About VFD, definitions, components, features and frequently asked questions
Variable frequency drive (VFD) is the application of frequency conversion technology and microelectronics technology to control the AC motor power control equipment by changing the frequency of the motor power supply.
The VFD is mainly composed of rectification (AC to DC), filtering, VFD (DC to AC), brake unit, drive unit, and detection unit micro-processing unit. The VFD adjusts the voltage and frequency of the output power supply by breaking the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed control. In addition, the VFD has many protection functions. , such as over-current, over-voltage, overload protection and so on. With the continuous improvement of industrial automation, VFD have also been widely used.
Working principle of VFD
The main circuit is the power conversion part of the frequency modulation power supply for the asynchronous motor. The main circuit of the VFD can be roughly divided into two categories: The voltage type is the VFD that converts the DC of the voltage source to AC, and the filter of the DC loop is the capacitor.The current type is a frequency converter that converts the current from a direct current source to an alternating current, and the direct current loop filter is an inductor. It consists of three parts. It converts the power-frequency power source into a "rectifier" for the DC power, absorbs the "fluid-wave circuit" of the voltage ripple generated by the converter and the VFD, and converts the DC power (battery and storage battery) into AC (usually 220V, 50Hz sine wave) VFD.
A large number of diode-based converters are used to convert the power-frequency power source to a DC power source. It is also possible to use two sets of transistor converters to construct a reversible converter, which can be regenerated due to its reversible power direction.
Smooth wave circuit
The rectifier-rectified DC voltage contains a ripple voltage of 6 times the power supply frequency, and the pulsating current generated by the inverter also causes the DC voltage to fluctuate. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the ripple voltage (current). When the capacity of the device is small, if the power supply and the main circuit configuration device have a margin, a simple flat wave circuit can be omitted from the inductor.
In contrast to a rectifier, an inverter converts DC power to AC power of a desired frequency, and turns on and off 6 switching devices at a determined time to obtain a 3-phase AC output. Take the voltage type pwm inverter as an example to illustrate the switching time and voltage waveforms.
The control circuit is the circuit that provides the control signal to the main circuit (voltage, frequency adjustable) of the asynchronous motor. It has the frequency and voltage "operation circuit", the main circuit "voltage and current detection circuit", the motor "speed detection "Circuit", a "driver circuit" that amplifies the control signal of the arithmetic circuit, and a "protection circuit" of the inverter and the motor.
(1) Operation circuit: Comparing the external speed and torque commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
(2) Voltage and current detection circuit: It is isolated from the main circuit potential to detect voltage and current.
(3) Drive circuit: A circuit that drives a main circuit device. It is isolated from the control circuit so that the main circuit device turns on and off.
(4) Speed detection circuit: The signal of the speed detector (tg, plg, etc.) mounted on the asynchronous motor shaft machine is the speed signal, which is sent to the calculation circuit. According to the instruction and operation, the motor can be operated at the command speed.
(5) Protection circuit: Detect the voltage, current, etc. of the main circuit. When an abnormality such as overload or overvoltage occurs, in order to prevent damage to the inverter and the asynchronous motor.
The function of the VFD
Frequency conversion and energy saving
VFD’S energy saving is mainly reflected in the application of fans and pumps. In order to ensure the reliability of production, all kinds of production machinery are left with a certain margin when designing and using power drives. When the motor cannot be operated at full load, the excess torque increases the consumption of active power in addition to the power drive requirements, resulting in a waste of electrical energy. The conventional speed regulation method for fans, pumps and other equipment is to adjust the amount of air supply and water supply by adjusting the baffle and valve opening of the inlet or outlet. The input power is large, and a large amount of energy is consumed in the closure process of baffles and valves. in. When using variable frequency speed regulation, if the flow requirements are reduced, the requirements can be met by reducing the pump or fan speed.
The function of the motor using the VFD is to regulate the speed and reduce the starting current. In order to produce a variable voltage and frequency, the device first converts the alternating current of the power supply to direct current (DC). This process is called rectification. The device that converts direct current (DC) to alternating current (AC) is scientifically termed "inverter". The general inverter is an inverter power source that inverts the DC power supply to a certain fixed frequency and a certain voltage. For inverters with reversed frequency and adjustable voltage, we call them VFD. VFD output waveform is analog sine wave, mainly used in three-phase asynchronous motor speed control, also known as frequency converter. For the variable-frequency inverters that require higher waveforms in instrumentation testing equipment, the waveforms should be sorted and the standard sine wave can be output, which is called variable-frequency power supply. The general variable frequency power supply is 15-20 times the price of the frequency converter.
