Page 637 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 623

r

r
c
c

m

r

m
c m
c
c c

r
c

P1 - Load line
Br
@20 °C
No permanent Ba
loss @140 °C
P2 - load line Bb

Bc
Knee point
Be
Bd
Permanent loss c
of magnetization
due to knee point

c
(d)
FIGURE 8.13: Magnetic properties of ferromagnetic materials are not linear. (a) In soft
magnetic materials,the B and H are small, hence they have small hysteresis. They retain a
c
r
very small portion of their magnetization when the external field is removed. They are
temporarily magnetized, not permanently. (b) In hard magnetic materials,the B r and H are
c
large (with large hysteresis) which means the material retains a large magnetization after
external field is removed. This is the case for permanent magnetic materials. (c) Permanent
magnets (PM) in electric actuator applications operate primarily in the second quadrant of the
B-H curve. Rare-earth PMs (i.e., samarium-cobalt mixtures, neodymium mixtures) have almost
linear characteristics in the second quadrant. (d) Temperature effect on demagnetization curve
of permanent magnets and the “knee point. Source: Adapted from www.hitachi-metals.co.jp/e.”