Circuit Description; Primary Signals; Constant Meter Factor (Cmf) Pc Assembly - ABB K-MAG 10D1476 Instruction Manual

10D1476 K-MAG MAGNETIC FLOWMETER INSTRUCTION MANUAL

5.0 Circuit Description

5.1 Primary Signals

The Magnetic Flowmeter body houses two signal electrodes and the flux producing magnet coils, as
shown schematically in Figures 5-1. All primary intraconnection wiring is terminated at the
CMF/ZERO pc board located in the base of the meter housing.
The primary provides two output signals to the associated Signal Converter; one, an electrode signal
that contains the flow rate information, and two, the reference signal which is proportional to the
magnet excitation current. Theoretically, this reference signal is proportional to the flux density in the
metering section. The reference voltage is derived across a precision "constant meter factor" resis-
tance network that is connected in series with the magnet coils. Changes in magnet drive voltage,
which cause a variation of flow signal, will simultaneously cause a proportional variation of the
reference voltage. The circuitry will provide an exact ratio and thereby provide immunity to power
supply variation. The magnet coil drive circuitry is contained in the Signal Converter.
The (gated) magnet driver operates at a frequency that permits magnetic flux in the primary to reach
a steady state level during the last 50% of each half period of magnet excitation. By using sampling
techniques, the flow (differential mode) signal is measured only during the intervals that magnetic flux
is constant (dφ / dt = 0).
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. A thorough discussion of Signal
Converter operation is provided in the instruction manual supplied with the particular Signal Con-
verter.
5.2 Constant Meter Factor (CMF) PC Assembly (686B623)
A circuit schematic of the primary CMF board is shown in Figure 5-3. The CMF Assembly provides
several functions. These include:
1. Establishing interconnections between the Flowmeter internal wiring and the Converter.
2. Permitting adjustment of meter calibration factors to a fixed value for each nominal size primary.
3. Permitting adjustment of primary zero.
4. Establishing proper wiring connections for remotely mounted Converters.
5-1