26-02-2011, 10:31 AM
DC MOTOR CONTROL USING CHOPPER
INTRODUCTION
Development of high performance motor drives are very essential for industrial applications. A high performance motor drive system must have good dynamic speed command tracking and load regulating response. DC motors provide excellent control of speed for acceleration and deceleration. The power supply of a DC motor connects directly to the field of the motor which allows for precise voltage control, and is necessary for speed and torque control applications. DC drives, because of their simplicity, ease of application, reliability and favorable cost have long been a backbone of industrial applications. DC drives are less complex as compared to AC drives system. DC drives are normally less expensive for low horsepower ratings. DC motors have a long tradition of being used as adjustable speed machines and a wide range of options have evolved for this purpose. Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque. DC regenerative drives are available for applications requiring continuous regeneration for overhauling loads. AC drives with this capability would be more complex and expensive. Properly applied brush and maintenance of commutator is minimal. DC motors are capable of providing starting and accelerating torques in excess of 400% of rated [3]. D.C motors have long been the primary means of electric traction. They are also used for mobile equipment such as golf carts, quarry and mining applications. DC motors are conveniently portable and well fit to special applications, like industrial equipments and machineries that are not easily run from remote power sources D.C motor is considered a SISO (Single Input and Single Output) system having torque/speed characteristics compatible with most mechanical loads. This makes a D.C motor controllable over a wide range of speeds by proper adjustment of the terminal voltage. Now days, Induction motors, Brushless D.C motors and Synchronous motors have gained widespread use in electric traction system. Even then, there is a persistent effort towards making them behave like dc motors through innovative design and control techniques. Hence dc motors are always a good option for advanced control algorithm because the theory of dc motor speed control is extendable to other types of motors as well [3]. Speed control techniques in separately excited dc motor:
By varying the armature voltage for below rated speed.
By varying field flux should to achieve speed above the rated speed.
Different methods for speed control of DC motor:
Traditionally armature voltage using Rheostatic method for low power dc motors.
Use of conventional PID controllers.
Neural Network Controllers.
Constant power motor field weakening controller based on load-adaptive multi- input multi- output linearization technique (for high speed regimes).
Single phase uniform PWM ac-dc buck-boost converter with only one switching device used for armature voltage control. Using NARMA-L2 (Non-linear Auto-regressive Moving Average) controller for the constant torque region. Large experiences have been gained in designing trajectory controllers based on self-tuning and PI control. The PI based speed control has many advantages like fast control, low cost and simplified structure. This thesis mainly deals with controlling DC motor speed using Chopper as power converter and PI as speed and current controller.
CHOPPER
2.1. DC CHOPPER A chopper is a static power electronic device that converts fixed dc input voltage to a variable dc output voltage. A Chopper may be considered as dc equivalent of an ac transformer since they behave in an identical manner. As chopper involves one stage conversion, these are more efficient [2]. Choppers are now being used all over the world for rapid transit systems. These are also used in trolley cars, marine hoist, forklift trucks and mine haulers. The future electric automobiles are likely to use choppers for their speed control and braking. Chopper systems offer smooth control, high efficiency, faster response and regeneration facility [2]. The power semiconductor devices used for a chopper circuit can be force commutated thyristor, power BJT, MOSFET and IGBT.GTO based chopper are also used. These devices are generally represented by a switch. When the switch is off, no current can flow. Current flows through the load when switch is “on”. The power semiconductor devices have on-state voltage drop of 0.5V to 2.5V across them. For the sake of simplicity, this voltage drop across these devices is generally neglected [2]. As mentioned above, a chopper is dc equivalent to an ac transformer, have continuously variable turn’s ratio. Like a transformer, a chopper can be used to step down or step up the fixed dc input voltage
2.2. PRINCIPLE OF CHOPPER OPERATION A chopper is a high speed “on" or “off” semiconductor switch. It connects source to load and load and disconnect the load from source at a fast speed. In this manner, a chopped load voltage as shown in Fig. is obtained from a constant dc supply of magnitude Vs. For the sake of highlighting the principle of chopper operation, the circuitry used for controlling the on, off periods is not shown. During the period Ton, chopper is on and load voltage is equal to source voltage Vs. During the period Toff, chopper is off, load voltage is zero. In this manner, a chopped dc voltage is produced at the load terminals
2.3. CONTROL STRATEGIES [2] The average value of output voltage Vo can be controlled through duty cycle by opening and closing the semiconductor switch periodically. The various control strategies for varying duty cycle are as following: 1. Time ratio Control (TRC) and 2. Current-Limit Control. These are now explained below.
2.3.1. Time ratio Control (TRC) In this control scheme, time ratio Ton/T(duty ratio) is varied. This is realized by two different ways called Constant Frequency System and Variable Frequency System as described below:
1. CONSTANT FREQUENCY SYSTEM [2]
In this scheme, on-time is varied but chopping frequency f is kept constant. Variation of Ton means adjustment of pulse width, as such this scheme is also called pulse-width-modulation scheme.
2. VARIABLE FREQUENCY SYSTEM [2]
In this technique, the chopping frequency f is varied and either (i) on-time Ton is kept constant or (ii) off-time Toff is kept constant. This method of controlling duty ratio is also called Frequency-modulation scheme.
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