Frequency converter common sense

- Sep 19, 2018-

1. What is a frequency converter?


The frequency converter is generally a power control device that converts the power frequency power source into another frequency by using the on/off function of the power semiconductor device.


2. What is the difference between PWM and PAM?


PWM is an abbreviation of English PulseWidthModulation, which changes the pulse width of a pulse train according to a certain rule to adjust the output value and waveform.


PAM is an abbreviation of PulseAmplitudeModulation in English. 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 inverter can be roughly divided into two types: the voltage type is a frequency converter that converts the direct current of the voltage source into an alternating current, the filtering of the direct current circuit is a capacitor, and the current type is a frequency converter that converts the direct current of the current source into an alternating current. The filtering of its DC loop is an inductor.


4. Why does the voltage and current of the inverter change in proportion?


The torque of the asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing in the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux is too large, and the magnetic circuit is saturated. The motor will be burned. Therefore, the frequency and voltage should be changed in proportion, that is, the frequency of the inverter is controlled while changing the frequency, so that the magnetic flux of the motor is kept constant to avoid the occurrence of weak magnetic and magnetic saturation. This type of control is mostly used for energy-saving inverters such as fans and pumps.


5. When the motor is driven by the commercial frequency power supply, the current increases when the voltage drops. For the inverter drive, if the voltage drops when the frequency decreases, does the current increase?


When the frequency drops (low speed), if the same power is output, the current increases, but under a certain torque, the current hardly changes.


6. What is the starting current and starting torque of the motor when the inverter is running?


The inverter is operated, and the frequency and voltage are increased correspondingly with the acceleration of the motor. The starting current is limited to 150% of the rated current (125% to 200% depending on the model). When starting directly with the commercial frequency power supply, the starting current is 6 to 7 times, so mechanical and electrical impact will occur. It can be started smoothly with the inverter drive (starting time becomes longer). The starting current is 1.2~1.5 times of the rated current, and the starting torque is 70%~120% of rated torque. For the inverter with automatic torque boosting function, the starting torque is 100% or more, and it can start with full load.


7. What does V/f mode mean?


The voltage V also decreases proportionally as the frequency decreases. This problem has been explained in 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 memory device (ROM) of the controller, which can be selected by a switch or a dial.


8. How does the torque of the motor change when V and f are changed proportionally?


When the frequency is lowered to completely reduce the voltage proportionally, since the AC resistance becomes small and the DC resistance does not change, the torque generated at the low speed tends to decrease. Therefore, given V/f at low frequencies, the output voltage is increased somewhat in order to obtain a certain starting torque, which is called an enhanced starting.


It can be implemented in various ways, such as an automatic method, selecting a V/f mode, or adjusting a potentiometer.


9. In the manual, the shift range is 60~6Hz, which is 10:1. Is there no output power below 6Hz?


The power can still be output below 6 Hz, but according to the conditions of the motor temperature rise and the starting torque, the minimum use frequency is about 6 Hz. At this time, the motor can output the rated torque without causing serious heat generation. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz according to the model.


10. For the combination of general motors, the torque is required to be above 60 Hz. Is it ok?


Usually it is not possible. Above 60Hz (also in the mode above 50Hz) the voltage is constant, generally constant power characteristics, when the same torque is required at high speed, you must pay attention to the choice of motor and inverter capacity.


11. What does it mean to open a ring?


A speed detector (PG) is provided for the motor device to be used, and the actual speed is fed back to the control device for control, which is called "closed loop". If the PG is not operated, it is called "open loop". General-purpose inverters are mostly open-loop, and some models use PG feedback.


12. What should I do if the actual speed is different for a given speed?


When the ring is open, even if the inverter outputs a given frequency, the motor's speed will fluctuate within the range of rated slip (1%~5%) when the motor is running with load. For the requirement that the speed regulation accuracy is relatively high, even if the load changes, it is required to operate at a speed close to the given speed, and the inverter (optional) with the PG feedback function can be used.


13. If the motor with PG is used, can the speed accuracy be improved after feedback?


The inverter with PG feedback function has improved accuracy. However, the value of the speed accuracy depends on the accuracy of the PG itself and the resolution of the inverter output frequency.


14. What does the stall prevention function mean?


If the given acceleration time is too short, the output frequency of the inverter changes far more than the change of the speed (electrical angle frequency), the inverter will trip due to the overcurrent, and the operation stops. This is called stall. In order to prevent the stall from running, the magnitude of the current is detected for frequency control. When the acceleration current is too large, the acceleration rate is appropriately slowed down. The same is true when decelerating. The combination of the two is the stall prevention function.


