Low voltage dc servo motor servo motor speed control, often say is he excitation brushless dc motor speed control, according to the dc motor speed equation, the speed n = (U - the armature voltageInternal impedance voltage current Ia * Ra)÷(Constant Ce * air-gap flux & Phi; )Because the armature resistance Ra is very small, so the voltage current Ia * resistance Ra≈ 0, such speed n = (The armature voltage U)÷(Constant Ce * air-gap flux & Phi; )As long as in the air gap flux & Phi; Safe, adjusts the armature voltage U, can adjust the dc motor speed n; Or in the armature voltage U safe adjusts the air-gap flux & Phi; , the same can adjust the speed of the motor n, the former is called constant torque control, which is called constant power speed regulation. Constant torque form, to stick to the air gap flux & Phi; Safe, dc motor stator and rotor magnetic field is orthogonal circumstances, does not affect each other. Insist on & Phi; Safe, as long as the guarantee that the excitation coil current safe to a value. In theory to a constant current source to control excitation coil current is relatively perfect, but because of the current source is bad, and generally to the excitation coil applied voltage to a safe value, also can approximate let field current, so that the air gap flux & Phi; Safe. If is the permanent magnet dc servo motor, with permanent magnets to replace the excitation coil, permanent magnetic flux is safe, so don't hold the heart. Simply adjust the voltage, is not satisfied with the load sway is fierce, so the introduction of cascade speed regulation system, and tested the motor current and rotating speed, depart out of the current inner loop and speed loop ring ring, using PID algorithm, useful satisfied the speed of load sway case, make dc motor speed control operation features very & other; Hard & throughout; , that is, the rotational speed of maximum torque will not be shaken, completed the real constant torque output. This kind of control method, has been the communication speed regulating system modeled on the other side, for vector control frequency converter, is modeled on this method. If current loop inner ring only, still can directly control the motor torque output must, with different tensile and bending control requirement. Armature voltage control in thyristor and IGBT these have not been created before, to control also is not a simple work, after all, power is larger, the early is through a generator, dc power to control after adjusting the generator of flux can control the generator output voltage, so as to adjust the armature voltage range. In the thyristor SCR are created in the future, after communication for thyristor to the input voltage, using the phase shift trigger skills control thyristor conduction Angle, can communication electric rectifier into pulsating dc, because the dc motor is large inductive load, pulsating direct current will be large inductance buffer settle down. The dc voltage can be adjusted, and the proportion of thyristor conduction Angle into must. This speed skill is very sophisticated secure, later period in the last century has been widely industrial application. Other field effect tube and IGBT device appeared in the future, such as the low voltage dc servo motor speed will also be able to do much more precise, to be able to use PWM chopper, let the output dc voltage is very safe, so the dc motor speed shake is very small, if make the rotor of the motor long point, moment of inertia of small, plus the position loop, will also be able to complete the accurate positioning control, this is the so-called dc servo system. Constant power low voltage dc servo motor speed control method is the so-called weak magnetic speed, the speed control method, is essentially constant torque speed control method is a kind of remedy, mainly in some occasions, demand more wide speed regulation, for example, some of the longmen bed, demand for motor processing time feed is very slow, the torque should be high; And come back when the torque was despised to run very fast, at this time feed with constant torque speed form, and return to time with the weak magnetic speed control method, the motor in maximum power is a constant. Also some electric vehicles, low speed up hill to run slowly, in a big demand torque, small resistance, flat road and want to run very fast, this time also need use constant power speed regulation, similar to the mechanical shift or lowered ratio method to speed. Generally weak magnetic speed control, is not suitable for permanent magnet motor, thus flux & Phi; Unable to control alone. To weak magnetism, is directly reduce the air gap flux & Phi; Intrusive, this time can drop excitation coil current, generally in using thyristor excitation coil or field effect tube these back to do a PI adjustment output a current source to complete. Weak magnetic speed, the higher the motor speed, motor output torque is smaller, the demand is take care of that, and generally do not unlimited reduce down, about can operate in about 90% of the additional exciting current.
