Application of variable frequency power supply in fan testing

1 Introduction

Due to the inconsistency of power grid indicators in countries around the world, export electrical appliance manufacturers need power supplies to simulate power grid conditions in different countries, and provide engineers with pure, reliable, low harmonic distortion, high-stability frequency and voltage regulation rate sine wave power output in design and development, production line testing, product testing for quality assurance, life, over-high voltage/low voltage simulation testing, etc. However, when frequency converters are used in laboratories, testing is greatly difficult, and problems such as increased vibration, increased electromagnetic noise, increased temperature rise, and inability of test instruments to work properly are prominent. Therefore, frequency converters that can provide pure sine wave output have become indispensable equipment in laboratories.

2 Application of frequency converter

The inverter industry is developing rapidly, and the industrialization scale of inverter products is growing. After the advent of AC inverters in the 1960s, they were widely used in industrialized countries in the 1980s. In the 1990s, as people's awareness of energy conservation and environmental protection increased, the application of inverters became more and more popular.

The energy saving of frequency converter is mainly reflected in the application of fans and water pumps. In order to ensure the reliability of production, various production machinery are designed with a certain margin when equipped with power drive. When the motor cannot run at full load, in addition to meeting the power drive requirements, the excess torque increases the consumption of active power, resulting in a waste of electric energy.

At the same time, after using variable frequency speed regulation, fans and pumps can control the starting current of the motor, reduce the voltage fluctuation of the power line, require less power at startup, have controllable acceleration function, operating speed and torque limit, and have controlled stopping methods such as deceleration stop, free stop, deceleration stop + DC braking, which can greatly reduce energy consumption and save mechanical transmission components such as gear boxes.

3 Problems caused by inverter application in laboratory

With the growth of export products, the voltage level and frequency vary around the world. The power frequency in other countries and regions is mostly 60hz, and the voltage is 400v and 415v. Due to the lack of relevant knowledge of inverter technology and the lack of comprehensive knowledge of the measurement method of the inverter electrical characteristics, we began to use the inverter to provide the required 60hz power supply. During the test process, many problems were found:

(1) Fan speed and voltage changes. When the 60Hz power supply is output, the motor speed is not 1.2 times that of 50Hz. When the input voltage is adjusted to 380V, the output voltage is different from the input voltage, and the difference is large. This is because the inverter uses inverter technology. General inverters are variable frequency and variable voltage (vvvf). When the frequency changes, the voltage will also change proportionally, as shown in Figure 1.

(2) Increased electromagnetic noise during motor operation. The inverter outputs a rectangular wave voltage with high-frequency components, so the motor will generate harsh high-frequency noise, which is significantly louder than the fan noise, making noise measurement difficult.

(3) The motor temperature rise increases. The inverter output voltage waveform is not a sine wave, but a distorted wave. The motor current at rated torque is about 10% higher than that at power frequency, so the temperature rise is slightly higher than that at power frequency (low-order current harmonics increase copper loss, and high-order harmonics increase iron loss), which brings inconvenience to the motor temperature rise test and causes large deviations in the temperature rise test results.

(4) Ordinary electromagnetic and digital test instruments fail. The input and output currents of the frequency converter contain high-frequency harmonic components. The electromagnetic field generated by the high-order harmonic current has the ability to radiate, causing other equipment (especially communication equipment) to be disturbed by the electromagnetic wave signals received. Therefore, electromagnetic instruments and digital instruments cannot be used to directly measure AC voltage and current. Rectifier instruments must be used for more accurate measurements.

In response to the problems, we had many discussions and analyses with the engineers of the inverter manufacturer, and made reasonable adjustments to the parameters of the inverter. We adjusted the carrier frequency to reduce electromagnetic noise and lower the temperature rise of the motor. However, electromagnetic noise and motor temperature rise are contradictory in the presence of carrier frequency, and it is impossible to achieve the effect of completely reducing or even eliminating them.

4 The difference between inverter and variable frequency power supply

In order to solve the problems of noise, temperature rise, testing, etc. when using the inverter, it is necessary to carefully analyze the working principle of the inverter and try to minimize the impact. In the process of learning and analysis, we found a more ideal laboratory power supply equipment-the variable frequency power supply. The following compares the principles and differences between the inverter and the variable frequency power supply to facilitate engineering and technical personnel to make a reasonable choice and avoid detours when choosing variable frequency equipment.