15. What is the significance of the models that can be given separately for the acceleration time and deceleration time, and the acceleration and deceleration time?


Acceleration and deceleration can be given separately. It is suitable for short-time acceleration, slow deceleration, or for small machine tools where the production tact time is strictly required. However, for fan drive and other occasions, the acceleration/deceleration time is longer. The acceleration time and deceleration time can be given together.


16. What is regenerative braking?


If the motor is lowered during operation, the motor becomes an asynchronous generator and operates as a brake. This is called regenerative (electrical) braking.


17. Can you get more braking power?


The energy regenerated from the motor is stored in the filter capacitor of the inverter. Due to the relationship between the capacity of the capacitor and the withstand voltage, the regenerative braking force of the general-purpose inverter is about 10% to 20% of the rated torque.


If the optional brake unit is used, it can reach 50%~100%.


18. Please explain the protection function of the inverter?


Protection functions can be divided into the following two categories:


(1) The correction operation is automatically performed after detecting the abnormal state, such as overcurrent stall prevention, and regeneration overvoltage stall prevention.


(2) After detecting the abnormality, the PWM control signal of the power semiconductor device is blocked, so that the motor automatically stops. Such as overcurrent cutoff, regenerative overvoltage cutoff, semiconductor cooling fan overheating and instantaneous power failure protection.


19. Why does the protection function of the inverter operate when the clutch is continuously loaded?


When the load is connected by the clutch, at the moment of connection, the motor changes sharply from the no-load state to the region where the slip ratio is large, and the large current flowing causes the inverter to over-current trip and cannot operate.


20. In the same factory, the large motors move together, and the inverter stops during operation. Why?


When the motor starts, the starting current corresponding to the flow rate and capacity, the transformer on the stator side of the motor produces a voltage drop. When the motor capacity is large, the voltage drop has a great influence. The inverter connected to the same transformer will make an undervoltage or instantaneous stop. Judgment, and thus the protection function (IPE) action sometimes causes the operation to stop.


21, what is the frequency conversion resolution? has no meaning?


For digitally controlled inverters, even if the frequency command is an analog signal, the output frequency is given in a given order. The smallest unit of this level difference is called the frequency conversion resolution.


The frequency conversion resolution usually takes a value of 0.015~0.5Hz. For example, the resolution is 0.5Hz, then the upper of 23Hz can be changed to 23.5, 24.0Hz, so the motor's action is also stepped. This creates problems for applications like continuous take-up control. In this case, if the resolution is about 0.015 Hz, and the level difference of the four-stage motor is 1 r/min or less, it can be sufficiently adapted. In addition, some models have different resolutions and output resolutions.


22. Is there a limit to the installation direction when installing the inverter?


The structure of the inside and the back of the inverter takes into account the cooling effect. The relationship between the upper and lower sides is also important for ventilation. Therefore, for the unit type, the longitudinal position is placed in the disc and hung on the wall, and installed as vertically as possible.


23. Is it possible to use a soft start without directly applying the motor to a fixed frequency inverter?


It is ok at very low frequencies, but if the given frequency is high, the conditions for direct start of the commercial power supply are similar. A large starting current (6~7 times rated current) will flow, and the motor will not start because the inverter cuts off the overcurrent.


24. What problems should I pay attention to when the motor runs over 60Hz?


Pay attention to the following when operating over 60Hz:


(1) It is necessary to operate the machine and the device at this speed (mechanical strength, noise, vibration, etc.).


(2) The motor enters the constant power output range, and its output torque should be able to maintain the work (the output power of the shaft such as the fan and pump increases proportionally to the cube of the speed, so pay attention when the speed is slightly increased).


(3) The problem of bearing life is to be fully considered.


(4) For motors with medium capacity or higher, especially 2-pole motors, it is necessary to discuss with the manufacturer carefully when operating above 60 Hz.


25. Can the inverter drive the gear motor?


Depending on the structure of the reducer and the way it is lubricated, there are a number of issues to be aware of. In the structure of the gear, 70~80Hz is usually considered as the maximum limit. When oil lubrication is used, continuous operation at low speed is related to the damage of the gear.


26. Can the inverter be used to drive a single-phase motor? Can I use a single-phase power supply?


Single-phase motors are basically not available. For the single-phase motor of the governor switch-start type, it will burn out under the speed range below the operating point.


Auxiliary winding; for capacitor starting or capacitor operation, capacitor explosion will be induced. The power supply of the inverter is usually three-phase, but for small-capacity, it also has a single-phase power supply.


