation materials has stabilized after one minute, so it is required to determine the insulation performance by the insulation resistance value after one minute of pressurization.
15. what is the function of motor low voltage protection?
Answer: when the power supply bus voltage of the motor decreases for a short time or is interrupted for a short time and then restored, in order to prevent the power supply voltage from being seriously reduced when the motor is started, the low-voltage protection is usually installed on the secondary motor. When the power supply bus voltage drops to a certain value, the low-voltage protection action will cut off the secondary motor, so that the bus voltage can be quickly restored, so as to ensure the self starting of the important motor.
16. what are the possible reasons why the induction motor can not start?
Answer: (1) power supply: A. no power: the operating circuit is disconnected, or the power switch is not closed. b. One or two phases are de energized. c. The voltage is too low.
(2) Motor itself: A. open circuit of rotor winding. b. The stator winding is open circuit. c. Stator and rotor windings have short circuit fault. d. The stator and rotor rub against each other.
(3) Load: a. the load is too heavy. b. The mechanical part is jammed.
50 hp motor low rpm dynamo price in pakistan
17. what is the effect of broken rotor bars on squirrel cage induction motors during operation?
Answer: broken rotor bars often occur in squirrel cage motors due to poor quality of cast aluminum or poor welding quality of copper cage. After the broken bar, the electromagnetic torque of the motor decreases, resulting in the decrease of the speed. The stator current varies from large to small, because the broken bar destroys the structural symmetry and the electromagnetic symmetry, so that the stator magnetic field with relative motion with the rotor will have different responses when penetrating the magnetic flux from different parts of the rotor surface, resulting in the stator current varying from large to small. At the same time, the broken bar will also make the fuselage vibrate, which is caused by the uneven magnetic tension around the whole stator bore, and the periodic buzzing sound is also generated.
18. what conditions should the running motor stop running immediately?
Answer: in case of any of the following conditions during the operation of the motor, the operation shall be stopped immediately:
(1) personal accident.
⑵ the motor smokes and catches fire, or one phase is disconnected for operation.
(3) there is strong friction sound inside the motor.
(4) DC motor commutator has serious ring fire.
(5) the motor vibrates strongly and the bearing temperature rises rapidly or exceeds the allowable value.
(6) the motor is flooded.
19. what are the possible reasons for the sudden change of the sound of the running motor and the current value indicated by the ammeter rising or falling to zero?
A: the possible reasons are as follows:
⑴ one phase of stator circuit is disconnected.
⑵ system voltage drops.
(3) winding turn to turn short circuit.
(4) the end ring of squirrel cage rotor winding has cracks or poor contact with copper (aluminum) strip.
(5) the rotor core of the motor is damaged or loose, and the rotating shaft is bent or cracked.
(6) some parts of the motor (such as bearing end cover, etc.) are loose or the connection between the motor base and the foundation is not fastened.
(7) uneven air gap between stator and rotor of motor exceeds the specified value.
50 hp motor low rpm dynamo price in pakistan
20. when the motor is started, what happens when it must be stopped after closing?
Answer: ⑴ when the maximum direction of motor ammeter exceeds the return time and does not return;
(2) the motor does not turn into buzzing or fails to reach the normal speed;
⑶ the machinery of the motor is seriously damaged;
(4) strong vibration of the motor exceeds the allowable value.
(5) the motor starting device is on fire and smoking;
(6) personal accident occurs in motor circuit.
(7) when starting, smoke or sparks appear inside the motor.
21. inspection items during normal operation of motor?
Answer: ⑴ the sound is normal without burning smell.
⑵ the motor voltage and current are within the allowable range, the vibration value is less than the allowable value, and the temperature of each part is normal.
(3) cable head and grounding wire shall be in good condition.
(4) the brush and commutator of wound motor and DC motor shall not be overheated, too short or burned, and the surface temperature of resistance shall not exceed 60 ℃.
(5) the oil color and oil level are normal.
(6) the cooling device shall operate well, and the temperature difference between inlet and outlet air shall not be greater than 25 ℃, and the maximum temperature shall not exceed 30 ℃.
22. how to change the rotation direction of three-phase motor?
Answer: the rotation direction of the motor rotor is determined by the rotation direction of the rotating magnetic field established by the stator, and the direction of the rotating magnetic field is related to the phase sequence of the three-phase current. This changes the current phase sequence, that is, changes the direction of the rotating magnetic field, that is, changes the rotation direction of the motor.
