Q&A About Frequency Converter

Q1.What is a frequency converter?

The use of power semiconductor devices on and off the role of the power frequency power supply to another frequency of the power control device, can realize the AC asynchronous motor soft start, frequency control, improve the operating accuracy, change the power factor, overcurrent/overvoltage/overload protection and other functions of the control of the power device.

Q2.What is the resistance value of the inverter’s give potentiometer?

The resistance value is generally 1KΩ to 10KΩ.

Q3.The difference between PWM and PAM?

PWM (Pulse Width Modulation): Pulse width modulation, according to a certain law to change the pulse width of the pulse train, in order to regulate the output and the waveform of a modulation method.

PAM (Pulse Amplitude Modulation): Pulse amplitude modulation, is a modulation method to change the pulse amplitude of the pulse train according to a certain law to adjust the output value and waveform.

Q4.What is the difference between voltage type and current type?

Voltage type is an inverter that converts the DC of voltage source into AC, and the filter of DC circuit is capacitor.

The current type is the inverter that converts the DC of the current source into AC, and the DC circuit filter is inductor.

Q5.The reason why the voltage of inverter changes proportionally with the frequency?

The electromagnetic torque of any motor is the result of the interaction of current and magnetic flux, and the current is not allowed to exceed the rated value, otherwise it will cause the motor to heat up.

Therefore, if the magnetic flux decreases, the electromagnetic torque must also decrease, resulting in lower load carrying capacity.

From the formula E = 4.44 * K * F * N * Φ can be seen, in frequency conversion speed control, the motor’s magnetic circuit with the operating frequency fX is in a fairly large range of changes, it is easy to make the motor’s magnetic circuit is seriously saturated, resulting in serious distortion of the waveform of the excitation current, resulting in a very high peak current spike.

Therefore, the frequency and voltage should be changed proportionally, that is, change the frequency while controlling the output voltage of the inverter, so as to keep the magnetic flux of the motor at a certain level, and avoid the generation of weak magnetism and magnetic saturation phenomenon. This control method is mostly used in fan and pump energy-saving inverter.

Q6.What is the starting current and starting torque of the motor when running with frequency converter?

When running with frequency converter, the frequency and voltage will be increased accordingly with the acceleration of the motor, and the starting current will be limited to below 150% of the rated current (125%-200% depending on the type of machine).

When starting directly with industrial frequency power supply, the starting current is 6-7 times of the rated current, therefore, it will produce mechanical and electrical shock.

The use of inverter drive can be started smoothly (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 the rated torque; for the frequency converter with automatic torque enhancement function, the starting torque is more than 100%, and it can be used to start with full load.

Q7.When V and f are changed proportionally, what kind of change is there in the torque of the motor?

If the voltage is lowered proportionally when the frequency is decreased, the torque generated at low speed will tend to decrease because the AC impedance becomes smaller while the DC resistance remains unchanged.

Therefore, given V/f at low frequency, the output voltage should be increased in order to obtain a certain starting torque, and this compensation is called enhanced starting.

Various methods can be used to realize, there are automatic methods, select V / f mode or adjust the potentiometer and other methods.

Q8.It is written in the instruction manual that the variable speed range is 60-6Hz, i.e. 10:1, so there is no output power below 6Hz?

In the 6Hz below can still output power, but according to the motor temperature rise and the size of the starting torque and other conditions, the minimum frequency of use take 6Hz or so, at this time the motor can output the rated torque without causing serious heating problems.

The actual output frequency of inverter (starting frequency) is 0.5-3Hz according to the type of machine.

Q9.What should be done when the actual speed deviates from the given speed?

In open loop, even if the frequency converter outputs the given frequency, when the motor is running with load, the speed of the motor varies within the range of the rated rotation rate (1%-5%).

For the requirement of high speed regulation accuracy, even if the load change is also required to run at a speed close to the given occasion, can be used with PG feedback function of the inverter (option).

Q10.If the motor with PG is used, can the speed accuracy be improved after feedback?

Inverter with PG feedback function can improve the accuracy. But the value of speed accuracy depends on the accuracy of PG itself and the resolution of inverter output frequency.