Discussion on the Problems in Design of Frequency Conversion Speed ​​Regulation System

1 Introduction

Frequency conversion technology was born to meet the needs of stepless speed regulation of AC motors. Since the second half of the 1960s, power electronic devices have developed from SCR (thyristor), GTO (gate turn-off thyristor), BJT (bipolar power transistor), MOSFET (metal oxide field effect transistor), SIT (static induction transistor), SITH (static induction thyristor), MGT (MOS control transistor), MCT (MOS control transistor) to today's IGBT (insulated gate bipolar transistor) and HVIGBT (high voltage insulated gate bipolar thyristor). The update of devices has promoted the continuous development of power conversion technology. Since the 1970s, the research on pulse width modulation variable voltage and frequency (PWM-VVVF) speed regulation has attracted great attention. In the 1980s, the PWM mode optimization problem, as the core of frequency conversion technology, attracted people's strong interest, and many optimization modes were obtained, among which the saddle wave PWM mode had the best effect. Since the second half of the 1980s, VVVF inverters have been put on the market and widely used in developed countries such as the United States, Japan, Germany, and the United Kingdom.

2 Efficiency analysis of variable frequency speed regulation system

2.1 Efficiency and loss of inverter

The efficiency of the frequency converter refers to its own conversion efficiency. As for the two forms of frequency converters. Although the AC-AC frequency converter has a higher efficiency, its frequency modulation range is limited and its application is restricted. At present, the common frequency converter is mainly AC-DC-AC type. Its working principle is to first convert the industrial frequency AC into DC through a rectifier, and then use an inverter to convert it into AC of the required frequency. Therefore, the loss of the frequency converter consists of three parts, with rectification loss accounting for about 40%, inverter loss accounting for about 50%, and control loop loss accounting for 10%. The first two losses vary with the capacity, load, and topology of the frequency converter, while the control loop loss does not vary with the capacity and load of the frequency converter. The frequency converter adopts modern power electronic technologies such as high-power self-shutoff switching devices. Its rectification loss and inverter loss are smaller than the rectification loss in traditional electronic technology. According to the information provided in the literature [1>, when the frequency converter is running at rated state, its efficiency is 8%~96%, which is improved as the power of the frequency converter increases.

2.2 Changes in motor efficiency after variable frequency speed regulation

After frequency conversion speed regulation, the various losses and efficiency of the motor change. According to motor theory, the losses of the motor can be divided into core loss (including hysteresis loss and eddy current loss), bearing friction loss, windage loss, stator winding copper loss, rotor winding copper loss, stray loss and so on.

The expression of hysteresis loss in the core is:

It shows that the hysteresis loss Pn is proportional to the alternating frequency f of the magnetic flux and to the αth power of the amplitude of the magnetic flux density Bm. For general silicon steel sheets, when Bm=0.8～1.6W/m2, α=2.

According to the fan and pump theory, the relationship between the flow rate Q, the required motor shaft power P and the speed n is: Q∝n; P∝n3; P∝Q3

After frequency conversion speed regulation, the hysteresis loss reduction rate is slower than the motor active power reduction rate, and the loss proportion increases.

The expression of eddy current loss is: Pe∝af2;

In the formula, a=（Bm）2d2/rw; Bm is the amplitude of magnetic flux density; d is the core thickness; rw is the equivalent resistance of eddy current circuit.

Bearing friction loss: Pz∝f1.5

Windage loss: Pf∝f3

The copper loss of the stator winding and the rotor winding has no direct relationship with the power supply frequency f, but high-order harmonics and pulsating current increase the copper loss of the motor.

Stray loss and additional loss: Regardless of the type of frequency converter, after frequency conversion, in addition to the fundamental wave, harmonics are generated. The torque direction of many of these additional higher harmonics is opposite to that of the fundamental wave torque. In addition, higher harmonics will also increase eddy current losses. In summary, after frequency conversion speed regulation, the proportion of hysteresis loss, eddy current loss, bearing friction loss, stator and rotor copper loss and stray loss in the power of the motor increases. Relevant literature points out that after frequency conversion speed regulation, the motor current increases by 10% and the temperature rise increases by 20%.

3 Reasonable selection of inverter control method

The control method is the key to determine the performance of the inverter. There are many brands of low-voltage general-purpose inverters on the market, including more than 50 brands from Europe, the United States, Japan and China. When choosing an inverter, don't think that the higher the grade, the better. In fact, as long as the characteristics of the load meet the use requirements, it can be used according to the quantity and economically. The parameters in the attached table are for reference when selecting.

4 Selection of torque control inverter and related issues

Based on the advantages of convenient speed regulation, energy saving and reliable operation, variable frequency speed regulators have gradually replaced traditional pole-changing speed regulation, electromagnetic speed regulation and voltage regulation. Several years after the introduction of PWM flux vector control frequency converters, frequency converters using DTC control technology (direct torque control frequency converters) appeared at the end of 1998. ABB's ACS600 series is the first generation of frequency converters using DTC technology. It can accurately control speed and torque in an open-loop manner, and its dynamic and static indicators are better than PWM closed-loop control indicators.

