Six misunderstandings and Response strategy in the use of frequency converter

- Sep 27, 2017 -

Misunderstanding 1: In the inverter output circuit connected to the electromagnetic switch, electromagnetic contactor

In practical applications, some occasions need to use the contactor to switch the inverter: such as when the inverter frequency switch to the frequency of the state to run, or when using a drag two, a motor failure, the inverter turned to another Taiwan motor and so on. So many users will think that the inverter output circuit installed electromagnetic switch, electromagnetic contactor is a standard configuration, is safe to disconnect the power of the way, in fact, this approach there is a big hidden dangers.

【Disadvantages:】 When the inverter is still running, the contactor first disconnect, suddenly interrupt the load, the surge current will over-current protection action, will give the rectifier inverter main circuit to produce a certain impact. Serious, and even cause the inverter output module IGBT damage. At the same time, in the induction motor load, the emotional magnetic field energy can not be quickly released, will produce high voltage, damage the motor and connecting cable insulation.

【Response strategy】: the inverter output side directly connected with the motor cable, the normal starting and stopping the motor can be triggered by triggering the inverter control terminal to achieve the effect of soft soft stop. If it is necessary to use the contactor on the output side of the inverter, it must be necessary to control the interlock between the output of the inverter and the contactor to ensure that the contactor can only operate when the inverter is not output.

Misunderstanding 2: When the device is out of service, disconnect the inverter AC input power

In the equipment outage, many users are accustomed to disconnect the inverter AC input power switch, that is more secure, can also save energy.

【Disadvantages:】 This approach, the surface seems to be able to protect the inverter from the role of power failure impact. In fact, the inverter for a long time without electricity, coupled with the impact of environmental protection on the scene, will cause the internal circuit board damp and slow oxidation, the gradual emergence of short circuit phenomenon. This is the inverter power outages for some time, once again when the power transmission will be reported to the reasons for soft failure.

【Response strategy】 In addition to equipment maintenance, the inverter should be a long time in a live state. In addition, should also open the inverter cabinet up and down the fan, placed in the cabinet desiccant or install automatic temperature and humidity control heater, keep the ventilation and dry environment.

Misunderstanding 3: open air or dust environment installed in the inverter control cabinet with sealed type

In some factories and mines, basement, open-air installation of the inverter control cabinet, will withstand such as high temperature, dust, humidity and other harsh environments of the harsh test. To this end, many users will use the sealed type of frequency conversion cabinet. Although this to a certain extent, can play a rain, dust effect, but also brought the problem of poor heat dissipation of the inverter.

【Disadvantages:】control cabinet seal will make the inverter due to lack of ventilation and cooling capacity caused by internal components overheating, thermal components to protect the action, resulting in fault trip, the equipment was forced outage.

【Response strategy】 in the upper part of the inverter control cabinet installed breathable rain cover, and with a dust filter, and as an exhaust port. The lower part of the same slot is installed with a filter fan, as the air inlet. Can form air circulation, while filtering the dust in the environment. Cooling air flow direction: from the bottom to the top. The horizontal installation distance between the inverter should be not less than 5mm, into the inverter cooling air temperature can not exceed +40 degrees Celsius. If the ambient temperature is above +40 degrees Celsius for a long time, consider installing the drive in a small room with air conditioning.

In the control box, the inverter should normally be installed in the upper part of the box, absolutely not allowed to heat the element or easy to heat the components close to the bottom of the inverter installation.

Misunderstanding 4: In order to improve the voltage quality, in the inverter output parallel power factor compensation capacitor

Some enterprises due to the use of electricity capacity constraints, voltage quality can not be guaranteed, especially large-scale use of electricity equipment, will cause the factory bus voltage drop, load power factor significantly decreased. To improve the voltage quality, the user usually in the inverter output parallel to the power factor compensation capacitor, hoping to improve the motor power factor.

【Disadvantages:】Connect the power factor compensation capacitor to the surge absorber on the motor cable (between the drive unit and the motor). Their effects not only reduce the motor's control accuracy, but also on the output side of the drive unit. Variable voltage, causing permanent damage to the ACS800 drive unit. If the power factor compensation capacitor is connected in parallel to the three-phase input line of the ACS800, it must be ensured that the capacitor and the ACS800 are not charged at the same time to avoid surge voltage damage to the inverter. The current flowing into the inverter flows into the capacitor that improves the power factor, and can not be started due to the charging current causing the inverter overcurrent (OCT).

【Response strategy】 the capacitor removed after the operation, as to improve the power factor, the input side of the inverter access AC reactor is effective.

Misunderstanding 5: the use of circuit breakers as the inverter thermal overload and short circuit protection, the effect is better than the fuse Circuit breakers have a more complete protection, has been widely used in power distribution equipment, a great replacement of the traditional fuse trend. Now many manufacturers of complete sets of frequency control equipment, are basically equipped with circuit breakers (air switch), in fact, there are some security risks.

【Disadvantages:】in the power cable short circuit failure, the circuit breaker protection action trip due to the circuit breaker itself, the inherent action time and delay, this period will short-circuit current into the inverter internal, resulting in damage to components.

【Response strategy】 As long as the cable is selected according to the rated current, the drive unit can protect itself, input terminals and motor cables to prevent thermal overload, and do not require additional thermal overload protection equipment. The configuration fuse will protect the input cable in the event of a short circuit, reduce device damage and prevent damage to the connected equipment when the drive is internally shorted.

Check the configuration of the fuse action time should be less than 0.5 seconds. The operating time depends on the fuse type (gG or aR), the power supply network impedance, the cross-sectional area of the power cable, the material and the length. When using a gG fuse for more than 0.5 seconds of operating time, the fast melt (aR) can in most cases reduce the operating time to an acceptable level. The fuse must be of no delay type.

The circuit breakers do not provide fast enough protection for the drive because they are slower than the fuses. Therefore, when fast protection is required, a fuse should be used instead of a circuit breaker.

Misunderstanding 6: inverter selection only need to consider the load power

Many users in the purchase of the inverter, usually only according to the drive motor power to match the inverter capacity. In fact, the motor driven by the load is not the same, the requirements of the inverter is not the same.

【Disadvantages:】due to the load characteristics of the motor there are differences, if not fully consider the comprehensive factors, may cause improper use of the inverter damage, and because not equipped with the necessary braking unit and filter, may cause security risks.

【Response strategy】 for the characteristics and type of load, the rational use of the inverter capacity and configuration.

(1) the fan and the pump is the most common load: the most simple requirements of the inverter, as long as the inverter capacity is equal to the motor capacity (air compressor, deep water pump, sediment pump, fast changing music fountain to increase capacity ).

(2) crane load: This type of load is characterized by a great impact on the start, so the inverter has a margin. At the same time, in the weight of the elbow, there will be energy feedback, so to use the brake unit or the use of common bus way.

(3) uneven load: some load sometimes light, sometimes heavy, this time should be in accordance with the heavy load situation to select the inverter capacity, such as rolling mill machinery, crushing machinery, mixer and so on.

(4) large inertia load: such as centrifuges, presses, cement mill rotary kiln, such a large inertia load, so the start may be oscillating, the motor deceleration energy recovery ... ... should be slightly larger capacity of the inverter Speed up the start to avoid oscillation. With the brake unit to eliminate the feedback power.

Concluding remarks

Inverter with other intelligent equipment (PLC, DCS system) with, can achieve multiple control strategies and closed-loop regulation, which itself also has a more complete protection. But in the practical application and installation environment, but there are many errors. Face the contradiction, to avoid risks, rational use, is to improve the efficiency of the inverter and the life of the key.

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