VFD can not be power saving everywhere, there are many occasions with frequency conversion does not necessarily save power. As an electronic circuit, the VFD itself also consumes power (about 3-5% of rated power). A 1.5-hp air conditioner itself consumes about 20-30 W, which is equivalent to a long lamp. It is a fact that the VFD runs at the power frequency and has power-saving features. But his preconditions are:
First, high power and fan/pump load;
Second, the device itself has power-saving features (software support);
This is the three conditions that reflect the power saving effect. In addition, it does not matter if the festival does not save energy. If you do not add the precondition that VFD frequency energy-saving operation, it is exaggerated or commercial hype. Know the truth, you will use him cleverly to serve you. Must pay attention to the use of occasions and the use of conditions to properly apply, or blindly, credulous and "cheated."
Power Factor Compensation Energy Saving
Reactive power not only increases the line loss and heat generation of the equipment, but more importantly, the reduction of the power factor leads to the reduction of the active power of the power grid. A large amount of reactive power is consumed in the line. The equipment is inefficiently used and the waste is serious. The frequency conversion speed controller is used. Later, because of the internal filter capacitor of the VFD, thus has reduced the reactive power loss, has increased the active power of the electrical network.
Soft Start Energy Saving
1: The hard start of the motor will cause serious impact on the power grid, and it will also impose excessive demands on the capacity of the power grid. The large current and vibration generated during start-up will cause great damage to the baffle and valve. The service life of the equipment and piping is extremely high. unfavorable. After the inverter energy-saving device is used, the use of the soft-start function of the VFD will start the starting current from zero, and the maximum value will not exceed the rated current, reducing the impact on the power grid and the requirements for the power supply capacity, and prolonging the use of equipment and valves. life. Save equipment maintenance costs.
2: In theory, the VFD can be used in all mechanical devices with motors. When the motor starts, the current will be 5-6 times higher than the rated value. This will not only affect the service life of the motor but also consume more power. The system design will leave a certain margin in the motor selection, the speed of the motor is fixed, but in the actual use of the process, sometimes run at a lower or higher speed, so the frequency conversion transformation is Very necessary. VFD can realize motor soft start and compensation power factor
The basic composition of the VFD
The VFD is usually divided into 4 parts: rectification unit, high capacity capacitance, inverter and controller.
Rectifier unit: Converts a fixed frequency AC power to DC power.
High-capacity capacitors: Store converted electrical energy.
Inverter: An electronic switch composed of a high-power switch transistor array converts direct current into square waves of different frequencies, widths, and amplitudes.
Controller: According to the set program, control the amplitude and pulse width of the output square wave, so that it is superimposed to approximate sinusoidal alternating current and drive the AC motor.
VFD’s common frequency given ways are: operator keyboard given, contact signal given, analog signal given, pulse signal given and given the way of communication. Each of these frequency-giving methods has its own advantages and disadvantages. It must be selected according to actual needs.
The low-voltage general-purpose inverter output voltage is 380-650V, the output power is 0.75-400kW, and the operating frequency is 0-400Hz. Its main circuit adopts an AC-DC-AC circuit. Its control mode has gone through the following four generations.
The first generation
1U/f=C sinusoidal pulse width modulation (SPWM) control method:
Its characteristic is that the control circuit has a simple structure, low cost, good mechanical characteristics and hardness, and can meet the smooth speed regulation requirements of general transmission. It has been widely used in various fields of the industry. However, this kind of control method is in the low frequency, because the output voltage is lower, the torque is influenced by the voltage drop of the resistance of the stator more prominently, make the output maximum torque reduce. In addition, the mechanical characteristics of the DC motor is not as hard as any of them. The dynamic torque capability and static speed regulation performance are still not satisfactory. The system performance is not high, the control curve changes with the load, the torque response is slow, and the motor rotation The moment utilization rate is not high, and the performance is degraded due to the existence of the stator resistance and the dead time effect of the inverter at low speed, and the stability becomes worse. Therefore, people have also developed vector control frequency control.
The second generation
Voltage space vector (SVPWM) control method:
It is based on the premise that the three-phase waveforms are generated as a whole. For the purpose of approximating the ideal circular rotating magnetic field trajectory of the air gap of the motor, a three-phase modulation waveform is generated at a time, and the control is performed in such a way that the inward polygon approaches the circle. After being used in practice, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; through estimating the amplitude of magnetic flux through feedback, eliminating the influence of stator resistance at low speed; and closing the output voltage and current to improve the dynamic accuracy and stability. However, there are many control circuits and no adjustment of torque is introduced. Therefore, the system performance has not been fundamentally improved.
