SOLUTION

The triac is a three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on. The triac also differs from the SCR in that either a positive or negative gate signal trigger it into conduction. Thus the triac is a three-terminal, four layers, directional, semiconductor device that controls ac power whereas an SCR controls dc power or forward biased half cycles of ac in a load. Because of its bidirectional conduction property, the triac is widely used in the field of power electronics for control purposes.

In Figure a, the control circuit of triac can be adjusted to pass the desired portions of positive and negative half cycle of ac supply through the load R_L.

In Figure (b), the triac passes the positive half-cycle of the supply from θ₁ to 180° i.e. the shaded portion of the positive half cycle. Similarly, the shaded portion of the negative half cycle will pass through the load. In this way, the alternating current and hence ac power flowing through the load can be controlled..

Why triac is not used for inductive loads?

1. For inductive load, the triac must be turned off when the voltage becomes zero in the positive half-cycle before the negative voltage is reapplied to triac. This negative voltage also forwards biases triac for a negative half cycle.
2. With inductive load, the triac conducts after π and due to the application of negative voltage in-next cycle, it does not turn off at all.
3. Thus the commutation of triac is not successful for inductive loads.
4. For inductive loads, it would be necessary to replace triacs with antiparallel SCRs when firing angle α increases and current continues to conduct longer duration after it.