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CONTROLLINGDC MOTOR

CONTROLLINGDC MOTOR

CONTROLLINGDC MOTOR

CONTENTSPAGESINTRODUCTION. . ... .. . . . . . . ..2DC MOTORS3THEORY OF OPERATION3 Mathematical model.4Feedback Control Principles ...6 PERFORMANCE CRITERIA.6PROPORTIONAL + INTEGRAL CONTROLER..7Analog PI Controller...8CONTROLLING WITH OPEN LOOP SYSTEM .9CONTROLLING WITH CLOSED LOOP SYSTEM.........................................................12PART I.12THEORETICAL PART..........................................................................................................................................................................13EXPERIMENTAL PART.......................................................................................................................................................................13Part A: LOAD TORQUE CONSTANT..................................................................................................................................................13Part B: CONTROL SPEED...................................................................................................................................................................15PART II...16EXPERIMENTAL PART......................................................................................................................................................................17CONCLUSION..19GENERAL CONCLUSION...... 20INTRODUCTIONDirect current (dc) motor has variable characteristics and is used extensively in variable-speed drives. Dc motors can provide a high starting torque and it is also possible to obtain speed control over wide range. The methods of speed control are normally simpler and less expensive than those of ac drives. Dc motor plays a significant role in modern industrial drives. Both series and separately excited dc motor are normally used in variable-speed drives, but series motors are traditionally employed for traction applications and require more maintenance than do ac motors. With recent advancement in power conversion, control techniques, and microcomputer, the ac motor drives are becoming increasingly competitive with dc motor drives. Although the future trend is toward ac drives, dc drives are currently used in many industries. It might be a few decades before the dc drives are completely replaced by ac drives.Direct-current motors are used also in and position-control systems where good dynamic response and steady-state performance are required. Examples are in robotic drives, printers, machine tools, process rolling mills, paper and textile industries, and many others. Control of a dc motor, especially of the separately excited type, is very straightforward, mainly because of the incorporation of the commentator within the motor. The commentator brush allows the motor-developed torque to be proportional to the armature current if the field current is held constant. Classical control theories are then easily applied to the design of the torque and other control loops of a drive system.DCMOTORSTHEORY OF OPERATION:The invention of the electrical motor comes simply from the phenomenon that: a current carrying conductor will create a force when placed in a magnetic field; the direction of the force is perpendicular to the both, magnetic field and current.An electrical motor uses this force in such a way to cause a rotating motion; this can be achieved by placing a wire in loop (coil) called armature windings that can rotate about an axis in a magnetic field, with a comutator and brushes to supply current to the armature as shown in Figure(1).Figure -1-Electrical motorOperation representationSo whenever the rotor turns half revolution, the current flow will change direction in the armature windings which maintain the rotating force in the same direction.One of the most important operating parameters is the torque, which is directly proportional to the magnetic field and the armature current; the motor torque can be expressed as:T K IT A(1.1)Where:T = motor torqueKT = a constant based on the motor constructionIA = armature current= magnetic fluxAlso another motor parameter can be seen when the motor is running, that opposes the line voltage and its effect to cancel out some of the line voltage, so the actual voltage available to the armature is:V A V in CEMF IA Vin CEMF(1.2)RA Where:VA = actual voltage available to the armatureVin = line voltage supplied to the motorCEMF = voltage generated within the motorRA = armature resistance Mathematical model:Before we can control a system we must understand in mathematical term how the system behaves without control, this is called system modelling and it is done to obtain the motor transfer function.The basic characteristics that affect servo motors are; the inertial load J on the out put shaft (rotor+ load), and the viscous friction on the motor bearings referred as a constant b which is found to be proportional to the shaft speed.So using Newtons law we get:T ti ( ) J JbT t( )Where:: The shaft positionJ: The load moment of inertia (load + rotor)B: viscous friction constantFrom eq (1.1) we get:(1.3)

Jb K IT A( )tSo its transfer function is:(1.4)

KIs js(b)(1.5)

