Power control range solves SWIR dilemma

Edited by the Engineeringtalk editorial team May 29, 2002

CD Automation has developed a new digital family of solid-state relays and thyristors for powering short-range infra-red lamps.

Short-wave infra-red (SWIR) lamps are commonly used in the heating and processing of such materials as plastics, glass, metals, ceramics and paper.

SWIR lamps have the advantage of transmitting high temperatures to localised areas with minimal heat dissipation.

Due to the lamp's low inertia it can respond very quickly to power load demands and provides a safe, cost effective means of heat treatment.

The lamp's low inertia however also causes the main problem of using SWIR elements, that of high initial inrush current when cold.

When the lamp is cold it exhibits low resistance which results in currents of up to six times the nominal being drawn, without attention this phenomenon could cause the controlling thyristor's fuse to fail.

The common solution to this problem is to employ a power attenuation technique called phase angle thyristor firing that controls the power by adjusting the portion of the AC supply cycle allowed for conduction.

The more power required the more the conduction angle is advanced until 100% of the cycle is conducting power.

In phase angle mode the first complete cycle could cause a current surge and blow the fuse, so a current limiting technique must also be employed.

Current limiting allows the phase angle proportioning to increase from zero until a preset current limit is reached, then the angle is regulated to keep the current within safe limits.

Using phase angle with current limiting is an effective method of controlling SWIR lamps but has four main drawbacks.

Interference and harmonics could be generated by the rapid switching in mid-cycle which may require filtering to meet with local regulations.

Due to the inertia of the current transformer and other associated passive components in the current limit circuitry it can take up to 100ms for this current protection to become effective.

The basic principal of phase angle is to reduce the voltage from the thyristor, which in turn reduces the energy to the lamp and increases the time taken to reach operating temperature.

Abnormal current levels will remain until the lamps resistance increases with temperature; therefore reducing the voltage results in prolonged elevated currents.

For the lamp to operate effectively it needs to be switched at full power and as quickly as possible to ensure the element does not get cold between cycles.

As you can see from using phase angle and current limit this cannot be achieved.

CD Automation has used its 20 years experience in power control design and manufacture to develop a new digital family of solid-state relays and thyristors.

After extensive testing and experience in the field of SWIR lamps the company has designed a range of products that reduce the level of inrush current and allows the user to select the firing mode that best suits the application.

A computerised sizing programme has also been developed that enables Paragon Alliance to specify the correct product that will overcome the complexities of controlling SWIR lamps, while maximising their efficiency.

(This was Engineeringtalk's Top Story on 28 May 2002).