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.

10 Common Problems

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 CompensationDepending 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 LoadAlthough 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 LoadAlthough 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 LoadThe 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 MotorThe 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 MotorIt 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-upThe rated current of the inverter should be greater than the sum of the maximum operating current of several motors.

B. Starting Time in TurnThe 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 Mode1) Radiatedradiated Interference 2) Conducted Interference

Anti-jamming MeasuresInterference 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.

The complete article can be view at the Inverter Application Instruction.