Basic Operating Principle; Signal Voltage Generation; Magnet Coil Drive Circuits - ABB CK-MAG 10D1477 Instruction Manual

10D1477 MAGNETIC FLOWMETER INSTRUCTION MANUAL

4.1 Basic Operating Principle

4.1.1 Signal Voltage Generation

The operating principle of the Model 10D1477 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 graphically illustrates the basic operating principle. Consider magnetic field, "B", being
generated in a plane which is perpendicular 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
will be induced within this 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 voltage is the summation of all incremental voltages developed within each liquid particle
that passes under the influence of the magnetic field.
This may be expressed mathematically as:
(Equation #1)
= 1 BDV
E
s
α
where,
E = induced electrode voltage
s
B = magnetic field strength
D = meter pipe diameter
V = liquid velocity
α = dimensionless constant
The metered liquid constitutes a continuous series of conductive liquid discs moving through a
magnetic field. The more rapid the rate of liquid flow, the greater the instantaneous value of signal
voltage as monitored at the meter electrodes.

4.1.2 Magnet Coil Drive Circuits

In most conventional magnetic flowmeters the integral magnet coils are driven directly by the
customer's 50/60 Hz power service. The design of the Series 10D1477 magnetic flowmeter uses
magnet drive circuits which are alternately energized bi-directionally at a low frequency rate as
commanded by the associated Converter/Driver assembly.
4-2
",
s