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

Update:08 May 2020

1: Torque increase
This function increases the output voltage of frequency converter (mainly at low frequency) to compensate the loss of output torque caused by voltage drop on stator resistance, so as to improve the output torque of motor.
The technology of improving the low speed output torque of motor
With vector control, the output torque of the motor at low speed, such as 1Hz (for 4-pole motor, the speed is about 30R / min), can reach the output torque of the motor at 50Hz (the maximum is about 150% of the rated torque).
For the conventional V / F control, the voltage drop of the motor increases with the decrease of the motor speed, which leads to the lack of excitation, so that the motor can not obtain enough rotation force. In order to compensate for this deficiency, the inverter needs to increase the voltage to compensate for the voltage drop caused by the decrease of motor speed. This function of frequency converter is called torque rise (* 1).
The torque raising function is to increase the output voltage of the frequency converter. However, even if the output voltage is increased a lot, the motor torque can not be increased corresponding to its current. Because the motor current contains the torque component and other components (such as excitation component) generated by the motor.
Vector control distributes the current value of the motor to determine the value of the current component of the motor that produces the torque and other current components (such as excitation component).
Vector control can optimize the 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 to improve the temperature rise of the motor at low speed.
Condition of converter braking
1: Concept of braking
It refers to the flow of electric energy from the motor side to the converter side (or power supply side), when the motor speed is higher than the synchronous speed.
The energy of the load is divided into kinetic energy and potential energy. Kinetic energy (determined by speed and weight) accumulates with the movement of the object. When kinetic energy is reduced to zero, the thing is at a standstill.
The method of mechanical brake device is to use the brake device to convert the kinetic energy of the object into friction and energy consumption.
For frequency converter, if the output frequency is reduced, the motor speed will follow the same frequency reduction. At this time, the braking process will be generated. The power generated by braking will return to the converter side. These power can be consumed by resistance heating.
When the load is used for lifting, the energy (potential energy) shall also return to the converter (or power supply) side for braking.
This method of operation is called "regenerative braking" and can be applied to converter braking.
During deceleration, if the generated power is not consumed by heat consumption, the method of returning energy to the power side of the converter is called "power return regeneration method". In practice, this application requires the option of "energy feedback unit".
2: How to improve the braking ability?
In order to use heat dissipation to consume regenerative power, it is necessary to install a braking resistor on the converter side.
In order to improve the braking ability, it is not expected to solve the problem by increasing the capacity of frequency converter. Please select options such as "braking resistor", "braking unit" or "power regeneration converter" to improve the braking capacity of the frequency converter.
3. When the rotation speed of the motor changes, what is the output torque?
The starting torque and the maximum torque of the inverter drive are smaller than those of the direct power frequency drive.
We often hear the following saying: "when the motor is powered by power frequency power supply, the starting and acceleration impact of the motor is very large, and when the inverter is used for power supply, these impacts will be weaker.". If the motor is started with a large voltage and frequency, for example, if the power is directly supplied by the power grid, a large starting impulse (large starting current) will be generated. When using frequency converter, the output voltage and frequency of frequency converter are gradually added to the motor, so the torque generated by the motor is less than the torque value of power grid. Therefore, the starting current of the motor driven by the frequency converter is smaller.
In general, the actual data that the torque generated by the motor will decrease with the decrease of frequency (the decrease of speed) will be given in some frequency converter manuals.
Through the use of flux vector control inverter, it will improve the motor torque shortage at low speed, even in the low speed area, the motor can output enough torque.
When the frequency converter is adjusted to more than 20% of the rated frequency, the output torque of the motor will be reduced
Generally, the motor is designed and manufactured according to the rated frequency and voltage, and its rated torque is also given in this voltage range. Therefore, the speed regulation under the rated frequency is called constant torque speed regulation. (t = te, P < = PE) when the output frequency of the frequency converter is greater than the rated frequency (for example, the motor in China is greater than 50Hz), the torque generated by the motor will decrease in a linear relationship inversely proportional to the frequency.
When the motor is running at a speed greater than 20% of the rated frequency, the size of the motor load must be taken into account to prevent the insufficient output torque of the motor.
For example, the torque generated by a motor with a rated frequency of 50Hz at 100Hz is reduced to about 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)

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