01
frequency converter
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency power into electrical energy of another frequency. It can realize functions such as soft starting of AC asynchronous motor, variable frequency speed regulation, improved operating accuracy, power factor change, and overcurrent/overvoltage/overload protection.
02
Differences between PWM and PAM
PWM is an abbreviation for Pulse Width Modulation, a modulation method that adjusts the output quantity and waveform by changing the pulse width of a pulse train according to a certain pattern. PAM is an abbreviation for Pulse Amplitude Modulation, a modulation method that adjusts the output quantity and waveform by changing the pulse amplitude of a pulse train according to a certain pattern.
03
What are the differences between voltage-source and current-source voltage ...
The main circuit of a frequency converter can be broadly divided into two categories: voltage-source frequency converters convert DC voltage to AC voltage, and the DC circuit filter is a capacitor; current-source frequency converters convert DC current to AC current, and the DC circuit filter is an inductor.
04
Why does the voltage of a frequency inverter change proportionally to its frequency?
The electromagnetic torque of any electric motor is the result of the interaction between current and magnetic flux. The current must not exceed the rated value, otherwise the motor will overheat. Therefore, if the magnetic flux decreases, the electromagnetic torque will also decrease, leading to a reduction in the load-carrying capacity.
As can be seen from the formula E=4.44*K*F*N*Φ, during variable frequency speed regulation, the magnetic circuit of the motor changes within a considerable range with the operating frequency fX. This can easily cause severe saturation of the motor's magnetic circuit, resulting in severe distortion of the excitation current waveform and the generation of a very high peak current.
Therefore, the frequency and voltage must be changed proportionally. That is, while changing the frequency, the output voltage of the frequency converter is controlled to keep the magnetic flux of the motor constant and avoid the occurrence of weak magnetic field and magnetic saturation. This control method is mostly used in energy-saving frequency converters for fans and pumps.
05
When an electric motor is driven by a mains frequency power supply, a decrease in voltage will increase the current; for a frequency converter drive, if a decrease in frequency also causes a decrease in voltage, will the current increase?
When the frequency decreases (low speed), the current increases if the output power is the same, but the current remains almost unchanged under the condition of constant torque.
06
What are the starting current and starting torque of the motor when operating with a frequency converter?
When using a frequency converter, the frequency and voltage increase accordingly as the motor accelerates, limiting the starting current to below 150% of the rated current (125%~200% depending on the model). When starting directly with a mains frequency power supply, the starting current is 6~7 times the rated current, resulting in mechanical and electrical shocks. Frequency converter drives allow for smooth starting (though starting time is 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 frequency converters with automatic torque boosting, the starting torque is over 100%, allowing for full-load starting.
07
What does V/f mode mean?
The voltage V decreases proportionally as the frequency decreases, as explained in answer 4. The ratio of V to f is predetermined by taking into account the motor characteristics. Several characteristics are usually stored in the controller's memory (ROM), which can be selected using a switch or dial.
08
How does the motor torque change when V and f are changed proportionally?
When the frequency decreases, the voltage is reduced proportionally. Since the AC impedance decreases while the DC resistance remains constant, this tends to reduce the torque generated at low speeds. Therefore, at low frequencies, given a V/f, the output voltage needs to be increased slightly to obtain a certain starting torque; this compensation is called enhanced starting. Various methods can be used to achieve this, including automatic methods, selecting a V/f mode, or adjusting a potentiometer.
09
The instruction manual states the speed range is 60~6Hz, or 10:1. Does that mean there's no output power below 6Hz?
While power can still be output below 6Hz, the minimum operating frequency is around 6Hz, considering factors such as motor temperature rise and starting torque. At this frequency, the motor can output rated torque without causing serious overheating problems. The actual output frequency (starting frequency) of the inverter ranges from 0.5Hz to 3Hz, depending on the model.
10
Is it possible to maintain a constant torque for typical motor combinations operating at frequencies above 60Hz?
