The frequency converter is mainly composed of rectification (AC to DC), filtering, re-rectification (DC to AC), brake unit, drive unit, and detection unit micro-processing unit.
1. Why can the rotation speed of the motor be freely changed?
*1: r/min
Motor rotation speed unit: number of rotations per minute, can also be expressed as rpm.
Example: 2-pole motor 50Hz 3000 [r/min]
4-pole motor 50Hz 1500 [r/min]
Conclusion: The rotational speed of the motor is proportional to the frequency
The motor referred to in this article is an induction AC motor, and most of the motors used in the industry are of this type. The rotational speed of an induction type AC motor (hereinafter simply referred to as a motor) is approximately determined by the number of poles and frequencies of the motor. The principle of the motor determines that the number of poles of the motor is fixed. Since the pole value is not a continuous value (a multiple of 2, for example 2, 4, 6 poles), it is generally uncomfortable and the speed of the motor is adjusted by changing this value.
In addition, the frequency can be adjusted outside the motor and then supplied to the motor, so that the rotation speed of the motor can be freely controlled.
Therefore, the inverter with the purpose of controlling the frequency is the preferred device for the motor speed control device.
n = 60f/p
n: Synchronous speed f: Power frequency p: Motor pole pair number Conclusion: Changing the frequency and voltage is the best motor control method
If only the frequency is changed without changing the voltage, the motor will be out of voltage (over-excitation) when the frequency is reduced, causing the motor to be burned out. Therefore, the inverter must change the voltage while changing the voltage. When the output frequency is above the rated frequency, the voltage cannot continue to increase. The highest value is equal to the rated voltage of the motor.
For example: In order to reduce the rotational speed of the motor by half, change the output frequency of the inverter from 50Hz to 25Hz. At this time, the output voltage of the inverter needs to be changed from 400V to about 200V.
2. What happens to the output torque when the speed (frequency) of the motor changes?
*1: Power frequency power supply Power supply (commercial power supply) supplied from the grid
*2: Starting current When the motor starts running, the output torque of the inverter is less than the starting torque and maximum torque when the inverter is driven, and the starting and accelerating shock is large when the motor is directly driven by the power frequency power supply. When using frequency converters, these shocks are weaker. Power frequency direct start will produce a large starting starting current. When using the inverter, the output voltage and frequency of the inverter are gradually added to the motor, so the motor starting current and impact are smaller.
In general, the torque generated by a motor decreases with decreasing frequency (decrease in speed). Reduced actual data is given in the manual of the frequency converter.
By using a magnetic flux vector controlled inverter, the shortage of torque at low speeds of the motor will be improved, and the motor may output sufficient torque even in the low speed range.
3. When the frequency converter speeds to a frequency greater than 50Hz, the output torque of the motor will decrease
The usual motor is designed and manufactured at a voltage of 50Hz. The rated torque is also given in this voltage range. Therefore, the speed regulation below the rated frequency is called constant torque speed regulation. (T=Te, P<=Pe)
When the output frequency of the inverter is greater than 50Hz, the torque generated by the motor decreases in a linear relationship inversely proportional to the frequency.
When the motor is running at a frequency speed of more than 50Hz, the size of the motor load must be taken into consideration to prevent the output torque of the motor from being insufficient.
For example, the torque generated by a motor at 100 Hz is reduced to approximately 1/2 of the torque at 50 Hz.
Therefore, the speed regulation above the rated frequency is called constant power speed regulation. (P=Ue*Ie)
4. Frequency converter above 50Hz application
As we all know, for a specific motor, its rated voltage and rated current are constant.
If the rated values ​​of the inverter and motor are: 15kW/380V/30A, the motor can work above 50Hz.
When the speed is 50Hz, the output voltage of the inverter is 380V, and the current is 30A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage and current of the inverter can only be 380V/30A. The output power is obviously unchanged. So we call it constant power speed regulation.
What about the torque situation at this time?
Because P = wT (w: angular velocity, T: torque). Because P is constant, w increases, so the torque will be reduced accordingly.