27. What is the power consumed by the inverter itself?


It is related to the model, operating status, frequency of use, etc. of the inverter, but it is difficult to answer. However, the efficiency of the inverter below 60 Hz is about 94%~96%. According to this, the loss can be estimated, but the built-in regenerative braking type (FR-K) inverter, if the braking loss is also taken into account, the power consumption It will become larger, and attention must be paid to the design of the operation panel.


28. Why can't I use it continuously in the whole area of 6~60Hz?


Generally, the motor is cooled by the blades on the outer fan or the rotor end ring mounted on the shaft. If the speed is lowered, the cooling effect is lowered, so that the same heat as the high-speed operation cannot be withstood, and the load torque at a low speed must be reduced, or A large-capacity inverter is combined with a motor or a dedicated motor.


29. What should I pay attention to when using a motor with a brake?


The brake excitation circuit power supply should be taken from the input side of the frequency converter. If the brake is operating when the inverter is outputting power, it will cause an overcurrent cut. Therefore, the brake should be actuated after the inverter stops outputting.


30. I want to use the inverter to drive the motor with the capacitor for improving the power factor. The motor does not move. Please explain why.


The current flowing into the inverter improves the power factor. Because the charging current causes the inverter overcurrent (OCT), it cannot be started. As a countermeasure, please operate the capacitor after removing it, or even improve the power factor. On the input side of the inverter. It is effective to connect an AC reactor.


31. How long is the life of the inverter?


Although the inverter is a stationary device, it also has consumption devices such as a filter capacitor and a cooling fan. If they are regularly maintained, it is expected to have a life of more than 10 years.


32. The inverter has a cooling fan inside. What is the direction of the wind? What if the fan is broken?


For small capacity, there are also models with no cooling fan. For fans with a fan type, the direction of the wind is from bottom to top. Therefore, where the inverter is installed, do not place mechanical equipment that interferes with suction and exhaust at the upper and lower parts. Also, do not place hot parts on the top of the inverter. When the fan fails, it is protected by the fan stop detection or overheat detection on the cooling fan.


33, the filter capacitor is a consumable, then how to judge its life?


As a capacitor used for the filter capacitor, the electrostatic capacity is gradually reduced with time, and the electrostatic capacity is periodically measured to determine the lifetime based on 85% of the rated capacity of the product.


34. Is there a limit to the installation direction when installing the inverter?


It should be basically stored in the disk. The problem is that the disk with a fully enclosed structure has a large outer shape, a large space, and a relatively high cost. The measures are:


(1) The design of the disc should be directed to the heat dissipation required by the actual device;


(2) using an aluminum fin, fin coolant to increase the cooling area;


(3) Use a heat pipe.


In addition, models have been developed that can be exposed on the back of the frequency converter.


35, want to improve the speed of the original conveyor belt, running at 80Hz, how to choose the capacity of the inverter?


The reference speed is 50 Hz, and the constant power output characteristic is above 50 Hz. When the constant torque characteristic load such as the conveyor belt is increased, the capacity needs to be increased by 80/50 ≈ 1.6 times. The motor capacity also increases like a frequency converter.


Introduction to inverter structure and fault judgment


The inverter is a device that converts a commercial power supply (50 Hz or 60 Hz) into an AC power supply of various frequencies to realize variable speed operation of the motor. The control circuit completes the control of the main circuit, the rectifier circuit converts the alternating current into direct current, the direct current intermediate circuit smoothes the output of the rectifier circuit, and the inverter circuit inverts the direct current into alternating current. For a frequency converter such as a vector control inverter that requires a large amount of calculations, sometimes a CPU for torque calculation and some corresponding circuits are required.


1. A rectifier that is connected to a single-phase or three-phase AC power source to produce a pulsating DC voltage.


2. The intermediate circuit has the following three functions:


a. Make the pulsating DC voltage stable or smooth for use by the inverter.


   b. Power is supplied to each control line through the switching power supply.


c. Filter or brakes can be configured to improve drive performance.


3. An inverter that converts a fixed DC voltage into an AC voltage of variable voltage and frequency.


4. A control circuit that transmits signals to the rectifier, intermediate circuit, and inverter, and it also receives signals from these portions. Its main components are: output drive circuit, operation control circuit. The main functions are:


a. A signal is used to switch the semiconductor device of the inverter.


b. Provide various control signals for operating the inverter.


c. Monitor the working status of the inverter and provide protection.


If the person who operates the inverter and the peripheral control device in the field can judge and deal with some common fault conditions, the work efficiency can be greatly improved, and some unnecessary


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