Normally, it's not possible. Above 60Hz (and some modes above 50Hz), the voltage remains constant, exhibiting largely constant power characteristics. However, at high speeds, to achieve the same torque... 11. What does "open-loop" mean?
A closed-loop control system uses a speed detector (PG) to feed back the actual speed to the control unit. A system operating without a PG is called an open-loop control system. Most general-purpose frequency converters are open-loop, although some models offer PG feedback as an option. Sensorless closed-loop control calculates the actual motor speed based on a mathematical model and magnetic flux, essentially using a virtual speed sensor to create a closed-loop control.
12
What should be done if the actual rotational speed deviates from the given speed?
In open-loop operation, even if the inverter outputs a given frequency, the motor speed will vary within the rated slip range (1%~5%) when the motor is under load. For applications requiring high speed control accuracy and operation close to the given speed even with load variations, an inverter with PG feedback function (optional) can be used.
13
If a motor with a PG (post-processor) is used, will the speed accuracy be improved after feedback?
Inverters with PG feedback function offer improved accuracy. However, the speed accuracy depends on the accuracy of the PG itself and the resolution of the inverter's output frequency.
14
What does stall prevention function mean?
If the given acceleration time is too short, the change in the inverter's output frequency will far exceed the change in speed (electrical angular frequency), causing the inverter to trip due to overcurrent and stop operation; this is called stall. To prevent stall and allow the motor to continue running, the current magnitude must be detected for frequency control. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same applies during deceleration. The combination of these two measures constitutes the stall function.
15
What is the significance of having models where acceleration and deceleration times can be set separately, and models where both acceleration and deceleration times can be set together?
For machine types where acceleration and deceleration can be set separately, this is suitable for short-time acceleration and slow deceleration applications, or for small machine tools where strict production cycle time needs to be set. However, for applications such as fan drives, where acceleration and deceleration times are relatively long, acceleration and deceleration times can be set together.
161
What is regenerative braking?
If the command frequency is reduced while the electric motor is running, the motor will operate as an asynchronous generator and act as a brake. This is called regenerative (electric) braking.
17
Is it possible to achieve greater braking force?
The energy regenerated from the motor is stored in the inverter's filter capacitor. Due to the capacitance and voltage rating of the capacitor, the regenerative braking force of a general-purpose inverter is approximately 10% to 20% of the rated torque. If an optional braking unit is used, it can reach 50% to 100%.
18
Please explain the protection functions of the frequency converter.
Protection functions can be divided into the following two categories:
(1) Automatically perform corrective actions after detecting abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention.
(2) Upon detecting an abnormality, the PWM control signal of the power semiconductor device is blocked, causing the motor to stop automatically. Examples include overcurrent cutoff, regenerative overvoltage cutoff, semiconductor cooling fan overheating protection, and instantaneous power outage protection.
19
Why does the inverter's protection function activate when the load is connected using a clutch?
When a clutch is used to connect a load, at the moment of connection, the motor changes drastically from an unloaded state to a region with high slip. The large current flowing through it causes the inverter to trip due to overcurrent and become unable to operate.
20
Why do the frequency converters stop running when large motors in the same factory start together?
When a motor starts, a starting current corresponding to its capacity will flow through it. The transformer on the stator side of the motor will generate a voltage drop. When the motor capacity is large, this voltage drop will have a greater impact. The frequency converter connected to the same transformer will make a judgment of undervoltage or momentary stop. Therefore, sometimes the protection function (IPE) will be activated, causing the motor to stop operating.
twenty one
What is variable frequency resolution? What is its significance?
For digitally controlled frequency converters, even if the frequency command is an analog signal, the output frequency is given in steps. The smallest unit of this step difference is called the frequency conversion resolution. Frequency conversion resolution is typically set to 0.015~0.5Hz. For example, with a resolution of 0.5Hz, 23Hz can be changed to 23.5Hz or 24.0Hz, thus the motor's operation also follows in steps. This causes problems for applications like continuous winding control. In such cases, a resolution of around 0.015Hz is sufficient for a 4-pole motor, where one step difference is less than 1 r/min. Additionally, some models have a different given resolution than their output resolution.
twenty two
Are there any restrictions on the installation direction when installing a frequency converter?