Direct torque control uses the measured motor current and DC voltage as inputs to the adaptive motor model. The model generates a set of accurate torque and flux actual values ​​every 25μs. The torque comparator and flux comparator compare the actual values ​​of torque and flux with the given values ​​of torque and flux to give the optimal switch position. It can be seen that it is through the measurement of torque and flux, the switching state of the inverter circuit is adjusted immediately, and then the torque and flux of the motor are adjusted to achieve the purpose of precise control.

4.1 Selection principles

Before selecting the model, you must first determine the maximum input power required by the machine (i.e. the minimum rated power of the motor) based on the machine's requirements for speed (maximum, minimum) and torque (starting, continuous and overload).

P=n. T/9950（kW）

Where: P—Input power required by the machine (kW)

n—Mechanical speed (r/min)

T—Maximum torque of the machine (Nm)

Then, select the number of poles and rated power of the motor. The number of poles of the motor determines the synchronous speed, which is required to cover the entire speed range as much as possible to make the continuous load capacity higher. In order to make full use of the equipment potential and avoid waste, the motor can be allowed to exceed the synchronous speed for a short time, but it must be less than the maximum speed allowed by the motor. The torque is the maximum torque of the equipment under starting, continuous operation, overload or maximum speed. Finally, determine the parameters and model of the inverter based on the inverter output power and rated current slightly greater than the motor power and rated current.

It should be noted that the rated capacity and parameters of the inverter are marked for a certain altitude and ambient temperature, generally referring to an altitude below 1000m and a temperature below 40℃ or 25℃. If the operating environment exceeds this regulation, the resulting derating factor should be considered before determining the model based on the inverter parameters.

4.2 Selection of inverter capacity

The selection of general frequency converter includes two aspects: the type and capacity of the frequency converter. The general principle is to first ensure the reliable realization of process requirements and then save money as much as possible.

According to the control function, general frequency converters can be divided into three types: ordinary function type u/f control frequency converter, high performance type u/f control frequency converter with torque control function (also called non-trip frequency converter) and vector control high performance frequency converter. The type of frequency converter should be selected according to the load requirements. For fans, pumps and other square torque (TL∝n2), the load torque is small at low speed, and ordinary function type frequency converters can usually be selected. For constant torque loads or machinery with high static speed requirements, it is ideal to use high performance frequency converters with torque control function. Because this type of frequency converter has large low speed torque, high static mechanical characteristics, is not afraid of load impact, and has excavator characteristics. FRENIC5000G11/P11 of Fuji Corporation of Japan and SAMCO-L series of Sanken Corporation belong to this category. There are also examples of using ordinary frequency converters. In order to achieve constant torque speed regulation with a large speed ratio, the method of increasing the capacity of the frequency converter is often used. For production machinery that requires high precision, good dynamic performance and fast response (such as papermaking machinery, rolling mills, etc.), vector control high-function general-purpose inverters should be used. Yaskawa's VS-616G5 series and Siemens' 6SE7 series inverters belong to this category.

Most inverter capacities can be expressed from three perspectives: rated current, available motor power, and rated capacity. The latter two items are given by the inverter manufacturer based on the standard motor produced by the country or company, or are reduced with the inverter output voltage, which makes it difficult to accurately express the inverter's capacity. When selecting an inverter, only the inverter's rated current is a key quantity that reflects the load capacity of a semiconductor inverter device. The basic principle for selecting an inverter is that the load current does not exceed the inverter's rated current. It should be emphasized that before determining the inverter capacity, the process conditions and motor parameters of the equipment should be carefully understood. For example, the rated current of submersible electric pumps and wound rotor motors is greater than the rated current of ordinary squirrel cage asynchronous motors. The roller motors commonly used in the metallurgical industry not only have a much larger rated current, but also allow a short-term stalled working state, and most roller drives are multi-motor drives. The total load current should not exceed the rated current of the inverter in a fault-free state.

The inverter supplies pulsating current to the motor. When the motor is in rated operation, the current of the inverter is larger than that of the power grid. Therefore, the inverter current or power should be one level higher than the motor current or power, generally:

Pnv≥1.1Pn

Where: PNV—inverter rated power, kW;

PN—rated power of motor, kW

5. Issues that should be paid attention to in the design of variable frequency speed regulation

5.1 Load matching problem

The maximum energy saving of pump load is to match the model and capacity with the actual load, including the matching of pump and motor, to avoid "big horse pulling a small cart", and the general design margin should be controlled within 10%. In the design of industrial systems in my country, there is often a phenomenon of leaning on the upper gear, adding layers of code, and preferring big to small. It can be said that from the process point of view, the coefficient is added when the flow rate is proposed, the coefficient is added when the pump is selected, and the coefficient is added when the motor is selected, so that the actual operation efficiency of the pump in some industrial systems is extremely low. If the variable frequency speed regulation is added when designing the pump load to achieve the purpose of energy saving, it actually adds a frequency converter, which nominally increases new energy loss and investment. At present, many energy-saving benefit analyses of variable frequency speed regulation often ignore the efficiency of the frequency converter, etc. This simple theoretical calculation has a distorted effect.

