Page 32 - Industrial Power Engineering and Applications Handbook

Theory, performance and constructional features of induction motors 1/13
(u) Eddy current loss, L,: magnetism, @ or B. For instance, at no current, i.e.
Because of the skin effect when H = 0, there would still be a residual magnetic
field in the circuit as indicated by ob and this would
t,z .f? .B? account for the hysteresis loss.
L, = (1.12)
P See the curve abcda in the shape of a loop, which
i.e. = B', other parameters remaining same. is drawn at different values of H after the magnetic
core was magnetized up to its saturation level oa.
where This is known as a hysteresis loop and the magnetic
,f = supply frequency area represented by daebd as the energy stored. This
t, = thickness of steel laminations. To reduce such energy is not released in full but by only a part of it
losses, particularly in large motors, it is imperative (bae) back to the magnetizing circuit when the
to use thinner laminations. To economize on the magnetic field H (or current) is reduced to zero. The
size and bulk of windings, because of the skin loss of this energy (dab) is termed hysteresis loss
effect, motor manufacturers adopt the method and appears as heat in the magnetizing circuit, i.e.
of transposition when manufacturing stator the stator and the rotor of an induction motor. This
windings. See also Section 28.8.4. loss may be attributed to molecular magnetic friction
B = flux density (magnetostriction) and is represented by
p = resistivity of the steel laminations.
Lh = f. (B,)'.s to (1.13)
(h) Hysteresis loss, Lh i.e. = (B,)' '
To understand this, consider the rotor as magnetized
up to a certain level u, as illustrated in Figure 1.10. Eddy current and hysteresis losses are complex
When the current (i.e. flux 9 or flux density B since quantities and can be estimated in a laboratory, on no
I = 4 B 0~ H etc.) is reduced to zero, the magnetic load, in the form of power input, less I,,,?.R and friction
circuit would still have a quantum of residual and windage losses etc. as shown in Section 11.5. Based

Non-linear mag-
Linear mag- netization (tending
netization to saturation)
----- &I--
t

-H 0 +H
H (-I,,,) Amp/metre
f-
Intensity of magnetlc held-
Figure 1.10 Hysteresis loop and magnetizing curve illustrating hysteresis loss