Carbolite STF Series - Manual (Page 9)

STF 14-16
LMF08 – 1.13
9
4.5 Power Adjustment
The furnace control system incorporates electronic power limiting. Power is supplied to the
elements in bursts of about 1/3rd second duration. This prevents overheating of the elements. The
power limit is programmed into the furnace controller and may be calculated as a percentage as
follows:
powerlimit = (100 x targetpower x ohms) / volts
2
where targetpower is the wattage given on the furnace rating label
ohms is the overall resistance of the combined element circuits (see example)
volts is the voltage across each element circuit (e.g. 240 for a 415/240V supply)
example:
2-phase furnace with 3 elements of 2.9 ohms each in series per phase, running on
380/220V, with a target power of 7000W:
ohms = 2.9 x 3/2 = 4.35
powerlimit = (100 x 7000 x 4.35) / (220 x 220) = 63%
A table of power limit settings is given in section 7.7. These are the settings for the furnace with
new elements as supplied.
Occasionally the power limit is set to zero to permit demonstration of the controls without the
heating elements taking power. In this case the power limit is accessible to the operator and may
be reset to its standard value or the value that it had before setting to zero.
4.6 Element Aging
Silicon carbide elements gradually increase in resistance with use: a process known as aging. Their
heating power reduces correspondingly. To ensure sufficient power to the furnace it may be
necessary to adjust the power limit, and, ultimately, to replace the elements - taking care to
readjust the power limit.
If the furnace does not reach temperature, or is slow, measure the element resistances and calculate
the required power limit using the method given above. If it exceeds 100%, the elements have too
high a resistance and need replacing: check your data and calculations with Carbolite.
See the separate manual supplied for how to adjust the controller. Always record the setting when
first making an adjustment. If new elements are fitted, return the adjustments to the original value,
or remeasure and recalculate.