Principle of frequency converter controlling motor speed
The frequency converter mainly consists of rectification (AC to DC), filtering, and re-rectification (DC to AC) , braking unit, drive unit, detection unit, micro-processing unit, etc.
The motor referred to in this article is an induction AC motor, and most of the motors used in industry are of this type. The rotational speed of an induction AC motor (hereinafter referred to as the motor) is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is determined by the working principle of the motor. Since the pole value is not a continuous value (it is a multiple of 2, for example, the number of poles is 2, 4, 6), it is generally not possible 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 with the purpose of controlling frequency is the preferred equipment as the motor speed regulating 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, causing the motor to be burned out. Therefore, when the frequency converter changes the frequency, it must change the voltage at the same time. 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: power supply (commercial power supply) provided by the power grid.
Starting current: When the motor starts running, the output current of the inverter.
The starting torque and maximum torque when driven by a frequency converter are smaller than those driven directly by industrial frequency power supply.
When the motor is powered by industrial frequency power supply, the starting and acceleration impacts are great, but when the frequency converter is used for power supply, these impacts are weaker. Direct starting at power frequency will produce 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 generated by the motor decreases as the frequency decreases (the speed decreases). The actual data of reduction will be explained in some inverter manuals.
By using the inverter with magnetic flux vector control, 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 frequency converter adjusts the speed to a frequency greater than 50Hz, the output torque of the motor will decrease
Usually the motor is designed and manufactured according to the 50Hz voltage, and its rated torque is also at this voltage given within the range. Therefore, speed regulation under the rated frequency is called constant torque speed regulation. (T=Te,P<=Pe)
When the output frequency of the inverter is greater than 50Hz, the torque generated by the motor must be Decreases linearly inversely with 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 is reduced to approximately 1/2 of the torque generated at 50Hz.
Therefore, the speed regulation above the rated frequency is called constant power speed regulation (P=Ue*Ie).
2. Application of frequency converters above 50Hz
As we all know, for a specific motor, its rated Voltage and rated current are unchanged.
If the inverter and motor ratings 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 remains unchanged and w increases, the torque will decrease accordingly.
We can also look at it from another angle:
The stator voltage of the motor 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 remain unchanged, E also remains unchanged.
And E=k*f*X, (k: constant, f: frequency, X: magnetic flux), so when f is from 50–>60Hz, X will decrease accordingly.
For the motor, T=K*I*X, (K: constant, I: current, X: magnetic flux), so the torque T will decrease as the magnetic flux X decreases.
At the same time, when it is less than 50Hz, since I*R is very small, when U/f=E/f does not change, the magnetic flux (X) is constant. Torque T is proportional to current. This is why the overcurrent capability of the frequency converter is usually used to describe its overload (torque) capability. It is also called constant torque speed regulation (the rated current remains unchanged–>the maximum torque remains unchanged).
Conclusion: When the frequency converter output frequency increases from above 50Hz, the output torque of the motor will decrease.
3. Other factors related to output torque
The heating and heat dissipation capabilities determine the output current capability of the inverter, thereby affecting the output torque capability of the inverter.
Carrier frequency: Generally, the rated current marked on the inverter is the value that can ensure continuous output at the highest carrier frequency and the highest ambient temperature. By reducing the carrier frequency, the motor’s current will not be affected. But the heating of components will be reduced.
Ambient temperature: It is like not to increase the inverter protection current value just because it detects that the ambient temperature is relatively low.
Altitude: As the altitude increases, the heat dissipation and insulation performance will be affected. All have an impact. Generally, the distance below 1000m can be ignored. The above is sufficient to reduce the capacity by 5% for every 1000 meters.
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 frequencies) to compensate for the output rotation caused by the voltage drop on the stator resistance. torque loss, thereby improving the output torque of the motor.
Technology to improve the insufficient output torque of the motor at low speed
Using vector control, the motor can be controlled at low speed, such as (without speed sensor) 1Hz (for a 4-pole motor, its speed is about The output torque at 30r/min) can reach the torque output by 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 results in the motor not being able to obtain sufficient rotational force due to insufficient excitation. In order to compensate for this deficiency, the voltage in the frequency converter needs to be increased to compensate for the voltage drop caused by the reduction in 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 motor torque cannot be increased correspondingly with its current. Because the motor current contains the torque component produced by the motor and other components (such as excitation components).
Vector control distributes the current value of the motor to determine the values of the motor current component that generates torque and other current components (such as excitation components).
Vector control can optimize compensation by responding to the voltage drop at the motor end, allowing the motor to produce large torque without increasing the current. This function is also effective in improving the temperature rise of the motor at low speed.
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