CycloConverters

AC RegulatorsINTRODUCTIONBy connecting a pair of thyristors or Triac in reverse parallel manner, between AC supply and the load, voltage applied to the load can be controlled. This type of power controller is known as an AC voltage controller or AC regulators.

AC voltage regulators converts fixed mains voltage directly to variable alternating voltage without a change in the frequency.

The important applications are: Speed control of polyphase induction motors, domestic and industrial heating, light controls, on-load transformer tap changing, static reactive power compensators, etc.. Earlier, the devices used for these applications were auto transformers, tap-changing transformers, magnetic amplifiers, saturable reactors, etc. INTRODUCTIONNow, thyristor and Triac a.c. regulators have replaced them in most of the applications because of high efficiency, compact size, flexibility in control, etc. The a.c. regulators are also suitable for closed loop control because of low control power and fast response.

Since the a.c. regulators are phase-controlled converters, thyristors and Triacs are line commutated and as such no complex commutation circuitry is required in these controllers.

The main disadvantage of these regulators is the presence of harmonics in the supply current and load voltage waveforms, particularly at lower output voltage levels.ClassificationThe a.c. voltage controllers can be classified as Single-phase controllers and Three-phase controllers.

Each type of controllers can be subdivided into(a) Unidirectional or half-wave control and(b) Bidirectional or full wave control.SINGLE-PHASE A.C. REGULATORHalf-Wave A.C. Voltage RegulatorDue to the presence of Diode D1, the control range is limited and the effective RMS output voltage can only be varied between 70.7 and 100%. It can be observed that positive half-cycle is not identical with negative half-cycle for both voltage and current waveforms. As a result, DC component is introduced in the supply and load circuits, which is undesirable.Since the power flow is controlled during the positive half-cycle of input voltage, this type of controller is also known as a unidirectional controller. This type of controller is only suitable for low power resistive loads, such as heating and lighting.

SINGLE-PHASE A.C. REGULATORPower circuit diagram of a single phase half wave a.c voltage regulator using one thristor in anti parallel with one diode.

SINGLE-PHASE A.C. REGULATOR

SINGLE-PHASE A.C. REGULATORThree possible configurations of single-phase a.c. voltage controllers.

SINGLE-PHASE A.C. REGULATORFull-wave (Bidirectional) A.C. Voltage Controllers

Figure (a) uses two thyristors connected in antiparallel. In this circuit, isolation between control and power circuit is a must because the cathodes of two thyristors are not connected to a common point. Figure (b) employs four diodes and one thyristor. In this circuit, isolation between control and power circuit is not mandatory. This scheme, therefore, offers a cheap a.c. voltage controller. Figure (c) shows the Triac based a.c. voltage regulator. This circuit configuration is suitable for low power applications where the load is resistive or has only a small inductance. The triggering circuit for the Triac need not be isolated from the power circuit.

Single-phase a.c. voltage controller with resistive (R) load Thyristors T1 and T2 are forward biased during positive and negative halfcycle, respectirely. During positive half-cycle, T1 is triggered at a firing angle .T1 starts conducting and source voltage is applied to load from to : At ; both eo, io fall to zero. Just after ; T1 is subjected to reverse bias and it is, therefore, turned-off. During negative half-cycle, T2 is triggered at ( + ). T2 conducts from ( + ) to 2.Soon after 2, T2 is subjected to a reverse bias and it is, therefore, commutated. From zero to , T1 is forward biased, therefore V T1 = es. From to , T1 conducts, VT1 is therefore about 1V. After ; T1 is reverse biased by source voltage, therefore, VT1 = es from to ( + a).Single-phase a.c. voltage controller with resistive (R) load SINGLE-PHASE A.C. REGULATOR Examination of this figure reveals that for any value of , each thyristor is reverse biased for / seconds.If es = 2 Es sin t is the input voltage, and the firing angles of thyristors T1 and T2 are equal (1= 2 = ), the RMS output voltage can be obtained from.

Thus by varying from to 0, the RMS output voltage can be controlled from RMS input voltage zero to Es .

SINGLE-PHASE A.C. REGULATORHarmonics of output quantities and input current: The waveforms for output quantities (voltage eo and current io) and input current is non-sinusoidal. These waveforms can be described by Fourier series. As the positive and negative half-cycles are identical, d.c. component and even harmonics are absent.The output voltage eo can be, therefore, represented by Fourier series as

SINGLE-PHASE A.C. REGULATOR

SINGLE-PHASE A.C. REGULATOR

These Equations can be used to evaluate the magnitude of harmonics for which n =3,5,7,.It cannot be used to calculate the fundamental component be cause substitution for n=1 leads to indefined expressions.The fundamental frequency, which has the same freq. as the supply voltage, can however be obtained by substituting n=1 in the basic integration expression for An and Bn and eo can be obtained. This yeilds,SINGLE-PHASE A.C. REGULATOR

SINGLE-PHASE A.C. REGULATORWhen a.c. voltage controller is used for the speed control of a single phase induction motor, only fundamental component is useful in producing the torque.

The harmonics in the motor current merely increase the losses and therefore heating of the induction motor.

