Soft starter with a small size, torque can be adjusted, start the smooth impact of a small and have the advantages of soft stop function has been more and more applications, there is a trend to replace the traditional auto-reduced voltage, star - angle starter. As the soft starter is a new development in recent years to start the equipment, in the design, installation, commissioning and use of the lack of guiding norms and regulations. We have also encountered some practical technical problems in the installation and commissioning of soft starters. For example: different starting load soft starter selection, soft start impact current and overcurrent protection value with the soft start equipment capacity and transformer capacity relationship and other issues.
1, soft starter introduction
At present, the market common soft starter mainly electronic, magnetron and automatic liquid resistance type. Electronic to thyristor voltage regulator type for the majority. Frequency converter in a sense is also a soft starter, and is able to truly realize the soft start starter, just the cost is higher.
Thyristor soft starter is connected in series in the power supply and motor between the three groups of positive and negative parallel thyristors, through the microcomputer control of the triggering angle to achieve AC voltage regulation. Thyristor soft starter starting mode has a ramp voltage type, sudden jump plus ramp voltage type and current limiting type and so on can choose.
The magnetron soft starter is a soft starter composed of a three-phase saturated reactor connected in series between the power supply and the motor using the magnetic amplifier principle. Starting through the digital control board to adjust the magnetic amplifier control winding of the excitation current, change the saturated reactor reactance value to adjust the starting voltage drop, to realize the motor soft start.
Regardless of the thyristor soft starter or magnetron soft starter can only adjust the output voltage during startup to control the voltage drop during startup, limiting the purpose of starting current. General soft starter can not adjust the power supply frequency, also can not be like a frequency converter from zero frequency and zero voltage to start the motor, to realize the shock-free start. In fact, the soft starter in the start of the equipment or to produce a certain inrush current; ramp voltage type control soft starter start the voltage, current change curve shown in Figure 1. Thyristor-type soft starter using ramp voltage start, start to make the soft starter output an initial voltage (initial voltage between 80 ~ 280V can be adjusted), so that the motor to produce enough to overcome the static friction of the mechanical equipment initial torque, dragging the equipment began to rotate, the startup current for the Is. under the control of the microcomputer, continue to increase the output voltage so that the motor accelerated. When the output voltage of the soft starter is close to the rated voltage, the motor has reached the rated speed, and Is is reduced to the load current In. At the end of the starting time t1, the soft starter outputs the rated voltage and sends out a bypass signal to close the bypass contactor, the soft starter stops outputting the voltage, and the motor is transferred to normal operation. The initial torque of the soft starter can be adjusted by giving the initial voltage and starting time, and the starting current can be controlled within 2~4.5 times of the rated motor current.
Low-voltage soft starter stopping methods are mainly free stopping, soft stopping, braking stopping three. The traditional motor stopping method commonly used free parking, but there are many applications, free parking will produce a big problem, such as high-rise building pumping system, if the use of free parking, will produce a huge "water hammer" effect, so that the pipeline, pump damage. Therefore, the use of soft parking can eliminate the free parking brought about by this anti-inertial shock. In the stopping moment t2 issued by the stopping command, the motor terminal voltage from Un slowly down, in the voltage drop instant motor current will have a small current shock, and then motor current will be with the voltage drop and drop until the motor stops.
2, low-voltage soft starter selection should pay attention to the problem
2.1 low-voltage soft start cabinet electrical wiring and component arrangement
General low-voltage soft start cabinet electrical wiring shown in Figure 2. Cabinet electrical components are arranged in the order shown in the diagram, to the main wiring briefly do not cross, to facilitate the connection of copper rows. Soft starter installed in the right side of the contactor is not affected by the heat of other components. Soft starter and control cabinet sidewalls and other components spacing requirements ≥ 100mm, to facilitate soft starter heat dissipation.
2.2 soft starter selection
In addition to technology, performance, price comparison, but also to consider the equipment site grid capacity, equipment startup load, start the degree of frequency and other specific conditions.
For pumps to start a lighter load of equipment, you can choose a simple function, lower price, easy to operate soft starter. This type of equipment according to the rated power of the motor, select the sample of the same capacity of the soft starter can meet the needs. For large fans, crushers and other heavy start load equipment, should be used to start more functions, current-limited start function, their own protection is more complete soft starter. Especially large power equipment (200KW or more), it is best to use the start function is more complete high-performance soft starter.
2.3 Isolator and fuse selection
Soft start cabinet isolation appliances, you can choose to isolate the switch can also choose to have the isolation function of the molded case circuit breaker. Small power soft start cabinet should choose isolation switch fuse combination of knife fuse switch. Not only play a role in isolation and protection, but also reduce the cost of the project. The rated current of the isolation switch is greater than the rated current of the motor can meet the operating requirements.
