Bias/Manual Control; Bias Function; Introduced - Ametek DPA1 Series Installation, Operation And Troubleshooting Instructions

Operation
3-9.

BIAS/MANUAL CONTROL.

3-10.
The BIAS/ MANUAL control is located on the Firing Circuit Module. The control performs two
functions, bias and manual. The bias function is used when a non-zero (4 to 20 milliampere output span, for
example) based process controller is connected to the PHASE/AMP and is defined between the "BIAS MAX"
and "0" markings on the dial. The manual function is used when a process controller is not available or
desirable and is defined between the "0" and "MAN MAX" Markings on the dial.
3-11.

Bias Function.

3-12.
The bias function of the control establishes the point at which the PHASE/AMP will start delivering
power to the load when a process controller is connected to the unit. If, for example, it is desired that the unit
should deliver power only when the output from the process controller exceeds a given "x" milliampere level,
the control must be adjusted to provide an offset bias current of the "-x" milliamperes.
purpose of the bias portion of the BIAS/MANUAL control is to force a process controller which exhibits
nonlinearities in the low end of its output span to operate over the linear portion of its output span.
3-13.
To illustrate the effect of this offset bias on the output span established by the GAIN control see figures
3-2 and 3-3. In figure 3-2, no offset bias is introduced. In this case, the PHASE/AMP will turn on when the
output of the process controller exceeds 0 milliamperes and will deliver power to the load at a level
proportional to the process control output span. In figure 3-3, A–2 milliamperes of offset bias have been
introduced (by turning the BIAS/MANUAL control slightly counter clockwise from the "0" marking). The result is
an upward shift of the 12 milliampere output span, from the process controller, such that the PHASE/AMP will
not turn on until the output of the process controller exceeds 2 milliamperes.
Figure 3-2 Controller-Output Span with No Offset Bias Current Introduced
Figure 3-3 Controller Output Span With A –2 Milliamperes of Offset Bias Current

Introduced

16 AMETEK HDR Power Systems
Phase/AMP Series DPA1/DCPA1
In essence, the
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