Functional Description; Basic Operating Principle; Signal Voltage Generation - ABB K-MAG 10D1476 Instruction Manual

10D1476 K-MAG MAGNETIC FLOWMETER INSTRUCTION MANUAL

4.0 FUNCTIONAL DESCRIPTION

The Magnetic Flowmeter body houses two signal electrodes and the flux producing magnet coils, as
shown schematically in Figure 4-1. All primary intraconnection wiring is terminated at a printed circuit
assembly located in the base of the meter housing (see Figure 3-2).
The primary provides two output signals to the associated Signal Converter:
• an electrode signal that contains the flow rate information
• 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" resistance 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.

4.1 Basic Operating Principle

4.1.1 Signal Voltage Generation

The operating principle of the Model 10D1476 Magnetic Flowmeter is based upon Faraday's Law of
Induction which states that the voltage induced across any conductor as it moves at right angles
through a magnetic field will be proportional to the velocity of that conductor. This principle finds
common application in direct and alternating current generators. Essentially, the Magnetic Flowme-
ter constitutes a modified form of a generator.
FIGURE 4-1. BASIC OPERATING PRINCIPLE
Figure 4-1 graphically illustrates the basic op-
erating principle. Consider magnetic field, "B",
being generated in a plane which is perpen-
dicular to the axis of the meter pipe. Further,
consider a disc of the metered liquid as a
conductor; the transverse length, "D", is equal
to the meter pipe diameter. Since the velocity,
"V", of the liquid disc is directed along the axis
of the meter pipe, a voltage, "E
duced within this liquid which is mutually per-
pendicular to the direction of the liquid
velocity and the flux linkages of the magnetic
field; i.e., in the axial direction of the meter
electrodes. This electrode voltage is the sum-
mation of all incremental voltages developed
within each liquid particle that passes under
the influence of the magnetic field.
", will be in-
s
4-1