Frequency Converter Knowledge: Application

Several issues that need to be paid attention to in the application of inverter in engineering

1 Introduction

With the increasing prosperity of the general inverter market, excluding OEM imported inverters, the annual consumption of general inverters in China exceeds RMB 2.5 billion. The installation, commissioning, daily maintenance and repair workload of inverters and their ancillary equipment has increased dramatically, causing significant direct and indirect losses to users. This article analyzes the causes of the above problems from the aspects of application environment, electromagnetic interference and anti-interference, power grid quality, motor insulation, etc. based on the actual application of a large number of users, and puts forward some suggestions for improvement.

2. Work environment issues

In the actual application of frequency converters, except for a few domestic customers who have dedicated machine rooms, most of them install frequency converters directly on industrial sites to reduce costs. The work site is generally dusty and hot, and in the south there is also the problem of high humidity. There is also metal dust in the cable industry, corrosive gases and dust in the ceramics, printing and dyeing industries, and explosion-proof requirements in coal mines and other occasions. Therefore, corresponding countermeasures must be made according to the on-site conditions.

2.1 Basic requirements for inverter installation design

(1) The inverter should be installed inside the control cabinet.

(2) The inverter is best installed in the middle of the control cabinet; the inverter should be installed vertically, and large components that may block the exhaust and air intake should be avoided directly above and below .

(3) The minimum distance between the upper and lower edges of the inverter and the top, bottom, or partition of the control cabinet, or large components that must be installed, should be greater than 300mm.

Basic requirements for installing inverters in cabinets

(4) If a special user needs to remove the keyboard during use, the keyboard hole on the inverter panel must be strictly sealed with tape or replaced with a fake panel to prevent a large amount of dust from entering the inverter.

(5) The inverter should be maintained regularly and internal dust should be cleaned in time.

(6) Other basic installation and use requirements must comply with the relevant instructions in the user manual; if you have any questions, please contact the corresponding manufacturer's technical support personnel in a timely manner.

2.2 Design requirements for dustproof control cabinets

When using the inverter in dusty places, especially places with a lot of metal dust and flocs, it is very necessary to take correct and reasonable protective measures. Proper dust prevention measures are very important to ensure the normal operation of the inverter. The overall requirements are that the control cabinet should be sealed as a whole, and ventilation should be carried out through specially designed air inlets and outlets; there should be a protective net and protective top cover outlet on the top of the control cabinet; there should be a bottom plate and air inlet and wire inlet holes at the bottom of the control cabinet, and a dust net should be installed.

(1) The air duct of the control cabinet should be designed reasonably and exhaust air smoothly to avoid the formation of vortex in the cabinet and the accumulation of dust in fixed positions.

(2) A protective cover should be installed above the air outlet on the top of the control cabinet to prevent debris from falling in directly; the height of the protective cover should be reasonable and should not affect the exhaust. A protective net should be installed on the side air outlet of the protective cover to prevent flocculent debris from falling in directly.

(3) If the exhaust method is adopted on the top side of the control cabinet, a protective net must be installed on the air outlet.

(4) Make sure that the axial flow fan on the top of the control cabinet rotates in the correct direction and draws air outward. If the fan is installed outside the top of the control cabinet, make sure there is enough height between the protective cover and the fan; if the fan is installed inside the top of the control cabinet, the screws required for installation must use anti-return springs to prevent the fan from falling off and causing damage to the components and equipment in the cabinet. It is recommended to install plastic or rubber vibration-damping washers between the fan and the cabinet to greatly reduce the noise caused by the fan vibration.

(5) The front and rear doors and other joints of the control cabinet should be sealed with sealing gaskets or sealants to prevent dust from entering.

(6) All air inlets and cable inlets on the bottom and side panels of the control cabinet must be equipped with dust screens to prevent flocculent debris from entering. The dust screen should be designed to be detachable for easy cleaning and maintenance. The mesh of the dust screen should be small enough to effectively block fine flocculent debris (similar to the mesh of ordinary household mosquito and fly screens); or the appropriate mesh size can be determined according to the specific situation. The joints between the dust screen and the control cabinet should be handled tightly.

(7) The control cabinet must be regularly maintained to clean up dust, lint and other debris inside and outside in a timely manner. The maintenance cycle can be determined according to the specific situation, but should be less than 2 to 3 months; for places with severe dust, the recommended maintenance cycle is about 1 month.

Installation requirements for dustproof control cabinets

2.3 Design requirements for moisture and mildew-proof control cabinets

Most inverter manufacturers do not carry out special treatment to prevent moisture and mildew on their internal printed circuit boards and metal structural parts. If the inverter is in this state for a long time, the metal structural parts are prone to rust. For the conductive copper busbar, the rust process is further aggravated when it is operated at high temperatures. The small copper wires on the microcomputer control board and the drive power board will be damaged due to rust. Therefore, for applications in humid and corrosive gas environments, there must be basic requirements for the internal design of the inverter. For example, the printed circuit board must be sprayed with three-proof paint, and the structural parts must be treated with nickel-chromium plating and other treatment processes. In addition, other active, effective and reasonable measures to prevent moisture and corrosive gases are also needed.

