Phase angle control is a thyristor firing technique that controls the conduction angle of an AC signal which in turn controls the average AC voltage across the load. By regulating the AC Voltage, the AC current drawn by the load is controlled. Thyristor Power Controllers incorporate Phase Angle Control Technique.

Zero Crossover control is a thyristor firing technique that varies the ON time and OFF time of the thyristor device. This is achieved by the means of PWM logic pulses generated by the electronic firing cards. This provides for a transient free, noiseless power control without any harmonic distortions. Thyristor Power Switches incorporate Zero Crossover Control Technique.

Power delivery is controlled by controlling the conduction angle of the AC signal. Only a portion of the AC voltage signal is allowed to pass through conducting Thyristor. This portion is proportional to the control signal received by the triggering card. The remainder of the signal is blocked. By varying the level of the signal allowed to pass through, the average voltage across the load gets regulated, which causes the load to proportionally draw the required current.

Phase angle control allows for very precise control of power delivered to the load. It also results in power saving as only the required current is utilized.

Libratherm recommends that Phase angle control / Thyristor power controllers be used in the following situation:

• Electrical power saving is a concern for the user.
• The system remains ON for a very long period of time.
• Consistency and stability of the process is of utmost importance.
• Power lines get burdened by frequent ON-OFF operations.
• The incoming voltage is not stable.
• Enhancing the life of heaters is a major concern.

Power control is achieved through varying the time period for which power is delivered to the load. The AC Signal is turned ON and OFF when the signal crosses the zero mark. This ensures that no transient noise is generated. The Zero crossover control card features a cycle time parameter which determines the period for which the Thyristor will conduct proportional to the control signal. (For a cycle time of 10 sec and 50% control signal, the thyristor will conduct for 5 sec and block for 5 sec).

Libratherm recommends that Zero crossover / Thyristor power switches be used in the following situation:

• Simple Temperature Control application.
• Frequent ON-OFF operations do not burden the power lines.
• Power factor is a major concern.
• Quickly replace existing contactor based solution.