01. What is a frequency converter?
A frequency converter is an electrical device that adjusts the frequency of a power supply using power semiconductor devices to turn it on and off. It can perform various functions, including soft starting, regulating frequency conversion speed, increasing operating accuracy, adjusting power factor, and providing overcurrent, overvoltage, and overload protection.
02. What are the differences between PWM and PAM?
PWM stands for Pulse Width Modulation and is a technique for adjusting the output and waveform by changing the width of the pulses in a pulse train.
PAM stands for Pulse Amplitude Modulation and is a method of adjusting the output value and waveform by changing the amplitude of pulses in a pulse train according to a specific law.
03. What is the difference between voltage mode and current mode?
The main circuit of a frequency converter can be classified into two types:
Voltage type frequency converter converts DC voltage source to AC. The DC circuit filter in this type of frequency converter is a capacitor.
The current mode frequency converter, on the other hand, converts the DC current source to AC. The DC loop filter in this type of frequency converter is an inductor.
04. Why does the frequency converter voltage change proportionally to the frequency?
The electromagnetic torque of a motor is generated by the interaction between current and magnetic flux. It is essential to keep the current within the nominal value to prevent the motor from overheating.
If the magnetic flux decreases, the electromagnetic torque will also decrease, leading to a reduction in the motor's load 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 significantly with the operating frequency fX, which can easily cause saturation of the magnetic circuit, leading to serious distortions of the excitation current waveform and high peak current.
To avoid weak magnetic field and magnetic saturation, it is important to change the frequency and voltage proportionally, that is, control the output voltage of the frequency converter while changing the frequency to maintain a certain level of magnetic flux in the motor.
This control mode is commonly used in energy-saving frequency converters for fans and pumps.
When a motor is driven by an electrical frequency power source, the current increases as the voltage decreases. However, in the case of frequency converter drive, if the voltage also decreases when the frequency decreases, will the current also increase?
When the frequency decreases (at low speed), if the same power is maintained (constant power), the current will increase. However, if the condition of a certain torque is maintained (constant torque), the current remains almost unchanged.
06. What is the starting current and starting torque of the motor when using a frequency converter?
The frequency converter is used during operation by gradually increasing the frequency and voltage of the motor. The starting current is limited to less than 150% of the rated current (varying from 125% to 200% depending on different models).
On the other hand, when starting directly with an industrial frequency power supply, the starting current can reach 6 to 7 times the nominal current, causing mechanical and electrical impacts.
When using frequency converter drive, the starting process becomes smoother, with starting current of 1.2 to 1.5 times the rated current and starting torque of 70% to 120% of the rated torque.
For frequency converters with automatic torque increase function, the starting torque exceeds 100% and allows full load starting.
07. What does V/f mode mean?
As the frequency decreases, the voltage (V) also decreases proportionally. This relationship between V and f was explained previously in answer 4.
The proportional relationship between V and f is predetermined based on the engine characteristics. Typically, several feature options are stored in the controller's memory device (ROM) and can be selected using a switch or dial.
08. How does the engine torque change when changing V and f proportionally?
When the voltage is reduced in proportion to the decrease in frequency, the ground torque generated at low speed tends to decrease as the AC impedance becomes smaller and the DC resistance remains unchanged.
To obtain a certain starting torque at low frequency, the output voltage must be increased. This compensation is known as enhanced departure.
This can be achieved through several methods, including an automatic method, selecting a V/f mode, or adjusting a potentiometer.
09. The manual says that the speed range is 60~6Hz, that is, 10:1, so there is no power output below 6Hz?
Although power can still be output below 6 Hz, the minimum usable frequency is around 6 Hz, considering factors such as increased engine temperature, starting torque, and other conditions. At this frequency, the motor can produce the rated torque without causing significant heating problems.
The actual output frequency (starting frequency) of the frequency converter varies from 0.5 to 3 Hz depending on the model.
10. For General Motors combination above 60 Hz, a certain torque is also required. Everything is fine?
Normally, no. When the voltage is above 60Hz (there are also modes above 50Hz), it has a constant power characteristic, requiring the same torque at high speed.
