Function of Intergroup Reactor In the Cycloconverter

Ratio of input frequency to output frequency

• If the ratio of input frequency to output frequency is not integer, the short circuit current flows through input side. 
• The firing angle of the P – converter and N – converter should be adjusted in order to stop generation of short circuit current. 
• If the load current is discontinuous, it is complicated to adjust firing angle of the converters. 
• The circulating current system is used in order to control firing angle of the converters. 
• If the SCRs of the P – converter operates at firing angle α_p, the firing angle of SCRs of the N – converter should be such that

         α_p + α_n = 180^o

• Therefore, the P – converter operates in the rectifier mode and N – converter operates in inverter mode. 
• The average output voltage of the rectifier group is equal to the back emf of the inverter group and this will prevent circulation current flows between converters. 
• However, the harmonic current flows between two converters due to instantaneous voltage of the both converters are not equal. 
• The harmonic current is limited by external reactor or impedance.        

Function of Intergroup Reactor

• The harmonic current is reduced by using inter group reactor as shown in the figure A.
• The load is connected at one end at the centre point of inter group reactor. 
• The value of reactance should be 4nX_L in order to reduce low frequency output current.

          N = Harmonic order number and

        X_L = Half reactance of reactor

• The SCRs of the rectifier group should be blocked for selected time in order to reduce circulating current between two converters or gate pulse of SCRs of rectifier group should be remove for selected time period. 
• There is a current sensing device connected at the output terminal of the Cycloconverter and it detects the direction of current. 
• The current sensing device generates control signal according to direction of current. 
• The gate pulse of SCRs is removed in case of overload and fault condition in order protect the SCRs.

Advantages of Circulating Current Scheme

• The advantages of circulating current scheme are as given follows. 
• The load current flows in any direction ( Either from load to Cyclo converter or Cyclo converter to load ). 
• The effect of continuous current or discontinuous current does not depends upon firing angle of SCRs and output voltage.

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Advantages, Disadvantages of the Cycloconverter

What is Cycloconverter?

The Cycloconverter is single stage AC to AC converter in which input constant voltage, constant frequency AC supply is converted into variable voltage, variable frequency AC supply. In this article, advantage, disadvantage and applications of Cycloconverter is given. 

Advantages 

• The cyclo converter is a single stage converter therefore its efficiency is very high. 
• The cyclo converter operates on line commutation ( except step up

Step up Cycloconverter

 

Power Circuit of Step up Cycloconverter

• It is Cycloconverter in which the output frequency is greater than the input frequency of the supply. 
• It is only Cycloconverter in which commutation of SCRs is done by forced commutation. 
• The power circuit of the step up Cycloconverter is shown in the figure A.
• There are two group of SCRs in which one group ( SCR T1 and SCR T2 ) generates positive half cycle and other group ( SCR T3 and SCR T4 ) generates negative half cycle. 

Operation of Step up Cycloconverter

The operation of the Cycloconverter is explained as below.

Positive half cycle

• The SCR T1 is turned on during positive half cycle of the alternating supply during time 0 to t₁ therefore the load current flows through path A – SCR T1 – D – LOAD – C. 
• The output voltage becomes positive. The SCR T1 is turned off by forced commutation at time t₁ and SCR T4 is turned on. 
• The load current flows through path C – LOAD – D – SCR T4 – B. The output voltage becomes negative during this time.

Negative half cycle

• The SCR T2 is turned on during negative half cycle ( at π time ) of alternating cycle therefore the load current flows through path B – SCR T2 – D – LOAD – C . 
• The output voltage becomes positive. The SCR T4 is tuned off at time t₄ by forced commutation and SCR T2 is turned on. 
• The output voltage becomes negative during this time interval. 
• The SCR T3 is turned off at time t₅ by forced commutation and SCR T2 is again turned on. 
• This will result in output voltage becomes positive. The cycle completes in this way. The output frequency ( from figure B )

       f₀ = 1 / ( t₂ – t₁ ) Hz

• The output frequency of the Cycloconverter f₀  is always greater than the supply or input frequency.

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Types of Cycloconverter

There are following types of Cycloconverter.

According to output frequency

Step up Cycloconverter

• It is a Cycloconverter in which output frequency is greater than the input frequency of the input supply.

