外文翻譯---直流電機(jī)速度控制-其他專業(yè)(編輯修改稿)
2025-02-24 09:41 本頁面
【文章內(nèi)容簡介】 ply by mounting a copper or aluminium disc on the shaft and rotating it in a magic field. The problem of removing the heat generated is severe in large system as the temperature of the shaft, bearing, and motor will be raised if prolonged braking is applied. In dynamic breaking, the stored energy is a resistor in the circuit. When applied to small DC machines, the armature supply is disconnected and a resistor is connected across the armature (usually by a relay, contactor, or thyristor). The field voltage is maintained, and braking is applied down to the lowest speed. Induction motors require a somewhat more plex arrangement, the stator windings being disconnected from the AC supply and reconnected to a DC supply. The electrical energy generated is then dissipated in the rotor circuit. Dynamic braking is applied to many large AC hoist system where the braking duty is both severe and prolonged. Any electrical motor can be brought to a standstill by suddenly reconnecting the supply to reverse the direction of rotation (reverse current braking). Applied under controlled conditions, this method of braking is satisfactory for all drivers. Its major disadvantage is that the electrical energy consumed by the machine when braking is equal to the stored energy in the load. This increases the running cost significantly in large drives. 3 Equal pulse width PWM law VVVF (Variable Voltage Variable Frequency) installs in the early time is uses PAM (Pulse Amplitude Modulation) to control, its inventor part which the technology realizes but only can output the frequency adjustable squarewave voltage not to be able to adjust the pressure. The pulse width PWM law are precisely in order to overe, is in the PWM law which the PAM law this shorting development es the simplest one kind. It is each pulse width equal pulse row took the PWM wave, through a change pulse row cycle may the frequency modulation, the change pulse width or the duty factor may adjust the pressure, uses the suitable control method then to cause the voltage and the frequency coordination change. Is opposite in the PAM law, this method merit simplified the electric circuit structure, enhanced the input end power factor, but simultaneously also has in the output voltage besides the fundamental wave, but also contains the big harmonic ponent. 4 Stochastic PWM That time the high efficiency transistor mainly for the bipolarity Daring ton triode, the carrier frequency generally did not surpass 5kHz, the vibration which the electrical machinery winding electromagism noise and the overtone created has aroused people39。s interest. In order to obtain the improvement, the stochastic PWM method arises at the historic moment. Its principle is the stochastic change turnon frequency causes the electrical machinery electromagism noise to be limited to approximately the belt white noise (in linear frequency coordinate system, various frequencies energy distribution is even
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