The frequency converter is a power source that can change the frequency and voltage. The frequency converter is composed of AC-DC-AC (modulated wave) and other circuits. The three-phase AC power of the power grid is rectified into pulsating DC through a three-phase bridge, and then filtered into smooth DC through electrolytic capacitors and inductors. Finally, it is inverted into three-phase AC power with adjustable voltage and frequency through an inverter. The standard name of the frequency converter is variable frequency speed regulator.

The input circuit of the inverter is a circuit that charges the filter capacitor after full-wave rectification of the three-phase AC power supply. The input current always appears near the amplitude value of the voltage in the form of a discontinuous shock wave, as shown in Figure 2. The output circuit of the inverter is an inverter bridge circuit that converts DC into a three-phase modulated AC voltage with continuously adjustable frequency. Its output voltage waveform is a sinusoidal modulated SPWM wave, as shown in Figure 3.

The harmonic voltage drop caused by high-order harmonic current on the line causes the grid voltage waveform to be distorted. This distortion affects other loads, resulting in reduced efficiency of electrical equipment, increased noise, and even resonance in some equipment, causing equipment overheating. It also causes serious electromagnetic interference to instrumentation and telecommunications equipment, and has serious consequences for the accuracy of test equipment and the reliability of data. Therefore, we need to find a more sophisticated power supply device to replace the inverter, and the variable frequency power supply is a very suitable choice.

The variable frequency power supply converts the AC power in the industrial power grid into a pure sine wave through ac-dc-ac conversion, and the output frequency and voltage are adjustable within a certain range. It is different from the frequency converter used for motor speed regulation and also different from the ordinary AC voltage stabilized power supply. The characteristics of an ideal AC power supply are stable frequency, stable voltage, zero internal resistance, and a pure sine wave voltage waveform (no distortion). The variable frequency power supply is very close to the ideal AC power supply, with an output phase deviation within ±2°, a frequency stability rate ≤0.01%, a load voltage regulation rate within the range of ±0.5%, a waveform distortion ≤2% (resistive load), a maximum response time of 2ms, an efficiency ≥85%, and an input and output non-fuse switch. The electronic circuit quickly detects overvoltage, overcurrent, overload, overtemperature & short circuit and automatically trips protection and alarms. Therefore, advanced and developed countries are increasingly using variable frequency power supplies as standard power supplies to provide the best power supply environment for electrical appliances and facilitate objective assessment of the technical performance of electrical appliances.

The variable frequency power supply can carry loads with various impedance characteristics, including common loads such as inductors, resistors, and rectifiers. It should be noted that the power capacity of the variable frequency power supply required varies greatly depending on the load type.

Power capacity selection method:

Resistive: Power capacity = 1.1 × load power

Inductive: Power supply capacity = load starting current / load rated current × load power

Rectification: Power capacity = load current crest factor/1.5×load power

Mixed type: Please select according to the proportion of different loads.

Note: For inductive loads such as refrigerators and air conditioners, the power capacity should be selected according to the starting power. The starting power is generally 5-7 times the rated power.

By choosing a reasonable variable frequency power supply, you can simulate the voltage and frequency of various parts of the world and obtain reliable laboratory data. For the most commonly used motors, you should choose according to the inductive load. The maximum power of the motor used in our factory is 3kw, and the starting current is about 3 times the rated current, so choosing a power supply capacity of 15kva is sufficient. Of course, the price of the inverter and the variable frequency power supply is not in the same order of magnitude, and the price of the variable frequency power supply is about 10 times that of the inverter.

5 Conclusion

Through the research and analysis of the principles of frequency converters and variable frequency power supplies, we have mastered the output power characteristics of frequency converters and variable frequency power supplies, understood the many problems caused by the use of frequency converters in laboratories, and provided a basis for the laboratory to select reasonable variable frequency power supply devices. Especially for mechanical engineering and technical personnel with less electrical knowledge, reasonable selection of laboratory power supply devices can provide guarantees for product performance testing and type testing, and ensure the smooth operation of products around the world.