McMurry Bedford Commutated Half Bridge Inverter (Complementary Commutation)

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McMurray Bedford Commutated Half Bridge Inverter (Complementary Commutation) 

Q. Sketch circuit diagram of McMurry Bedford Inverter. How commutation of SCA is done?

Circuit diagram of McMurry Bedford Commutated Half Bridge Inverter :

  • The single phase half bridge Inverter using Mc Murry Bedford commutation is as shown in Figure A. The operation of the commutation circuit is explained for the half bridge configuration but the same principle of operation applies for the full bridge configuration as well.
  • This type of commutation is also known as complementary commutation or voltage commutation.
  • The SCRs S1 and S2 both are main SCRs. C1,
    C    2 are the commutating capacitors L1 and L2 are identical closely coupled coils and D1 and D2 are the feedback diodes.

Operation of McMurry Bedford Commutated Half Bridge Inverter :

Mode I (t0 – t1) : 

  • In the beginning of mode I, S1 is turned on. The load current flows from the upper half of the dc supply through S1, IL load as shown in Figure B.
McMurry Bedford Half Bridge Circuit
Figure A

    McMurry Bedford Half Bridge Circuit Mode I
    Figure B
  • This current produces a positive voltage across the lone SCR S2 is reverse biased. 
  • The voltage across the commutating inductances almost 0. The load voltage is therefore positive equal to V / 2. The voltage on capacitor C2 reaches  end of this interval +V at the end of this interval.

Mode II (t1 – t2) : 

  • At instant  t1, SCR S2 is turned on. This will apply the voltage across C2, directly across the inductance L2.
  • Thus VL2 = V. The capacitor C1 discharges through L2 and S2 as well as the load to maintain the load current.
  • The current through L2 will induce a voltage into L1 with the polarities as shown in Figure C . 
  • The capacitor C1 charges through L2 and S2  as shown in Figure C.
McMurry Bedford Half Bridge Circuit Mode II
Figure C
McMurry Bedford Half Bridge Circuit Mode III
Figure D
Mode III (t2 – t3) :
  • When VC1 = V and VC2 = 0 the capacitor discharge current IC2 drops down to zero.
  • However the current through will be maintained due to self induced voltage into it as shown in Figure D.
  • The return path for this current is through D2. The load is inductive in nature. So it will try to maintain the current through the load by inducing a self induced voltage in the load.
  • The stored energy in the inductive load is then fed back to the lower half of the dc source. (See Figure D) through diode D2

Mode IV (t3 – t4) :

  • When the stored energy in is completely dissipated, IL2 = 0. This will turn off SCR S2.
  • But the energy feedback from load to lower half of the dc supply continues through D2.
  • The load voltage is negative – V / 2 but the load current remains positive.
  •  After this SCR2 can be triggered to reverse the direction of the flow of current.
  • The sequence of events continues as explained for the first half of the total period of the output waveform.
    McMurry Bedford Half Bridge Circuit Mode IV
    Figure E
Volatge and Current Waveforms for McMurry Bedford Inverter
Figure F
McMurry Bedford Full Bridge Inverter
Figure G
  • SCR2  can be turned off by turning SCR1 on. The SCR2 again will be turned off due to reverse voltage appearing across it. So this is known as the voltage commutation.
  • The advantage of this type of commutation circuit is that it does not require any additional SCRs for commutating the main SCRs.
  • The same McMurry Bedford commutation can be used for the single phase bridge inverter as shown in (Figure G).

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