Due to the soft starter in the thyristor surge Joule integral (I2t) value is limited, the choice of circuit breakers to do short-circuit protection device can not effectively protect the thyristor components. It is recommended to choose a fast fuse as a short-circuit protection device. Fast fuse can choose aR or NGT type semiconductor protection fuse. Selection of fast fuse generally do not have to do breaking capacity calibration, because the aR-type fuse rated breaking capacity of 50KA, NGT fast fuse breaking capacity of 120KA, to meet the general needs of power distribution projects. In addition, the fuse also has a current-limiting function, the protection of the thyristor is more reliable than the circuit breaker.
The principle of selection of the rated current of the fast fuse is that the equipment will not fuse when starting, the installation of the equipment must be reliable when the minimum short-circuit current fuse. Specific selection can be based on the load nature of the equipment and motor start-up current, access to the fuse manufacturer to provide the fuse time - current characteristic curve, I2t value and thyristor I2t value for calculation and selection. In the absence of the above information can also be calculated according to the following empirical formula:
Ifn ≥ (1.8~2.0)*Ie (A)
Ifn-rapid fuse rated current (A)
Ie-motor rated current (A)
2.4 Bypass contactors Selection of bypass contactor
At the end of soft start, the motor is already running on the rated voltage, so the AC contactor selected according to the rated current of the motor can meet the requirements. It should be noted that in the cabinet wiring, to the soft starter and contactor connected to the same, do not connect the wrong phase sequence.
2.5 Selection of overload protection device
Soft start device overload protection device should be selected with overload protection, phase failure protection and temperature compensation function of the thermal overload relay. Specific selection, to make the motor's operating current within the range of the thermal component rectification current. Work easily overloaded equipment, to make the motor rated current value close to the lower limit of the range of thermal component rectification current.
2.6 transformer load capacity and protection setting value verification
In addition to the above requirements in the selection of soft start devices, but also pay attention to the load capacity of the transformer for the equipment power supply. If the transformer is close to full load beforehand, be careful to choose soft start equipment. Especially to increase the power of the larger equipment, it is more important to check the transformer load capacity and protection of the set value;
Increase the soft start equipment, the transformer secondary side of the circuit breaker short-circuit short-delay off the set value of Ir2:
Ir2 ≥ 1.1 (IL + 1.35 * K * Ie) A
Where: IL -The load current of the transformer during normal operation A
K-The ratio of the starting current of the newly added soft-start equipment to the rated current of the motor (see Table 1);
Ie-The rated current of the newly added motor A.
The calculated Ir2 should be less than Transformer secondary side circuit breaker is now the actual short-circuit short-delay tripper setting value; otherwise, in the new addition to the equipment startup, the transformer secondary side circuit breaker to break trip. Resulting in soft starter selection failure.
Table 1 typical equipment soft start effect and start current reference value
Application of machinery type Selected function Executed function Starting current (%) Starting time (sec)
Centrifugal pump Standard start Reduce the impact, eliminate water hammer 300 5~15
Screw compressor Standard start Reduce the impact, prolong the life of machinery 300 3~20
Centrifugal compressors Standard startup Reduces shocks and extends mechanical life 350 10~40Piston compressors Standard startup Reduces shocks and extends mechanical life 350 5~10
Conveyor belt transporters Standard + jumps Smooth startup, reduces shocks 300 3~10
Fans Standard startup Reduces shocks and extends mechanical life 300 10~40
Mixer standard startup Reduce startup current 350 5~20
Mill heavy-duty startup Reduce startup current 450 5~60
3, typical applications
Example: a city's second heat source plant added four 250KW circulating pumps, two working and two standby, transformer capacity of 1250kVA, 10/0.4kV, in order to reduce motor startup current. kV, in order to reduce the motor starting current and avoid excessive investment in capacity increase, we use soft starter according to user requirements. As the load is pumps, the start is not very frequent, so the cost-effective SS2-250 type soft starter. The isolation switch adopts HD17-630/3, the fuse adopts NT4-1250A, the bypass contactor adopts CJ29-500/3, and the thermal relay adopts JRS2-630/3. It has been running normally for two years, and it can meet the user's requirements well.
