Operation of an AC-AC converter with inductive load
Operation of an AC-AC converter with Inductive load:
Fig: Operation of a single phase PAC with an inductive load
Fig:Load current for a single phase AC-AC converter with a R_L load. Vs -supply voltage, iss -steady state current component , itr - transient current component and iload - load current (= iss itr).
- The current builds up from zero in each cycle. It quenches not at the zero crossing of the applied voltage as with the resistive load but after that instant.
- The supply voltage thus continues to be impressed on the load till the load current returns to zero. A single-pulse trigger for the TRIAC or the anti-parallel SCR has no effect on the devices if it (or the anti-parallel device) is already in conduction in the reverse direction.
- The devices would fail to conduct when they are intended to, as they do not have the supply voltage forward biasing them when the trigger pulse arrives.
- A single pulse trigger will work till the trigger angle α > φ, where φ is the power factor angle of the inductive load. A train of pulses is required here. The output voltage is controllable only between triggering angles φ and 180o.
- The load current waveform is further explained in Fig. 26.6. The current is composed of two components. The first is the steady state component of the load current, iss and the second, itr is the transient component.
- With an inductance in the load the distinguishing feature of the load current is that it must always start from zero. However, if the switch could have permanently kept the load connected to the supply the current would have become a sinusoidal one phase shifted from the voltage by the phase angle of the load, φ.
- This current restricted to the half periods of conduction is called the 'steady-state component' of load current iss. The 'transient component' of load current itr, again in each half cycle, must add up to zero with this iss to start from zero. This condition sets the initial value of the transient component to that of the steady state at the instant that the SCR/TRIAC is triggered.
- When a device is in conduction, the load current is governed by the equation
- The instant when the load current extinguishes is called the extinction angle, β. It can be inferred that there would be no transients in the load current if the devices are triggered at the power factor angle of the load. The load current I in this case is perfectly sinusoidal.