(1) The control cabinet can be installed in a separate, enclosed machine room with air conditioning. This method is suitable for situations where there are many control devices and the cost of building a machine room is lower than that of enclosing the cabinet separately. In this case, the control cabinet can adopt the above dustproof or general environment design.

(2) Use an independent air inlet. A separate air inlet can be located at the bottom of the control cabinet and connected to the clean external environment through an independent enclosed trench. This method requires the installation of a dust screen at the air inlet. If the trench is longer than 5m, consider installing a blower.

(3) A desiccant that absorbs moisture or an active material that absorbs toxic gases can be installed in the closed control cabinet and replaced in the near future.

3 Interference issues

3.1 Interference of the frequency converter on the microcomputer control board

In the control systems of injection molding machines, elevators, etc. , microcomputers or PLCs are often used for control. During system design or modification, attention must be paid to the interference of the frequency converter on the microcomputer control board. Since the microcomputer control board designed by the user is generally of poor craftsmanship and does not meet the international EMC standards, the conduction and radiation interference generated after the frequency converter is used often leads to abnormal operation of the control system, so necessary measures need to be taken.

(1) Good grounding. The grounding wire of the motor and other high-voltage control systems must be reliably grounded through the grounding bus. The shielding ground of the microcomputer control board should preferably be grounded separately. For some occasions with severe interference, it is recommended to connect the shielding layer of the sensor and I/O interface to the control ground of the control board [3].

(2) Adding EMI filters, common mode inductors , high-frequency magnetic rings, etc. to the input power supply of the microcomputer control board is low-cost and can effectively suppress conducted interference. In addition, in situations where radiation interference is severe, such as when there are GSM or PHS stations around, a metal mesh shielding cover can be added to the microcomputer control board for shielding.

Anti-interference measures for power supply of microcomputer control board

(3) Adding an EMI filter to the inverter input can effectively suppress the conducted interference of the inverter to the power grid. Adding input AC and DC reactors L1 and L2 can improve the power factor and reduce harmonic pollution, with good overall effect. In some cases where the distance between the motor and the inverter exceeds 100m, it is necessary to add an AC output reactor L3 on the inverter side to solve the leakage current protection caused by the output wire to the ground distribution parameters and reduce the external radiation interference. An effective method is to use steel pipe threading or shielded cable method, and reliably connect the steel pipe shell or cable shielding layer to the ground. Please note that when the AC output reactor L3 is not added, if the steel pipe threading or shielded cable method is used, the distributed capacitance of the output to the ground is increased, which is prone to overcurrent. Of course, in practice, only one or several of these methods are generally adopted.

Measures to reduce the interference of frequency converter to external control equipment

(4) Electrically shield and isolate the analog sensor detection input and analog control signal. In the design process of the control system composed of the inverter, it is recommended not to use analog control, especially when the control distance is greater than 1M and it is installed across the control cabinet. Because the inverter generally has multi-speed settings, switch frequency input and output, it can meet the requirements. If analog control must be used, it is recommended to use shielded cables and achieve remote grounding on the sensor side or the inverter side. If the interference is still serious, DC/DC isolation measures need to be implemented. You can use a standard DC/DC module, or use V/F conversion, optical coupling isolation and then use the frequency setting input method.

3.2 The inverter's own anti-interference problem

When there are high-frequency impact loads such as welding machines, electroplating power supplies, electrolytic power supplies, or slip ring power supplies near the inverter power supply system, the inverter itself is prone to protection due to interference. Users are recommended to take the following measures:

(1) Add inductors and capacitors to the input side of the inverter to form an LC filter network .

(2) The power supply line of the inverter is directly supplied from the transformer side.

(3) If conditions permit, a separate transformer can be used.

(4) When using external switch control terminals for control, it is recommended to use shielded cables when the connection line is long. When the control line and the main circuit power supply are buried in the trench, in addition to the control line must use shielded cable, the main circuit line must use steel pipe shielding to reduce mutual interference and prevent malfunction of the inverter.

(5) When using external analog control terminals for control, if the connection line is within 1M, use shielded cables for connection and implement single-point grounding on the inverter side; if the line is long and there is severe on-site interference, it is recommended to install a DC/DC isolation module on the inverter side or use V/F conversion and frequency command setting mode for control.

