外文翻译--直流电机速度控制-精品.doc

1、Speed Control of DC MotorRegulator SystemsA 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 removed ,the motor m

2、ust 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 maintain must ou

3、tput 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 BrakingIn many speed control system, e.g., rolling mills mine winders, etc., the load has to b

4、e 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 this is not feasible

5、with large speed controllers since it is difficult and costly to remove the heat generated. The various methods of electrical braking avaiable are:(1) Regenerative braking.(2) Eddy current braking.(3) Dynamic braking.(4) Reverse current braking(plugging).Regenerative braking is the best method, thou

6、gh 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 energy is delivered. Providing the

7、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 invert to regenerate. Induction

8、 motor driver can regenerate if the rotor shaft is driven faster than speed of the rotating field. The advent of low-cost variable variable-frequency supplies from thyristor inverters have brought about considerable charges in the use of induction motors in variable speed drives. Eddy current brakin

9、g can be applied to any machine, simply by mounting a copper or aluminium 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, bearing, and motor will be raised if prolonged braking is applied.In

10、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

11、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 to many large AC hoist system wher

12、e 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 ma

13、jor 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.Equal pulse width PWM law VVVF (Variable Voltage Variable Frequency) installs in the early time is uses PAM (Pu

14、lse Amplitude Modulation) to control, its inventor part which the technology realizes but only can output the frequency adjustable square-wave voltage not to be able to adjust the pressure. The pulse width PWM law are precisely in order to overcome, is in the PWM law which the PAM law this shortcomi

15、ng development comes 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 f

16、requency 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 component.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