Variable torque is the load torque increases with the motor speed increases, is a non-linear changes, such as fan pumps.
Constant torque is the load torque does not increase with the motor speed increases, generally relative to the constant power control. Such as belt conveyor hoist and other mechanical load.
VF control is the inverter output frequency and output voltage ratio is constant or proportional. For example: 50Hz when the output voltage of 380V, 25Hz when the output voltage is 190V constant flux control; torque is not controllable, the system is only a closed-loop physical volume closed loop control system, he can only control the motor speed.
According to the principle of the motor can be seen, three-phase asynchronous motor stator per phase electromotive force RMS:
E1 = 4.44 f1N1Φm
Where:
E1 - the effective value of the electromotive force induced by the air gap flux in each phase of the stator, V;
f1 - stator frequency, Hz;
N1 - effective turns per stator winding;
Φm - per pole magnetic flux
It can be seen from the formula, Φm value determined by the E1 / f1, but because E1 is difficult to direct control, so the electromotive force is high, can ignore the stator leakage impedance drop, and the stator phase voltage U1 instead. Then to ensure that Φm unchanged, as long as U1 / f1 is always a certain value can be. This is the base frequency to the basic speed of the basic situation, for the constant voltage ratio (constant magnetic flux) control, are constant torque speed.
The reference frequency is the highest frequency of the constant torque governor zone. The voltage corresponding to the reference frequency is the reference voltage, which is the highest voltage of the constant torque speed regulation zone. When the frequency is adjusted below the fundamental frequency, the voltage will change with frequency , But the ratio of the two does not change. When the frequency is above the fundamental frequency, the frequency can be adjusted from the fundamental frequency to the upper limit frequency value. However, since the motor stator can not exceed the rated voltage of the motor, the voltage will not change with the frequency, and the reference voltage will be kept constant. The magnetic flux must be weakened with the frequency increase, the corresponding reduction in torque, power basically remain unchanged, are constant power speed zone.
Vector control, the output current divider and torque current and control, torque control, the system is a torque to do the inner ring, the speed of the outer ring to do the double closed-loop control system. It can control the speed of the motor, you can also control the motor torque. The speed control (ASR) at the time of vector control is such that the deviation of the speed command and the speed detection value (PG feedback or speed estimation value) is zero by operating the torque command.
Speed control when V / f control of PG is controlled By operating the output frequency, the deviation between the speed command and the speed detection value (PG feedback or speed estimation value) is zero.
The principle of vector control is to control the principle of DC motor. According to the dynamic mathematical model of asynchronous motor, the stator current vector is decomposed into excitation component and torque component by a series of coordinate transformations, and the torque current component and excitation component of the motor are controlled separately , In the rotor magnetic field after the realization of the magnetic field and torque decoupling, so as to control the purpose of asynchronous motor torque, so that the induction motor is close to the control of DC motor performance.
Specifically, the stator current vector of the asynchronous motor is decomposed into the current component (excitation current) and the current component (torque current) generating the magnetic field, respectively, and the amplitude and phase between the two components are controlled simultaneously Control the stator current vector, so called this control method known as the vector control mode.
Vector control points are speed sensor vector control and no speed sensor vector control two, the former high precision of the latter low precision.
Vector control system without speed sensor operation mode, we must first solve the motor speed and rotor flux position angle on-line identification problem. Commonly used methods are based on the identification of the stator current signal identification method, there are simultaneously using the current detection signal and voltage detection signal identification method, as well as based on the current detection signal and the inverter switch control signal reconstruction signal method.
