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Frequency selection and configuration points
Visitors:4355  Date Time: 7/14/2011 10:27:27 PM

1 Introduction
    
As power electronics technology continues to evolve and progress, along with the proposed new control theory and improvement of the AC variable speed drives, especially high performance frequency control gear has been developed rapidly. In recent years, the frequency of falling prices, but its function has continued to improve and expand, it is now widely used from some of the hundreds of watts of household equipment until some of the thousands of kilowatt-class large-scale industrial gear drive. AC variable speed from the original only for fans, pumps used in various types of speed the transition to the requirements of high precision, fast response, high-performance timing indicators industrial field. Promote the use of a large number of the inverter, energy-saving, labor-saving, automation and increase productivity, improve quality, reduce maintenance and improve comfort, and many have made application results have attracted worldwide attention. However, the drive, after all, one implication of the past two decades a variety of emerging high-tech electronic product, in order to make it play very good application effect, it must make its selection and configuration in-depth research.
    
2 The drive selection
    
The correct selection of frequency electronic control systems for machinery and equipment is essential to normal operation. Select the inverter, the first mechanical device in accordance with the type of load torque characteristics, speed range, the static speed accuracy, starting torque and the use of environmental requirements before deciding what kind of control and selection of protective structure, the most suitable inverter. The so-called right is the actual process machinery and equipment to meet production requirements and use cases under the premise of the application of the drive is the best value for money.
    
2.1 load torque characteristics of mechanical equipment
    
In practice, people often will produce mechanical torque characteristics under different load, divided into three types: constant torque load, constant power load and fluid class load.
    
2.1.1 Constant torque load
    
In this type of load, the load torque TL has nothing to do with the speed n, the total TL any constant speed or almost constant load power is increased with the load and linear speed increase. Conveyor, mixer, extruder and feed mechanism of mechanical equipment such as load and friction type cranes, hoists, elevators and other gravity loads, are constant torque load.
    
Constant torque converter drag the nature of load, low speed output torque is large enough and have enough overload. If you need to slow-speed when the next long run should be considered the standard cage induction motor cooling capacity to avoid the motor temperature is too high.
    
2.1.2 Constant Power Load
    
Demand for this type of load torque TL is characterized by generally inversely proportional to the rotational speed n, but its product is similar to that power remains unchanged. Metal cutting machine tool spindle and the rolling mill, paper machine, film production line coiler, coil etc., are constant power load.
    
The nature of the load is constant power speed range on certain terms. When the speed is very low, limited by the mechanical strength, TL can not be infinitely increased, at low speed into the nature of constant torque. Load of constant power and constant torque zone area of ??the drive program selection has a great influence. Motor speed in constant flux, the maximum allowable output torque constant, is constant torque speed; in weak magnetic speed, the maximum allowable output torque and speed is inversely proportional to, are constant power speed. If the motor constant torque and constant power speed range with constant torque load is consistent and constant power range, the so-called "matching" of cases, the capacity of the motor and inverter capacity are minimal.
    
2.1.3 Fluid class load
    
Type of load torque and speed is proportional to the quadratic, power and speed proportional to the cube. All kinds of fans, pumps and oil pumps, are typical of fluid class load.
    
Fluid class load by inverter to regulate air flow, flow, can greatly save energy. As the fluid class load power demand at high speed rapid growth, and the load is proportional to the cube of speed, it should not make such loads over frequency operation.
    
2.2 Select the appropriate load characteristics according to the frequency control mode
    
The inverter is now on the market a wide range of functions are also increasingly powerful, the performance of the inverter speed is increasingly becoming the merits of the determinants of performance, in addition to the manufacturing process of the drive itself, "a priori" conditions, the frequency converter What kind of control is also very important. The following table reviews the recent performance of various frequency control characteristics.
    
In summary, induction motor variable frequency control selection of different control methods, you can get different performance characteristics of the speed characteristics.
    