23. what are the regulations for motor bearing temperature?
Answer: when the ambient temperature is +35 ℃, the sliding shaft shall not exceed 80 ℃, and the flow shaft shall not exceed 100 ℃. (in case of poor grease quality, it shall not exceed 5 ℃).
24. how is the motor insulation resistance specified?
Answer: (1) 1000v--2500v megger shall be used to measure the insulation resistance of 6kV motor, and its value shall not be less than 6m Ω.
(2) The insulation resistance of 380V motor shall be measured with a 500V megger, and the value shall not be less than 0.5m Ω.
(3) The absorption ratio R60 "/r15" of the motor with a capacity of more than 500kW shall not be less than 1.3, and shall not be less than 1/2 of the previous measured value compared with the previous time under the same conditions. If it is lower than this value, it shall be reported to the relevant leaders.
(4) When the motor is out of service for more than 7 days, the insulation shall be measured before starting, and the insulation of the standby motor shall be measured once a month.
(5) In case of abnormal conditions such as water spray and steam admission of the motor, the insulation must be measured before starting.
25. what are the regulations and precautions for the running motor?
Answer: (1) under the rated cooling condition, the motor can operate according to the rated data specified on the nameplate of the manufacturer. It is not allowed to operate blindly if the limit is not clear.
(2) The maximum monitoring temperature of motor coil and iron core shall be implemented according to the regulations of the manufacturer. If the manufacturer does not specify it, it shall be implemented according to the following table. The motor shall not exceed this temperature rise under any operating conditions.
Insulation class a class B class E class F
Measuring method thermometer resistance thermometer resistance thermometer resistance.
50 hp motor low rpm dynamo price in pakistan
Stator winding temperature rise 506060757080
Temperature rise 8595100110105115
Stator core temperature rise 607580
Temperature rise 95100115
The values in the above table are specified when the ambient temperature is 35 ℃
(3) The allowable temperature of motor bearing shall comply with the regulations of the manufacturer. If there is no manufacturing regulation, the following regulations shall be followed:
a. For plain bearings, do not exceed 80 ℃.
b. For rolling bearings, it shall not exceed 100 ℃ (in case of poor grease quality, it shall not exceed 85 ℃)
(4) Generally, the motor can operate within the range of - 5% to + 10% of the rated voltage change, and its rated output remains unchanged.
(5) When the motor operates at rated output, the unbalance rate of phase to phase voltage shall not be greater than 5%, and the three-phase current difference shall not be greater than 10%.
(6) When the motor is running, the vibration measured at each bearing shall not exceed the provisions in the following table:
Motor speed vibration value (double amplitude) mm
3000rpm 0.05
1500rpm 0.085
1000rpm 0.10
0.12 at or below 750rpm
In addition to strictly implementing various regulations during the operation of the motor, the following problems shall be paid attention to:
(1) The current of the motor shall not exceed the allowable value under normal conditions, and the difference of three-phase current shall not be greater than 10%.
(2) Sound and smell: the sound of the motor shall be normal and uniform without noise during normal operation; There is no burnt smell or smoke smell near the motor. In case of abnormal sound, burnt smell or smoke, measures shall be taken to deal with it.
(3) Working condition of bearing: mainly refers to lubrication, whether the lubricating oil is normal, whether the temperature is high, and whether there are sundries.
(4) Other conditions: such as whether the cooling water system is normal, whether the brush on the slip ring of the wound motor operates normally, etc.
26. what happens when the motor is running and should it stop running immediately?
Answer: (1) personal accident.
(2) The motor smokes and catches fire, or one phase is disconnected for operation.
(3) There is strong friction sound inside the motor.
(4) DC motor commutator has serious ring fire.
(5) The motor vibrates strongly and the bearing temperature rises rapidly or exceeds the allowable value.
(6) The motor is flooded.
27. under what circumstances can the standby motor be started first and then the faulty motor be stopped?
Answer: in case of the following conditions, the standby motor set can be started in advance for the important auxiliary motor, and then the faulty motor can be stopped:
(1) Abnormal sound or burnt smell in the motor.
(2) Sparks or smoke appear in the motor or in the starting regulating device.
(3) The stator current exceeds the operating value.
(4) Strong vibration occurs.
(5) An unacceptable increase in bearing temperature occurs.
