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Inverter Application: Instruction to 10 Common Problems



This technical article will introduce 10 common problems you might encounter with when apply inverter in your project.



What is Inverter?


This video explains what inverter is and which inverter you need in your project.


1. Leakage Circuit Breakers are Prone to Tripping When Using Variable-Frequency Drive.

The output waveform of the ac drive contains higher harmonic, and the leakage current will be generated between the motor and the cable between the inverter and the motor, what’s more, the leakage current is much larger than that of the motor driven by the power frequency.


The leakage current at the output side of the inverter is about three times that of the power frequency operation, in addition, adding the leakage current of the motor. The operation current of the selected leakage protector should be 10 times greater than that of the leakage current at the power frequency. 


2. The Temperature Rise of the Motor Higher Than That of the Power Frequency When the AC Drive is Used.

The output voltage waveform of the inverter is not sinusoid wave, but distorted wave, the motor current under rating torque is about 10% more than the power frequency, so the temperature rise is slightly higher than the power frequency.


3. How to Adjust Torque Boost.

A. When the torque boost setting is too high and the load is very small, the current will increase due to the magnetic flux saturation of the motor core, and the variable-frequency drive may run overcurrent protection. Therefore, in order to improve the motor efficiency, the setting should be reduced when the load is lightened.


B. For heavy load, the voltage drop loss caused by stator winding and motor cable can be compensated by increasing the torque-boost setting value.


4. Carrier Frequency and How to Adjust It.

A. The output voltage of the SPWM converter is a series of pulses whose pulse frequency is equal to the carrier frequency.


B. In the current of the motor, there is a strong harmonic component of the carrier frequency, which will cause the oscillation of the iron core of the motor and emit noise. If the frequency of the noise is equal to the inherent oscillation frequency of the motor core, the noise will increase.

In order to reduce it, the frequency inverter can adjust the carrier frequency in a certain range to avoid the resonance frequency of the noise.


C. Harmonic component of carrier frequency has strong radiation, which will cause electromagnetic interference to external electronic equipment.


D. From the point of view of improving the current waveform, the higher the carrier frequency, the smoother the current waveform. However, the electromagnetic interference to the outside is also stronger.


E. The higher the carrier frequency is, the less the motor noise is, but the greater the switching loss of power device is, the more serious the frequency converter is. The lower the carrier frequency, the greater the motor noise, and the switching loss of the inverter is lower too.


5. DC Brake

(1) It is used to control the precise parking of some equipments, to avoid "crawling" at low speed, and to start the function at the time of shutdown.


(2) Since the frequency conversion speed control system always starts from the lowest frequency, if the motor starts with a certain speed, and the frequency converter does not set the speed tracking function, the overcurrent or overvoltage will appear.


6. Should the Rating Frequency of the Load Motor be the Same as That of the Motor?

This function parameter:  the fundamental frequency


A. If the fundamental frequency is set below the rated frequency of the motor, the motor voltage will increase, and the output voltage will increase will lead to the increase of the magnetic flux of the motor, making the saturation of the flux, the distortion of the exciting current, and the occurrence of a very large peak current. As a result, the converter tripped because of overcurrent.


B. If the fundamental frequency is higher than the rated frequency of the motor, the voltage and load capacity of the motor will decrease.


Difference Compensation

Depending on the magnitude of the load current, the output frequency of the ac drive (internal improvement, actual display constant) is appropriately increased to compensate for the increase in the rotational difference due to the increase in the load.


7. AVR Function

When the power network voltage drops, the reference frequency is reduced automatically and the flux K*U/F is constant, so as to ensure the load capacity of the motor unchanged.


Kinds of Common Load:

1)Constant Torque Load

Although the rotational speed is different, resistance torque load is basically constant. The output power is proportional to the rotational speed, like the belt conveyer.


2)Constant Power Load

Although the rotational speed is different, load power is basically constant. The output torque is proportional to the rotational speed. Like a winding device, such as a thin film or sheet.


3)Square Load

The resistance torque load is proportional to the square of the rotational speed. Such as fans and pumps.


8. Frequency Control of Several Special Motors

(1) Wound Rotor Asynchronous Motor

The rotor winding of a wound rotor asynchronous motor is a set of star-schema three-phase windings. The end points of the three-phase windings are connected to the three collector rings, through it to collect the brush and the external resistor (starting or adjusting speed).


After adopting the frequency converter to adjust the speed, the rotor winding does not need to connect the resistor, so the terminal of the three-phase winding can be connected directly with the wire.


(2) Magnetic Brake Motor

It is composed of ordinary motor and magnetic brake. The motor and the magnetic brake are connected to the power supply at the same time, and the armature of the electromagnet is absorbed, which makes the motor rotor rotate freely.


After cutting off the power supply, the excitation winding of the brake powers off and the rotor stops quickly. The excitation winding circuit of the electromagnet should be connected to the input side of the frequency converter after adopting the frequency converter, and turned on at the same time as motor.


