Myth 1: connect the electromagnetic switch and the electromagnetic contactor in the inverter output circuit
In practical application, some occasions need to use contactor to switch frequency converter: for example, when the frequency converter fails, switch to the power frequency state, or when one motor fails, the converter turns to drag another motor and so on. So many users will think that in the frequency converter output circuit to install electromagnetic switch, electromagnetic contacter is the standard configuration, is a safe way to disconnect the power supply, in fact, there are big hidden dangers.
Disadvantages: when the inverter is still running, the contactor will be disconnected first, and the load will be suddenly interrupted. The surge current will make the overcurrent protection action, and will produce certain impact on the rectifier inverter main circuit. Serious, even make inverter output module IGBT caused damage. At the same time, when the inductive motor is loaded, the inductive magnetic field energy cannot be released quickly, which will produce high voltage and damage the insulation of the motor and connecting cable.
Coping strategy: connect the output side of the frequency converter directly with the electric motor cable. The normal starting and power failure motivation can be realized by triggering the control terminal of the frequency converter to achieve the soft starting and stopping effect. If the contactor must be used on the output side of the variable-frequency governor, necessary control interlocks must be applied between the output of the variable-frequency governor and the action of the contactor, so that the contactor can only operate when the variable-frequency governor has no output.
Myth 2: equipment normal shutdown, disconnect inverter ac input power
In the normal shutdown of equipment, many users used to disconnect the inverter ac input power switch, think that it is safer, also can save energy.
Disadvantages: this approach, on the surface, seems to be able to protect the inverter from the impact of power failure. In fact, frequency converter for a long time without electrification, coupled with the impact of site humidity, will cause the internal circuit board moisture and slow oxidation, gradually appear short circuit phenomenon. This is why soft faults are frequently reported when the inverter is switched off for a period of time.
Countermeasures: in addition to equipment maintenance, should make the inverter in a long time on the state. Besides, still should open the fluctuation fan of frequency conversion control cabinet, place desiccant inside cabinet or install automatic temperature and humidity to control heater, keep ventilated and environment dry.
Myth 3: open air or dust environment installed inverter control cabinet with sealing type.
Inverter control cabinets installed in some factories, basements and open air will be subjected to the harsh test of such harsh environment as high temperature, dust and humidity. For this, a lot of users can choose the frequency conversion cabinet of sealing type. In this way, although it can prevent rain and dust to a certain extent, it also brings the problem of inverter heat dissipation.
Disadvantages: the tight sealing of the control cabinet will cause the frequency converter to overheat the internal components due to insufficient ventilation and heat dissipation ability, and the thermal sensitive components will be protected, resulting in fault tripping and forced shutdown of the equipment.
Countermeasures: install a breathable rain-proof cover on the upper part of the inverter control cabinet, with dustproof filter screen, and serve as the exhaust port at the same time. The lower part is also slotted to install the fan with filter screen as the air inlet. Can form air circulation, and filter the dust in the environment at the same time. Cooling air flow direction: from the bottom to the top. The horizontal installation distance between frequency converters shall be no less than 5mm, and the cooling air temperature into the frequency converters shall not exceed +40 ℃. If the ambient temperature is above +40 ℃ for a long time, the frequency converter should be installed in a small air-conditioned room.
In the control box, the frequency converter should be installed on the upper part of the box. It is absolutely not allowed to install the heating element or heating element close to the bottom of the frequency converter.
Myth 4: to improve the voltage quality, put the power factor compensation capacitor in parallel at the output end of the converter
Due to the limitation of power capacity, voltage quality cannot be guaranteed in some enterprises, especially when large electrical equipment is put into use, the bus voltage in the plant station will decrease, and the load power factor will obviously decline with the decline. In order to improve the voltage quality, the power factor compensation capacitor is usually connected in parallel at the output end of the converter, hoping to improve the power factor of the motor.
Disadvantages: connect the power factor compensation capacitor and surge absorber to the motor cable (in the transmission unit and motor
), their influence will not only reduce the control accuracy of the motor, but also form transient voltage on the output side of the transmission unit, causing permanent damage to the ACS 800 transmission unit. If the power factor compensation capacitor is in parallel on the ACS 800 three-phase input line, it must be ensured that the capacitor and the ACS 800 are not charged at the same time to avoid voltage surge damage to the converter. Inverter current flow to improve the power factor of the capacitor, due to its charging current caused by inverter over current (OCT), so can not start.
Countermeasures: remove the capacitor and run it. As for improving the power factor, it is effective to connect the AC reactor on the input side of the converter.
Mistake 5: choose the circuit breaker as the inverter thermal overload and short circuit protection, the effect is better than the fuse
The circuit breaker has perfect protection function and has been widely used in power distribution equipment. Now many manufacturers of complete sets of frequency conversion speed control equipment, are basically equipped with circuit breakers (air switches), in fact, there are some safety risks.
Disadvantages: when the power cable is short-circuit fault, the circuit breaker protection trip will cause delay due to the inherent operating time of the circuit breaker itself, during which the short-circuit current will be introduced into the inverter, causing damage to the components.
Solution: as long as the cable is selected according to the rated current, the inverter transmission unit can protect itself, the input end and the motor cable to prevent thermal overload. No additional thermal overload protection equipment is required. Equipped with a fuse will protect the input cable in case of short circuit, reducing device damage and preventing damage to connected equipment in case of short circuit inside the transmission.
Check the configured fuse action time should be less than 0.5 seconds. The operating time depends on the type of fuse (gG or aR), the supply network impedance, the cross-sectional area, material, and length of the power cable. When the gG fuse exceeds the 0.5 second action time, fast melting (aR) reduces the action time to an acceptable level in most cases. The fuse must be of no delay type.
Circuit breakers do not provide fast enough protection for transmission devices because they react more slowly than fusing devices. Therefore, when fast protection is required, a fuse should be used instead of a circuit breaker.
Myth 6: inverter selection only needs to consider the load power
When purchasing frequency converter, many users usually only match the frequency converter capacity according to the power of the driving motor. In fact, the motor driven by the load is not the same, the inverter requirements are not the same.
Disadvantages: due to the difference of the load characteristics of the motor, if comprehensive factors are not fully considered, the inverter may be damaged due to improper use. At the same time, the failure to equip the necessary brake unit and filter may cause safety risks.
Countermeasures: according to the characteristics and types of load, select the capacity and configuration of frequency converter reasonably.1) fan and water pump are the most common load: the requirement for frequency converter is the most simple, as long as the frequency converter capacity is equal to the motor capacity (air compressor, deep water pump, sediment pump, fast-changing music fountain needs to increase the capacity).
2) crane load: this type of load is characterized by great impact when starting, so it requires the frequency converter to have a certain margin. At the same time, there will be energy feedback when the elbow is under the weight, so use brake units or use Shared bus.
3) uneven load: some load sometimes light, sometimes heavy, at this time should be in accordance with the circumstances of heavy load to choose the frequency converter capacity, such as rolling machinery, crushing machinery, mixer, etc.
4) large inertia load: such as centrifuge, punch, rotary kiln of cement factory, such load inertia is very large, so may oscillate when starting, motor deceleration when energy feedback. A frequency converter with a higher capacity should be used to speed up the start and avoid oscillations. With the brake unit to eliminate the feedback power.
With the cooperation of other intelligent equipment (PLC, DCS system), frequency converter can realize multiple control strategies and closed-loop regulation, and it also has relatively perfect protection function. But in the practical application and installation environment, there are many mistakes. Facing up to the contradiction, avoiding the risk and using reasonably are the key to improve the frequency converter efficiency and service life.