機械專業(yè)外文翻譯----直流電動機調速控制-文庫吧
2025-04-17 00:26 本頁面
【正文】 apacity, the consequent rise in terminal voltage will be small during the short periods of regeneration. In the WardLeonard method of speed control of DC motors, regenerative braking is inherent, but thyristor drives have to be arranged to invert to regenerate. Induction motor drives can regenerate if the rotor shaft is driven faster than speed of the rotating field. The advent of lowcost variablefrequency supplies from thyristor inverters have brought about considerable changes in the use of induction motors in variable speed drives. Eddy current braking can be applied to any machine, simply by mounting a copper or aluminum 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, bearings, and motor will be raised if prolonged braking is applied. In dynamic braking, the stored energy is dissipated in 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 systems where the braking duty is both severe and prolonged. DC Motor Speed Control The basis of all methods of DC motor speed control is derived from the equations: ???E aaRIEU ?? the terms having their usual meanings. If the IaRa drop is small, the equations approximate to ???U or ??U? 。 Thus, control of armature voltage and field flux influences the motor speed. To reduce the speed to zero, either U=0 orΦ=∞.The latter is inadmissible。 hence control at low speed is by armature voltage variation. To increase the speed to a high value, either U is made very large or Φis reduced. The latter is the most practical way and is known as field weakening. Combinations of the two are used where a wide range of speed is required. A SingleQuadrant Speed Control System Using Thyristors A singlequadrant thyristor converter system is shown in the moment the reader should ignore the rectifier BR2 and its associated circuitry (including resistor R in the AC circuit)
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