The Third Generation
Vector Control (VC) Mode:
The vector control frequency conversion speed regulation method is that the asynchronous motor in the three-phase coordinate system stator current Ia, Ib, Ic, through the three-phase-two-phase transformation, equivalent to two-phase static coordinate system under the alternating current Ia1Ib1, and then through According to the rotational orientation of the rotor field, the DC current Im1, It1 (Im1 corresponds to the excitation current of the DC motor; It1 corresponds to the armature current proportional to the torque) in the synchronous rotating coordinate system, and then imitates the DC motor The control method obtains the control quantity of the direct current motor and realizes the control of the asynchronous motor through the corresponding coordinate inverse transformation. The essence of this is that the AC motor is equivalent to a DC motor, which independently controls the speed and magnetic components. By controlling the rotor flux linkage, and then decomposing the stator current to obtain two components of torque and magnetic field, the orthogonal or decoupled control is achieved through coordinate transformation. The proposed vector control method has epoch-making significance. However, in practical applications, because the rotor flux linkage is difficult to observe accurately, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the control process of the equivalent DC motor is more complicated, making the actual control effect difficult to achieve the ideal analysis. result.
The Fourth Generation
Direct torque control (DTC) method:
In 1985, Prof. DePenbrock of Ruhr University in Germany proposed the direct torque control frequency conversion technology for the first time. This technology solves the above-mentioned deficiencies of vector control to a large extent, and has been rapidly developed with novel control concepts, concise and clear system structure, and excellent dynamic and static performance. The technology has been successfully applied to high-power AC drives for electric locomotive traction. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the flux linkage and torque of the motor. It does not require that the AC motor be equivalent to a DC motor, thus eliminating many of the complex calculations in the vector rotation transformation; it does not need to mimic the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.
VVVF frequency conversion, vector control frequency conversion, direct torque control frequency conversion is a kind of AC-DC-AC frequency conversion. The common disadvantages are the low input power factor, large harmonic currents, large energy storage capacitors required in the DC circuit, and the inability to feed back the energy back to the grid, which means that four-quadrant operation is not possible. For this reason, matrix-type AC-AC frequency conversion came into being. Because the matrix-type AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the need for bulky, expensive electrolytic capacitors. It can achieve a power factor of l, input current is sinusoidal and can operate in four quadrants, and the power density of the system is large. Although this technology is not yet mature, it still attracts many scholars to conduct in-depth research. The essence of this is not to indirectly control the current and flux linkage, but to directly use the torque as the controlled variable. The specific method is:
1. Control the stator flux into the stator flux observer to realize a speed sensorless method;
2, automatic identification (ID) rely on accurate motor mathematical model, automatic identification of motor parameters;
3. Calculate the actual value corresponding to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. Calculate the actual torque, stator flux, and rotor speed for real-time control;
4. Realize the Band-Band control. The Band-Band control of the flux linkage and torque generates the PWM signal to control the inverter switching state.
Matrix-AC-AC frequency conversion has a fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), high torque accuracy (<+3%); also has a higher start Torque and high torque accuracy, especially at low speed (including 0 speed), can output 150% to 200% torque.
VVC control principle:
The control principle of VVC is to apply the principle of vector modulation to a fixed voltage source PWM inverter. This control is based on an improved motor model that better compensates for load and slip.
Because the active and reactive current components are important to the control system, controlling the voltage vector angle can significantly improve the dynamic performance in the 0-12Hz range, while in the standard PWM U/F drive 0-10Hz range There are generally problems.
Using the SFAVM or 60° AVM principle to calculate the switching mode of the inverter, the pulsation of the air-gap torque can be made very small (compared to a frequency converter using synchronous PWM).
The development history of VFD
The birth background of frequency conversion technology is the extensive demand for stepless speed regulation of AC motors. The traditional DC speed regulation technology is limited due to its high volumetric failure rate.
After the 1960s, thyristors and their upgraded products were commonly used in power electronic devices. However, its speed control performance can not meet the needs. In 1968, high-tech companies represented by Danfoss began to mass-produce VFD and opened a new era of industrialization of VFD.
Since the 1970s, the pulse-width-modulated variable-voltage-variable-frequency (PWM-VVVF) speed control system has achieved breakthroughs. After the 1980s, the improvement of microprocessor technology made it easy to implement various optimization algorithms.
In the mid to late 1980s, the VVVF VFD technology of the developed countries such as the United States, Japan, Germany, and Britain was put into practical use, and the commodities were put into the market and widely used. The earliest VFD may have been developed by the Japanese who bought British patents. However, the United States and Germany rely on the advantages of electronic component production and electronic technology, and high-end products quickly seize the market.