(6) When using external communication control terminals, it is recommended to use shielded twisted pair cables and ground the shield layer on the inverter side (PE). If the interference is very serious, it is recommended to connect the shield layer to the control power ground (GND). For RS232 communication, pay attention to the control line should not exceed 15m. If it needs to be extended, the communication baud rate must be reduced accordingly. When it is about 100m, the baud rate that can communicate normally is less than 600bps. For RS485 communication, terminal matching resistance must also be considered. For high-speed control systems using fieldbus , the communication cable must use a dedicated cable and adopt a multi-point grounding method to improve reliability.

4. Grid quality issues

In high-frequency impact loads such as welding machines, electroplating power supplies, electrolytic power supplies, etc., the voltage often flickers; in a workshop, when there are hundreds of inverters and other capacitive rectifier loads working, the harmonics of the power grid are very large, which seriously pollutes the quality of the power grid and has a considerable destructive effect on the equipment itself. At the least, it cannot operate continuously and normally, and at the worst, it causes damage to the input circuit of the equipment. The following measures can be taken:

Centrally rectified DC common bus power supply mode

(1) In situations with high-frequency impact loads such as welding machines, electroplating power supplies, and electrolytic power supplies, it is recommended that users add reactive static compensation devices to improve the power factor and quality of the power grid.

(2) In workshops where frequency converters are concentrated, it is recommended to use centralized rectification and DC common bus power supply. It is recommended that users use 12-pulse rectification mode. The advantages are small harmonics and energy saving , which is particularly suitable for occasions where frequent starting and braking, electric operation and power generation operation are carried out simultaneously.

(3) Install a passive LC filter on the input side of the inverter to reduce input harmonics and improve the power factor. This has low cost, high reliability and good effect.

(4) Installing an active PFC device on the input side of the inverter has the best effect, but the cost is higher.

5 Motor leakage, shaft voltage and bearing current issues

The motor model of the inverter-driven induction motor, Csf is the equivalent capacitance between the stator and the housing, Csr is the equivalent capacitance between the stator and the rotor, Crf is the equivalent capacitance between the rotor and the housing, Rb is the resistance of the bearing to the shaft; Cb and Zb are the capacitance and nonlinear impedance of the bearing oil film.

Under high-frequency PWM pulse input, the voltage coupling of the distributed capacitance in the motor forms a system common-mode loop, which causes problems of ground leakage current, shaft voltage and bearing current.

The leakage current is mainly generated between the PWM three-phase power supply voltage and the extremely instantaneous unbalanced voltage and the ground through Csf. Its size is related to the dv/dt size of PWM and the switching frequency, and its direct result will cause the leakage protection device to operate. In addition, for old motors, due to their poor insulation materials and long-term aging, some have insulation damage after frequency conversion. Therefore, it is recommended that insulation tests must be performed before the conversion . The insulation requirements for new variable frequency motors are one level higher than those for standard motors.

Bearing current mainly exists in three forms: dv/dt current, EDM (Electr ic Discharge Mac hining) current and loop current. The magnitude of the shaft voltage is not only related to the coupling capacitance parameters of each part of the motor, but also to the pulse voltage rise time and amplitude. The dv/dt current is mainly related to the rise time tr of PWM. The smaller tr is, the larger the amplitude of the dv/dt current is; the higher the inverter carrier frequency is, the more dv/dt current components there are in the bearing current. The EDM current appears with a certain degree of contingency. Only when the bearing lubricating oil layer is broken down or contact occurs inside the bearing, the charge (1/2 Crf×Urf) stored in the electronic rotor-to-ground capacitance Crf is discharged to the ground through the bearing equivalent circuit Rb, Cb and Zb in a spark-like manner, causing the bearing finish to decrease, reducing the service life, and seriously causing direct damage. The degree of damage mainly depends on the shaft voltage and the size of the electronic rotor-to-ground capacitance Crf.

Loop current occurs in the loop between the grid transformer ground, the inverter ground, the motor ground, and the motor load and the earth ground (such as water pump loads). Loop current mainly causes conducted interference and ground interference, and has little effect on the inverter and motor. The way to avoid or reduce the loop current is to reduce the impedance of the ground loop as much as possible. Since the inverter ground wire (PE inverter) is generally connected to the motor ground wire (PE motor 1) at one point, the motor ground cable diameter must be as thick as possible to reduce the resistance between the two. At the same time, the ground wire between the inverter and the power supply uses a ground copper busbar or a special grounding cable to ensure good grounding. For loads such as submersible deep well pumps, the ground impedance ZE motor 2 may be less than the sum of the ZE transformer and the ZE inverter, which is easy to form a ground loop. It is recommended to disconnect the ZE inverter for good anti-interference effect.

Connecting a sine wave filter composed of an inductor and RC in series at the output end of the inverter is an effective way to suppress shaft voltage and bearing current. Currently, many manufacturers can provide standard filters.