13-05-2011, 02:20 PM
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INTRODUCTION
Efficient control of D.C motor speed is very economical to any industries for that reason we are using full wave rectifier in which we are using triacs as switches because of bi-directional nature of triacs we are preferred instead of using thyrister as a switches. To obtain a four quadrant operation we need 8 switches if we use thyristers (this circuit called dual converter) and if we use triacs we need 4 switches only.
At the same time armature voltage control is preferred because above and below speeds are possible, in the control circuit most of the components are operational amplifiers. It is easy to get the required pulses by using pulses using op-amp.
Power supply for the op-amp is also used. This is most efficient and useful circuit from that we are getting +15v, -15v. The possible ways of controlling of dc motor are Armature voltage control and field control. In armature voltage control there are some types of control methods. But there are some disadvantages because of lot of applications in so many areas D.C motors are became popular. If we provide most efficient control of speed it is more economical for such reasons we are going for this method.
CHAPTER -1
1.COMPLETE BLOCK DIAGRAM:
The below circuit shows the total block diagram .which gives the total four quadrant operation of DC-separately excited motor . this circuit having different parts in order to get the required out put.
2.OPERATION:
The main aim of the project is to get the four quadrant operation of D.C separately excited motor. This operation is achieved simply by a single switch s1. There is mainly two possibility condition of s1 i.e. either it is in open condition or in closed condition.
1. When the switch is in open condition then we get motor running condition i.e. either forward motoring or reverse motoring.
2. When the switch is in closed condition then we get motor breaking condition i.e. either forward regenerative breaking or reverse regenerative breaking.
From the above discussion there is a possibility of four conditions.
1. Switch s1 open forward motoring
2. Switch s1 open reverse motoring
3. Switch s1 closed forward regenerative breaking
4. Switch s1 closed reverse regenerative breaking.
1. Switch s1 open forward motoring
Keep initially the switch s1 is in open condition. Then the reference value changes from 0 to -ve value i.e. the value of firing angle changes from 0 to 900. That means during the +ve half cycle TRIACS (T1, T3) are turned on and for –ve half cycle (T2, T4) are turned on. That means voltage applied across the armature of the D.C motor is +ve and the current direction is as shown in the diagram. Then the motor running in the forward direction.
From the equation
N= KN(Vr- IaRa)/¢
Here Vr=(2Vmcosα)/π
Vm= peak to peak value of input supply
α = firing angle and
Back emf E= Vr- IaRa
During the forward motoring operation the firing angle changes from 0 to 900 that means there is no possibility of getting –ve voltage across the armature of D.C motor.
When starting D.C motor initially we kept the firing angle near to 900 because the starting voltage across the armature must be limited to avoid the high starting current.
Speed Control:-
The Speed in forward motoring is achieved by varying the reference value that means firing angle is varied. As the firing angle is increases the speed of the motor decreased. We can vary the speed above and below rated value.
N= KN [(2Vm cosα)/π- IaRa] /¢
2.Switch s1 closed forward regenerative
When switch S1 is closed the reference value changes to high value. This reference value is applied to the comparator then the output of comparator changes from low to high value. This output is given to clock of the JK flip-flop. Then the value of Q and Q1 are changed from clock changed from low to high only. That means the gate pulses applied to the TRIAC pairs gets interchanged. That means during +ve half cycle (T2, T4) receive signals and during –ve half cycle (T1, T3). At the same time if you see the charging circuit the reference value is applied to the comparator and it is greater than Zero. The output of this comparator is high and which charges the capacitor. Whenever the capacitor voltage is greater than 5V The output of next comparator output changes i.e. high which resets the D-flip-flop. Then the ‘Q¯’ is going high. This high value is applied at the Triac driving circuit. Which gives 15V at the 2nd terminal of (2, 3) moc3021 opt isolators. That means the voltage at the two terminals of ‘LED’ is 15V. So LED does not glow and hence Triac is not getting control signal to the gate.
Here the firing angle value becomes greater than 900. Hence the back emf is greater than supply voltage then machine try to run in the reverse direction. This motors supplies energy to the supply. This mode is called forward regenerative mode.
E= Vr+IaRa
Vr= +(2Vmcosα)/π (α > 900)
E= -K
N= KN(-Vr + IaRa)/¢
Speed also negative
3.Switch S1 open Reverse motoring
When the switch s1 is opened the value of Vr becomes less. Before opening the switch we must keep ‘α ‘ near to 900 in order to apply less voltage at starting .Here during +ve cycle (T2, T4) are getting pulses and during –ve cycle (T1, T3) are getting pulses. So that the voltage applied across the armature is negative and current direction is opposite to the forward motoring. So the motor runs in the reverse direction. Torque developed is in reverse direction. This mode of operation is called reverse motoring.
Vr= -(2Vmcosα)/π
N= KN(-Vr + IaRa)/¢
-E= (-Vr) + IaRa
By varying the firing angle ‘α’we can vary the rectifier output so that vary the speed of the D.C motor.
N= KN [-(2Vm cosα)/π + IaRa] /¢
If ‘α ‘decreases the speed of the motor increases in reverse direction.
4. Switch S1 closed Reverse regenerative
Whenever switch s1 is closed the Vr is goes to high and α becomes greater than 900 that means here the pulses applied to the Triac pairs gets interchanged. At the same time the voltage at the points b and c are become high i.e. 15V. After the time ‘tl’ again the point band c become low. Then the Triacs conducts but now the power flows from motor to the supply mains. This mode of operation is called Reverse regenerative breaking.
Vr= +(2Vmcosα)/π (α > 900)
Vr is negative
-E= Vr+ IaRa
N= KN [(2Vm cosα)/π + IaRa]