After entering the 21st century, China domestic VFD have gradually emerged and are now gradually taking over the high-end market. Shanghai and Shenzhen have become front-line positions for the development of China domestic VFD, and a number of well-known domestic VFD such as Inovance VFD, Inventronics inverter, Anbangs inverter, and Oerlikon inverter have emerged. Among them, Anbangxin VFD was established in 1998 and is one of the earliest manufacturers of VFD in China. For more than ten years, Anbang Xinren has built a strong foundation of culture and supported growth. The company passed the ISO9000 quality system certification of the TUV organization earlier and was awarded the “National High-tech Enterprise”. It has been rated as “China's drive customer satisfaction for many years” for many years. Ten domestic brands."
DC electric drive and AC motor drive were born in the 19th century. It has a history of more than 100 years and has become the main driving device of power machinery. Due to technical problems at the time, in a long period of time, a DC motor was basically used in a drive system that required speed control.
The following drawbacks of DC motors are due to structural reasons:
1. Due to the presence of commutation sparks in DC motors, it is difficult to apply them to harsh environments where flammable and explosive gases are present.
2. The brush and commutator need to be replaced on a regular basis, and the maintenance is difficult and the service life is short;
3, the structure is complex, it is difficult to manufacture large capacity, high speed and high voltage DC motor.
Related questions and answers
1.What is a VFD
The VFD is a power control device that uses the on-off function of the power semiconductor device to convert the power frequency power source to another frequency, and can realize the soft start, frequency conversion speed regulation of the AC asynchronous motor, increase the operation accuracy, change the power factor, and overcurrent/ Overvoltage / overload protection and other functions.
2.The difference between PWM and PAM
PWM is the English Pulse Width Modulation (pulse width modulation) abbreviation, according to a certain rule to change the pulse width of the pulse train to adjust the output and waveform of a modulation method. PAM is English Abbreviation of Pulse Amplitude Modulation (Pulse Amplitude Modulation). It is a modulation method that changes the pulse amplitude of a pulse train according to a certain rule to adjust the output value and waveform.
3.What is the difference between voltage type and current type?
The main circuit of the VFD can be roughly divided into two kinds: The voltage type is the frequency converter which converts the direct current of the voltage source into alternating current, the filter of the direct current loop is the electric capacity; The electric current type is the frequency converter which transforms the direct current of the current source into alternating current, Its DC loop filter is an inductor.
4.Why does the VFD's voltage change in proportion to the frequency?
The electromagnetic torque of any motor is the result of the interaction of the current and the magnetic flux. The current is not allowed to exceed the rated value, otherwise it will cause the motor to generate heat. Therefore, if the magnetic flux decreases, the electromagnetic torque must also be reduced, resulting in a reduction in the load carrying capacity.
It can be seen from the formula E=4.44*K*F*N*Φ that in the frequency conversion speed regulation, the magnetic circuit of the motor changes with the operating frequency fX in a relatively large range, and it is very easy to make the magnetic circuit of the motor serious. Saturation causes the waveform of the excitation current to be severely distorted, resulting in a very high spike current.
Therefore, the frequency and voltage must be changed proportionally, that is, the inverter output voltage should be controlled while changing the frequency, so that the magnetic flux of the motor can be kept constant to avoid the generation of weak magnetic and magnetic saturation phenomena. This kind of control method is mostly used in fans, pumps and energy-saving VFD.
5.When the motor is driven by an industrial frequency power supply, the current increases when the voltage drops. For the VFD drive, if the voltage drops when the frequency drops, does the current increase?
When the frequency drops (low speed), the current increases if the same power is output, but the current does not change substantially under the condition of constant torque.
6.When using the VFD to run, what is the starting current and starting torque of the motor?
With the VFD running, the starting current is limited to 150% or less of the rated current (125% to 200% depending on the model) as the motor speeds up the frequency and voltage accordingly. When using industrial frequency power supply to start directly, the starting current is 6~7 times of the rated current. Therefore, mechanical and electrical impact will occur. The VFD drive can be started smoothly (the starting time becomes longer). The starting current is 1.2~1.5 times the rated current and the starting torque is 70%~120% of the rated torque. For the VFD with the torque automatic enhancement function, the starting torque is more than 100%, and it can be started with full load.
7.What does the V/f mode mean?
When the frequency drops, the voltage V also decreases proportionally. This problem has been explained in the answer 4. The proportional relationship between V and f is predetermined in consideration of the characteristics of the motor. Usually, there are several characteristics in the controller's storage device (ROM) that can be selected with a switch or a dial.