Step down Cycloconverter

• It is a Cycloconverter in which output frequency is less than the input frequency of the input supply.

According to supply voltage

• Single Phase to Single Phase
• Centre Taped Transformer Configuration

Bridge Configuration

• Three Phase to Three Phase Cycloconverter
• Three Phase to Single Phase Cycloconverter

Single Phase to Single Phase Cycloconverter

• It is a Cycloconverter in which output frequency may be higher or lower than the input frequency. 
• The input and output of this type of Cycloconverter is single phase alternating. There are two configurations in this type of Cycloconverter.

Single Phase to Single Phase Centre Taped Configuration

• The power circuit of single phase to single phase center taped configuration is shown in the figure A. 
• There are two groups of SCRs. The positive half cycle of the output voltage is generated by one group of SCRs ( SCR T1 and SCR T2 ).
• Similarly, the negative half cycle of the output voltage is generated by second group of SCRs ( SCR T3 and SCR T4 ). 
• The output frequency generates by this type of Cycloconverter may be 1/3 that of input frequency.  
• The load is connected between point C and D as shown in the figure A.

Operation : Resistive Load

Positive Group

• The point A is made positive with respect to point C in the positive half cycle of the supply voltage. 
• The SCR T1 is turned on during this interval. The SCR T2 remains turns off when point B is negative with respect to point C. 
• The load current flows through path A – SCR T1 – D – Load – C – A during this interval.
• The SCR T1 automatic turn off during negative cycle of the alternating supply because point A is made negative with respect to point C. 
• As the SCR T2 is turned on, the load current flows through path point B – SCR T2 – D – Load – C – B.
• The SCR T1 again turns on during positive half cycle and SCR T2 automatic turns off during that time. 
• The load current flows from supply to load during three half cycle of the alternating supply ( Figure D ). 
• The voltage across load is equal to one half cycle which is made by combination of three half cycle of input supply. 
• The output voltage of the Cycloconverter is adjusted by controlling firing angle of SCR T1 and SCR T2.

Negative Group

• The SCR T3 and SCR T4 receive gate pulse and gate pulse of SCR T1 and SCR T2 remove in this mode. 
• When point B is made positive with respect to point C, SCR T3 is switched on in spite of SCR T2 and load current flows through path C – LOAD – D – SCR T3 – A – C. 
• The direction of load current flows through path load to supply side.
• The point A is made positive with respect to point C in the next half cycle and at that time SCR T3 automatic turns off and SCR T4 is turns on resulting load current flows through path C – LOAD – D – SCR T4 – B – C. The SCR T3 turns on and SCR T4 turns off in the next half cycle resulting voltage across load is equal to one negative half cycle equivalent to three half cycle of alternating supply. 
• The load current flows through load to supply side.
• Finally, we conclude that we receive only one complete cycle at the load side for three complete cycles at the input side therefore the output frequency is one – third that of input frequency. The output voltage of the Cyclo converter is adjusted by controlling the firing angle of the SCRs.

Operation : Inductive Load

• The load current continues to flow in spite of complete half cycle due to energy stored in the inductor when load is inductive. 
• This will result in SCR does not turn off after completion of half cycle and load voltage becomes negative.
• When positive half cycle of the supply complete and negative half cycle starts, the SCR T3 turns on by applying gate pulse but it does not turns on because the conduction of SCR T1 of positive converter continue due to inductive load. 
• The SCR T1 reverse biases the SCR T3. This will result in no voltage available at the load point during this half cycle. ( or we can say that the voltage across load becomes zero until SCR T1 does not turns off. ) 
• The voltage across load becomes zero when conduction of Cyclo converter shift from positive half cycle to negative half cycle. 
• This will generate distortion in the output voltage waveform and its depend upon firing angle of SCRs and power factor of load.

Effect of ratio of Input frequency and Output frequency

• When the ratio of input frequency and output frequency does not integer, the conduction of positive group SCR continue in the last half cycle but at that time conduction of SCR T3 or SCR T4 of negative group starts.
• This will result in short circuit and load voltage becomes zero. 
• The time duration of short circuit is less than half cycle of the input supply. 
• The inductor is connected at the input side in order to limit the short circuit current.

