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English

USE OF TRIACS ON INDUCTIVE LOADS By J BELLIN

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Niveau: Supérieur, Doctorat, Bac+8
APPLICATION NOTE AN307/0289 USE OF TRIACS ON INDUCTIVE LOADS By J. BELLIN ? Although triac circuits are now well known by desi- gners. The use of these components for inductive loads requires certain precautions which should not be neglectedof optimum use is to be made of them. That is the purpose of this article which reviews the various triac controlmodesandrecalls theprinciples which guarantee its correct operation. PHENOMENA OCCURING WHEN THE CIRCUIT IS CLOSED. The triac is known as a component which is essen- tial in controlling power from an AC source (mains). In most cases, the circuit has an inductive compo- nent : either because of thenatureof the load itself : motors, transformers, ballast inductance ; or be- cause of the source impedance : utilization of the secondaryofa transformer,lengthof the supply line, etc. On inductive loads, the operating conditions vary considerably, when closing the circuit, depend- ing on the control mode (gate current, polarity and width) and synchronization of the firing. In order to build an optimal control circuit it is indispensable to analyse the various possibilities. FIRING CONTROL SIGNAL The triac is fired by a gate current Ig > Igt whose du- ration should enable the main current to reach the triac holding current value (IL). The width of the control signal is determined by the rate of increase of the main current (di/dt), limited by the load inductance and by the choice of the firing quadrant.

  • zero mains

  • gate control

  • than previously

  • mains voltage

  • main current

  • voltage peak

  • firing


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APPLICATION NOTE
USE OF TRIACS ON INDUCTIVE LOADS By J. BELLI N Although triac circuits are now well known by desi- ing on the control mode (gate current, polarity and gners. The use of these components for inductive width) and synchronization of the firing. In order to loads requires certain precautions which should not build an optimal control circuit it is indispensable to be neglected of optimum use is to be made of them. analyse the various possibilities. That is the purpose of this article which reviews the FIRING various triac control modes and recalls the principles which guarantee its correct operation. CONTROL SIGNAL PHENOMENA OCCURING WHEN THE CIRCUIT The triac is fired by a gate current Ig > Igt whose du-IS CLOSED. ration should enable the main current to reach the The triac is known as a component which is essen- triac holding current value (IL). tial in controlling power from an AC source (mains). The width of the control signal is determined by the In most cases, the circuit has an inductive compo- rate of increase of the main current (di/dt), limited by nent : either because of the nature of the load itself : the load inductance and by the choice of the firing motors, transformers, ballast inductance ; or be- quadrant. cause of the source impedance : utilization of the The loading current, IL, is highest in the second secondaryof a transformer,length of the supply line, quadrant (A2positive with respect to A1, Ignega-etc. On inductive loads, the operating conditions tive) : (figure 1-a). vary considerably, when closing the circuit, depend-
Figure 1 :Width of Control Signal Required as a Function of the Firing Quadrant (a) ; width of Control Signal Required as a Function of the Moment of Firing (b).
Q uadrants Po larity A2 G with Respect to A1
AN307/0289
a
I I I III I V
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b
D89AN307-01
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