Hysteresis

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

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 domainsbefore applying magnetic field

The domainsafter 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: 2.IRPcu

- Eddy Current loss:

ehc PPP - Core losses:

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.

222 .... BtfVKP ee

Eddy current losses

Eddy current losses

How do we reduce Eddy current losses

LAMINATED

SOLID

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