11. What does open circuit mean?
The motorized device used is equipped with a speed detector (PG) that feeds the actual speed back to the control device for control, called “closed loop”. On the other hand, a motorized device without PG operation is called “open loop”.
Most frequency converters operate in open loop mode, although some machines offer the option of PG feedback.
Closed-loop control mode without speed sensor calculates actual motor speed using a predetermined mathematical model and magnetic flux, effectively forming closed-loop control with a virtual speed sensor.
12. What if the actual speed deviates from the indicated speed?
In an open-loop system, even though the frequency converter outputs a specific frequency, the motor speed may change within the range of the rated slip rate (1% to 5%) when it is running under load.
For applications that require high speed regulation accuracy and require the motor to operate close to the specified speed even if the load changes, a frequency converter with PG feedback function can be used (as an optional feature).
13. If PG motor is used, can speed accuracy be improved after feedback?
Frequency converter with PG feedback function improves accuracy. However, speed accuracy depends on both the accuracy of the PG and the output frequency resolution of the frequency converter.
14. What does stall prevention mean?
If the specified acceleration time is too short and the frequency converter output frequency changes much faster than the change in speed (electrical angular frequency), the frequency converter may trip and stop running due to overcurrent, known as locking.
To avoid stalling and ensure that the motor continues to run, it is necessary to monitor the current and control the frequency.
If the acceleration current becomes too high, the acceleration rate must be reduced accordingly. The same applies during deceleration.
The combination of these actions is known as the stall function.
15. What is the meaning of having models with acceleration time and deceleration time given separately and with acceleration and deceleration time given together?
Acceleration and deceleration can be specified separately. This is suitable for short-duration accelerations and slow decelerations, or for small machine tools where the production tapping time needs to be strictly defined.
However, for fan drives and other applications with long acceleration and deceleration times, the acceleration and deceleration times may be specified together.
16. What is regenerative braking?
If the motor command frequency decreases during operation, the motor becomes an asynchronous generator and acts as a brake, known as regenerative (electrical) braking.
17. Can you get more braking force?
The energy generated by the motor during regenerative braking is stored in the frequency converter filter capacitor.
However, the regenerative braking force of a general frequency converter is limited to about 10% to 20% of the rated torque due to the relationship between the capacitor capacity and the withstand voltage.
By using an optional brake unit, the regenerative braking force can be increased by 50% to 100%.
18. Describe the protection function of the frequency converter?
Protection functions can be divided into two categories:
(1) Automatically performs corrective actions after detecting an abnormal state, such as overcurrent blocking prevention and regeneration overvoltage blocking prevention.
(2) Blocks the PWM control signal of the power semiconductor device after detecting an abnormality, causing the motor to stop automatically. Examples include overcurrent cutout, regeneration overvoltage cutout, semiconductor cooling fan overheating, and instantaneous power failure protection.
19. Why does the frequency converter protection function work when the clutch is used to connect the load?
When a clutch is used to connect the load, the engine suddenly goes from a no-load state to an area with a high slip rate at the time of connection. The resulting large current flow causes the frequency converter to trip due to overcurrent, preventing operation.
20. In the same factory, when large motors move together, the frequency converter stops during operation. Why?
When the motor starts, a starting current proportional to its capacity will flow, causing a voltage drop in the transformer on the stator side of the motor. If the motor has a large capacity, this voltage drop can have a significant impact.
A frequency converter connected to the same transformer can detect an undervoltage or trigger an instantaneous stop. As a result, the protection function (IPE) may be activated, causing operation to stop.
21. What is frequency conversion resolution? What's the point?
For digitally controlled frequency converters, even if the frequency command is an analog signal, the output frequency is set in increments. The smallest unit of this increment is called the frequency conversion resolution, which is typically 0.015 to 0.5 Hz.
For example, if the resolution is 0.5 Hz, the frequency can be changed in 0.5 Hz increments, such as from 23 Hz to 23.5 Hz and 24.0 Hz, causing the motor to also operate in increments.