The internal and rear structure of the frequency converter takes into account the cooling effect, and the vertical relationship is also important for ventilation. Therefore, for unit-type inverters that are inside the panel or mounted on the wall, the vertical position is taken, and they are installed as vertically as possible.
twenty three
Is it possible to directly connect the motor to a frequency converter with a fixed frequency without using soft starting?
It's possible at very low frequencies, but if the given frequency is high, the conditions are similar to direct starting with a mains frequency power supply. A large starting current (6-7 times the rated current) will flow through it, and the motor will not start because the frequency converter cuts off the overcurrent.
twenty four
What precautions should be taken when a motor is running at frequencies exceeding 60Hz?
The following precautions should be taken when operating at frequencies above 60Hz:
(1) The machinery and equipment must be able to operate at this speed with sufficient capacity (mechanical strength, noise, vibration, etc.).
(2) When the motor enters the constant power output range, its output torque must be able to maintain operation (the shaft output power of fans, pumps, etc. increases proportionally to the cube of the speed, so attention should be paid when the speed increases slightly).
(3) The issue of bearing life should be fully considered.
(4) For medium-capacity motors and above, especially 2-pole motors, it is necessary to discuss with the manufacturer carefully when operating at frequencies above 60Hz.
Depending on the structure and lubrication method of the speed reducer, several issues need to be considered. For gears, a maximum speed limit of 70-80Hz is generally considered. When using oil lubrication, continuous operation at low speeds can lead to gear damage.
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Can a frequency converter be used to drive a single-phase motor? Can it use a single-phase power supply?
Basically, it's unusable. For single-phase motors with speed controller switch start, it will burn out the auxiliary winding when operating below the speed limit; for capacitor-start or capacitor-run motors, it will induce capacitor explosion. Frequency converters are typically three-phase, but some small-capacity models operate on single-phase power.
26
How much power does the frequency converter itself consume?
It depends on the type of inverter, its operating status, and the frequency of use, but it's difficult to answer definitively. However, inverters below 60Hz have an efficiency of approximately 94% to 96%, from which losses can be estimated. But for regenerative braking (FR-K) inverters, if braking losses are also taken into account, the power consumption will increase, which must be considered in the design of the control panel, etc.
27
Why can't it operate continuously across the entire 6~60Hz frequency range?
Generally, motors use external fans mounted on the shaft or blades on the rotor end rings for cooling. If the speed decreases, the cooling effect decreases, and therefore the motor cannot withstand the same heat generation as when running at high speed. It is necessary to reduce the load torque at low speeds, or use a combination of a high-capacity frequency converter and the motor, or use a special motor.
28
What precautions should be taken when using a motor with a brake?
The power supply for the brake's excitation circuit should be taken from the input side of the frequency converter. If the brake operates while the frequency converter is outputting power, it will cause an overcurrent trip. Therefore, the brake should only be activated after the frequency converter has stopped outputting power.
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I want to use a frequency converter to drive a motor with a capacitor for improving the power factor, but the motor doesn't move.
The inverter's current flows into the capacitor used to improve the power factor. Because its charging current causes an overcurrent (OCT) in the inverter, it cannot start. As a countermeasure, please remove the capacitor and then operate the inverter. As for improving the power factor, connecting an AC reactor to the input side of the inverter is effective.
30
How long is the lifespan of a frequency inverter?
Although frequency converters are static devices, they also have consumable components such as filter capacitors and cooling fans. If they are maintained regularly, they can be expected to have a lifespan of more than 10 years.
31
The inverter has a built-in cooling fan. What is the direction of the airflow? What happens if the fan breaks down?