8.How does the motor torque change when proportionally changing V and F?
When the frequency is decreased, the voltage is completely proportionally reduced. Therefore, since the AC resistance becomes small and the DC resistance does not change, the torque generated at a low speed tends to decrease. Therefore, given V/f at low frequencies, the output voltage is raised somewhat in order to obtain a certain starting torque. This compensation is referred to as boost starting. Can use various methods to achieve, there are automatic methods, select V / f mode or adjust the potentiometer and other methods.
9.The manual says that the speed range is 60~6Hz, ie 10:1. So is there no output power below 6Hz?
Below 6Hz can still output power, but according to the motor temperature rise and starting torque size and other conditions, the minimum frequency of use to take about 6Hz, then the motor can output the rated torque without causing serious heating problems. The actual output frequency of the VFD (starting frequency) is 0.5~3Hz depending on the model.
10.For the general motor combination, the torque is also required to be above 60Hz. Is it possible?
Normally it is not possible. Above 60Hz (also more than 50Hz mode) voltage is constant, generally constant power characteristics, when the same torque is required at high speed
11.What does the so-called open-loop mean?
A speed detector (PG) is provided for the motor device to be used, and the actual rotational speed is fed back to the control device for control. This is called “closed loop”, and the operation without PG is called “open loop”. General-purpose VFD are mostly open-loop, and some models use options for PG feedback. The speed sensor-less closed-loop control method is based on the established mathematical model to calculate the actual speed of the motor based on the magnetic flux, equivalent to using a virtual speed sensor. Form closed loop control.
12.How does the actual speed deviate for a given speed?
In the open loop, even if the VFD outputs a given frequency and the motor is running with a load, the motor speed changes within the range of the rated slip (1% to 5%). For the requirement of higher speed control accuracy, even if the load change requires operation near a given speed, a VFD (option) with PG feedback function can be used.
13.If using a motor with PG, can the speed accuracy be improved after feedback?
The inverter with PG feedback function has improved accuracy. However, the speed accuracy value depends on the accuracy of the PG itself and the resolution of the inverter output frequency.
14.What does the stall prevention feature mean?
If the given acceleration time is too short, the output frequency of the VFD will change far beyond the change of the speed (electrical angle frequency). The VFD will trip due to the overcurrent and the operation will stop. This is called stalling. In order to prevent the stall from continuing to operate the motor, it is necessary to detect the magnitude of the current for frequency control. When the acceleration current is too large, the acceleration rate is appropriately slowed down. This is also true during deceleration. The combination of the two is a stall function.
15.What is the meaning of the model that can be given the acceleration time and deceleration time separately, and the type of acceleration and deceleration time given together?
Acceleration and deceleration can be given for each type of machine, for short-time acceleration, slow deceleration occasions, or for small machine tools need to strictly specify the production of the tact time is appropriate, but for fan drive and other occasions, acceleration and deceleration time is longer, Acceleration time and deceleration time can be given together.
16.What is regenerative braking?
When the motor is running, if the command frequency is reduced, the motor becomes an asynchronous generator state operation and operates as a brake, which is called regenerative (electrical) braking.
17.Can you get a bigger brake?
The energy regenerated from the motor is stored in the filter capacitor of the VFD. Due to the relationship between the capacity of the capacitor and the withstand voltage, the regenerative braking force of the general VFD is about 10% to 20% of the rated torque. If using optional brake unit, it can reach 50%~100%.
18.Please explain the protection function of the VFD?
Protection features of VFD can be divided into the following two categories:
(1) Corrective actions are automatically performed after detection of abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention.
(2) Block the power semiconductor device PWM control signal after detecting the abnormality, and make the motor stop automatically. Such as over-current cut-off, regenerative over-voltage cut-off, semiconductor cooling fan overheating and instantaneous power failure protection.
19.Why does the protection function of the VFD operate when the load is connected with the clutch?
When the clutch is used to connect the load, the motor rapidly changes from the no-load state to the area where the slip rate is large at the moment of connection, and the large current that flows causes the VFD to trip overcurrent and cannot operate.
20. In the same factory, a large motor moves together and the VFD stops during operation. Why?
When the motor starts, the starting current corresponding to the capacity will flow. The transformer on the stator side of the motor will generate a voltage drop. When the motor capacity is large, the pressure drop will also have a large effect. The VFD connected to the same transformer will make undervoltage or instantaneous stop. As a result of the judgment, the protection function (IPE) may occasionally act to stop the operation.
