How the inverter works

The inverter is mainly composed of rectifying (AC to DC), filtering, rerectifying (DC to AC), braking unit, driving 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, which can also be expressed as RPM.
Example: 2 pole motor 50Hz 3000 [r/min]
4 pole motor 50Hz 1500 [r/min]
　　　　
Conclusion: The motor rotation speed is proportional to the frequency.

The motor mentioned in this article is an induction type AC motor. Most of the motors used in the industry are of this type. The rotating speed of an induction AC motor (hereinafter referred to as a motor) is almost determined by the number and frequency of the motor. The operation principle of the motor determines that the Pole Number of the motor is fixed. Because the pole number is not a continuous value (a multiple of 2, for example, the pole number is 2, 4, 6), it is generally not suitable to adjust the speed of the motor by changing the value.

In addition, the frequency can be adjusted outside the motor and then provided to the motor, so that the motor's rotation speed can be freely controlled.
Therefore, the inverter for frequency control is the preferred equipment for motor speed control equipment.
N = 60f/P
N: synchronization speed
F: Power Supply frequency
P: pole logarithm of the Motor
　　
Conclusion: changing the frequency and voltage is the optimal motor control method.

If only the frequency is changed but the voltage is not changed, the motor may be burned out due to overvoltage (over-excitation) when the frequency is reduced. Therefore, the inverter must change the voltage at the same time while changing the frequency. When the output frequency is above the rated frequency, the voltage cannot be increased. The maximum voltage can only be equal to the rated voltage of the motor.

For example, to reduce the rotation speed of a 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 200 V to about v.
　　
2. What is the output torque when the rotation speed (frequency) of the motor changes?

* 1: Power Frequency
Power supply provided by the power grid (commercial power supply)
* 2: starting current
The output current of the inverter when the motor starts to run
The starting torque and maximum torque of the inverter must be less than that of the direct drive by the power frequency power supply.
When the power frequency power supply is used, the starting and acceleration impacts greatly. When the inverter is used for power supply, these impacts will be weaker. Power Frequency Direct start will generate a large starting current. When the inverter is used, the output voltage and frequency of the inverter are gradually added to the motor, so the starting current and impact of the motor are smaller.
Generally, the torque produced by the motor decreases with the decrease in frequency (lower speed. The actual data for reduction is described in some inverter manuals.
The Inverter Controlled by the magnetic flux vector can improve the torque deficiency at low speed of the motor, and even the motor can output enough torque in the low speed zone.
　　
3. When the speed of the inverter reaches a frequency greater than 50Hz, the output torque of the motor will be reduced.

Generally, a motor is designed and manufactured at a voltage of 50Hz, and its rated torque is also provided within this voltage range. Therefore, the speed regulation at the rated frequency is called the constant torque speed regulation (t = tE, P <= PE)
When the inverter output frequency is greater than 50Hz, the torque generated by the motor will decrease linearly in inverse proportion to the frequency.
When the motor runs at a speed greater than 50Hz, the motor load must be considered to prevent the motor output torque from being insufficient.
For example, the torque generated by the motor at 1/2Hz is reduced to about of the torque generated at 50Hz.
Therefore, the speed regulation above the rated frequency is called the constant power speed regulation. (P = ue * IE)
　　
4. Application of the inverter over 50Hz

As you know, for a specific motor, its rated voltage and rated current remain unchanged.
For example, the inverter and motor rated values are 15kW/380 V/30a, and the motor can work at 50Hz or above.
When the speed is 50Hz, the output voltage of the inverter is 380 V 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 380 V/30a. obviously, the output power remains unchanged. so we call it constant power speed regulation.

What is the torque?
Because p = wt (W: angular velocity, T: Torque). Because p remains unchanged and W increases, the torque decreases accordingly.
We can also look at it from another perspective:
Stator Voltage U = e + I * r of the motor (I is the current, r is the electronic resistance, e is the induction potential)
We can see that E remains unchanged when u and I remain unchanged.
E = K * f * X, (K: constant, F: frequency, X: magnetic flux), so when F is from 50 --> 60Hz, X decreases accordingly.
For the motor, t = K * I * X, (K: constant, I: Current, X: magnetic flux), so the torque T will decrease with the magnetic flux X.
At the same time, if I * r is small when it is less than 50Hz, and U/F = E/F remains unchanged, the magnetic flux (X) is a constant. the torque T is proportional to the current. this is why the inverter's Overcurrent Capability is usually used to describe its overload (torque) capability. it is also called constant torque speed control (the rated current remains unchanged --> the maximum torque remains unchanged)
Conclusion: When the inverter output frequency increases from 50Hz to above, the motor output torque will decrease.