For small-capacity models without cooling fans, and for models with fans, the airflow is from bottom to top. Therefore, avoid placing any mechanical equipment above or below the inverter installation location that might obstruct airflow. Also, do not place heat-sensitive components above the inverter. In case of fan failure, protection will be provided by fan stop detection or overheat detection on the cooling fan.
32
Filter capacitors are consumables, so how do you determine their lifespan?
As a capacitor used for filtering, its capacitance decreases slowly over time. The capacitance is measured periodically, and its lifespan is judged based on the point at which it reaches 85% of the product's rated capacity.
33
Are there any restrictions on the installation direction when installing a frequency converter?
Ideally, the contents should be primarily stored within a tray. However, the problem is that trays with a fully enclosed structure are large in size, take up a lot of space, and are relatively expensive. Solutions include:
(1) The design of the tray should be based on the heat dissipation requirements of the actual device;
(2) Increase the cooling area by using aluminum heat sinks, finned coolants, etc.;
34
What is the function of the DC reactor in a frequency converter?
Reduce high-order harmonic interference of the input current and improve the power factor of the input power supply.
35
What is the function of a sinusoidal filter in a frequency converter accessory?
Sine filters allow frequency converters to operate with longer motor cables and are also suitable for circuits with an intermediate transformer between the frequency converter and the motor.
36
What is the resistance value of the potentiometer for the frequency converter?
The resistance value of the potentiometer for frequency converters is typically between 1KΩ and 10KΩ.
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What are the types of interference in frequency converters and how are they typically handled?
1. Methods of dissemination:
(1) Radiated interference;
(2) Conducted interference
1. Anti-interference measures: For interference signals propagated by radiation, the main methods to weaken them are through wiring and shielding of the radiation source and the affected lines. For interference signals propagated through lines, the main methods are to install filters, reactors, or magnetic rings on the input and output sides of the frequency converter. Specific methods and precautions are as follows:
(1) Signal lines and power lines should cross perpendicularly or be laid in separate slots.
(2) Do not use wires of different metals to connect to each other.
(3) The shielding tube (layer) should be reliably grounded and ensure continuous and reliable grounding along its entire length.
(4) Twisted-pair shielded cables must be used in signal circuits.
(5) The grounding point of the shielding layer should be as far away from the inverter as possible and separate from the grounding point of the inverter.
(6) The magnetic ring can be used on both the input and output power lines of the frequency converter. The specific method is as follows: wind the input lines together in the same direction 4 times, and wind the output lines together in the same direction 3 times. When winding, be careful to keep the magnetic ring as close to the frequency converter as possible.
(7) Generally, shielding and other anti-interference measures can be taken for the equipment and instruments that are interfered with.
38
To increase the speed of an existing conveyor belt and operate it at 80Hz, how should the capacity of the frequency converter be selected?
The power consumed by the conveyor belt is directly proportional to the rotational speed. Therefore, if you want to operate at 80Hz, the power of the frequency converter and the motor must be increased proportionally to 80Hz/50Hz, which means increasing the capacity by 60%.
39
What are the differences 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. Furthermore, the synchronous 60° PWM method integrated into the ASIC circuitry determines the optimal switching time of the inverter semiconductor devices (IGBTS).
40
Why can't a frequency converter be used as a variable frequency power supply?
A variable frequency power supply consists of AC-DC-AC-filtering circuits, resulting in pure sine waves for both its output voltage and current, closely resembling an ideal AC power supply. It can output the grid voltage and frequency of any country in the world. In contrast, a frequency converter (also known as a variable frequency drive) is composed of AC-DC-AC (modulated wave) circuits. Its output voltage waveform is a pulsed square wave with numerous harmonic components. The voltage and frequency change proportionally and cannot be adjusted separately, failing to meet the requirements of an AC power supply. Therefore, it is generally unsuitable for use as a power source and is typically only used for speed control of three-phase asynchronous motors.
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