4, adjustment and trial
Soft start control cabinet installation is completed to be carefully checked. According to the design drawings to check whether the wiring is correct, whether the connection is reliable. Because the thyristor and other electronic devices are not allowed to do insulation resistance test, so only with a digital multimeter high resistance gear to check the insulation of the thyristor in the soft starter. New soft starter (cold) each group of thyristor input and output terminals measured resistance value should indicate about 1.3MΩ, phase to phase (phase is not connected to the control circuit), relative to the ground measured resistance value should indicate infinity. After checking and confirming that there is no error, connect a small-power motor to the output of the soft starter and set the starting mode, initial voltage, starting time and stopping time and other technical parameters. Before connecting the motor to the soft starter, the insulation status of the motor and cable must be checked with a 500V insulation resistance tester. Only if the insulation resistance complies with the relevant regulations, it is authorized to connect the motor to the output of the soft starter for starting test. The first soft start before the machinery to manpower disk, check the machinery with or without "card blocking" phenomenon, and then equipment startup test and adjust the startup parameters, can be delivered to the use.
In short, the soft starter starting torque and can be adjusted, the equipment startup time is short, with soft stop function, less components, small amount of maintenance, to complete the more difficult to start the equipment, is the performance of a better motor startup control equipment, should be popularized and applied.
Introduction of the principle of frequency converter
Frequency converter is the use of power semiconductor devices on and off the role of power frequency power supply will be converted to another frequency of the power control device. The frequency converter we use nowadays mainly adopts the AC-DC-AC mode (VVVF frequency converter or vector control frequency converter), which firstly converts the AC power of industrial frequency into DC power through rectifier, and then converts the DC power into the AC power with controllable frequency and voltage for supplying to the motor. The circuit of inverter is generally composed of four parts: rectifier, intermediate DC link, inverter and control. Rectifier part of the three-phase bridge uncontrolled rectifier, inverter part of the IGBT three-phase bridge inverter, and the output for the PWM waveform, the intermediate DC link for filtering, DC energy storage and buffer reactive power.
Selection of frequency converter:
The following points should be determined when selecting the frequency converter:
1) The purpose of using frequency conversion; constant voltage control or constant current control, etc..
2) The type of load of the frequency converter; such as vane pumps or displacement pumps, etc., pay special attention to the performance curve of the load, the performance curve determines the way and method of application.
3) Frequency converter and load matching problem;
I. Voltage matching; the rated voltage of the frequency converter and the rated voltage of the load match.
II. Current matching; for ordinary centrifugal pumps, the rated current of the inverter matches the rated current of the motor. For special loads such as deep water pumps need to refer to the motor performance parameters to determine the maximum current inverter current and overload capacity.
III. Torque matching; this situation is possible in constant torque loads or when there is a deceleration device.
4) When inverter is used to drive high speed motor, due to small reactance of high speed motor, the increase in high harmonics leads to increase in output current value. Therefore, the selection of inverter for high-speed motors, its capacity should be slightly larger than the selection of ordinary motors.
5) If the frequency converter is to be run on long cables, measures should be taken to suppress the influence of the long cable coupling capacitance on the ground to avoid insufficient output of the frequency converter, so in this case, the capacity of the frequency converter should be enlarged by one step or output reactor should be installed on the output side of the frequency converter.
6) For some special applications, such as high temperature, high altitude, this will cause the inverter capacity reduction, the inverter capacity should be enlarged by one gear.
Inverter control schematic design:
1) First confirm the installation environment of the inverter;
I. Operating temperature. Frequency converter inside is a high-power electronic components, very susceptible to the influence of the operating temperature, the product is generally required to 0 ~ 55 ℃, but in order to ensure safe and reliable work, the use of should be considered to leave a margin of error, it is best to control the temperature below 40 ℃. In the control box, the frequency converter should generally be installed in the upper part of the box, and strictly comply with the installation requirements of the product specification, absolutely do not allow the heat-generating components or easy to heat the components installed close to the bottom of the frequency converter.
II. Ambient temperature. When the temperature is too high and the temperature change is large, the inverter is prone to internal condensation phenomenon, its insulation performance will be greatly reduced, and may even cause a short-circuit accident. If necessary, desiccant and heater must be added to the box. In the water treatment room, the general water vapor is heavier, if the temperature change is large, this problem will be more prominent.
III. Corrosive gases. Use of the environment if the concentration of corrosive gases is large, not only will corrode the component leads, printed circuit boards, etc., but also accelerate the aging of the plastic device, reducing the insulation properties.
IV. Vibration and shock. Control cabinets equipped with inverters are subject to mechanical vibration and shock, which can cause poor electrical contact. Huai'an thermoelectricity on such a problem. In addition to improving the mechanical strength of the control cabinet, away from the source of vibration and shock sources, should also use anti-vibration rubber pads to fix the control cabinet outside and inside the electromagnetic switches and other components that generate vibration. After a period of operation, the equipment should be inspected and maintained.