5. Other factors related to the output torque

The output current of the inverter depends on the heat supply and heat dissipation capabilities, thus affecting the output torque of the inverter.
Carrier Frequency: Generally, the rated current of the inverter is based on the highest carrier frequency. The constant output value can be ensured at the highest ambient temperature to reduce the carrier frequency. The current of the motor will not be affected. However, the heat of components is reduced.
Ambient temperature: it is like not increasing the protection current value of the inverter when the ambient temperature is low.
Altitude: the increase in altitude affects both heat dissipation and insulation performance. Generally, less than 1000 m can be ignored. You can drop the temperature by 5% every M.
　　
6. How does Vector Control improve the output torque of a motor?

* 1: Torque increase
This function increases the output voltage of the inverter (mainly in low frequency) to compensate the output torque loss caused by the voltage drop of the stator resistance, thus improving the output torque of the motor.
$ Technologies to improve low-speed output torque of Motor
With "vector control", the motor can be at low speed, for example (when there is no speed sensor) 1Hz (for a 4 pole motor, its speed is about 30r/min) the output torque can reach the torque output by the motor at 50Hz (about 150% of the rated torque at the maximum ).
For conventional V/F control, the voltage drop of the motor increases with the speed of the motor, which leads to insufficient excitation, so that the motor cannot obtain enough rotation force. In order to compensate for this deficiency, the inverter needs to increase the voltage to compensate for the voltage drop caused by the speed reduction of the motor. This function of the inverter is called "torque boost" (* 1 ).
The torque increase 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 correspondingly increased with its current. Because the motor current contains the torque and other components (such as the excitation component) produced by the motor ).
"Vector Control" allocates the current value of the motor to determine the current component of the motor generating torque and the value of other current components (such as the excitation component.
"Vector Control" allows the motor to generate a large torque without increasing the current by responding to the voltage drop of the motor. This function is also effective for improving the temperature rise at low speed of the motor.

1. What is a frequency converter?

A frequency converter is an electric energy control device that converts a power frequency power supply to another frequency by using the breaking function of power semiconductor devices.

2. What are the differences between PWM and Pam?

PWM is the abbreviation of pulse width modulation (Pulse Width Modulation). The pulse width of the pulse column is changed according to a certain law to adjust the output and waveform.
Pam is the abbreviation of pulse amplitude modulation (pulse amplitude modulation). It is a modulation method that changes the pulse amplitude of the pulse column according to a certain law to regulate the output value and waveform.

3. What is the difference between the voltage type and the current type?

The main circuit of the inverter can be divided into two types: the voltage type is to convert the DC of the voltage source to the AC inverter, And the filtering of the DC circuit is a capacitor; the current type is to convert the DC of the current source into an AC inverter, and its DC Circuit filters the rock inductance.

4. Why is the inverter voltage proportional to the current?

The torque of an asynchronous motor is generated by the interaction between the motor's magnetic flux and the rotor's internal overcurrent. At the rated frequency, if the voltage decreases only the frequency, the magnetic flux will be too large, magnetic Circuit saturation, will burn the motor in severe cases. Therefore, the frequency and voltage must change proportionally, that is, when the frequency is changed, the output voltage of the inverter is controlled at the same time, so that the magnetic flux of the motor remains certain and the phenomenon of Weak Magnetic and magnetic saturation is avoided. This control mode is mostly used for Fan and pump energy-saving inverters.

5. When the motor is driven by a power frequency power supply, the current increases when the voltage drops. If the frequency decreases when the inverter is driven, does the current increase?

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

6. How is the starting current and starting torque of the motor when the inverter is used?

The inverter is used for operation. As the motor accelerates, the frequency and voltage are increased accordingly. The starting current is limited to less than 150% of the rated current (125% ~ 200% ). When the power frequency power supply is used for Direct start, the starting current is 6 ~ As a result, it will generate a mechanical and electrical impact. The inverter drive can be smoothly started (longer start time ). The starting current is 1.2 ~ of the rated current ~ 1.5 times, starting torque is 70% ~ 120% rated torque. For inverters with the auto-enhanced torque function, the starting torque is more than 100% and can be started with full load.

7. What does the V/F mode mean?

The voltage V also decreases proportionally when the frequency drops, which is already explained in Answer 4. The proportional relationship between V and F is pre-determined based on the characteristics of the motor. Generally, there are several features in the Controller's storage device (ROM), which can be selected using a switch or a scale disk.

