There is a difference between ac frequency conversion and dc frequency conversion
Ac conversion speed regulation, change the number of the electric motor, the characteristic is fixed gear, also is high middle and low Dc speed control, it is the first plastic for dc ac, and then through a controlled back to ac inverter, is characterized by speed range is big, but because of the plastic, inverter will have a conversion efficiency. For example, the speed of communication is 100 percent, 70 percent, 40 percent speed 3, The range of dc speed adjustment is 30% ~ 100% So in 100%, 70%, 40%, near the three gears runtime ac frequency conversion efficiency will be better than dc, but when the compressor to run only need in 55% can meet the requirements, the ac frequency conversion will be run at a speed of 70%, this time, small dc variable frequency energy consumption is not communication ^_^ I'm not a professional Let's do it
Dc motor brush and the cause of the volume, restricting its scope of application of frequency control of motor speed can be developed by the fan and water pump, is due to the needs of the fan and water pump energy saving, frequency control of motor speed is the best choice, but I personally think that the price of electricity and the inverter's own, compared to the energy saving is pointless, because of the frequency converter investment back, at least need 5 to 6 years, in the five or six years, working condition of don't know what will happen. Therefore, inverter is best applied to speed regulation, and start on the performance and torque adjustment is not very demanding, and the occasion, this is the reason in general use frequency control of motor speed. So you can say that if you use dc speed control controllers to control ac motors that's the best, you can do that, and it's very difficult right now
To compare the differences, you must first understand the structure of the two. In order to realize the dc motor speed regulation, there are several ways: (1) in the winding internal series resistance, the method of reducing I increases the loss of the drag system, the mechanical performance is "soft", and now it is not necessary. (2) regulating the armature winding voltage U, which is by far the best method of regulating speed, has the following characteristics: A) the mechanical properties of the speed adjustment are constant, and the speed landing only changes with the armature voltage. B) to achieve stepless speed adjustment, zero speed adjustment C) because the main flux is constant, the motor rated current is constant, so the speed control process has a "constant torque" characteristic.
Ac frequency conversion speed regulation than dc speed regulation is widely used because of ac motor, is not a principle of frequency control of motor speed has the superiority, frequency control of motor speed can only be applied to speed regulation, and to control the torque is unable to be precise, the reason is very simple, dc speed control of the armature and the coupling of the excitation is not, is separate, so to the armature current and exciting current to achieve precise control. And ac speed regulation, armature current and exciting current are coupled, is unable to achieve accurate control, while the current vector control, frequency control of motor speed has also is using the modern control theory, by the vector transformation, the ac motor in the coupling of the armature current and exciting current, thus carries on the control, the principle of dc speed regulating is simulation. But it is difficult to control the dc speed control. Therefore, in mill, paper and so on, the torque demand is very high, dc speed adjustment is also widely used. For speed control only, current frequency conversion speed is the characteristic of realistic dc speed adjustment, because the advantage of ac motor is that dc motors cannot.
