Dynamic "fast response" DC motors are rectified constrained and excessively high di/dt is not allowed. The asynchronous motor is only constrained by the capacity of the inverter, and the multiple of the forced current can be very high, so the speed echoes quickly, generally reaching the millisecond level, which has surpassed the DC motor in terms of speed.
At low frequencies, torque is generally added as VFD (VVVF Operation). At low frequencies, the torque is often lower than the extra torque, and it cannot operate at full load below 5Hz. Vector-operated VFDs, on the other hand, maintain the magnetic flux and the torque is linearly related to it, allowing the motor to rotate higher than the extra torque at very low frequencies.
Flexibility of control: DC motors often choose separate excitation, series excitation, compound excitation and other methods according to different load targets. They have different handling characteristics and mechanical characteristics. In the vector control system of asynchronous motors, the same motor can output different characteristics. By using different function generators as flux regulators in the system, the mechanical characteristics of separately excited or series excited DC motors can be obtained.
With vector control, the output torque of the motor at low speeds, such as (without speed sensor) 1 Hz (about 30 r/min for 4-pole motors) can reach the output torque of the motor at 50 Hz (up to about 150% of the extra torque).
With conventional V/F control, the voltage drop of the motor increases as the speed of the motor decreases, resulting in a lack of excitation and the motor cannot obtain a satisfactory rotational force. To compensate for this deficiency, VFDs require progressive voltages to compensate for the voltage drop caused by motor slowdowns. This function of the VFD is called "torque improvement".
The torque boost function is to add the output voltage of the VFD. However, even if the output voltage is added a lot, the motor torque cannot be added to its current accordingly. Since the motor current includes the torque weight and other weights (e.g. excitation weight) that occur in the motor.
The vector control distributes the current value of the motor, and then confirms the value of the motor current weight and other current weights (such as excitation weight) where the torque occurs.
Vector control can be optimally compensated by echoing the voltage drop on the motor side, allowing the motor to generate high torque without adding current. This function is also effective in improving the temperature rise of the motor at low speeds.
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