There are several methods commonly used to control power to electric heaters. Although some may seem “cheap and easy,” the total costs are not always considered.

MECHANICAL RELAYS/CONTACTORS

The up front cost for a simple relay is low on low power levels. As the power level goes up, the price rises. Since the mechanical relays will get dirty, arc and burn out, the relay has to be replaced every so many cycles. The labor cost associated with this is often overlooked. Due to the slow cycle time inherent for mechanical relays (30 seconds minimum) control is poor, heated process may be damaged and heater life is shortened due to thermal shock.

MERCURY DISPLACEMENT RELAYS

(MDR) The up front cost of a mercury displacement relay, or MDR, is low at low power. The MDR can cycle faster than a mechanical relay, and will not burn out as quickly. MDR’s do have to be mounted in one direction. However, the MDR will explode if overheated due to cycling too fast, or if overpowered. This results in a Haz-Mat problem. Control is still poor compared with solid state devices. The heater is still receiving full line voltage for several seconds at a time.

SOLID STATE RELAYS

(SSR) The costs of low power Solid State Relays (SSRs), is also low. However, there are several cost factors forgotten about with SSRs. Common to all solid state devices, they generate heat and have to have heat sinks. Also, SSRs generate more heat than SCRs due to their packaging, and come without lugs that make a solid electrical connection for higher power levels. The MOVs and fuses needed for protection against electrical shorts and spikes have to be customer installed. Again, this is a hidden cost. Finally, when looking at the SSR data, you must always DERATE THE DEVICE. Manufacturers of SSRs have derating curves published in their catalogs. However, please note that all SSRs are rated for the name plate power rating at 25ºC/77ºF ambient temperature. So a 50 Amp SSR is not good for 50 Amps if the temperature in the enclosure or work area is above 25ºC/77ºF (how hot is your facility in July?). The single surge current and voltage spike ratings are not as strong as an SCR. The SSR can cycle once a second and deliver better performance to both process and heaters than mechanical means. However, SSRs can only turn cycles on or off.

SILICON CONTROLLED RECTIFIERS

(SCR) The costs of Silicon Controlled Rectifier (SCR) Controls, is higher at low power, but level out with other methods at mid to high power. All Avatar SCRs come equipped with proper heat sinks, semi-conductor fuses and power lugs installed as an integrated package. With standard MOVs and DV/DT protection for line noise and voltage spikes, an SCR control can operate for decades. There is no routine maintenance to carry out. The level of control that can be achieved with a SCR is unattainable with any other devices.

Phase angle firing and zero voltage switched control offers 0.08 second response times on 60Hz power lines. Heater life can be extended by 7 times. Power to heaters is infinitely variable. Soft Start, Current Limit and Voltage Limit Options give advantages for controlling heaters. Soft Start allows the use of high inrush heaters. Current Limit gives control of high temperature elements for furnaces. Voltage Limit prevents temperature overshoot and extends heater life. Peak power is reduced, saving operators utility costs.

Still having trouble deciding the right power control for your application? Contact Avatar Instruments for help.

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