Circuit Description - ABB MAG-X 3000 Series Instruction Manual

INSTRUCTION MANUAL 10DX3111A&G Sizes 14 - 24 Inches

5.0 CIRCUIT DESCRIPTION

Flowmeters of the pulsed DC type operate on the principle that unwanted electrode signals occur
while the magnetic flux is changing. Accordingly, the signal converters have been designed to
capture the electrode voltage only during that portion of the excitation cycle when the magnetic flux
is constant. This interval occurs during the last 25% of each half excitation cycle. By using sampling
techniques, the flow (differential mode) signal is measured only during the intervals that magnetic
flux is constant.
 
dΦ
= 0
 
dt
 
Therefore, zero-instability due to changing flux is eliminated by use of the MAG-X design concept
(sampling technique), providing a meter totally free of zero drift. Pulsed DC operation of a magmeter
system eliminates those variables capable of causing drift of the meter zero point. A thorough
discussion of Signal Converter operation is provided in the Instruction Manual supplied with the
particular Signal Converter.
Models 10DX3111A & G flanged flowmeter primaries contain two flux producing coils wired in series
and a pair of diametrically opposed electrodes mounted at 90 degrees to the coil flux plane (refer to
FIGURE 5-1 below). Meter coils are excited with approximately +10 volts of pulsed DC. A precision
current sensing network is connected in series with the coils. The current sense network produces
what ABB Automation refers to as a "Reference Voltage", which is typically +70 millivolts. The
reference voltage is directly proportional to the strength of the magnetic field in the measuring tube
and is measured by the signal converter. The reference voltage must be measured, since any
variation in reference voltage will also produce a proportional change in electrode signal voltage,
assuming an unchanged flow velocity. The current sense (reference network) may be in the Primary
or the Secondary, depending on the model number and/or the design level.
FIGURE 5-1. SIMPLIFIED MAGMETER SYSTEM BLOCK DIAGRAM
5-1