This can create problems for applications that require continuous winding control. In these cases, a resolution of about 0.015 Hz is recommended. With this resolution, the difference of one stage in a 4-stage motor is less than 1R/min, providing sufficient accuracy. Please note that the resolution specified for some models may not match the actual output resolution.
22. Are there any restrictions on the installation direction when installing the frequency converter?
The cooling effect of the frequency converter is considered in the design of its internal and rear structure. Proper ventilation also depends on the orientation of the unit.
Therefore, it is recommended to install panel-type units and wall-mounted units vertically whenever possible.
23. Is it possible to put the motor directly into a fixed frequency converter without soft starting?
It is possible to start a motor with a very low frequency, but if the specified frequency is high, it is similar to starting directly with an industrial frequency power supply. This will result in a large starting current (6 to 7 times the rated current), causing the frequency converter to trip due to overcurrent and preventing the motor from starting.
24. What should be paid attention to when the motor runs above 60Hz?
The following items must be taken into consideration when operating above 60Hz:
(1) Ensure that machines and devices are capable of operating at this speed, taking into account factors such as mechanical resistance, noise and vibration.
(2) When the motor reaches the constant power range, its output torque must be sufficient to maintain operation. Note that the power output of the fan, pump, and other shafts increases proportionally to the cube of the speed, so be careful when the speed increases.
(3) Consider the impact on bearing life.
(4) For motors of medium or greater capacity, especially 2-pole motors, it is important to consult the manufacturer before operating above 60Hz.
When using a reducer, several aspects must be taken into consideration, depending on its structure and lubrication method.
In gear structures, a maximum limit of 70 to 80Hz must be considered.
When using oil lubrication, continuous operation at low speed may cause gear damage.
25. Can the frequency converter be used to drive a single-phase motor? Can I use single phase power supply?
Basically, no. For single-phase motors with governor switch type starters, the auxiliary winding may burn out in the speed regulation range below the operating point.
For capacitor start or capacitor operation mode, capacitor explosions may occur.
The power supply for frequency converters is normally three-phase, but for small capacities, single-phase power supply can also be used.
26. How much energy does the frequency converter itself consume?
The efficiency of a frequency converter depends on several factors, including model, operational status and frequency of use. It's difficult to provide a definitive answer.
However, it is estimated that the efficiency of frequency converters operating below 60 Hz is approximately 94% to 96%. This can be used as a basis for calculating losses.
It is important to note that energy consumption may be higher when considering loss during braking.
Designing an effective operator panel is also crucial and should receive close attention.
27. Why can't it operate continuously in the entire 6~60Hz area?
Generally the motor is cooled by an external fan installed on the shaft or by blades on the rotor end ring.
If the speed is reduced, the cooling effect will also decrease, making it unable to withstand the same level of heat as it would during high-speed operation.
To avoid this, it is necessary to reduce the load torque at low speed, use a high-capacity frequency converter in combination with the motor, or choose a special motor designed for low-speed operation.
28. What should be observed when using the engine with a brake?
The power supply for the brake excitation circuit must be taken from the input side of the frequency converter.
If the brake is applied while the frequency converter is still outputting power, this may result in an overcurrent cutout.
To avoid this, it is important to ensure that the brake is only applied after the frequency converter stops supplying power.
29. If you want to use a frequency converter to drive a capacitor motor to improve the power factor, but the motor does not move, explain why.
Regarding the impact of the frequency converter capacitor on the effective power factor after removing the frequency converter, measures must be taken to improve the power factor caused by the current flowing into the frequency converter capacitor.
30. What is the useful life of the frequency converter?
Although the frequency converter is a static device, it also includes consumable components such as filter capacitors and cooling fans.
With proper maintenance, these components are expected to have a useful life of more than 10 years.
31. There is a cooling fan hidden in the frequency converter. What is the wind direction? What happens if the fan breaks?
For small capacity models with or without fans:
For models with fans, air flows from bottom to top, so it is important not to place any mechanical equipment that could obstruct suction and exhaust in the upper and lower parts of the place where the frequency converter is installed.
Furthermore, it is also important to avoid placing heat-sensitive components above the frequency converter.
