Carbolite nanodac - Manual (Page 7)

nanodac™
5
MC27 –EN–1.04
3. Cascade
3.1. P RINCIPLE OF C ASCADE C ONTROL
When cascade control is fitted, the Load Loop in the Nanodac senses the temperature of the
workload, and the Element Loop in the Nanodac senses the element temperature. Depending on
the oven or furnace model, the Load Thermocouple may be in a fixed position, or may be movable.
The Element Thermocouple is fixed in a predetermined position near the heating elements
If movable, the Load Thermocouple should be placed as near as possible to the work to be heated.
The Load Loop communicates with the Element Loop, calling for heat according to the load
temperature and the current program or setpoint. The Element Loop regulates the heat according
to element temperature and the requests from the Load Loop.
The element temperature could, in principle, vary between the furnace or oven maximum and
minimum, regardless of the working temperature of the load. In practice, the cascade control
system is configured to limit the element temperature to a band around the load temperature,
typically of ±50
0
C of the setpoint temperature.
The Load Loop is always accessible to the operator. The Element Loop may or may not be located
out of sight inside the control compartment. If the Element Loop is visible to the operator, the
display may be disregarded.
In a three-zone furnace for which the “three zone cascade” option has been ordered there are
three Element Controllers, all receiving the same control instructions from the Load Controller
3.2. O PERATION
The operator may ignore all the complications of the system, and program the Load Loop
according to the standard instructions for that controller. Home display is top view (Load Loop)
bottom view (Element loop). By pressing page, separate views of load and element loops will
show.
The Element Loop should never be adjusted. Note that the elements usually run at a higher
temperature than the load.
Because the details of the customer’s cascade application (in particular the nature of the load) are
generally not known, the feed forward parameter (Cascade Trim) is made easily accessible on the
cascade trim page. (Refer to instrument operating instructions). Its default setting is ±500 C. If
overshoot of the load temperature occurs – often a problem at low temperatures – then Cascade
Trim can be reduced in order to limit the amount of overshoot. If Cascade Trim is reduced too
much, however, the load may not reach the desired temperature. If the load does not reach the
desired temperature or is slow in the final stages, Cascade Trim can be increased: a maximum of
750 C is suggested.
When attempting to control at very low temperatures it can become a problem to achieve stable
control because of excessive power being supplied during heating. The available power can be
limited using the power limit parameter OP.Hi, and make a note of the original settings before
adjusting. The manual for the furnace or oven may also list standard settings for this parameter.
The parameter is normally set to 100% and contact Carbolite Engineering if this value needs to be
amended.
3.3. C AUTION
If the Load Thermocouple is removable, leave it in the furnace or oven chamber
whenever possible.
Operating with the Cascade Control active and with the Load Thermocouple in
an unheated position (such as lying on the work bench) may cause the furnace
or oven to heat up to its maximum temperature, wasting power and possibly
shortening element life.