User`s manual

7800 Series Hysteresisgraph System User's Manual
Theory of Operation 2-1
CHAPTER 2
THEORY OF OPERATION
2.0 GENERAL
This chapter provides a theory of operation relating to the operation of the Lake Shore 7800 Series
Hysteresisgraph System. An introduction is provided in Paragraph 2.1, theory of operation in Paragraph 2.2, and
yoke and magnetization conditions in Paragraph 2.3.
2.1 INTRODUCTION
The 7800 Series Hysteresisgraph measures basic DC properties of permanent magnet materials when a magnetic
field is applied to a test sample in a closed magnetic circuit. The closed circuit is created when the sample (and
concentric-coil sensor) are placed in the electromagnet air gap and the iron pole caps are brought into contact with
the sample; the yoke frame functions as a flux return path. Application of current from the bipolar power supply
generates the applied field, H, in the electromagnet gap. Software for the 7800 Series controls the applied field by
using changes in B flux data and H field data for feedback control.
The Lake Shore B, H concentric coil sensor is designed for the greatest possible accuracy in hysteresisgraph
measurement. The inner B coil measures flux from the sample, while two outer H coils in series opposition
measure the applied field that surrounds the sample. Proximity of the sensing coil array to within 2 millimeters of
the sample surface provides the most accurate loop shape determination possible. Sensing coil output is a time-
varying voltage, –v(t). Two Lake Shore Model 480 Fluxmeters process these signals by electronic integration to
determine total flux, Φ. Area compensation of the B channel and similar processing of the H channel determine
flux density and applied field, respectively:
change in
flux density
area
compensation
integrated
coil voltage
compensation for H flux
linked by B coil
Symbol Unit Definition
∆B
T Change in magnetic flux density
N
B
– B coil turns
A
sample
m
2
Area of sample
A
coil
m
2
Area of B coil
t
1
, t
2
sec Start and end time
µ
0
H/m Permeability of free space
H A/m Applied field measured independently
The Model 480 Fluxmeters feature automatic drift compensation for low drift in the acquisition of B and H
signals. This simplifies the measurement process and eliminates the need for operator intervention in the drift-
nulling process. A Lake Shore Model 450 Gaussmeter and Hall effect probe provide an alternate method for
measuring applied field, H. Magnetic induction, B, plotted against the varying applied field, H, defines the normal
induction hysteresis loop; B
i
or J is calculated from the B curve and plotted against applied field, H, to define the
intrinsic induction loop. Recoil loops and other functions can also be plotted. In the 7800 Series, advanced sensing
coil design, high-performance electronics, and innovative system integration provide unsurpassed flexibility,
versatility, and ease of use in magnetic hysteresisgraph measurement.