Single Phase to Single Phase bridge configuration

• The center tapping transformer is not required in this configuration. The SCR T1 to SCR T4 works as positive group ( P – converter ) and SCR T5 to SCR T8 works as negative group ( N – converter ). 
• If the P – converter and N – converter conducts simultaneously, the supply is short circuited. 
• It should be noted that when P – group SCRs conducts, the N group SCRs remains in off condition and vice versa. 
• The output voltage becomes symmetrical if the firing angle of the both converter SCRs are kept same.

Operation : Positive Group

• The SCR T1 and SCR T3 are turned on during positive half cycle of the alternating supply resulting load current flows through path A – SCR T1 – LOAD – SCR T3 – B. 
• The SCR T1 and SCR T3 naturally turns off due to negative half cycle of the alternating supply. 
• When SCR T2 and SCR T4 are turned on, the load current flows through path B – SCR T2 – LOAD – SCR T4 – A.  
• The load current flows through supply to load during positive half cycle of the alternating supply. 
• There are two positive half cycle at the output due to one positive and one negative half cycle of the alternating supply.

Negative Group

• The SCR T5 and SCR T7 are turned on during positive half cycle of the alternating supply and load current flows through path A – SCR T7 – LOAD – SCR T5 – B. 
• The direction of load current in the N – converter is opposite to that of current flow in the P – converter. 
• The SCR T5 and SCR T7 naturally turns off due to negative half cycle of the alternating supply. 
• When SCR T6 and SCR T8 are turned on, the load current flows through path B – SCR T8 – LOAD – SCR T6 – A. 
• The direction of load current reverses and there are two negative half cycle at the output due to one positive and one negative half cycle of the alternating supply at the input.

• The input and output waveforms of the bridge configuration Cycloconverter is shown in the figure. 
• When there are two complete cycles at the input sides, there is only one complete cycle at the output side resulting we can say that the output frequency is one half to that of input frequency. 
• If the input frequency is 50 Hz, the output frequency will be 25 Hz. 
• The waveform of the output voltage is adjusted by adjusting firing angle of the SCRs.

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Principal of Cyclo converter

Cycloconverter

• It is an apparatus or device which convert constant AC voltage, constant frequency input supply into variable voltage, variable frequency supply.

Principle of operation

• The Cycloconverter is made of two converters in which one converter works as a positive converter and second converter works as negative converter. The firing angle of both converters are set such that
• α_P +α_N  = π

        Where

• α_P = Firing angle of positive converter and
• α_N = Firing angle of negative converter
• The firing angle of both converters is set such that it will change the output frequency. 
• The P – converter and N – converter is made by series connection of variable alternating supply and diode. 
• The output voltage of each converter becomes equal in the P – type converter and N – type converter of the ideal Cycloconverter. 
• The output voltage at the terminal of the Cycloconverter is either of the alternating voltage of the converter because each converter has equal alternating voltage amplitude, phase as well as frequency.

                        

• The power flows from either load to converter to converter to load by controlling the firing angle of each converter. 
• The unidirectional property of the P – converter and N – converter results in P – converter works in positive half cycle of alternating supply and N – converter works in negative half cycle of alternating supply. 
• It does not depend upon phase between load voltage and load current.

Effect of Load Displacement On Working of Cycloconverter

• The voltage and current waveform for the ideal Cycloconverter for different load displacement is shown in the figure B. ( Displacement angle – It is angle between fundamental current and voltage of load current.)

Load displacement angle 0^o

• The current flows through converters when both converters are operated in the rectifier mode. 
• Figure B shows waveforms for each converter.

Load displacement angle 180^o

• Both converters operate in inverting mode when the load displacement angle is 180^o. 
• As the load is regenerative, the power flows from load terminal to output terminal of Cycloconverter.

Load displacement angle 80^o lagging

• The converter operates in the rectifier mode during 100^o of the positive half cycle from its first zero. 
• The power flows from Cycloconverter to load during this mode. 
• The converter operates in the inverter mode during 80^o of the remaining positive half cycle of the alternating supply.

Load displacement angle 80^o leading

• The converter operates in the inverter mode during 80^o of the positive half cycle from its first zero. 
• Similarly, the converter operates in the rectifier mode in the 100^o of the remaining positive half cycle.

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