1、附录 D 外文翻译 附录 D 外文翻译 Speed Control of DC Motor Abstract Conditioning system is characterized in that output power to maintain stability. Different speed control system can use a different brake system, high starting and braking torque, quick response and quick adjustment range of degree requirements
2、of DC drive system, the use of the electric braking mode. Depends on the speed control of DC motor armature voltage and flux. To zero speed, or U = 0 or = . The latter is impossible, it only changes through the armature voltage to reduce speed. To speed to a higher value can increase or decrease the
3、 U . Keyword DC Speed Feedback Brake Regulator Systems A regulator system is one which normally provides output power in its steady-state operation. For example, a motor speed regulator maintains the motor speed at a constant value despite variations in load torque. Even if the load torque is remove
4、d, the motor must provide sufficient torque to overcome the viscous friction effect of the bearings. Other forms of regulator also provide output power； A temperature regulator must maintain the temperature of, say, an oven constant despite the heat loss in the oven. A voltage regulator must also ma
5、intain the output voltage constant despite variation in the load current. For any system to provide an output, e.g., speed, temperature, voltage, etc., an error signal must exist under steady-state conditions. Electrical Braking In many speed control systems, e.g., rolling mills, mine winders, etc.,
6、 the load has to be frequently brought to a standstill and reversed. The rate at which the speed reduces following a reduced speed demand is dependent on the stored energy and the braking system used. A small speed control system (sometimes known as a velodyne) can employ mechanical braking, but thi
7、s is not feasible with large speed controllers since it is difficult and costly to remove the heat generated. The various methods of electrical braking available are:附录 D 外文翻译 (1) Regenerative braking. (2) Eddy current braking. (3) Dynamic braking. (4) Reverse current braking(plugging) Regenerative
8、braking is the best method, though not necessarily the most economic. The stored energy in the load is converted into electrical energy by the work motor (acting temporarily as a generator) and is returned to the power supply system. The supply system thus acts as a”sink”into which the unwanted ener
9、gy is delivered. Providing the supply system has adequate capacity, the consequent rise in terminal voltage will be small during the short periods of regeneration. In the Ward-Leonard method of speed control of DC motors, regenerative braking is inherent, but thyristor drives have to be arranged to
10、invert to regenerate. Induction motor drives can regenerate if the rotor shaft is driven faster than speed of the rotating field. The advent of low-cost variable-frequency supplies from thyristor inverters have brought about considerable changes in the use of induction motors in variable speed drive
11、s. 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 magnetic 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
12、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,
13、and braking is applied down to the lowest speed. Induction motors require a somewhat more complex 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
14、 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 aa RIEU the terms having their usual meanings. If the IaRa drop is small, the equations approximate to U or
15、U 。 Thus, control of armature voltage and field 附录 D 外文翻译 flux influences the motor speed. To reduce the speed to zero, either U=0 or=.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 is red
16、uced. 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 Single-Quadrant Speed Control System Using Thyristors A single-quadrant thyristor converter system is shown in Fig.1.For the moment the reader shoul
17、d ignore the rectifier BR2 and its associated circuitry (including resistor R in the AC circuit), since this is needed only as a protective feature and is described in next section. Fig.1 Thyristor speed control system with current limitation on the AC side Since the circuit is a single-quadrant con
18、verter, the speed of the motor shaft (which is the output from the system) can be controlled in one direction of rotation only. Moreover, regenerative braking cannot be applied to the motor； in this type of system, the motor armature can suddenly be brought to rest by dynamic braking (i.e. when the
19、thyristor gate pulses are phased back to 180o, a resister can be connected across the armature by a relay or some other means). Rectifier BR1 provides a constant voltage across the shunt field winding, giving a constant field flux. The armature current is controlled by a thyristor which is, in turn, controlled by the pulses applied to its gate. The armature speed increases as the pulses are phased forward (which reduces the delay angle of firing), and the armature speed
