Functional Description; Basic Operating Principle; Signal Voltage Generation - ABB COPA-XM 3000 Series Instruction Manual

Series 10DX3000 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.
The Flowmeter 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 10D1475 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 flowmeter
constitutes a modified form of a generator.
FIGURE 4-1. BASIC OPERATING PRINCIPLE
Figure 4-1 graphically illustrates the
basic operating principle. A magnetic
field, "B", is being generated in a plane
which is perpendicular to the axis of the
meter pipe. A disk of the metered liquid
can be considered as a conductor. The
transverse length "D" is equal to the
meter pipe diameter. Since the velocity
"V" of the liquid disk is directed along
the axis of the meter pipe, a signal volt-
age, "E ", will be induced within this
s
liquid which is mutually perpendicular
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 volt-
age is the summation of all incremental
voltages developed within each liquid
particle that passes under the influence
of the magnetic field.
4-1