For heating and lighting loads, however, both fundamental and harmonics are useful in producing the a.c. controlled power. In such applications, RMS value of the output voltage Eo is of interest.SINGLE-PHASE A.C. REGULATORPower factor:

Assuming that the source voltage remains sinusoidal even though non-sinusoidal current is drawn from it, the power factor is given by

The above equation gives the definition of power factor when the source voltage is sinusoidal but the current is non-sinusoidal. For the present case, another expression for PF can be obtained as follows:SINGLE-PHASE A.C. REGULATORThe maximum values of RMS output voltage and current occurs at = 0 and are given by E and E/R, respectively. Since harmonics are absent at = 0 these are also the maximum values of fundamental RMS voltage and current. If these quantities are used to normalize various voltages and currents, then PF = normalized output voltage = normalized source (or load) currentThe power factor is poor for low values of the load current and appreciable amount of harmonics is present. For example, at a per unit value of 0.4 of the load current, third harmonic becomes comparable to the fundamental and at a p.u. value of 0.2, fifth and seventh harmonics also have comparable values. Since the source current and load current waveforms are identical, the curves of harmonics components and fundamental are also applicable to the source current.SINGLE-PHASE A.C. REGULATOR

Plot of normalised fundamental component of load current, normalised values of harmonics in load current and power factor against nonalised RMS load current. SINGLE-PHASE A.C. REGULATOR with inductive (RL) loadDuring the interval zero to ; thyristor T1 is forward biased. At t = , T1 is triggered and io = iT1 starts building up through the load.

At ; load and source voltages are zero but the current is not zero because of the presence of inductance in the load circuit. Thyristor T1 will continue to conduct until its current falls to zero at t=.

Angle is called as the extinction angle. The load is subjected to the source voltage from to .

At ,when io is zero, T1 is turned-off as it is already reversed biased. After the commutation of T1 at , a voltage of magnitude Em sin at once appears as a reverse bias across T1 and as a forward bias across T2.

From to + , no current exists in the power circuit. Thyristor T2, is turned on at (+)>.

SINGLE-PHASE A.C. REGULATORCurrent io = iT2 starts building up in the reversed direction through the load. At 2, eo and es are zero, but iT2 = io is not zero .

At (++), iT2 = 0, and T2 is turned off because it is already reverse biased. At (++), Em sin (++) appears as a forward bias across T1 and a reverse bias across T2.From(++) to (2+), no current exists in the power circuit.

At (2+), T1 is turned on and current starts building up as before.

SINGLE-PHASE A.C. REGULATOR with RL-load

SINGLE-PHASE A.C. REGULATORThe Expression for load current is and the angle can be obtained as follows:

es = Em sin(t) = R.io + L (diodt)

The solution of this equation is of the form

io= (EmZ ). sin(t ) + Ae-(R/L)t

From these equations, one can obtain a relationship between and for a given value of .

SINGLE-PHASE A.C. REGULATOR with RL-load SINGLE-PHASE A.C. REGULATOR versus curves for various values of .shows that as is decreased, the conduction angle increases. The waveforms of current io reveals that for < ,current iT1 through T1, flows from to (+) = , and T1 remains OFF from (+) up to (+). At (+), current iT2 through T2 flows from (+) to (++).

T2 remains OFF from (++) to (2 + ). At (2 + ), T1 is turned-on.

With progressive decrease in , may become equal to .Under this condition, when is just equal to , T1 will be ON from to ( + ) and iT1 flows from to ( + ). Further, T2 will be ON from ( + ) to (2+ ) and current iT2 flows from + to (2 + ). Thus, when = , From 0 to T2 conducts.from to ( + ) T1 conductsfrom ( + ) to (2 + ) T2 conducts, and so on

This shows that load current will never become zero for any segment of time and therefore, for all the time load is connected to source.

Hence, for = , the load voltage is equal to sinusoidal source voltage provided the voltage drop in thyristors is neglected. Under these conditions, load behaves as if it is being fed directly by the a.c. source.

To determine the value of for which = and load is directly connected to a.c. source, consider that the RL load, with load phase angle , is connected directly to a.c. source. Under steady state, the load current will be a sine wave and lag behind the voltage wave by an angle .SINGLE-PHASE A.C. REGULATOR with RL-load The current is positive from to ( + ) and negative from ( + ) to (2 + ). If it is required to obtain the current waveform of Fig. c above, through the operation of power circuit of Fig. (a), then

From 0 to T2 conducts to ( + ) T1 conducts ( + ) to (2 + ) T2 conducts, and so on

Comparison of expressions reveals that when = , = .

When = , sin ( ) = 0 = sin or, ( ) =

and = ( ) =

This shows that for a single-phase a.c. voltage controller, waveforms of Fig. (b) are applicable only when > and that of Fig. (c) for .

SINGLE-PHASE A.C. REGULATOR with RL-load The rms output voltage can be found from

The RMS thyristor current can be found as

SINGLE-PHASE A.C. REGULATOR with RL-load 30SINGLE-PHASE A.C. REGULATORThe RMS output current can then be determined by combining the RMS current of each thyristor as

The average value of thyristor current can also be found from

31SINGLE-PHASE A.C. REGULATOROperation with .

Note that for and in Fig (c) for