Relationship between Torque and SpeedThe fact that the field flux of a PM motor remains constant regardless of speed makes for a very linear torque-speed curve. This is very desirable for control applications because it simplifies the control equations. Figure -2- shows a typical PM motor symbol and torque-speed curve. Notice that the curves are straight lines for both speed and current. The absence of field coils is apparent in the schematic [Figure -2 (a)], which shows the applied voltage feeding only the armature. The PM motor is easily reversed by reversing the polarity of the applied voltage. The torquespeed curve can be used to predict the performance of a motor under any load condition.Figure -2-A permanentant Motor.Feedback Control Principles :Broadly speaking, control systems can be classified into two groups: open-loop and closed-loop. In an open-loop system [Figure-3-(a)], no feedback is used, so the controller must independently determine what signal to send to the actuator. The trouble with this approach is that the controller never actually knows if the actuator did what it was supposed to do.Figure -3Open- and closed Loop systems.PERFORMANCE CRITERIA:Performance criteria are various measurable parameters that indicate how good (or bad) the control system is. These are divided into transient (moving) and steady-state (not changing) parameters.The exact path the controlled variable takes when going from one position to the next is called its transient response. Consider the behavior of the robot arm whose response is shown in Figure -4-; it is directed to move from 0 to 30, as shown by the dashed line. This type of command (changing instantaneously from one position to another) is called a step change. The actual response of the system is shown as a solid line. As you can see, there is a difference between the ideal path of the arm and the one it took. One major consideration is how fast the system picks up speed (called rise time). The real arm simply cannot move fast enough to follow the ideal path.Rise time (T) is usually defined as the time it takes for the controlled variable to go from 10 to 90% of the way to its new position. Another transient parameter is overshoot.Once the arm starts moving, its momentum will keep it going right on past where it was supposed to stop. Overshoot can be reduced by the controller but usually at the expense of a longer rise time. Settling time (Ts) refers to the time it takes for the response to settle down to within some small percentage (typically 2-5%) of its final value. In this case, it is the time it takes for the oscillations to die out. Rise time, settling time, and overshoot are all related; a change in one will cause a change in the others.The steady-state error (ESS) of the system is simply the final position error, which is the difference between where the controlled variable is and where it should be. InFigure -4-, ESS is shown as the position error after the oscillations have died out. This error is the result of friction, loading, and feedback-sensor accuracy. A sophisticated controller can reduce steady-state error to practically zero.Figure-4-TransientResponse.PROPORTIONAL + INTEGRAL CONTROLLER:Many control systems use a combination of the three types of feedback already discussed: Proportional + Integral(PI) Control. The foundation of the system is proportional control. Adding integral control provides a means to eliminate steady-state error but may increase overshoot. The response of the PI system can be described by Equation 11.6, which simply adds together the three components required:

Another form of this obtained by factoring out KP from each term, is

WhereOutput PI = output of the PI controllerKP = proportional control gainKI = integral control gain (often seen as 1/TI)E = error (deviation from the set point) (E T) = sum of all past errors (area under the error time curve) E/ t = rate of change of error (slope of the error curve) Analog PI Controller:

CONTROLLING WITH OPEN LOOP SYSTEMIn this part we are dealing with speed torque control system .we will have to see different relationship between them .we will have then to plot the speed , the armature current and the torque and we will see the behaviour of the torque with respect to the current since there is related to the current .Using XY graph we will plot the wm versus the torque in order to seen the influence of the variation of the speed on the torque .MATLAB SIMULINK is used as tool.EXPERIMENTAL PART:a)Designingthecontrolsystem:

b) Graphs out of the scope:

Remark: From the simulation result we remark that the speed increases until reaches the steady state conditions and remains constant, and the torque is proportional to the current, but practically this is not true because after few minutes of running the parameter of the motor will change, and this will effect on the torque-speed characteristics. For this reason this system not useful and not used in applications which implies to use another type of system which is the closed loop system.c) Speed torque plot :