9. Capacity Selection of a Single Inverter with Multiple Motors. 

A. Simultaneous Start-up

The rated current of the inverter should be greater than the sum of the maximum operating current of several motors.


B. Starting Time in Turn

The rated current of the converter shall be greater than the sum of the rated current of the motor other than the maximum motor plus the seven-times rated current of the maximum motor.


10. Interference Mode and Treatment of Inverter

Propagation Mode

1) Radiatedradiated Interference  2) Conducted Interference


Anti-jamming Measures

Interference signals propagating by radiation are weakened mainly by wiring and shielding the radioactive sources and the interfered lines.


For the interference signal propagating through the circuit, the filter, reactor or magnetic ring are added to the input and output side of the inverter.


The Specific Methods and Precautions are as Follows:


(1) Signal lines and power lines should be vertically crossed or slotted separately.

(2) Do not use different metal wires to connect to each other.

(3) Shielding tube (layer) should be reliably grounded and ensure continuous and reliable grounding across the whole length.

(4) Twisted-pair shielded cables should be used in signal circuits.

(5) Grounding contacts of the shield layer should away from the frequency converter as far as possible, and separated from the connecting location of the frequency converter.

(6) The magnetic ring can be used on the input power line and output line of the inverter. The method is as follows: the input line goes around four times in the same direction and the output line around three times in the same direction with magnetic rings. When winding the wire, the magnetic rings should close the frequency converter as far as possible.

7) Shielding and other anti-interference measures, such as the temperature control of injection molding machine, can be taken for the equipment.


FAQ


1. What does an inverter do?

Inverters are also called AC Drives, or VFD (variable frequency drive). They are electronic devices that can turn DC (Direct Current) to AC (Alternating Current). It is also responsible for controlling speed and torque for electric motors.


2. What is the purpose and function of an inverter?

An inverter converts the DC electricity from sources such as batteries or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.


3. What is inverter and how it works?

The first thing to keep in mind when it comes to enriching your understanding of the internal structure of an inverter device, is that the converter circuit converts alternating current (AC) coming from the power source into direct current (DC), and the inverter circuit changes the converted direct current (DC) back into alternating current (AC). They work as a set. 


4. Does inverter really save electricity?

An inverter is energy saving technology that eliminates wasted operation in air conditioners by efficiently controlling motor speed. ... Compared to non-inverter type air conditioners, air conditioners with inverters have less power loss and can save in energy.


5. What can you plug into an inverter?

A power inverter changes DC power from a battery into conventional AC power that you can use to operate all kinds of devices ... electric lights, kitchen appliances, microwaves, power tools, TVs, radios, computers, to name just a few.


6. How many hours can an inverter last?

Usually, you can expect your inverter battery to last anywhere from 5 to 10 hours when it is fully charged. Most inverters show an estimated as soon as they start powering the appliances.


7. How many watts inverter do I need for home?

Peak output is the wattage that an inverter can supply for short periods of time when the demand spikes, while continuous output is the limit for normal operation. If your devices draw a combined total of 600 watts, then you need to buy an inverter that has a continuous output rating of 600 watts


8. Is UPS and inverter same?

The UPS is the electric device that has a rectifier for providing the backup power to the system whereas the inverter converts DC into AC. The main function of the UPS is to store the electric supply whereas the inverter converts the AC power into DC power.


9. Where should I install an inverter in my house?

To install the inverter, place the inverter assembly on top of the main housing chassis in such a way that the inverter faces forward. Remember to remove power at the electricity board meter of the home.


10. What size inverter do I need to run a laptop?

Volts (120) x Amps = Watts.

For example if your DVD player draws 100 watts and your laptop another 100 watts, a minimum 300-watt inverter is recommended. If the item is motor driven, it requires additional start-up (surge) wattage (typically 2-3 times the continuous wattage required) to start the device.


11. What are the types of inverter?

There are 3 major types of inverters - sine wave (sometimes referred to as a "true" or "pure" sine wave), modified sine wave (actually a modified square wave), and square wave.


12. How long will a 12V battery last with an inverter?

For example: 12V 100Ah battery will be able to power 1000W inverter for ~30 minutes, 12V 200Ah battery will be able to power 2000W inverter for ~30 minutes, etc.


13. Is inverter An gate?

An Inverter is a Logic Gate that has only one Input, it outputs the opposite Logic State of its Input. The Inverter is also called NOT Gate.


14. What is the disadvantage of inverter AC?

If an inverter AC has lower capacity than requirement, the compressor runs at higher speeds for longer duration of time thereby increasing power consumption. On the other hand, if an inverter AC has higher capacity than heat load, it will run for short cycles and render the room over cooled and uncomfortable.


15. What is the difference between Eco mode and UPS mode in an inverter?

In eco-mode the load is normally powered by the bypass path, allowing raw mains power to supply the load, and the UPS inverter is engaged only when the utility mains fails. In eco-mode the UPS inverter operates in a “standby” mode. In principle, this is a simple change in the control software of the UPS.


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