2.3 Depending on the installation environment, select the inverter protective structure
    
Inverter protective structure to adapt to their installation environment, it is necessary to consider the environmental temperature, humidity, dust, pH, corrosive gases and other factors, which can drive long-term, safe, reliable operation is at stake. Most drive manufacturers provide the following common for users to choose the protective structure:
    
(1) open IP00, protecting it from the front of the body can not touch live parts inside the inverter, suitable for installation in control cabinet or in the electrical room screens, plates, shelves, especially the use of more concentrated than the inverter good, but it requires high installation environment.
    
(2) closed IP20, IP21, this protective cover all around the structure of the inverter can be in the building wall wall mount, which applies to most indoor installation environment.
    
(3) Sealed IP40, IP42, it applies to the industrial field where poor environmental conditions.
    
(4) closed type IP54, IP55, it has a dust and waterproof protective structure for industrial on-site environmental conditions are poor, there is water washout, corrosive gases, dust and certain occasions.
    
Of the drive capacity is calculated, see references 1, space is limited, this does not repeat.
    
3 The drive peripheral configuration points
    
3.1 inverter connected to the large-capacity power transformer (500KVA and above) power, or in connection with the same power transformer without the use of thyristor converter converter reactor, or have the same power line with a switch capacitor banks to improve power should be configured AC reactor or DC reactor, they also have to improve power supply side inverter input power factor and reduce the effect of high harmonic currents.
    
3.2 between the drive and power supply with short circuit and overload protection should be installed in low-voltage circuit breaker, AC contactor, in order to avoid an accident expand inverter fault occurs. Electronic control system of emergency stop control should enable the inverter power supply side of the AC contactor breaking, completely cut off the inverter power supply, ensure that equipment and personal safety.
    
3.3 Inverter Input R, S, T and output U, V, W can not be connected correctly. Inverter input R, S, T is the three-phase rectifier with input Duanxiang connection, and output U, V, W three-phase asynchronous motor is connected with the transistor inverter circuit. If they pick the wrong, ranging from frequency control can not be achieved, the motor will not run, and at the burning frequency.
    
3.4 at the start and stop the frequent occasions, do not pass with the main circuit power supply, control the drive off to start, stop, the inverter should be used on the control panel RUN / STOP button or the SF / SR control terminal. Because the drive is started, the first to give the DC circuit of large-capacity electrolytic capacitor is charged, if the frequency is bound to result in frequent start capacitor charging current limiting resistor with a serious fever, but also shorten the life of a large-capacity electrolytic capacitor.
    
3.5 inverter terminal "N" is the low end of the intermediate DC circuit, is strictly prohibited and three-phase four-wire power lines connecting the zero line or the earth, otherwise it will create three-phase rectifier bridge damage due to power converter circuit .
    
3.6 inverter output generally can not install the magnetic contactor, if must be installed, be sure to pay attention to the following conditions: If the drive is running in non-switching output side magnetic contactor; To switch contacts must wait until the drive is stopped output before they can be. Because, if the normal output switching converter output side of the contactor, the contactor will be disconnected instantly generate a high frequency over-voltage and easily damaged in the power electronic devices. Therefore, to switch the inverter output side of the contactor, the control must wait until after the motor stopped completely.
    
3.7 In case of built-in brake unit and the drive to be external braking resistor, braking resistor must pay attention to the proper wiring. Braking resistors to be connected between P and DB can not be connected to the P, N, between otherwise cause the inverter to the inverter three-phase rectifier bridge is not running on full load, the inverter does not work, system fixed resistors may also have burned.
    
3.8 Frequency of multi-speed motor, the operation can not be changed in the number of pole pairs. If the frequency control system, in order to expand the speed range and must use multi-speed motor, due to multi-speed motor is used to change the method of the stator winding connection, change the number of pole pairs, to achieve speed control purposes. If the drive is running in the motor windings to change the wiring, can cause a great impact on the current, the inverter overload trip, and even burned a serious accident. Therefore, the safety switch to multi-speed motor windings, we must wait until the inverter stops output to proceed.
    