In the event of fan failure, the frequency converter is protected by electric fan stop detection or cooling fan overheat detection.
32. The filter capacitor is a consumable, so how to evaluate its useful life?
For the capacitor used as a filter capacitor, its electrostatic capacity gradually decreases with time.
It is recommended to regularly measure the electrostatic capacity and evaluate its useful life based on whether it has reached 85% of the product's nominal capacity.
33. Are there any restrictions on the installation direction when installing the frequency converter?
Typically, the capacitor should be stored in a disc-shaped container.
However, fully enclosed disc-shaped containers can be quite large, take up a significant amount of space, and are relatively expensive.
To resolve these issues, the following measures can be taken:
(1) The disk design must take into account the heat dissipation requirements of the device;
(2) Aluminum fins and a finned coolant can be used to increase the cooling area.
34. What is the function of the DC reactor of the frequency converter?
To reduce high-order harmonic interference in the input current and improve the power factor of the input power supply.
35. What is the function of the sinusoidal filter connected to the frequency converter?
The sine filter allows the frequency converter to operate with a long motor cable and is also suitable for circuits that include an intermediate transformer between the frequency converter and the motor.
36. What is the resistance value of the given potentiometer of the frequency converter?
The resistance value of the potentiometer supplied with the frequency converter is normally in the range of 1K Ω to 10K Ω.
37. What are the frequency converter interference modes and how to deal with them?
- Communication modes:
(1) Radiation interference;
(2) Conducted interference.
- Anti-interference measures:
For interference signals transmitted through radiation, they can be effectively reduced by properly routing and shielding the radiation source and the disturbed line.
Interference signals transmitted through the line can be resolved by adding filters, ballasts or magnetic rings on the input and output side of the frequency converter.
Specific methods and precautions to reduce interference are as follows:
(1) Signal and power lines must be crossed or bundled vertically.
(2) Avoid connecting wires made of different metals together.
(3) The shielding layer must be properly grounded, and the grounding must be continuous and reliable throughout its entire length.
(4) A shielded twisted pair cable must be used in signal circuits.
(5) The grounding point of the shielding layer should be as far away from the frequency converter as possible and separate from the grounding point of the frequency converter.
(6) A magnetic ring can be used on the input power line and output line of the frequency converter.
The specific method for using a magnetic ring is as follows: The input line can be wound four times in the same direction, while the output line can be wound three times in the same direction.
It is important to keep the magnetic ring as close as possible to the frequency converter during winding.
(7) Furthermore, to prevent interference, shielding and other anti-interference measures can be implemented for disturbed equipment and instruments.
38. If you want to increase the speed of the original conveyor belt and operate at 80Hz, how to choose the capacity of the frequency converter?
The power consumed by the conveyor belt is directly proportional to its speed.
Therefore, if you want to operate at 80Hz, the power of the frequency converter and motor must be increased proportionally, which means a 60% increase over the 50Hz capability. This means that the capacity of the frequency converter and motor must be increased by 60%.
39. What is the difference between PWM and VVC+?
In VVC (variable voltage and frequency) control, the control circuit uses a mathematical model to calculate the ideal motor excitation in response to changes in motor load and compensates the load accordingly.
Additionally, the control circuit incorporates a 60° synchronous PWM method implemented in an ASIC (Application Specific Integrated Circuit) that determines the optimal switching time for the inverter semiconductor devices (IGBTs).
40. Why can't the frequency converter be used as a frequency conversion power supply?
The overall circuitry of a variable frequency power supply is made up of components such as AC constant current and AC filters, resulting in pure sine wave output voltage and current waveforms that closely resemble the ideal AC power supply.
It is capable of generating grid voltage and frequency for any country in the world.
On the other hand, the frequency converter consists of components such as AC constant current (modulation wave) and other circuits. The standard name for this device is variable frequency regulator.
However, the output voltage waveform of the frequency converter is a square pulse wave with numerous harmonic components. Voltage and frequency change proportionally at the same time and cannot be adjusted independently, making them unsuitable for use as a power supply.
It is normally only used to regulate the speed of a three-phase asynchronous motor.