21.What is the resolution of frequency conversion? What does it mean?
For digitally controlled VFD, even if the frequency command is an analog signal, the output frequency is given a step. The smallest unit of this differential is called the frequency resolution. The resolution of the frequency conversion is usually 0.015~0.5Hz. For example, if the resolution is 0.5Hz, then the upper side of 23Hz can be changed to 23.5, 24.0Hz, so the action of the motor is also followed step by step. This causes problems for the use of a continuous coil control. In this case, if the resolution is about 0.015 Hz, one step for a 4-stage motor is 1 r/min or less, and it can be sufficiently adapted. In addition, some models have different resolutions and output resolutions.
22.Is there any limit to the mounting direction when installing the VFD?
The structure of the inside and the back of the VFD considers the cooling effect. The relationship between the upper and the lower is also important for ventilation. Therefore, take the longitudinal position of the unit type in the tray and hung on the wall, and install it as vertically as possible.
23.Is it possible to use a soft start without putting the motor directly into a fixed frequency VFD?
It is possible to operate at very low frequencies, but if the given frequency is high then the conditions for direct start with the commercial frequency power supply are similar. A large starting current (6 to 7 times the rated current) will flow, and the motor cannot be started because the VFD cuts off the overcurrent.
24.What should you watch out for when the motor is running above 60Hz?
When operating over 60Hz, please note the following:
(1) Machines and devices must be fully operational at this speed (mechanical strength, noise, vibration, etc.).
(2) The motor enters the constant power output range, and its output torque must be able to maintain the work (shaft, pump and other shaft output power increase in proportion to the cube speed, so speed should be paid attention to when it rises slightly).
(3) The bearing life problem should be fully considered.
(4) For motors of medium-capacity and above, especially 2-pole motors, it is necessary to carefully discuss with the manufacturer when operating above 60Hz.
According to the structure and lubrication of the reducer, there are a number of issues that need attention. The maximum limit of 70~80Hz can be considered in the structure of the gear. When oil lubrication is used, the continuous operation at low speed is related to the damage of the gear.
26.Can the VFD be used to drive a single-phase motor? Can I use single-phase power?
Basically can not be used. For single-phase motors with governor switch start-up type, the auxiliary winding will be burned when the speed range is below the operating point; for capacitive start-up or capacitor operation, capacitor explosion will be induced. The frequency converter usually has a 3-phase power supply, but for a small capacity, it also has a single-phase power-running type.
27.How much power is consumed by the VFD itself?
It is related to the model, operating status, frequency of use of the VFD, but it is difficult to answer. However, the efficiency of VFD below 60Hz is approximately 94% to 96%, and losses can be estimated accordingly. However, if a regenerative braking (FR-K) VFD is incorporated, the loss during braking is also taken into account. Power consumption Will become larger, must pay attention to the operating panel design.
28.Why can't I use it continuously in the whole area of 6~60Hz?
General motors use blades mounted on the outer fan or rotor end ring of the shaft for cooling. If the speed is reduced, the cooling effect is reduced, so that the same heat generation as the high speed operation cannot be endured, the load torque at low speed must be reduced, or A large-capacity VFD is combined with a motor or a special motor is used.
29.What should you watch out for when using motors with brakes?
Brake excitation circuit power should be taken from the input side of the VFD. If the brake is operating when the VFD is outputting power, the overcurrent will be cut off. Therefore, the brake must be operated after the VFD stops outputting.
30.I would like to use a VFD to drive a motor with a power factor improving capacitor. However, the motor does not move. Please explain why.
The current of VFD flows into the capacitor for improving the power factor. Since the overcurrent (OCT) of VFD is caused by the charging current, it cannot be started. As a countermeasure, remove the capacitor and operate it. Improve the power factor at the input side of the VFD. Access to AC reactors is effective.
31.How long is the life of the VFD?
Although VFD are stationary devices, they also have consumer devices such as filter capacitors and cooling fans. If these VFD are regularly maintained, they are expected to have a lifetime of more than 10 years.
32.The VFD contains a cooling fan. What is the direction of the wind? What happens if the fan is broken?
There is also no cooling fan for small capacity. With a fan type, the direction of the wind is from the bottom up. Therefore, in the place where the VFD is installed, do not place mechanical devices that impede suction and discharge on the upper and lower parts. Also, do not place heat-sensitive parts above the VFD. When the fan fails, it is protected by fan stop detection or overheat detection on the cooling fan.
33.Filter capacitor is a consumable product, so how to judge its life?