V. Electromagnetic interference. Frequency converter in the work due to rectification and frequency conversion, around the generation of a lot of interference electromagnetic waves, these high-frequency electromagnetic waves on the nearby instruments, instruments have a certain degree of interference. Therefore, the cabinet instrumentation and electronic systems, should be selected metal shell, shielding the frequency converter on the interference of the instrument. All components should be reliably grounded, in addition to the electrical components, instruments and meters between the connection should be selected shielded control cable, and the shield should be grounded. If the electromagnetic interference is not handled well, often the whole system can not work, resulting in control unit failure or damage.
2) The distance between the inverter and the motor to determine the cable and wiring method;
I. The distance between the inverter and the motor should be as short as possible. This reduces the capacitance of the cable to ground and reduces the source of interference emission.
II. Control cables use shielded cables, power cables use shielded cables or from the inverter to the motor all shielded with a penetrating pipe.
III. motor cable should be independent of other cable routing, the minimum distance of 500mm. at the same time, should avoid the motor cable and other cables long distance parallel routing, so as to reduce the frequency converter output voltage rapid changes in electromagnetic interference. If the control cable and power cable cross, they should be crossed at an angle of 90 degrees as far as possible. With the frequency converter related analog signal line and the main circuit line separate routing, even in the control cabinet should be so.
IV. The analog signal line related to the frequency converter should preferably use shielded twisted-pair cable, and the power cable should use shielded three-core cable (whose specification should be larger than that of the ordinary motor cable) or follow the user's manual of the frequency converter.
3) Inverter control schematic;
I. Main circuit: the role of the reactor is to prevent the frequency converter generated by the high harmonics through the power supply input circuit to return to the power grid, thereby affecting other powered equipment, need to be based on the size of the capacity of the inverter to decide whether to add the reactor; filter is installed in the output of the inverter, to reduce the output of the frequency converter of the high harmonics when the distance from the frequency converter to the motor is relatively large. The filter should be installed when the distance from the inverter to the motor is far. Although the inverter itself has a variety of protection functions, but the phase loss protection is not perfect, the circuit breaker in the main circuit plays a role in overload, phase loss and other protection, selection can be made in accordance with the capacity of the inverter. The overload protection of the inverter itself can be used to replace the thermal relay.
II. Control circuit: with frequency inverter manual switching, in order to frequency inverter failure can be manually cut frequency operation, because the output can not add voltage, solid frequency and frequency to have interlock.
4) Grounding of the inverter;
Correct grounding of the inverter is an important means of improving system stability and suppressing noise. The smaller the grounding resistance of the grounding terminal of the frequency converter, the better, and the cross-section of the grounding wire is not less than 4mm, and the length is not more than 5m. The grounding of the frequency converter should be separated from the grounding point of the power equipment, and can not be **** ground. One end of the shield of the signal line is connected to the grounding end of the inverter, and the other end is floating. The inverter and the control cabinet are electrically connected to each other.
Frequency converter control cabinet design:
Frequency converter should be installed inside the control cabinet, the control cabinet should pay attention to the following issues in the design
1) Heat dissipation: the heat of the frequency converter is generated by the internal loss. In the frequency converter in each part of the loss is mainly in the main circuit, accounting for about 98%, the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the frequency converter, the frequency converter must be dissipated we usually use fan cooling; frequency converter internal fan can be the frequency converter box internal heat dissipation away, if the fan can not work properly, you should immediately stop the frequency converter operation; high-power frequency converter also need to add a fan on the control cabinet, the control cabinet air ducts should be reasonably designed, all the inlet should be set up to prevent dust nets, exhaust smooth, avoid the formation of eddy currents in the cabinet, and avoid the formation of eddy currents in the fixed cabinet. cabinet to form eddy currents, the formation of dust accumulation in a fixed position; according to the inverter manual ventilation to select the matching fan, fan installation should pay attention to the problem of anti-vibration.
2) Electromagnetic interference problems:
I. Inverter in the work due to rectification and frequency conversion, around the generation of a lot of interference electromagnetic waves, these high-frequency electromagnetic waves on the nearby meters, instruments have a certain degree of interference, and will produce high harmonics, this high harmonics will be through the power supply circuit into the entire power supply network, thus affecting other instruments. If the power of the frequency converter is very large accounted for more than 25% of the whole system, you need to consider the control of the power supply anti-interference measures.
II. When the system has high-frequency impact loads such as welding machines, electroplating power supply, the inverter itself will be protected because of interference, then consider the power quality of the entire system.