CONTROLLING WITH CLOSED LOOP SYSTEMPART I:In this part we are dealing with the control of the speed and the torque of the Dc motor using a feedback control system and a DC chopper .This feedback and DC chopper will help us to overcome the offset error during the control.MATLAB is used as tool.THEORETICAL PART:In many industrial applications it is required to convert a fixed voltage dc source into a variable voltage dc source .A dc chopper converts directly from dc to dc and it is known as a dc to dc converter .Choppers are widely used for traction motor control in electric automobiles, trolley cars They provide smooth acceleration control, high efficiency and fast dynamic response. Choppers can be used in regenerative braking of dc motors to return energy back in to the supply.In our work dc chopper are used as dc to dc converter in ON OFF feedback control system in order to control the speed and torque of the Dc machine .The operation of the Step Down chopper with a freewheeling diode explain the use of chopper in ON OFF feedback control system. Therefore the operation of the chopper can be divided in to two modes .During mode 1, the chopper is switched on and the current flows from the supply to the load .During mode 2, the chopper is switched off and the load current continues to flow through freewheeling diode.In our case since we are controlling the speed, the ON or OFF are activated by the difference between the measured value compared to the set point .Therefore if there is an error the controller is ON and a signal is outputted from the chopper to the DC machine .Otherwise the controller is off and no output from the chopper.EXPERIMENTAL PART:PartA: LOAD TORQUE CONSTANTIn this experiment the load torque is fixed and we give to the speed two values (initial value and final value) the we see the behaviour of the torque when the speed controller is trying to overcome the error in order to reach the desired final value.a) Designing the control system:

d) Graph

Remark: As it can be seen from the resulting graph the torque is remarkably affected by the speed error .Therefore when there s difference between the set point and the measured speed values the torque is destabilized and will return on its fixed value when the error will be corrected by the controller and the speed at its desired final value.NB.-Initial speed value is fixed at 80 rad/s-final speed value is fixed at 160 rad/s-The load torque is fixed at 5 N.mPartB: CONTROL SPEEDIn this part the speed is fixed and we vary the load torque and then we see variation of the seep with respect to the torque.a) Designing the control system:

b)Graph:

Remark: In this part we see the influence of the torque in the variation of the speed. Therefore as it has been verified mathematically the speed is inversely proportional to the torque.PART II:In this part we are going to improve the feedback speed control system seen in previous part just by adding PI controller in the feedback loop in order to try to remove the offset in speed controlEXPERIMENTAL PART:a)Designingthecontrolsystem:

b) Graph(speed and torque):

CONCLUSION:We have seen in our experimentation work two types of controlling a dc motor with following conclude:A. In part one we were controlling the speed and torque of the dc machine using a closed loop system. From this, as shown on XY graph we have shown the relationship between the speed and the torque.From here we have to specify the advantages of using MOSFET in this closed loop control system. Therefore MOSFET is a voltage controlled device with infinite input impedance which means that if a voltage is applied the input current to the MOSFET will be very low; thus it doesnt require much power .And it is also very simply drivenB. In this part we dealt with a simple feedback controlled system which controls the speed. The step down chopper has been used for this effect. This kind of system is not enough to control a signal which contains many oscillations (where in addition to the error there is an offset to the signal). That why there is a need to add a PI controllers in the feedback loop in order to overcome this offset.C. The PI controller has an advantage in control system since it helps us to correct continuously the error without oscillations. Today Fasy-logic controllers are built up to replace the PI controller because of its advantages since it doesnt need transfer function to be implemented.The control of the torque needs to know how the current is varying since the torque is related to the current .The current transformer could be used for this purpose.GENERAL CONCLUSION DC motors are widely used in applications requiring accurate speed controlfor Example, in servo systems. Having developed a circuit model and analysis methods for the DC generator, we can extend these results to DC motors, since these are in effect DC generators with the roles of input and output reversed. The closed-loop controller is a very common means of keeping motor speed at the required "set point" under varying load conditions. It is also able to keep the speed at the set point value where for example, the set point is ramping up or down at a defined rate. The essential addition to the previous system is a means for the current speed to be measured. MATLAB/SIMULINK is/was used because of the short learning curve that we require to start using it, its wide distribution, and its general-purpose nature. This demonstrate the advantages of using MATLAB for analyzing power system steady state behavior and its capabilities for simulating transients in power systems and power electronics, including control system dynamic behavior. According to the literature review and the results of this work, it is possible to outline the most favorable simulation environment for electrical machines, circuit and control systems. A fundamental assumption for doing so is that the emphasis of the simulation will be/is on the operation of the whole system and the interaction between the components, as well as on the behavior and design of each component separately. Therefore, the best solution must contain two or three different tools operating firmly together.REFERENCE[1]. P.C. Sen, Principles of Electric Machines and Power Electronics (2ndEdition), John WILEY and Sons Inc., 1989[2]. Power Electronics Hand Book by MUHAMMAD H. RASHID[3]. Pricinples and applications of electrical engineering.111