Inverters

Saturday, April 11, 2009

THYRISTOR INVERTER

 

 THYRISTOR INVERTER

 

The principle and design of this kind of inverter is based on the class D commutation. The topology is also reliable and works on transformer less operation. This inverter circuit has the following advantages:

 

  1. Capability of a wide range of frequency variation.
  2. Capability of excellent voltage regulation
  3. Low commutation losses
  4. Low no load losses
  5. Transformer less operation and therefore higher speed of response as well as higher full load operating efficiency.

   

 

The circuit diagram is shown below

 

 

 Auxiliary Impulse Commutated Inverter

  As shown above, the circuit consists of two main thyristors SCR1 and SCR2 two auxiliary thyristors SCR3 and SCR4, the feedback diodes D1 andD2 and the commutating circuit components L and C. The load R1 is connected between the pole point P and centre tap of DC supply. In order to explain the detailed circuit operation it is assumed that the circuit has attained the steady state while operation. It is also assumed that thyristor SCR1 is conducting and the current is flowing through the load from right to left, through upper half of the DC supply (V/2). The capacitor C is charged to its maximum voltage Vcmax with right hand side plate being positive, as shown. Now any time SCR1 has to be turned -- off, the anode is fired. The circuit consisting of C, L, T1 and T2 oscillates to reverse the charge across C. Thus, the net current through SCR1 is now the algebraic sum of the load current and the oscillatory cycle current. If the oscillatory cycle current is more than the load current, for a period of time higher than the turn  off time toff  of the device, the thyristor SCR1 will turn off the diode D1 in the mean time becomes forward biased. However, in case the charge across the plate of the capacitor is not fully reversed and SCR1 has recovered its blocking capability, the diode D1 will allow the oscillatory current to flow, helping C to reverse the charge. While the thyristor SCR1 is still conducting SCR2 is fired. The load current now transfers to SCR3 via SCR1, C and L, thereby helping the capacitor to develop its full charge in reversing direction (-Vcmax). As and when C is fully charged, the current through SCR1 dies down to zero. Thyristor SCR3 therefore turns off by itself. The load current now flows through SCR4 via the lower half of DC supply (V/2) from left to right. Thus, the direction of current through the load is reversed.

 

Any time when SCR2 is to be turned off, SCR4 is fired. The capacitor, which has been fully charged with –Vcmax across its plates, reverses through SCR3, SCR2 and L; thus pushing the oscillatory cycle current through SCR2 in the direction opposite to the load current. The commutation process repeats similar to that already described for commutation of SCR1. While SCR2 is still conducting fire SCR4 so as to make up the loss of charge across C (if any) during commutation of SCR2. As the end of this process when C is fully charged to Vcmax, the thyristor SCR2 turns off by itself, leaving SCR1 to continue conduction of current through the load from right to left. Thus, one complete cycle of operation is completed.

 

This inverter is very popular in all industrial applications and is therefore very widely used for variable speed drives. 

 

 

MODIFIED SINGLE- PHASE Mc MURRAY INVERTER

  The auxiliary impulse commutated inverter (also popularly known as Mc Murray inverter) discussed in the preceding sub-section is one of the circuit techniques that is used to generate the pulse width Modulated (PWM) voltage. The basic circuit can be used as a building block in either single phase as half bridge (to generate a square waveform) or as full bridge (to generate a quasi square waveform). The main objective to use this circuit as a full bridge is to generate a quasi square or stepped load voltage wave, rather than square wave. The later is preferred in an attempt to reduce harmonics in the output waveform.

 

The full bridge consists of two identical inverters. While inverter 1 consists ofT1 and T2 as its main thyristors; inverter 2 has its main thyristors as T3 and T4. The two half bridges inverter 1 and inverter 2 are independently fired with a phase displacement between the two. The stepped load voltage waveform obtained at the output of bridge inverter is shown below. Since the load is connected between the two poles P1 and P2, the voltage across the load results into a stepped (quasi square) waveform with much lower harmonic content as compared to that of the square wave inverter.

 

 


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