Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 58

powerful, having an energy product (BH) max from 200 to about 400 kJ m − 3 . They
also possess a high remanence Br, a relatively high coercivity and they are less
expensive than Sm-Co magnets; however, their cost is much higher than that of
other types of magnets.

3.2.1.5 Sm - Co Magnets

Their main characteristics are a high remanence Br, a high coercive eld H c and a

relatively high energy product, which varies between 140 and 250 kJ m − 3 . They are
the most expensive permanent magnets. They are less temperature sensitive than
Nd

B magnets, but very high temperature ranges are not of interest in magnetic
refrigeration or heat pumping. However, it may be very important in power-gen-
eration applications. These are the reasons why Sm

Fe

- -

Co magnets have not been
applied in any of the prototype magnetic refrigeration devices.
More information about permanent-magnet materials can be found in Cullity and
Graham [ 11 ] and Campbell [ 14 ].

-

3.3 Electromagnetic Coils

These types of magnets require an electric power source for their operation. They can
be divided into electromagnets, superconducting magnets, and the special domain of
electric pulsed magnets. In this section, we will brie

fl y describe the rst two types.

3.3.1 The Electromagnetic Coil

An electromagnet consists of a soft iron core surrounded by a number of turns of
an electric wire (see also Fig. 3.2 a). Figure 3.18 shows the case of an empty “ gap ” ,

surrounded by a single electric wire.
In the case of Eq. ( 3.3 ) we have de ned the magnetic eld at the centre of the

coil. Based on Fig. 3.18 and Eq. ( 3.1 ) we can de ne the magnetic fl ux density in the

axis of the circular coil with the steady current.
First, we will de ne the position vectors to be in the following relation:

r ¼ z a ¼ z k a cos u i a sin u j ð 3 : 69 Þ

where:

1
2

a 2 z 2 jj¼ ~ r r ¼ þ ð 3 : 70 Þ