Capacitors used as filter capacitors gradually decrease their electrostatic capacity over time, and periodically measure the electrostatic capacitance. The life is judged on the basis of 85% of the rated capacity of the product.
34.Is there any limit to the mounting direction when installing the VFD?
Should be basically stored in the disk, the problem is that the use of fully enclosed structure of the disk size, space, cost is relatively high. The measures are:
(1) The design of the disk should be based on the heat dissipation required by the actual device;
(2) Increase the cooling area with aluminum heat sinks, airfoil coolants, etc..
35.What is the role of DC reactor of VFD?
Reduce the harmonics of the input current and increase the power factor of the input power supply.
36.What is the role of the VFD accessory sine filter?
The sine wave filter allows the VFD to run with a longer motor cable, as well as a circuit with an intermediate transformer between the VFD and the motor.
37.How much is the resistance value of the given potentiometer of the VFD?
The resistance of the given potentiometer of the VFD is generally 1KΩ to 10KΩ.
38.Why can't VFD be used as variable frequency power supply?
The entire circuit of the variable frequency power supply is composed of AC current, AC current, and filtering. Therefore, the output voltage and current waveforms are pure sine waves, which are very close to the ideal AC power supply. It can output the grid voltage and frequency of any country in the world. The VFD is composed of alternating current, an alternating current (modulation wave) and other circuits, and the standard name of the VFD should be a frequency converter. The output voltage waveform is a pulse square wave, and there are many harmonic components. The voltage and frequency change at the same time and can't be adjusted separately. It does not meet the requirements of the AC power supply. In principle, the use of power supply can not be used, generally only for the three-phase asynchronous motor speed control.
39.What are the interference methods and how to deal with the VFD?
Transmission mode: (1) Radiated interference; (2) Conducted interference
Anti-interference measures: For the interference signal transmitted by radiation, it is mainly weakened by the wiring and shielding of the radiation source and the disturbed circuit. For the interference signal transmitted through the line, it is mainly processed by adding filters, reactors or magnetic rings on the input and output sides of the VFD. The specific methods and precautions are as follows:
(1) The signal line and the power line should be perpendicularly crossed or separated.
(2) Do not connect with different metal wires.
(3) The shielding tube (layer) should be reliably grounded and ensure continuous and reliable grounding of the entire length.
(4) Use twisted-pair shielded cables in the signal circuit.
(5) The grounding point of the shielding layer should be as far away as possible from the inverter and separated from the grounding point of the inverter.
(6) The magnetic ring can be used on the input power line and output line of the inverter. The specific method is: the input lines are wound in the same direction four times, and the output line can be wound three times in the same direction. When winding, pay attention to try to place the magnetic ring close to the inverter.
(7) Generally, shielding and other anti-jamming measures can be taken for the equipment being interfered with.
40.Want to increase the speed of the original conveyor belt and operate at 80Hz. How should the capacity of the VFD be selected?
The power consumed by the conveyor belt is proportional to the rotation speed. Therefore, if you want to operate at 80Hz, the power of the VFD and the motor must be increased in proportion to 80HZ/50HZ, that is, increase the capacity by 60%.
41.What is the difference between using PWM and VVC+?
In VVC, the control circuit uses a mathematical model to calculate the optimal motor excitation when the motor load changes and compensates for the load. In addition, the synchronous 60° PWM method integrated in the ASIC circuit determines the optimal switching time of the inverter semiconductor device (IGBTS).
Determine the switching time to follow the following principles:
1). The numerically largest phase maintains its positive or negative potential within 1/6 cycles (60°).
2). The other two phases are scaled so that the output line voltage remains sinusoidal and reaches the desired amplitude.
Unlike sinusoidal control PWM, VVC works on the digital output of the desired output voltage. This ensures that the inverter's output reaches the voltage rating, the motor current is a sine wave, and the operation of the motor is the same as when the motor is directly connected.
Since the constant of the motor (stator resistance and inductance) is taken into account when the inverter calculates the optimum output voltage, the best motor excitation can be obtained.
Because the inverter continuously detects the load current, the inverter can adjust the output voltage to match the load, so the motor voltage can adapt to the type of motor and follow the change of the load.
Selection of VFD
Select the type of inverter, in accordance with the type of production machinery, speed range, static speed accuracy, starting torque requirements, decided to choose the kind of control method of the inverter is the most appropriate. The so-called appropriate is not only to use, but also economic, to meet the basic conditions and requirements of the process and production.
