Transcript Machine Losses - GTU e

Hysteresis
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When we increased the current we observed, saturation.
What would happen if I decrease the current after
saturation?
The flux for a given H is higher when decreasing
Hysteresis Losses
Figure shows the effect within
ferromagnetic materials known
as hysteresis.
We start with an unmagnetized
sample at the origin (P1) where
both field strength and flux
density are zero. The field
strength is increased in the
positive direction and the flux
begins to grow along the dotted
path until we reach P2. This is
called the initial magnetization
curve.
Hysteresis Losses….contd
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If the field strength is now relaxed then
some curious behavior occurs. Instead of
retracing the initial magnetization curve
the flux falls more slowly. In fact, even
when the applied field is returned to zero
there will still be a remaining (remnant or
remanent) flux density at P3. It is this
phenomenon which makes permanent
magnets possible.
Can we explain the hysteresis phenomena?
All materials consist of small magnetic
domains.
When they are in a magnetic field the domains
are intended to be in line with the field.
The domains
before applying
magnetic field
The domains
after applying
magnetic field
When the magnetic field is removed, not all domains
are randomized again
Hysteresis loss
Hysteresis is not a serious problem when we have DC
excitation (the examples considered so far). It causes some
loss when we have AC excitation, called hysteresis loss.
If we have AC excitation, e.g. the current i is sinusoid, the
hysteresis happens at each cycle. The hysteresis loss is
proportional to the frequency and also depends on the area of
the hysteresis loop.
Other losses
- Copper loss:
Pcu  R.I
2
- Eddy Current loss:
- Core losses:
Pc  Ph  Pe
Eddy Current
Eddy current:
As we saw, a flux induces a voltage on a coil.
Q: Why not inducing a voltage on the core itself?
A: It actually does.
The result is eddy current. That is why the transformers core are laminated.
Pe  K e .V . f 2 .t 2 .B 2
Eddy current losses
Eddy current losses
How do we reduce Eddy current losses
SOLID
LAMINATED
Eddy current losses
Eddy current losses
Eddy current losses in windings
Eddy current losses in windings
Can be a problem with thick wires
- Low voltage machines
- High speed machines