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Phase/AMP Series DPA1/DCPA1
3-14.
Manual Function.
3-15.
The manual function of the control establishes the point at which the PHASE/AMP will start delivering
power to the load when a process controller is not connected to the unit. Output power is delivered to the load
by turning the control counter clockwise from the "0" marking on the dial. As the BIAS/MANUAL control is
advanced, power delivered to the load will increase. Full power will be delivered to the load when the control
has been rotated approximately 135 degrees from the "0" marking. Once the output power level has been set
in the manual mode, it will remain constant (assuming the load impedance does not vary) until a change in
control setting is made.
3-16.
OPTIONAL CONTROLS, INDICATORS, AND DEVICES.
3-17.
Optional devices and indicators consist of the Current Limit device, Current Indicator, and Voltage
Indicator. The operation of these devices and indicators is discussed in the paragraphs listed below.
Optional Current Limit device, paragraph 3-18
Optional Current Indicator, paragraph 3-25
Optional Voltage Indicator, paragraph 3-27
3-18.
OPTIONAL CURRENT LIMIT DEVICE.
3-19.
The optional Current Limit device is bolted to the panel of the PHASE/AMP. The device contains a
CURRENT LIMIT control which is used to establish the value of output current at which limiting action will
commence. Any value between2 5 percent and 125 percent of the rated current output can be selected by the
CURRENT LIMIT control. Generally, the Current Limit device is used in application where the PHASE/AMP is
required to drive a load exhibiting a large positive or negative temperature coefficient. This type of load may
demand current in excess of the rated maximum value of the PHASE/AMP at some point on the
temperature/resistance characteristic curve.
3-20.
Loads used in furnace and heating applications may utilize alloys of elements possessing a positive
temperature coefficient. Molybdenum (Mo), Platinum (Pt), Tantalum (Ta), and Tungsten (W) all fall within this
category.
Figure 3-4 illustrates the comparative resistivity of three of these metals as a function of
temperature. The resistivity/temperature characteristic of a typical Iron (Fe) base alloy is also shown in Figure
3-4 for contrast. The Iron based alloys commonly exhibit temperature coefficients very close to zero.
Molybdenum Tungsten, and Platinum exhibit much lower resistivity at 20 degrees C. than at 1000 degrees C.
Heating elements comprised of these metals will constitute a variable load which will demand more current at
low temperatures than at high temperatures. If a load of this type will draw more than 125 percent of the rated
output current from the PHASE/AMP, when cold, the optional Current Limit device should be used.
3-21.
A similar situation exists for load exhibiting a negative temperature coefficient. Figure 3-5 shows the
resistivity/temperature characteristic of a typical silicon carbide heating element. As temperature increases,
the resistance of the load (up to a minimum) decreases. If the resistance decreases to the point where the
current output drawn by the load exceeds 125 percent of the rated current output of the PHASE/AMP, the
optional current Limit device is required. The Current Limit will protect the unit from overload while the heating
element is coming up to temperature.
Document 9100030
AMETEK HDR Power Systems
Operation
17
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