- The motor to be controlled and the inverter itself
1) The number of poles of the motor. Generally, the number of motor poles is not more than (extremely appropriate, otherwise the inverter capacity must be properly increased. 2) Torque characteristics, critical torque, and acceleration torque. With the same motor power, the inverter specifications can be derated compared to the high overload torque mode. 3) Electromagnetic Compatibility. In order to reduce the main power interference, a reactor can be added to the input circuit of the intermediate circuit or the inverter, or a front isolation transformer can be installed. Generally, when the distance between the motor and the inverter exceeds 50m, a reactor, a filter or a shielded protective cable should be connected in series between them.
- Selection of inverter power
The efficiency of the system is equal to the product of the efficiency of the frequency converter and the efficiency of the motor. Only when both are operating at a higher efficiency, the system efficiency is higher. From the perspective of efficiency, when selecting the inverter power, the following points must be noted:
1) When the inverter power value is equivalent to the motor power value, it is most suitable to facilitate the inverter to operate at high efficiency values. 2) When the power rating of the inverter is not the same as the power classification of the motor, the power of the inverter should be as close to the power of the motor as possible, but it should be slightly larger than the power of the motor. 3) When the motor is frequently started, braked, or when it is under heavy-load starting and it is working more frequently, a large-scale inverter can be selected to use the inverter for long-term and safe operation. 4) After testing, the actual power of the motor does have surplus, and it may be considered to use an inverter whose power is less than the motor power, but it should be noted whether the instantaneous peak current will cause overcurrent protection. 5) When the power of the inverter and the motor is not the same, the setting of the energy-saving program must be adjusted accordingly in order to achieve a higher energy-saving effect.
- Selection of Inverter Box Structure
The cabinet structure of the inverter must be compatible with the environmental conditions, ie, temperature, humidity, dust, pH, corrosive gas and other factors must be taken into account. The following types of structure are commonly available for users to choose: 1) The open type IP00 itself has no chassis and is suitable for being installed in the electric control box or the screen, tray, and frame of the electric room, especially when multiple inverters are used together. It is better to use this type, but the environmental conditions are higher; 2) The closed type IP20 is suitable for general use, and there may be a small amount of dust or a little temperature and humidity; 3) The sealed type IP45 is suitable for the environment with poor industrial conditions. ;4) Closed type IP65 is suitable for poor conditions, water, dust, and certain corrosive gases.
- Determination of the capacity of the frequency converter
Reasonable capacity selection is itself a measure of energy saving and consumption reduction. According to available information and experience, there are three simple methods: 1) The actual power of the motor is determined. First, the actual power of the motor is measured to select the capacity of the inverter. 2) Formula method. When one inverter is used for multiple motors, it should satisfy: At least one motor starting current must be considered to avoid overcurrent tripping of the inverter. 3) Motor rated current method frequency converter. Inverter capacity selection process is actually an optimum matching process between the inverter and the motor. The most common and safer is to make the capacity of the inverter greater than or equal to the rated power of the motor, but the actual matching should consider the motor How much the actual power differs from the rated power is usually because the capacity selected by the equipment is too large, and the actual capacity required is small. Therefore, it is reasonable to select the frequency converter according to the actual power of the motor, to avoid excessive selection of the frequency converter and increase the investment. . For the light load category, the inverter current should generally be selected according to 1.1N (N is the rated current of the motor), or according to the maximum motor power matched with the output power rating of the inverter marked in the product.
- main power supply
1) Power supply voltage and fluctuations. Special attention should be paid to the setting of the low voltage protection setting of the inverter, because in practical use, the possibility of low voltage of the power grid is more likely. 2) Mains frequency fluctuations and harmonic interference. This interference will increase the heat loss of the frequency converter system, resulting in increased noise and reduced output. 3) The power consumption of the inverter and the motor during operation. When designing the system main power supply, both of the power consumption factors should be taken into consideration.
The development direction of the Chinese VFD
1: The entire high-voltage frequency converter market did not appear continuous explosive growth, but China's VFD brand has covered almost all areas, and the relative international brands have a cost-effective advantage. The market share of domestic-funded high-voltage frequency converters has exceeded 55%. From the corporate rankings, Hekang's frequency conversion increased by 13.2%, and the market accounted for 13%, which has already ranked No. 1 in the industry; Lide Huafu market accounted for 12%, Siemens accounted for Compared with 11%, ABB accounted for 9%, and Oriental Hitachi accounted for 5%. Guodian's four-dimensional development speed was faster, with a growth of 44% in 2012, and the industry accounted for close to 5%.
2: China's high-voltage VFD market has a broad space for development. With the expansion of the market and the diversification of customer demand, the functions of domestic VFD products are constantly improving and increasing, and the degree of integration and systemization are getting higher and higher.