1、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 of DC drive system,
2、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 U . Keyword DC Spee
3、d 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 removed, the motor must pr
4、ovide 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 the output vo
5、ltage 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., the load has to be
6、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 wi
7、th large speed controllers since it is difficult and costly to remove the heat generated. The various methods of electrical braking available are: (1) Regenerative braking. (2) Eddy current braking. (3) Dynamic braking. (4) Reverse current braking(plugging) Regenerative braking is the best method, t
8、hough 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 th
9、e 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. Inducti
10、on 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 drives. Eddy current braking can b
11、e 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 braking is applied. In dynami
12、c 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 t
13、o 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 to many large AC hoist syste
14、ms 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 U 。 Thus, control of armature
15、 voltage and field 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 reduced. The latter is the most practical
16、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 should ignore the rectifier BR2 and its asso
17、ciated 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 converter, 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.