3.9 Frequency mechanical brake system in the proper use. In the pulse-width modulation (PWM) inverter, its output frequency and output voltage ratio is a constant, that f / V = ??C. The output frequency is low, the output voltage is low, if the mechanical brake of the electromagnetic brake coil connected to the U, V, W side, the low speed when the drive is always in the mechanical brake to hold fast to the state, the drive due to overload trip, so the mechanical brake solenoid coil connected to the inverter can only input R, S, T end.
    
3.10 drive low speed characteristics and countermeasures. Conventional design of self-ventilated induction motor at rated operating conditions and requirements of the environmental temperature range, will not exceed the rated temperature rise, but at the frequency control system, the situation is somewhat different. Self-ventilated induction motors running at 20HZ the following, the rotor blades of the cooling capacity decreased, and then if the constant torque loads long run, will inevitably cause the motor temperature increases, the characteristics of the speed control system deterioration. So, when self-ventilated induction motor runs at low frequency and drag constant torque load, the cooling must take compulsory measures to improve the motor's cooling capacity to ensure the stability of frequency control system.
    
3.11 When the wiring between the inverter and the motor long, with the inverter output cable length increases, the distributed capacitance increased significantly, resulting in the capacitive frequency inverter output peak current is too large causes the inverter to trip protection Therefore you must use the output reactor or du / dt filter or sine filter and other equipment for the capacitive peak current limit.
    
Output filter reactor to compensate for long cable laying in the motor circuit when the capacitor charging current caused, but also inhibit the harmonics. In multi-motor drives into a group, you can access a single output filter reactor with a total cable length is the sum of each motor cable length. In theory, the power level allowed by the laying of different inverter motor cable length is different, and different drive manufacturers to allow the laying of the motor cable lengths are different. Therefore, the motor of the drive cable laying over long distances should be the installation of multi-output filter reactor should also refer to the frequency supplied by the manufacturer manual.
    
3.12 Do not install the inverter output side of the power capacitor, surge suppressor, and radio noise filter, which will cause the inverter to malfunction or damage to the capacitor and surge suppressor.
    
3.13 leakage current and its frequency response. As the frequency of the input and output wiring and motor winding distributed capacitance exists, but now they use a PWM inverter modulation, and therefore leakage current flowing through them, its value is proportional to the distribution of electric capacity and the inverter carrier frequency . Therefore, to reduce the frequency leakage current, one as short as possible connections between the inverter and the motor length, the second is to minimize the frequency of the carrier frequency.
    
In order to protect the personal safety equipment and, in the line-side inverter leakage circuit breakers installed. Elected to use a dedicated frequency leakage circuit breakers, its rated sensitivity current: I △ n ≥ 10 × (Ig1 + Ign + Ig2 + Igm); elected by a general leakage breaker, its rated sensitivity current: I △ n ≥ 10 × {Ig1 + Ign +3 × (Ig2 + Igm)}; the formula: Ig1-frequency power running the inverter input circuit leakage current, Ig2-frequency output power inverter circuit operation leakage current, Ign-inverter input side noise filter leakage current, Igm-frequency power supply leakage current when the motor is running. If the leakage current on the type of basic data are not identified, selected according to the following experience. Frequency-specific sensitivity of current leakage circuit breakers rated at 20mA each frequency to estimate the general sensitivity of current leakage circuit breakers rated at 50mA each frequency estimate.
    
4 Conclusion
    
With the continuous development of power electronics technology, improvement of AC variable speed technology is emerging as a superior control and speed performance, high efficiency, easy maintenance features, combined with its price falling, it's speed in the field of machinery and equipment increasingly widespread application, the program has become a preferred speed. However, the frequency of application, different from the past, electrical transmission characteristics. In this paper, widely used general-purpose inverters, inverter proposed in the selection and peripheral configuration issues that need attention. Project site selection of frequency control system, should be weighed, a reasonable choice. Only correct, the flexibility to make good use of the inverter, variable frequency drive systems to safe, reliable operation.

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