3) Protection issues need to pay attention to the following:
I. Waterproof condensation: If the inverter is placed in the field, need to pay attention to the inverter cabinet above the pipe flange or other leakage points, in the vicinity of the inverter can not have splash water, in short, the field cabinet protection level should be above IP43.
II. Dustproof: all air inlets should be set up to block the dust net flocculent debris into the dust net should be designed as a removable type, in order to facilitate cleaning, maintenance. The grid of the dust net is determined according to the specific conditions of the site, and the combination of the dust net around the control cabinet should be handled tightly.
III. Anti-corrosive gases: In the chemical industry, this situation is more common, this time you can put the frequency converter cabinet in the control room.
Frequency converter wiring specifications:
Signal line and power line must be separated: when using analog signals to remotely control the frequency converter, in order to reduce the analog interference from the frequency converter and other equipment, please separate the signal line of the control frequency converter and the strong power circuit (the main circuit and the downstream control circuit). The distance should be more than 30cm. The same wiring specification should be maintained even inside the control cabinet. The maximum length of the control loop line between this signal and the frequency converter shall not exceed 50m.
The signal line and power line must be placed in different metal pipes or inside metal hoses: the signal line connecting PLC and frequency converter is extremely susceptible to interference by the frequency converter and external equipment if it is not placed in a metal pipe; at the same time, since the frequency converter does not have any built-in reactors, the input and output level of the frequency converter Power line to the outside will produce strong interference, so place the signal line of the metal pipe or metal hose has to be extended to the control terminals of the frequency converter, in order to ensure that the signal line and the power line of the complete separation.
1) analog control signal line should use double stranded shielded wire, wire specifications for the 0.75mm2. In the wiring must pay attention to, cable stripping should be as short as possible (5-7mm or so), and at the same time after the stripping of shielding layer should be wrapped with insulating tape to prevent the shielding line from contacting with other equipment to introduce interference.
2) In order to improve the simplicity and reliability of the wiring, it is recommended to use crimp rod terminals on the signal lines.
The operation of the frequency converter and the related parameter settings:
The frequency converter has many setting parameters, each parameter has a certain selection range, the use of the frequency converter is often encountered due to the individual parameter settings are not appropriate, resulting in the inverter can not work properly.
Control mode: speed control, pitch control, PID control or other ways. After taking the control mode, generally according to the control accuracy, the need for static or dynamic identification.
Minimum operating frequency: that is, the minimum speed of motor operation, the motor runs at low speed, its heat dissipation performance is very poor, the motor runs for a long time at low speed, will lead to motor burnout. And at low speed, the current in its cable will also increase, which will also lead to cable heating.
Maximum operating frequency: the general frequency converter maximum frequency to 60Hz, some even to 400 Hz, high frequency will make the motor high-speed operation, which is for ordinary motors, its bearings can not be a long time to run over the rated speed, the motor's rotor can withstand such a centrifugal force.
Carrier frequency: the higher the carrier frequency is set, the higher the harmonic component is, which is closely related to the length of the cable, the heat of the motor, the heat of the cable, the heat of the frequency converter and other factors.
Motor parameters: the frequency converter in the parameters set motor power, current, voltage, speed, maximum frequency, these parameters can be obtained directly from the motor nameplate.
Frequency hopping: at a certain frequency point, there is a possibility of *** vibration phenomenon, especially when the whole device is relatively high; in the control of the compressor, to avoid the compressor's wheezing point.
Common failure analysis:
1) overcurrent fault: overcurrent fault can be divided into acceleration, deceleration, constant speed overcurrent. It may be due to the frequency converter acceleration and deceleration time is too short, sudden changes in load, uneven load distribution, output short circuit and other reasons. This can generally be done by extending the acceleration and deceleration time, reduce the sudden change of load, plus energy braking elements, load distribution design, check the line. If disconnecting the load inverter or overcurrent fault, indicating that the inverter inverter circuit has been ring, need to replace the inverter.
2) overload fault: overload faults include frequency conversion overload and motor overload. It may be acceleration time is too short, the grid voltage is too low, the load is too heavy and other reasons. Generally can be extended by extending the acceleration time, extend the braking time, check the grid voltage and so on. The load is too heavy, the selected motor and inverter can not drag the load, may also be caused by poor mechanical lubrication. If the former must be replaced with high-power motors and inverters; if the latter to overhaul the production machinery.
3) undervoltage: the inverter power input part of the problem, need to check before you can run.
Summary:
1) In short, in the design, installation, and use of frequency converter must follow the guidance of the frequency converter instruction manual.
2) Various electrical designers, on-site electrical commissioning personnel can improve this frequency converter reference on this basis.