BACKGROUND
Silicon Carbide heating elements (also known as "Glo-Bars") are often used for high temperature applications as they are relatively low cost. As with any load element there are limitations to their use, and this information sheet aims to provide insight into correct sizing for use with phase angle type controllers, as well as outlining the traditional control methods.
The traditional method of control for Silicon Carbide heating elements has been to use a step-down transformer with several voltage taps. These taps are used to correct for the increase in resistance, "aging" as it is called.
This method of control causes either full voltage or no voltage to be applied to the elements. When the elements are suddenly subjected to full voltage they may be subject to "thermal shock" due to the sudden application of full power. As the Silicon Carbide elements are brittle, thermal shock can be a significant problem, especially when the furnace is first started from the "cold" state. The thermal shock may cause reduced element life, or in extreme cases cause a fracture of the element
The main disadvantages of this method of control are:
LIFE CYCLE OF SILICON CARBIDE HEATING ELEMENTS
In general Silicon Carbide heating elements are gradually oxidised to produce silica, causing "aging", as its electric resistance increases due to silica being an insulator. This oxidisation occurs at about 800 °C and is accelerated by increased temperatures. For EREMA elements of types D3, E, F & U the expected life cycle will be reached when their resistance has increased by a factor of 3. For types SG & SGR this factor is 1.7 due to their special design.
The expected life of Silicon Carbide heating elements also decreases more rapidly when furnace temperatures exceed 1400 °C for types D3, E & F and 1600 °C for types SG & SGR.
It is therefore important to keep the temperature differential between the element and the furnace as low as possible, so as to obtain the maximum life from the elements.
It is also important to not mix new with old elements where possible due to their mis-matched resistances. The "new" element being of a lower resistance will tend to "use" most of the voltage and will be subject to accelerated "aging".
PHASE ANGLE CONTROL
Phase Angle Control uses SCR’s (or thyristors) to control the voltage to the load, and works by varying the point in the AC mains cycle where the SCR’s are turned on (see Figure 1). Many furnace manufacturers have successfully used a Phase Angle Power Controller with Silicon Carbide heating elements.
The Phase Angle Power Controller should have a "POWER LIMIT" function.
Figure 1
PHASE ANGLE CONTROL WITH SILICON CARBIDE ELEMENTS
Silicon Carbide heating elements are normally quoted with a "Starting Voltage" which is normally much lower than the mains supply voltage. The Silicon Carbide heating elements also increase their resistance by a factor of up to 3 during their life.
The impact of this is that when the elements are new, they will draw a larger current from the mains supply, as the power to the furnace is being supplied at a lower voltage.
If the starting voltage of Silicon Carbide heating element arrangement used is selected such that it is approximately the available mains supply voltage divided by a factor of between 2 to 3, then at the end of the life of the Silicon Carbide heating elements when they have a higher resistance, the voltage required for full furnace power will be the full mains supply voltage.
This optimises both the size of the Phase Angle Power Controller and the size of the mains supply.
If the Phase Angle Power Controller used has a "Power Limit" function, then the Controller will automatically limit the maximum power supplied to the Silicon Carbide heating elements, irrespective of the level of the control signal. This provides the two major benefits of protecting the Silicon Carbide heating elements from an applied power in excess of their W/cm2 maximum rating, and of automatically correcting the voltage to the Silicon Carbide heating elements as they age, a major drawback of traditional control methods.
As opposed to "on / off" contactor style control, Phase Angle Control applies a continuously variable voltage to the load. The problem of "thermal shock" due to the sudden application of full power is effectively removed as the power to the load is only applied slowly.
The benefits of Phase Angle Power Control are:
Silicon Carbide heating elements should never be used with "Time Proportioning" or "Burst" mode controllers unless a voltage matching transformer is used. The "Time Proportioning" or "Burst" mode controller should always be on the secondary side of the transformer.
The disadvantages of Phase Angle Power Control are:
The Phase Angle Power Controller (3 phase) may be run in "6-wire" configuration so as to allow a reduced size and cost of controller to be used. The disadvantage is that more cabling to the load is required.
Contact Practical Control Solutions for further information.