The frequency converter is mainly composed of rectification (AC to DC), filtering, re-rectification (DC to AC), braking unit, drive unit, and detection unit micro-processing unit.
The motors referred to herein are inductive AC motors, and most of the motors used in the industry are motors of this type. The rotational speed of an inductive 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, such as a pole number of 2, 4, 6), it is generally uncomfortable and the speed of the motor is adjusted by changing the value.
In addition, the frequency can be supplied to the motor after being adjusted outside the motor, so that the rotational speed of the motor can be freely controlled.
Therefore, the inverter for the purpose of controlling the frequency is the preferred device for the motor speed control device.
Conclusion: Changing frequency and voltage is the optimal motor control method
If the frequency is changed only without changing the voltage, the frequency will decrease and the motor will be over-voltage (overexcitation), causing the motor to be burned out. Therefore, the inverter must change the voltage at the same time while 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.
Power frequency power supply: power supply (commercial power supply) provided by the power grid
Starting current: The output current of the inverter when the motor starts running
The starting torque and maximum torque when the inverter is driven are less than that of direct power frequency power supply.
When the motor is powered by the commercial frequency power supply, the starting and acceleration shocks are large, and when the inverter is used for power supply, these impacts are weaker. A direct start of the power frequency produces a large starting current. 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.
Generally, the torque produced by the motor is reduced as the frequency decreases (the speed decreases). The reduced actual data is given in some of the drive manuals.
By using a flux vector controlled inverter, the torque of the motor at low speeds is improved, and even in the low speed range, the motor can output sufficient torque.
1. When the inverter speeds up to a frequency greater than 50Hz, the output torque of the motor will decrease.
The usual motor is designed and manufactured at a voltage of 50 Hz, and its rated torque is also given in this voltage range. Therefore, the speed regulation below 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 should decrease in a linear relationship inversely proportional to the frequency.
When the motor is running at a frequency greater than 50 Hz, the size of the motor load must be considered to prevent the motor from outputting insufficient torque.
For example, the torque generated by the motor at 100 Hz is reduced to approximately 1/2 of the torque at 50 Hz.
Therefore, the speed regulation above the rated frequency is called constant power speed regulation. (P=Ue*Ie)
2. Application of inverter above 50Hz
As you know, for a particular motor, its rated voltage and current rating are constant.
If the inverter and motor are rated: 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. If the output frequency is increased to 60Hz, the maximum output voltage of the inverter can only be 380V/30A. Obviously, the output power is unchanged. 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 does not change, w increases, so the torque will decrease accordingly.
We can also look at 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, I are unchanged, E does not change.
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: flux), so the torque T will decrease as the flux X decreases.
At the same time, when it is less than 50Hz, since I*R is 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 inverter is usually used. Overcurrent capability to describe its overload (torque) capability. It is called constant torque speed regulation (rated current is not changed -> maximum torque is constant)
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
The heat and heat dissipation capacity determine the output current capability of the inverter, which affects the output torque capability of the inverter.
Carrier frequency: Generally, the rated current of the inverter is the highest carrier frequency, and the value of continuous output can be guaranteed at the highest ambient temperature. The carrier frequency is reduced and the current of the motor is not affected. However, the heat of the components will decrease.
Ambient temperature: It is not like increasing the protection current value of the inverter because it detects that the ambient temperature is low.
Altitude: The increase in altitude has an effect on heat dissipation and insulation performance. Generally, it can be ignored below 1000m. It can be reduced by 5% per 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 torque loss caused by the voltage drop across the stator resistance, thereby improving the output torque of the motor.
Technology to improve the low output torque of the motor
Using vector control, the output torque of the motor at low speed, such as (without speed sensor) 1Hz (for a 4-pole motor with a speed of about 30r/min) can reach the torque of the motor at 50Hz power supply output (most It is 150% of 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 inverter needs to increase the voltage to compensate for the voltage drop caused by the motor speed reduction. 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 a lot of output voltage is increased, the motor torque cannot be increased corresponding to its current. Because the motor current contains the torque component generated by the motor and other components (such as the excitation component).
The vector control distributes the current value of the motor to determine the value of the motor current component and other current components (such as the excitation component) that produce the torque.
The vector control can be optimally compensated 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 for improving the temperature rise of the motor at low speeds.
Guangzhou Yunge Tianhong Electronic Technology Co., Ltd , https://www.e-cigaretteyfactory.com