8. How does the torque of the motor change when the ratio of V and F is adjusted?

When the frequency drops, the voltage is reduced proportionally. As the AC impedance becomes smaller and the DC resistance remains unchanged, the local torque will decrease at low speed. Therefore, if V/F is given at low frequencies, the output voltage must be increased to obtain a certain starting torque. This compensation is called enhanced startup. Various methods can be used for implementation, including automatic method, V/F mode, and potentiometer adjustment.

9. Write the variable speed range 60 ~ 6Hz, that is, 10:1, is there no output power below 6Hz?

The output power is still available below 6Hz, but the minimum usage frequency is about 6Hz according to the motor temperature rise and start torque, at this time, the motor can output the rated torque without causing serious fever problems. The actual output frequency (starting frequency) of the inverter is 0.5 ~ 3Hz.

10. If the combination of General Motors is more than 60Hz, the torque is required. Is it OK?

Generally, this is not acceptable. The voltage remains unchanged at a voltage of more than 60Hz (or more than 50Hz) and is generally constant power. When the same torque is required at high speed, you must pay attention to the Capacity Selection of the motor and inverter.

11. What does the so-called open-loop mean?

Set a speed generator (PG) for the motor device in use, and feed back the actual speed to the control device for control. It is called a "closed loop". If PG is not used, it is called an "open loop ". Most of the general inverters are open-loop, and some options can be used for PG feedback.

12. What should I do if the actual speed is different from the specified speed?

During open-loop, even if the inverter outputs a given frequency, when the motor is running with load, the motor speed is within the rated conversion rate range (1% ~ 5%) changes. The inverter with PG feedback function (optional parts) can be used for scenarios requiring high speed and even load changes that run at a speed close to the given speed ).

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

The inverter with PG feedback function improves the accuracy. However, the speed precision depends on the PG precision and the resolution of the inverter output frequency.

14. What does the stall prevention function mean?

If the specified acceleration time is too short and the output frequency of the inverter changes much more than the speed (angle frequency), the inverter will trip over the current and stop running. This is called a stall. In order to prevent the motor from being paused, it is necessary to check the current size for frequency control. When the acceleration current is too large, the acceleration speed is slowed down as appropriate. This is also true when slowing down. The combination of the two is the hold function.

15. What is the significance of specifying the options for acceleration time and deceleration time?

Acceleration and deceleration can be set separately for Short-Time acceleration and slow deceleration, or for small machine tools that require strict production cycle time, but for Fan transmission and other occasions, the acceleration and deceleration time are long, and the acceleration and deceleration time can be jointly specified.

16. What is regenerative braking?

If a motor reduces the instruction frequency during operation, the motor changes to an asynchronous generator and runs as a brake, which is called a regenerative (Electrical) brake.

17. Can we get a larger braking force?

The energy storage generated 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% ~ of the rated torque ~ 20%. If the selected parts brake unit is used, it can reach 50% ~ 100%.

18. What is the protection function of the inverter?

There are two types of protection functions:
(1) automatically corrected the abnormal state after detection, such as the prevention of overcurrent stalls and the prevention of regenerative overvoltage stalls.
(2) detect exceptions and block the PWM control signal of the power semiconductor device so that the motor will stop automatically. Such as over-current cut-off, over-voltage regeneration cut-off, semiconductor cooling fan overheating and transient power failure protection.

19. Why does the protection function of the inverter work when the clutch is used for continuous load?

When a clutch is used to connect to the load, the motor changes sharply from the no-load state to the area with a high conversion rate. The large current may cause the inverter to trip over the current and cannot run.

20. If a large motor works together in the same factory, the inverter stops working. Why?

When the motor starts, it will flow through the starting current corresponding to the capacity. The transformer at the stator side of the motor will generate a voltage drop. When the motor capacity is large, this pressure drop will also affect a lot, the inverter connected to the same transformer will make an undervoltage or transient Stop Judgment, so sometimes the protection function (IPE) Action, causing the stop operation.

21. What is the frequency conversion resolution? What is the significance?

For digitally controlled inverters, even if the frequency command is a analog signal, the output frequency is given in a level. The minimum unit of this level difference is called the frequency conversion resolution.
The frequency conversion resolution is usually 0.015 ~ 0.5 hz. for example, if the resolution is 23.5Hz, the upper part of the 23hz can be 24.0 or Hz, so the motor action also follows in a certain level. This will cause problems for the use of continuous take-up control. In this case, if the resolution is about 0.015hz, it can be fully adapted for a class-4 motor with a level difference of below 1R/min. In addition, the resolution given by some machines is different from the output resolution.

