Basic Operating Principle; Signal Voltage Generation; Magnet Coil Drive Circuits; Figure 1-4. Basic Operating Principle - ABB MINI-MAG 10D1475 Instruction Manual

10D1475 MINI-MAG MAGNETIC FLOWMETER INSTRUCTION BULLETIN

1.2.1 Basic Operating Principle

1.2.1.1 Signal Voltage Generation

The operating principle of the Bailey-Fischer & Porter 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
Bailey-Fischer & Porter Magnetic Flowmeter constitutes a modified form of a generator.
Magnet coil
Metering
Measurement
electrode

FIGURE 1-4. BASIC OPERATING PRINCIPLE

Figure 1-4 graphically illustrates the basic operating principle. A magnetic field, "B", is being gener-
ated 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 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)
E s =
where:
E = induced electrode voltage
s
B = magnetic field strength
D = meter pipe diameter
α = dimensionless constant
V = liquid velocity
Thus, the metered liquid constitutes a continuous series of conductive liquid disks 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.

1.2.1.2 Magnet Coil Drive Circuits

In many conventional Magnetic Flowmeters the integral magnet coils are driven directly by the
customer's 50/60 Hz power service. The design of the Bailey-Fischer & Porter Series 10D1475
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.
INTRODUCTION
1
BDV
α
E = induced signal voltage
S
B = magnetic flux density
D = electrode spacing
= (actual meter pipe diameter)
v = average flow velocity of fluid
q =volume flow rate
v
~ B ⋅ D ⋅ v
E
S
2
π
D
⋅ v
q =
v
4
E ~ q
S v
",
s
1-3