Single phase fully controlled half wave converter:
The circuit diagram of single-phase half-wave controlled rectifier with RL load and without freewheeling diode is shown in Fig. (a). The SCR T is forward biased only during positive half cycle whereas reverse biased during negative half cycle. Therefore, it is triggered in positive half cycles only. When the gate pulse is applied in positive half cycle with delay angle of as shown in waveform diagram (b), the SCR conducts and starts to carry the load current. Since the load is inductive (RL), the current lags behind the voltage. The load inductance maintains the load current and keeps SCR on even if the supply voltage is reversed. Thus every positive half cycle of load voltage is followed by some negative voltage till the current drops to zero. The negative voltage appearing across load reduces the average load voltage. For some sensitive loads, the negative voltage is undesirable. In such cases freewheeling diode is used to prevent the negative voltage across the load. Thus the use of freewheeling diode helps to increase the average load voltage.
Mode 1: (0 to α) (+ve half cycle) SCR anode is positive w. r. t. cathode but gate pulse is not applied therefore SCR is in off state though it is forward biased. The load current is zero & load voltage is also zero.
Mode 2: (α to π) (+ve half cycle + gate signal is applied at α) SCR is forward biased and gate signal is applied, therefore SCR turns on at α.
When SCR is triggered the load current will increase in a finite time through the inductive load. The supplied voltage from this instant appears across the load. Due to the inductive load the increase in current is gradual, energy is stored in inductor during α to π .
Mode3: (π to 2 π) (negative half cycle) During this part of negative half cycle, current continues to flow but falls and finally becomes zero when the energy stored in the inductance is dissipated in the load resistor and a part of the energy is feedback to the source, Hence due to energy stored in inductor, current continues to flow up to instant ωt = β. Thus during interval from ωt = π to ωt = β, the negative supply voltage appears across the load. When the load current becomes zero, due to negative supply voltage, the SCR is turnd off. At instant ωt = π+ α , when again pulse is applied, the SCR is turned on & the above cycle is repeated.