Basic Electronics

# AC Voltage Controller, Alternating Current Voltage Controller

## AC Voltage Controller:

An alternating current voltage controller or regulator converts a fixed AC voltage source to a variable AC voltage source and control the load power by varying the rms voltage of the load. The output frequency is always equal to the input frequency and can be used in industries for the speed control of 3 phase induction motor, ac power magnetic control and on load transformer tap changing. It is also used in the reactive power control. The simplest way to control the AC voltage to a load is by using an AC switch. This switch will be bidirectional switch like a triac or a pair of SCRs connected in antiparallel. Switch devices other than thyristors can also be used to implement bidirectional switches. For most purposes the control result is independent on the switch used. The practical limitations of the presently available triac ratings often make it necessary to use SCRs in very high power applications for which triacs might be used.

## AC Power control:

There are two basic methods for controlling the load power integral cycle control or power integral cycle control which will only changes the voltage level of the signal and frequency of the waveform will be remain same. The first method is suitable for systems with large time constant such as temperature control system. The load power can be connected to the source and load for a few complete cycles then disconnecting the source from the load for another number of cycles and repeating the switching cycle. The on and off periods of the relative duration of  the duty cycle d, is adjusted so that the average power delivered to the load meets some particular objective. In ideal circumstances the average power to the load can be controlled from 0% to 100%.

## Integral cycle control:

Integral cycle control will not be used if the loads have short time constant. Phase control can be used in these situations. In phase control the load connected to the supply for n cycles and disconnected from supply for “m” cycles of the integral cycle. The voltage at the load can be varied by altering the fire angle for each half cycle of a period. If ⍺=0 the output voltage is maximum when ⍺=π the output voltage is minimum.

Therefore the output voltage can be controlled to any value between zero and the source voltage. This process produces a phase controlled alternating output that is suitable for applications such as lightning and motor speed control.

## Single phase ac voltage full wave controller:

In single phase ac voltage full wave controller we use two thyristors in antiparallel which will be connected with ac input voltage. At output a resistor is connected the voltage that we are supplying from the input will be equal to Vm sin⁡ωt  which will provide the input in sinusoidal form. We will provide trigger pulse on both sides of the thyristors. In which thyristor 1 will work in positive half cycle and the thyristor 2 will work in negative half cycle. We will receive voltage across the resistor. ### Working:

When we will provide the source voltage positive voltage cycle the upper portion of the cycle will be positive and the lower portion will be negative. The current that will flow from the source voltage will be source current . This current will flow from thyristor 1. So in positive half cycle the thyrsitor 1 will conduct. After ⍺ angle at the gate of thyristor trigger pulse will be provided the thyristor will be in conducting position. ⍺ angle is the firing angle which need trigger pulse at the gate to activate. When it will be triggered it will starts conducting current the thyrsitor 2 will not conducting. The voltage will drop across the thyristor 2 but no voltage will drop across the thyristor 1. The current from the thyristor 1 will go to the resistor which will know as output current. The output wave form will be equal to   . Its value will be between 0 and π. We will not receive any voltage if we are not providing trigger voltage at the gate. In the diagram the point at which we have no voltage is known as firing angle ⍺. In diagram we are showing two wave one wave is the source voltage and other is the output voltage.  The thrysistor will give output voltage after the firing angle. The wave of the source voltage before the firing angle will not be occurs at the output. The thyristor will be conducting in T1 time period. The output current will be same as that of the voltage. During negative half cycle thyristor 1 will not conduct. Only thyristor 2 will conduct now we will again provide trigger pulse for the thyristor 2 after the firing angle the thyristor 2 will start conducting the current will go to the resistor. The wave form will be occur between π and 2π. The output current will be similar like output voltage. Now again when the positive cycle will occur thyristor will off and thyristor 1 will start working. ## Single phase control with RL load:

The voltage will be supply from the input which will be Vm sin⁡ωt. When the positive cycle will pass from the thyristor 1 and go towards the loads which are resistor and inductor. The inductor will start storing energy. The Ig1 represent the current of thyristor 1 and Ig2 represent the current of the thyristor 2 which will be in reverse bias. The thyristor will not conduct during ⍺ angle. When the pulse will end the output current will not zero. The current will be near to maximum because the inductor is connected at the output. The inductor will maintain the output current so that current is not zero suddenly. When the output current will be zero the output voltage will also be zero and when a pulse will be given at the trigger of the thyristor the output voltage will produce. This voltage will be equal to maximum voltage. The pulse in which the output current is negative while the voltage is positive is due to the thyristor 2 which will be in conducting state. When a negative cycle will come it will conduct at certain limit due to inductor. The current will flow until it become zero. When the current will become zero the voltage drop across thyristor 1 will be in positive and the voltage drop across thyristor 2 will be in negative. When a negative cycle occur the polarity of the supply will be change. When a negative cycle start the output current will decrease and a point will occur at which we will donate it with the β at which the output current will zero. The current from the inductor will start flowing which was stored by inductor and the current will start decreasing which is denoted be β. In this condition thyristor 1 will also be in conducting condition. The thyristor 1 will conductioning between ⍺ and β.
The current will be zero after β for certain time. Then current will be in reverse direction. When the negative half cycle will be finished the output current will not be zero and will to be near the maximum value. ## Two stage sequence control AC voltage controller with R load:

