The inverter controls the speed of the AC motor – Schneider

Most of the motors used in the industry are of this type. The rotational speed of an induction AC motor (henceforth 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, e.g. the pole number is 2, 4, 6), it is generally not suitable to adjust the speed of the motor by changing the value.

The inverter controls the speed of the AC motor – Schneider

The inverter controls the speed of the AC motor – Schneider

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 controlled freely.

Therefore, the inverter for the purpose of controlling the frequency is the preferred equipment for motor speed regulation equipment.

Conclusion: Changing the frequency and voltage is the best way to control the motor

If only the frequency is changed without changing the voltage, the motor may burn out due to overvoltage (overexcitation) when the frequency is reduced. Therefore, it is necessary to change the voltage at the same time as changing the frequency of the inverter. When the output frequency is above the additional frequency, the voltage cannot be further added, and the maximum voltage can only be equal to the additional voltage of the motor.

Power Frequency Power Supply: Power supply supplied by the grid (commercial power supply)

Starting current: The output current of the inverter when the motor is running at the beginning of the operation

The starting torque and maximum torque when driven by the inverter are less than those driven by direct industrial frequency power supply

The motor starts and accelerates when the power frequency power supply is very large, and when the inverter is used for power supply, these impacts are weaker. A large starting current will occur when the power frequency is directly started. When the inverter is used, the output voltage and frequency of the inverter are gradually added to the motor, so the starting current and impact of the motor are smaller.

In general, the torque generated by a motor decreases as the frequency decreases (speed decreases). The actual data for the reduction is explained in some inverter manuals.

The lack of torque in the motor at low speeds will be improved by using the inverter operated by magnetic flux vectoring, and the motor can output sufficient torque even at low speeds.

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

The general motor is designed and manufactured with a voltage of 50Hz, and its additional torque is also given within this voltage range. Therefore, the speed regulation under the extra frequency is called constant torque speed regulation. (T=Te, P<=Pe)

When the output frequency of the inverter is greater than the frequency of 50Hz, the torque of the motor decreases in a linear relationship inversely proportional to the frequency.

When the motor is running at a frequency speed greater than 50Hz, it is necessary to consider the size of the motor load to avoid the lack of motor output torque.

For example, the torque of a motor at 100 Hz is reduced to about 1/2 of that at 50 Hz.

Therefore, the speed regulation above the additional frequency is called constant power speed regulation. (P=Ue*Ie)

4. The application status of the inverter above 50Hz

We know that for a particular motor, the extra voltage and the extra current are constant.

For example, the additional value of the inverter and the motor are: 15kW/380V/30A, and 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 remains the same. That's why we call it constant power speed regulation.

What is the torque situation at this time?

Since P=wT (w: angular velocity, T: torque). Since P does not change, w is added, so the torque decreases accordingly.

Let's look at it from another angle:

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

It can be seen that when U,I does not change, E does not change.

E = k*f*X, (k: constant, f: frequency, X: magnetic flux), so when f is from 50 to > 60 Hz, X decreases accordingly

For motors, T=K*I*X, (K: CONSTANT, I: CURRENT, X: MAGNETIC FLUX), SO THE TORQUE T DECREASES WITH THE DECREASE OF MAGNETIC FLUX X.

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

Conclusion: When the output frequency of the inverter is increased from more than 50Hz, the output torque of the motor will decrease.

5. Other elements related to output torque

Heat generation and heat dissipation can determine the output current of the inverter, and then affect the output torque of the inverter.

Carrier frequency: Generally, the additional current marked by the inverter is the highest carrier frequency, and the maximum ambient temperature can ensure continuous output. By reducing the carrier frequency, the current of the motor will not be affected. However, the heat generation of the components will be reduced.

Ambient temperature: Just like the inverter maintenance current value will not be increased when the ambient temperature is detected to be relatively low.

Altitude: Altitude added, has an impact on heat dissipation and insulation performance. Generally, less than 1000m can be considered. More than 5% reduction per 1000 meters can be done.

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

*1: Torque progression

This function adds the output voltage of the inverter (mainly at low frequencies) to compensate for the loss of output torque caused by the voltage drop across the stator resistor, and then improves the output torque of the motor.

Improve the skills that the motor lacks in low-speed output torque

Using vector control, the output torque of the motor at low speeds, such as 1 Hz (for a 4-pole motor, the speed is about 30 r/min) can reach the torque of the motor at 50 Hz (up to about 150% of the additional torque).

With regard to conventional V/F operation, the voltage drop of the motor increases relative to the decrease in motor speed, which results in the motor not being able to obtain sufficient rotational force due to the lack of excitation. In order to compensate for this deficiency, progressive voltages are required in the inverter to compensate for the voltage drop caused by the reduction in motor speed. This function of the inverter is called torque progression (*1).

The torque advancement function is the output voltage of the progressive inverter. However, even if the output voltage is improved by a lot, the torque of the motor does not correspond to the progress of its current. Since the motor current includes the torque weight and other weights (e.g. excitation weight) incurred by the motor.

Vector control distributes the current value of the motor and then determines the current weight of the motor and other current weights (e.g. excitation weight) where the torque occurs.

Vector control can be optimized to compensate for the voltage drop at the motor end, allowing the motor to produce large torques without adding current. This feature is also effective for improving the temperature rise of the motor at low speeds.

The inverter controls the speed of the AC motor – Schneider

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