Why does dc motor reduce the voltage regulating ac motor?
The rotational speed of the ac motor is determined by the rotational speed of the rotating field, while the rotational speed of the rotating field is only related to the frequency and the magnetic poles. The frequency is fixed, and the magnetic poles are determined by the structure of the motor, which cannot be changed in the course of operation, so it is quite difficult to adjust the speed of the ac motor. The variable speed of dc motor can be obtained from the hardness of its mechanical properties, which is the relationship between the torque of the motor and the rotational speed. Beta = dM/dn. Type in the beta is hardness, it is the differential ratio of torque and rotational speed, beta is hard characteristics, on the other hand is soft features, need speed can be as long as the soft characteristics of dc motor, series motor, when the load increases, its speed will come down.
To understand "why do dc motors reduce voltage regulating ac motors?" The first thing you need to know about their speed is what the parameters are. Three-phase ac motor speed formula: n = (1 - s) 60 f/p, can be seen from the formula, the speed of the three-phase ac motor with slip s very logarithmic p, power frequency f, motor, voltage has nothing to do with work. Dc motor speed formula: n = (U - IaRa)/(Ke Φ), can be seen from the formula, the dc motor speed related to the armature voltage U and excitation flux Φ. From the above two formulas, the dc motor can be used to adjust the armature voltage or adjust the excitation voltage to achieve the speed adjustment. Not while ac motor, ac motor can only by adjusting the slip s of wound rotor motor, asynchronous motor power frequency f, multi-speed motors of logarithmic to achieve speed control purposes.
The magnetic field of the dc motor is generated by the excitation current through the magnetic pole coil, which has a stable magnetic field, so changing the armature voltage can change its speed. Ac motor can also use dropping speed, but the range of speed regulation is very small, mainly its magnetic field is produced by the power supply current, change the voltage, the magnetic field also changed, so a lower voltage, its torque is proportional to the square of the voltage, torque declines very quickly, so the speed range is very small, also reduces the load ability, therefore ac motor generally don't dropping speed.