We can also look at another perspective:
The stator voltage of the motor U = E + I*R (I is the current, R is the electronic resistance, E is the induced potential)
It can be seen that E is unchanged when U and I are unchanged.
And E = k*f*X, (k: constant, f: frequency, X: magnetic flux), so when f is 50-->60Hz, X will decrease accordingly. For the motor, T=K*I *X, (K: constant, I: current, X: magnetic flux), so the torque T will decrease as the magnetic flux X decreases.
At the same time, when the frequency is less than 50Hz, since I*R is small, when U/f=E/f is constant, the flux (X) is constant. The torque T is proportional to the current. This is why the inverter is usually used. Overcurrent capability to describe its overload (torque) capability. Also known as constant torque speed regulation (nominal current -> maximum torque constant)
Conclusion: When the frequency converter output frequency increases from above 50Hz, the output torque of the motor will decrease.
5. Other factors related to output torque
Heat generation and heat dissipation determine the output current capability of the inverter, which affects the output torque capability of the drive.
Carrier frequency: Generally, the rated current of the inverter is the highest carrier frequency, which can ensure the continuous output at the highest ambient temperature. If the carrier frequency is reduced, the motor current will not be affected. However, the heating of the components will be reduced.
Ambient temperature: As if the ambient temperature is detected to be low, increase the inverter protection current value.
Altitude: The increase in altitude will affect the heat dissipation and insulation performance. Normally, the following 1000m can not be considered. The above derating of 5% per 1000 meters is sufficient.
6. How does vector control improve the output torque capability of a motor?
*1: Torque boost This function increases the output voltage of the inverter (mainly at low frequencies) to compensate for the loss of output torque caused by the voltage drop across the stator resistance, thereby improving the output torque of the motor.
The technique of improving the low speed output torque of the motor using the "vector control" can make the motor at low speed, such as (without speed sensor) 1Hz (for a 4-pole motor, its speed is about 30r/min) can output torque The motor's torque output at 50Hz is reached (maximum is approximately 150% of the rated torque).
For the conventional V/F control, the voltage drop of the motor is relatively increased as the motor speed decreases, which results in the motor being unable to obtain sufficient rotational force due to insufficient excitation. In order to compensate for this deficiency, the inverter needs to increase the voltage to compensate for the voltage drop caused by the reduced motor speed. This function of the inverter is called "torque boost" (*1).
The torque boost function is to increase the output voltage of the inverter. However, even if a lot of output voltage is increased, the motor torque cannot be increased corresponding to its current. Because the motor current contains the motor generated torque component and other components (such as excitation components).
"Vector control" distributes the motor's current value to determine the value of the motor current component and other current components (such as the excitation component) that produce the torque.
"Vector control" can be optimized and compensated by the response to the voltage drop at the motor terminal, allowing the motor to produce large torque without increasing the current. This function is also effective for improving the temperature rise of the motor at low speed.
1. What is a frequency converter?
The frequency converter is a power control device that utilizes the on-off function of the power semiconductor device to convert the power frequency power source to another frequency.
What are the differences between PWM and PAM?
PWM is English Pulse Width Modulation (Pulse Width Modulation) (abbreviation of pulse width modulation), according to a certain rule to change the pulse width of the pulse train to adjust the amount of output and waveform adjustment method.
PAM is the abbreviation of English 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 the waveform.
3. What is the difference between voltage type and current type?
The main circuit of the frequency converter 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 stone inductance.
4. Why does the inverter's voltage and current change proportionally?
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 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 inverters.
5. When the motor is driven by the power frequency, the current will increase when the voltage drops. For the inverter drive, if the voltage drops when the frequency drops, then 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. What is the starting current and starting torque of the motor when using the inverter?
With the inverter 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 starting with the power frequency power supply directly, the starting current is 6~7 times. Therefore, mechanical and electrical impact will occur. The inverter 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 inverter 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 ratio between V and f is pre-determined in consideration of the characteristics of the motor. There are several characteristics in the controller's storage device (ROM) that can be selected with switches or dials.