In this we will use set of thyristor connected antiparallel means we will use four thyristor in this circuit. The source voltage will provide the ac voltage which will consist of primary winding and secondary winding. The secondary winding will be center tapped the two winding will be mutual connected. The voltage from the one winding will be V1= Vm sin⁡ωt and voltage across second winding will
V2= Vm sin⁡ωt
The circuit is connected with the resistive load at output. ### Working:

The thyristor T3 and T4 will only work when the ωt=0 means when the firing angle ⍺ will be equal to zero. The thyristor T1 and T2 will work when at particular angle the pulse is provide means there firing angle will be at certain phase. When we start the supply from the source voltage when positive cycle will occur and ωt=0 then thyristor 3 will conduct and the current will pass from thyristor 3. The voltage will be from the secondary winding V2 and the source current will flow from thyristor T3 when ⍺ will be equal to zero when the ωt will be equal to ⍺ then we will provide trigger pulse at the gate of T1 then thyristor 3 will off and thyristor one will conduct at this time then the voltage will be from V1+V2 and the source current will flow in T1. The wave will go in upward direction after trigger pulse. This is because first there is only one voltage which was V1 and now we have two voltages which are V1+V2 the complete secondary winding will work.
When a negative cycle will come from the source the primary and lower secondary will start working the voltage will be V2 when we will provide trigger pulse at T4 it will start conducting. The output voltage will start in reverse direction and the ωt will be equal to π. The output current will be also in reverse direction. When a trigger pulse will be given at the gate of the T2 then the output voltage will be equal to V1+V2. ## Multistage sequence control of Voltage:

The controller will be connected in multistage. There will be separate voltage at the secondary of the winding such that 1V at one winding, 2V at second winding and 8V at third winding. ### Working:

When the source voltage is applied the source current will start flowing from thyristor T3 and it will start working and with T3. The T1and T2 will start working at ⍺ angle. The voltage will be 10Vm which is the sum of 2 and 8 volt. In positive cycle the upper portion of the secondary winding when the trigger voltage will be applied at the thyristor T1 and will start conducting. Then the output voltage will be the sum of all winding voltages which are 1, 2 and 8V which is equal to 11V. when a negative cycle will occur the thyristor T4 will start conducting. The output voltage will be the sum of 2 and 8 which is equal to 10V. When trigger pulse will be applied at thyristor T2 and will start conducting at π+⍺. When T1 and T2 will conduct we will add all the winding voltage and when T3 and T4 will conduct we will add only two winding. ## Step-up midpoint cycloconverter:

In this type of converter the P1 and P2 are positive type of thyristors and N1 and N2 are negative type of thyristors and the load are centre tapped at the secondary winding. The cycloconveter work is to convert the frequency. As the name indicates it will increase the frequency. The output frequency will be more than the input frequency. The upper portion of the circuit is represented with A and the lower portion is represented with B. The source will provide the ac voltage. When the positive voltage from the source will occur the P1 and N2 will start conducting at one time. Similarly P2 and N1 will work in one time. In positive cycle A will become positive with respect to B. the current will flow from P1 to the load. P1 will be after ωt 1 by force commutation and the current direction will be change. N2 is also in conducting state A will negative with respect to B. the current will flow from point O and passes through N2 this process will be done during positive half cycle. The output voltage of N2 will be negative and we will donate at with ωt 2. Similar process will continue again P1 will start conducting this process will continue until the positive cycle not end.

Now when the negative cycle will occur the P2 will start conducting and the current will goes to the load. Then the current will pass through N1. ## Step-up bridge cycloconverter:

In this converter P1, P2, P3 and P4 thyristors are used for positive cycle and four other thyristors which are N1,N2, N3 and N4 for negative half cycle. The load is connected between them. In this type of converter the P1 and P2 will convert at one time, while N1 and N2 will convert at one time. They will conduct in parallel in positive half cycle. In the period between 0 and π four thyristor will conduct the source current will flow from positive thyristors and will move towards the P1 which is in conducting condition and is passes through the load. From where it will towards the P2 and again towards source. The output obtain during this period will be from 0 to ωt1. At ωt1 due to force commutation both the thyristor will be off. N1 and N2 will be in on condition. Now the current will flow from negative thyristor and will pass from N1 then towards load from where it pass through N2 and then towards the source. Now again force commutation will occurs and P1 and P2 will start conducting. Similar process will be used for negative cycle. When a negative cycle will occur P3 and P4 will start conducting. The point a will become negative with respect to point b. The current will start flowing in downward direction when the force commutation will occur P3 and P4 will off and N3 and N4 will start conducting. The current will flow from N3 then it will pass from the load. From load it will pass through N4 and then moves towards source. The wave will be form in negative envelop. 