Solutions summary for inductive load control

AN308

12/16

A.1

Triggering synchronized across the TRIAC

See Figure 12 part A for the schematic diagram.

A.1.1

Resistive load

Current and voltage are in phase, good synchronization. No fault over the whole adjustment

range.

A.1.2

Inductive load

The current lags by

π / 2. Two cases should be considered:

●

Broad conducting angle; narrow lag angle.

The time separating two conducting periods is very brief. The positive and negative

currents are very similar. Certain applications are covered by this case (for instance

speed-control circuit for ac motors).

●

Narrow conducting angle; broad lag angle.

The flow of current in one direction is a function of the control and thus of the duration

of the current flow in the previous polarity.

The TRIAC can be triggered at the end of the mains half-cycle. In this case no current

flows through the circuit and it acts as a rectifier.

A.1.3

Circuit characteristics

Advantages of the circuit are as follows:

●

Connection by two wires without polarity

●

Low power dissipated by the passive components

●

Excellent power variation circuit for resitive or slightly inductive loads

●

With highly inductive loads, the circuit can only operate satisfactorily within the limits of

a slight decrease in the conduction angle.

Disadvantages of the circuit are as follows

●

For inductive loads, there is a large current dissymmetry for a variation towards the

narrowest conduction angles. For this type of application the circuit cannot be used at

all.