9. Can a transformer be used as a load for a frequency converter?

Theoretically, yes, but practically it's impractical. Even without a transformer for voltage boosting, frequency converters should be available for circuits above 380V. For even higher voltages, there are circuits that directly convert 220V or 380V and then use a voltage multiplier to obtain the high voltage.

Frequency converters are primarily used for load driving (such as motors), rarely for power supply frequency conversion. Their function extends far beyond frequency conversion, including various protection functions. Using a frequency converter to obtain a variable frequency power supply is economically inadvisable; alternative frequency conversion circuits are recommended.

10. Can a frequency converter be adjusted to 1Hz? What is the highest possible frequency range?

If a frequency converter is used on a general AC asynchronous motor, adjusting it to 1Hz is close to DC, which is absolutely not allowed. The motor will operate at the maximum current limit of the frequency converter, causing severe overheating and potentially burning out.

Operating above 50Hz will increase the iron losses of the motor, which is detrimental to the motor. Generally, it's best not to exceed 60Hz (short-term exceedances are permissible), otherwise it will affect the motor's lifespan.

11. What is the working principle of the frequency adjustment resistor in a frequency converter? Why can adjusting the resistor change the frequency? 

The frequency adjustment resistor in a frequency converter is used to proportionally divide the 10V reference voltage of the frequency converter and then send it back to the main control board. The main control board then performs analog-to-digital conversion on the voltage returned by the resistor, reads the data, and then converts it into a proportional value of the rated frequency to output the current frequency. Therefore, adjusting the resistor value can adjust the frequency of the frequency converter. 

12. Can a frequency converter decouple the motor current? 

Can a frequency converter decouple the current? No! However, as long as the output frequency f and synchronous speed n1 keep the slip rate in the stable region or at the rated slip rate Se, it is equivalent to decoupling the motor current, because the rotor power factor is 1 at this time, and the rotor current is the torque current that needs to be decoupled and controlled! The frequency converter is a speed control device for asynchronous motors; it cannot exceed the mechanical characteristics of the asynchronous motor to perform any so-called control!

13. Why is the current large when an induction motor starts, and then decreases after starting?

When the induction motor is in a stopped state, from an electromagnetic perspective, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed-circuit rotor winding is equivalent to the short-circuited secondary coil of the transformer. There is no electrical connection between the stator winding and the rotor winding, only a magnetic connection; the magnetic flux forms a closed circuit through the stator, air gap, and rotor core.

At the moment of closing the circuit breaker, before the rotor begins to rotate due to inertia, the rotating magnetic field cuts the rotor windings at its maximum cutting speed—synchronous speed. This induces the highest possible electromotive force (EMF) in the rotor windings, causing a large current to flow through the rotor conductors. This current generates magnetic energy to counteract the stator magnetic field, much like the secondary magnetic flux in a transformer counteracts the primary magnetic flux.

To maintain the original magnetic flux compatible with the power supply voltage, the stator automatically increases its current. Because the rotor current is very large at this time, the stator current also increases significantly, sometimes reaching 4 to 7 times the rated current. This is the reason for the large starting current.

Why does the current decrease after starting? As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductors decreases, the induced EMF in the rotor conductors decreases, and the current in the rotor conductors also decreases. Consequently, the portion of the stator current used to counteract the magnetic flux generated by the rotor current also decreases, so the stator current decreases from large to normal. 

14. What is the effect of carrier frequency on the inverter and motor? The carrier frequency affects the inverter's output current:

(1) The higher the operating frequency, the larger the duty cycle of the voltage wave and the smaller the higher harmonic components of the current; that is, the higher the carrier frequency, the smoother the current waveform. 

(2) The higher the carrier frequency, the smaller the allowable output current of the inverter.

(3) The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitor (because Xc=1/2πfC), and the larger the leakage current caused by high-frequency pulses.

The carrier frequency affects the motor:

The higher the carrier frequency, the smaller the motor vibration, the lower the operating noise, and the less heat generated by the motor. However, the higher the carrier frequency, the higher the frequency of the harmonic current, the more severe the skin effect of the motor stator, the greater the motor loss, and the smaller the output power. 

15. Why can't an inverter be used as a variable frequency power supply?

The entire circuit of a variable frequency power supply consists of AC-DC-AC-filtering components, so its output voltage and current waveforms are pure sine waves, very close to an ideal AC power supply. It can output the grid voltage and frequency of any country in the world. A frequency converter is composed of AC/DC/AC (modulated wave) circuits, and its standard name should be variable frequency speed controller. Its output voltage waveform is a pulsed square wave with many harmonic components. Voltage and frequency change proportionally simultaneously and cannot be adjusted separately, which does not meet the requirements of AC power supplies.

In principle, it cannot be used as a power supply and is generally only used for speed control of three-phase asynchronous motors.

16. Why is the motor temperature rise higher when using a frequency converter than at mains frequency?

Because the output waveform of a frequency converter is not a sine wave, but a distorted wave. The motor current at rated torque is about 10% higher than at mains frequency, so the temperature rise is slightly higher. 

Another point: When the motor speed decreases, the motor cooling fan speed is insufficient, and the motor temperature rise will be higher.

17. What does the motor protection rating mean?

For example, an IP23 motor means that it can prevent the intrusion of solid objects larger than 12mm, prevent human fingers from touching internal parts, and prevent the intrusion of medium-sized foreign objects (diameter greater than 12mm). It can prevent water spray from entering or causing damage from water sprayed at an angle of less than 60 degrees to the vertical.

The IP (International Protection) rating system was drafted by the IEC (International Electrotechnical Commission). It classifies motors according to their dust and moisture protection characteristics. Foreign objects, including tools and human fingers, must not come into contact with the live parts inside the motor to avoid electric shock.

The IP protection rating consists of two digits. The first digit indicates the level of dust and foreign object protection, and the second digit indicates the degree of moisture and water resistance. A higher number indicates a higher level of protection.