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 be implemented by various methods, such as automatic method, V/f mode selection or potentiometer adjustment
9, in the description of the shift range of 60 ~ 6Hz, that is 10:1, then there is 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 inverter (starting frequency) is 0.5~3Hz depending on the model.
10, for the general motor combination is above 60Hz also requires a certain torque, is it possible?
Normally it is not possible. In the above 60Hz (also has the mode above 50Hz) the voltage does not change, basically is the constant power characteristic, when requests the identical torque in the high speed, must pay attention to the choice of the electrical machinery and the frequency changer capacity.
11, what is the meaning of the so-called open-loop?
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. This is called "closed loop", and the operation without PG is called "open loop." General-purpose inverters are mostly open-loop, and some models use options for PG feedback.
12. How to do when the actual speed has a deviation for a given speed?
In the open loop, even if the inverter 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 frequency converter (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 accuracy of the speed 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 will change far beyond the change of the speed (electrical angle frequency). The inverter 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 models with acceleration time and deceleration time given separately, and the type of acceleration/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. Is greater braking power available?
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 inverter 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 inverter?
Protection features 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, so that the motor will automatically stop. Such as over-current cut-off, regenerative over-voltage cut-off, semiconductor cooling fan overheating and instantaneous power failure protection.
19. Why does the inverter's protection function act when the clutch is continuously loaded?
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 inverter to trip overcurrent and cannot operate.
20. In the same factory, large-scale motors move together and the inverter 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 inverter 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 frequency conversion resolution? has no meaning?
For digitally controlled inverters, 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 inverter?
The structure of the inside and the back of the inverter 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 inputting the motor directly to a fixed frequency inverter?
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 inverter cuts off the overcurrent.
24, when the motor exceeds 60Hz operation should pay attention to what the problem?
Pay attention to the following matters when operating above 60Hz
(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, etc. shaft output power in proportion to the cube speed increase, so a little increase in speed should also pay attention to).
(3) Bearing life issues 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.
25, inverter can drive gear motor?
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, inverter can be used to drive a single-phase motor? Can I use single-phase power?
Machine 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 inverter itself?
It is related to the model, operating status, frequency of use of the inverter, but it is difficult to answer. However, the efficiency of inverters below 60Hz is approximately 94% to 96%, and losses can be estimated accordingly. However, if a regenerative braking (FR-K) inverter 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 6~60Hz region?
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 frequency converter is combined with a motor or a special motor is used.
29. What should you watch out for when using a motor with brakes?
Brake excitation circuit power should be taken from the input side of the inverter. If the brake is operating when the inverter is outputting power, the overcurrent will be cut off. Therefore, the brake must be operated after the inverter stops outputting.
30. I want to use a frequency converter to drive a motor with a capacitor for improving power factor. The motor does not move.
The inverter's current flowing into the capacitor for improving the power factor, due to the charging current caused by the inverter overcurrent (OCT), it can not start, as a measure, please remove the capacitor and run, or even improve the power factor, at the input side of the inverter Access to AC reactors is effective.
31. How long is the life of the inverter?
Although inverters are stationary devices, they also have consumer devices such as filter capacitors and cooling fans. If these inverters are regularly maintained, they are expected to have a lifetime of more than 10 years.
32. The inverter 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 inverter 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 inverter. When the fan fails, it is protected by fan stop detection or overheat detection on the cooling fan
33, filter capacitors for consumables, then how to determine 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 inverter?
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.;
(3) Use a heat pipe.
In addition, a type in which the reverse side of the inverter can be exposed has been developed.
35. Want to increase the speed of the original conveyor belt and operate at 80Hz. How to choose the capacity of the inverter?
Let the reference speed be 50Hz, and 50Hz or more constant power output characteristics. When a constant torque characteristic such as a conveyor belt increases the load, the capacity needs to be increased to 80/50≈1.6 times. The motor capacity also increases like a frequency converter.