22. Whether the installation direction is limited when the inverter is installed.

The internal and back structures of the inverter take into account the cooling effect, and the relationship between the upper and lower is also important for ventilation. Therefore, for the unit type in the disc and hanging on the wall, take the vertical position, as far as possible vertical installation.

23. Can I directly invest the motor into a fixed frequency inverter without using soft start?

It is possible at a very low frequency, but if the given frequency is high, it is similar to the conditions for direct start of power frequency power supply. Will flow through a large starting current (6 ~ 7 times rated current), because the inverter cut off the current, the motor cannot start.

24. When the motor exceeds 60Hz, what should I pay attention?

Note the following when running 60Hz
(1) The machinery and equipment should operate at this speed (mechanical strength, noise, vibration, etc ).

(2) When the motor enters the constant power output range, its output torque must be able to maintain the operation (the output power of the fan and Pump Shafts increases proportionally to the cubic speed, so pay attention to this when the speed is slightly higher ).
(3) bearing life issues should be fully considered.
(4) For Motor with medium capacity or above, especially 2 pole motor, it should be carefully discussed with the manufacturer during operation at 60Hz or above.

25. Can the inverter drive the gear motor?

Several problems should be noted according to the structure and lubrication method of the Cer. Generally 70 ~ can be considered in the gear Structure ~ 80Hz is the maximum limit. When lubricating with oil, continuous operation at low speed is related to gear damage.

26. Can a inverter be used to drive a single-phase Motor? Can single-phase power supply be used?

The host is basically unavailable. For single-phase Motor with the Governor switch starting, it will be burned out in the speed adjustment range below the working point
Auxiliary winding. capacitor explosion will be induced when the capacitor is started or the capacitor is running. The power supply of the inverter is usually three-phase, but for small capacity, it is also useful for single-phase power supply operation.

27. How much power is consumed by the inverter itself?

It depends on the type, running status, and frequency of the inverter, but it is very difficult to answer. However, the efficiency of inverters below 60Hz is about 94% ~ 96%, according to this can calculate the loss, but the internal regenerative brake (FR-K) inverter, if the brake wear is also taken into account, power consumption will become larger, for the operation of the disk design must pay attention.

28. Why not 6 ~ 60Hz continuous operation in all regions?

Generally, the Motor uses the blades on the external fan or rotor end ring mounted on the shaft for cooling. If the speed is reduced, the cooling effect decreases, so it cannot withstand the same fever as high-speed operation, the load torque at low speed must be reduced, the inverter with large capacity must be combined with the motor, or a dedicated motor must be used.

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 inverter. If the inverter is acting as a brake when the power is output, the current will be cut off. So the brake action should be performed after the inverter stops the output.

30. If you want to use a inverter to drive a motor with a capacitor to improve the power factor, the motor will not move.

The current inflow of the inverter to improve the power factor of the capacitor, because of its charging current caused by inverter over-current (OCT), so can not start, as a countermeasure, please remove the capacitor after running, it even improves the power factor and is effective when AC reactor is connected to the input side of the inverter.

31. How long is the life of the inverter?

Although the inverter is a static device, it also consumes devices like filter capacitors and cooling fans. If regular maintenance is performed on them, it is expected to have a life of more than 10 years.

32. There is a cooling fan in the inverter. What is the direction of the wind? What if the fan breaks down?

There is no cooling fan type for small capacity. There is a fan of the machine type, the direction of the wind is from the bottom up, so the local installation of the inverter, the upper and lower do not place to prevent suction, exhaust mechanical equipment. Also, do not place hot parts above the inverter. When a fan fails, the fan stops detection or cools the fan to prevent overheating.

33. How can we determine the lifetime of a filter capacitor as a consumable?

As a capacitor used by the filter capacitor, the electrostatic capacity decreases slowly over time. The static capacity is measured regularly to determine the life cycle when the rated capacity of the product reaches 85%.

34. Whether the installation direction is limited when the inverter is installed.

The problem is that the disk with fully enclosed structure has a large shape and space, and the cost is relatively high. The measures include:
(1) The design of the disk should aim at the heat dissipation required by the actual device;
(2) Use Aluminum Heat Sink and wing coolant to increase the cooling area;
(3) Use a heat catheter.
In addition, the type that can be exposed on the back of the inverter has been developed.

35. How can I choose to increase the speed of the original conveyor belt to operate at 80Hz?

Set the reference speed to 50Hz and 50Hz or above as the constant power output feature. The capacity needs to increase to 80/50 ≈ 1.6 times as the load growth rate of constant torque characteristics such as conveyor belts. The motor capacity is also increased like the inverter