With the development of frequency converters, the use of frequency converters has become more and more popular, but many times we just use frequency converters, but never understand it.

How does the inverter control the motor speed? When the motor is powered by a commercial frequency power supply, the shocks of starting and accelerating are very large, but when the motor is powered by a frequency converter, these shocks will be weaker. Power frequency direct starting will generate a large starting current.

When using a frequency converter, the output voltage and frequency of the frequency converter are gradually added to the motor, so the starting current and impact of the motor are smaller. Let's find out with the editor below.

The inverter is mainly composed of rectification (AC to DC), filtering, rectification again (DC to AC), braking unit, drive unit, detection unit, micro-processing unit, etc.

The motors referred to in this article are induction AC motors, which are the majority of motors used in industry. The rotational speed of an induction AC motor (hereinafter simply referred to as a motor) is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is fixed by the working principle of the motor. Since the pole value is not a continuous value (a multiple of 2, for example, the number of poles is 2, 4, 6), it is generally uncomfortable to adjust the speed of the motor by changing this value.

In addition, the frequency can be adjusted outside the motor and then supplied to the motor, so that the rotation speed of the motor can be freely controlled.

Therefore, the frequency converter for the purpose of controlling the frequency is the preferred equipment as the motor speed control equipment.

Conclusion: Changing frequency and voltage is the optimal motor control method.

If you only change the frequency without changing the voltage, the motor will be over-voltage (over-excitation) when the frequency is reduced, which may cause the motor to burn out. Therefore, the frequency converter must change the voltage at the same time as changing the frequency. When the output frequency is above the rated frequency, the voltage cannot continue to increase, and the maximum can only be equal to the rated voltage of the motor.

Industrial frequency power supply: The power supply (commercial power supply) provided by the grid.

Starting current: when the motor starts to run, the output current of the inverter.

The starting torque and maximum torque when driven by frequency converter are smaller than those directly driven by commercial frequency power supply.

When the motor is powered by a commercial frequency power supply, the shocks of starting and accelerating are very large, but when the motor is powered by a frequency converter, these shocks will be weaker. Power frequency direct starting will generate a large starting current. When using a frequency converter, the output voltage and frequency of the frequency converter 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 as the frequency decreases (speed decreases). The reduced actual data will be explained in some inverter manuals.

By using the frequency converter controlled by magnetic flux vector, the lack of torque at low speed of the motor will be improved, and the motor can output sufficient torque even in the low speed area.

1. When the inverter speed is adjusted to a frequency greater than 50Hz, the output torque of the motor will be reduced

Usually the motor is designed and manufactured according to the voltage of 50Hz, and its rated torque is also given over this voltage range. Therefore, the speed regulation below the rated frequency is called constant torque speed regulation. (T=Te, P<=Pe)

When the frequency converter output frequency is greater than 50Hz, the torque generated by the motor should be Decreases in a linear relationship inversely proportional to frequency.

When the motor runs at a frequency greater than 50Hz, the size of the motor load must be considered to prevent insufficient motor output torque.

For example, the torque generated by the motor at 100Hz should be reduced to about 1/2 of the torque generated at 50Hz.

So the speed regulation above the rated frequency is called constant power speed regulation (P=Ue*Ie).

2. The application of frequency converter above 50Hz

As you know, for a specific motor, its rated voltage and rated current are unchanged.

If the rated values of the inverter and the motor are both: 15kW/380V/30A, the motor can work above 50Hz.

When the speed is 50Hz, the output voltage of the inverter is 380V 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 380V/30A. Obviously the output power does not change, so we call it constant power speed regulation.

What is the torque situation at this time?

Because P=wT (w: angular velocity, T: torque). Because P is unchanged and w increases, the torque will decrease accordingly.

We can also look at it from another angle:

The stator voltage of the motor is U=E+I*R (I is the current, R is the electronic resistance, and E is the induced potential)

It can be seen that when U and I are unchanged, E is also unchanged.

And E=kfX, (k: constant, f: frequency, X: magnetic flux), so when f is changed from 50-->60Hz, X will be corresponding decrease.

For the motor, T=KIX, (K: constant, I: current, X: flux), so the torque T will decrease with the flux X Small and reduced.

At the same time, when it is less than 50Hz, since I*R is very small, when U/f=E/f is constant, the magnetic flux (X) is constant. The torque T is proportional to the current. This is why the overcurrent capability of the inverter is usually used to describe its overload (torque) capability. It is also called constant torque speed regulation (the rated current does not change --> the maximum torque does not change).

Conclusion: When the output frequency of the inverter increases from above 50Hz, the output torque of the motor will decrease.

3. Other factors related to output torque

Heating and heat dissipation capabilities determine the output current capability of the inverter, which affects the output torque capability of the inverter.

Carrier frequency: Generally, the rated current marked on the inverter is the value that can guarantee continuous output at the highest carrier frequency and the highest ambient temperature. Reduce the carrier frequency, the current of the motor will not be affected. But the heating of components will be reduced.

Ambient temperature: It seems that the protection current value of the inverter will not be increased when the ambient temperature is relatively low.

Altitude: The increase in altitude will affect the heat dissipation and insulation performance Both have an impact. Generally below 1000m can not be considered. It is enough to reduce the capacity by 5% every 1000 meters above.

4. How does vector control improve the output torque capability of the motor?

*1: Torque Boost

This function increases the output voltage of the inverter (mainly at low frequency) to compensate the output torque loss caused by the voltage drop on the stator resistance, thereby Improve the output torque of the motor.

Technology for Improving the Insufficient Output Torque of the Motor at Low Speed

Using vector control can make the motor run at low speed, such as (without speed sensor) 1Hz (for For a 4-pole motor, the output torque when the speed is about 30r/min) can reach the output torque of the motor at 50Hz power supply (the maximum is about 150% of the rated torque).

For conventional V/F control, the voltage drop of the motor increases relatively as the motor speed decreases, which leads to the fact that the motor cannot obtain sufficient rotational force due to insufficient excitation. In order to compensate for this deficiency, the frequency converter needs to increase the voltage to compensate for the voltage drop caused by the reduction of the motor speed. This function of the inverter is called torque boost (*1).

The torque boost function is to increase the output voltage of the inverter. However, even if the output voltage is increased a lot, the torque of the motor cannot be increased correspondingly to its current. Because the motor current contains the torque component and other components (such as excitation components) produced by the motor.

Vector control distributes the current value of the motor to determine the value of the motor current component that produces torque and other current components (such as excitation components).

Vector control can optimize the compensation by responding to the voltage drop at the motor end, allowing the motor to produce a large torque without increasing the current. This function is also effective for improving the temperature rise of the motor at low speed.