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 operating 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.
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 you only change the frequency but not the voltage, the motor may burn out due to overvoltage (overexcitation) when the frequency drops. Therefore, it is necessary to change the voltage of the inverter at the same time as changing the frequency. 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 provided 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 starting and acceleration impact of the motor is large when the power frequency power supply is used, and when the power supply is used by the inverter, these shocks 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 the motor decreases as the frequency decreases (speed decreases). Practical data for the reduction is illustrated 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 adjusted to a frequency greater than 50Hz, the output torque of the motor will decrease
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 drops 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 of 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 operate 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 about the torque?
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 point of view:
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.
Together, 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 value that can be guaranteed to continue to output at the highest carrier frequency and the highest ambient temperature. Drop the carrier frequency, and 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 protection current value will not be increased when the ambient temperature is 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 output torque loss 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 conventional V/F control, the voltage drop of the motor increases relative to the decrease in motor speed, which leads to the lack of excitation and the motor cannot obtain sufficient rotational force. In order to compensate for this deficiency, progressive voltages are required in the inverter to compensate for the voltage drop caused by the decrease in motor speed. This function of the inverter is called torque progression (*1).
The torque progress function is the output voltage of the progressive inverter. However, even with many advances in output voltage, motor torque does not correspond to the progress in its current. Since the motor current contains 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 checks the current weight of the motor and other current weights (such as excitation weight) where the torque occurs.
Vector control is optimally compensated in response to the voltage drop on the motor side, allowing the motor to generate large torques without adding current. This function is also effective in improving the temperature rise of the motor at low speeds.
Shenzhen Xingfengyuan Electromechanical specializes in the production and sales of stepper motors, stepper drives, servo motors, servo drives, agent Japan Tamagawa servo, Decos (TKS) planetary reducer and motion control products.
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