In industrial production, due to changes in production load and production seasons, the pump load is not constant, and sometimes the range of changes is still very large. Some power plants have large load changes during the day and night due to peak load regulation needs, and the output flow of the pump load will be lower than the rated flow after one year of use. The effect of using variable frequency speed regulation for pump loads with large flow changes is obvious, and the larger the load change range, the better the energy saving effect.

5.2 High-order harmonics

The high-order harmonics generated by the inverter will cause distortion of the grid voltage waveform, and the smaller the effective capacity of the grid and the larger the capacity of the inverter, the more serious this effect will be. This kind of pollution to the grid will make power capacitors, reactors, and transformers easily heat up, and produce electromagnetic resonance, additional losses in motors and generators, and malfunctions of relays. Various countries have corresponding regulations on voltage distortion and harmonic control. my country's GB12668-90 stipulates that the voltage distortion rate is less than 10%, any odd harmonics are not more than 5%, and any even harmonics are not more than 2%. After using the inverter, the national standard will be exceeded in some parts of the grid, so corresponding measures must be taken.

5.3 Motor selection

Since the carrier frequency of frequency conversion is high, the motor winding has to bear a high impulse voltage. In addition, the motor efficiency decreases, and the cooling effect after the speed decreases, which should be paid attention to. Therefore, after frequency conversion speed regulation, there are many cases of damage to the inverter and the motor. If conditions permit, YTSP and YSG series frequency conversion speed regulation special motors can be used.

The AC speed regulation transmission system composed of general frequency converters generally adopts standard asynchronous motors. When the PWM frequency converter is used to power the asynchronous motor, the stator current will inevitably contain high-order harmonics. The power factor and efficiency of the motor will be lower when running at no-load, and the iron loss will increase when running under load. The current of the motor under rated load increases by about 8%, and the temperature rise increases by about 20%. This is a problem that cannot be ignored for motors that work at full load or close to full load for a long time. Therefore, the motor capacity should be appropriately increased to prevent excessive temperature rise and affect the service life of the motor.

The heat dissipation capacity of the general standard squirrel cage asynchronous motor is considered at rated speed and self-fan cooling. When the motor is running at a constant torque load, its heat generation remains unchanged but the heat dissipation capacity at low speed becomes worse. You can use an additional constant speed cooling fan or use a motor with a higher insulation grade.

5.4 External Configuration of the Inverter and Issues to Note

(1) Select a suitable external fuse to protect the rectifier components from damage due to internal short circuit. After the inverter model is determined, if there is no fast fuse to protect the silicon components before the inverter internal rectifier circuit, a fuse and isolating switch that meet the requirements should be configured between the inverter and the power supply. Air circuit breakers cannot be used to replace fuses and isolating switches.

(2) Select the input and output cables of the inverter. Select a three-core or four-core shielded power cable with a suitable conductor cross-section according to the power of the inverter. In particular, the power cable from the inverter to the motor must be a shielded cable and as short as possible to reduce electromagnetic radiation and capacitive leakage current. When the cable length exceeds the output cable length allowed by the inverter, the stray capacitance of the cable will affect the normal operation of the inverter. For this reason, an output reactor must be configured. For control cables, especially I/0 signal cables, a shielded structure should also be used. The length of the connecting cable between the inverter's optional parts and the inverter must not exceed 10m.

(3) Install an AC reactor or EMC filter on the input side. According to the requirements of other equipment on the power grid quality at the inverter installation site, if the inverter has affected the normal operation of these equipment, an AC reactor or EMC filter can be installed on the input side of the inverter to suppress the harmonic distortion and conducted radiation caused by the switching of power components. If the neutral point of the transformer of the power grid connected to the inverter is not grounded, the EMC filter cannot be used. When the inverter drives the motor with a voltage of more than 500V, a dv/dt filter must be configured on the output side to suppress the inverter output voltage spike and voltage change, which is beneficial to protect the motor. At the same time, it also reduces the capacitive leakage current and the high-frequency radiation of the motor cable, as well as the high-frequency loss and bearing current of the motor. When using a dv/dt filter, it should be noted that the voltage drop on the filter will cause a slight reduction in the motor torque; the cable length between the inverter and the filter shall not exceed 3m.

6 Conclusion

The design of the variable frequency speed regulation system should be combined with the static and dynamic indicators of the system, with the mechanical characteristics of the load as the main design parameters. On the basis of paying attention to the selection of the inverter control method and the selection of the inverter, the practical problems in the design of the variable frequency speed regulation system should be solved so that the designed variable frequency speed regulation system has high static and dynamic indicators and high performance-price ratio.