Is it bad to operate a a PM motor core in saturation? 1

[nickw1881]

If I use ND magnets with a Br of over 1T, it is almost impossible to keep the thin parts of the field and armature core out of saturation, without making the field core and armature core very nearly as thick as the magnets are wide. This would make the motor extremely heavy. Is it "bad" to operate the core of a permanent magnet motor in saturation?

 

[jpts]

Hi

When you increase the iron flux density beyond saturation, its permeability will start to decrease (iron becomes less ferromagnetic). This means that the realuctance of the flux path increases, which can be seen in smaller air gap flux density and smaller induced voltage. Magnets will produce constant magnet field strength [kA/m], but decreasing permeability will decrease the flux density (flux density defines the induced voltage). Flux density=Permeability*magnet field strength

So, core saturation will decrease your induced voltage. Usually slight saturation is still ok to minimize the yoke thickness, but excessive saturation will cause too big drop in induced voltage. There is a tradeoff; if you increase the iron dimensions (so that they will not saturate), you will get more flux in air gap anr more voltage, or consequently you can reduce the magnet thickness to produce same flux. As the magnets are quite expensive, it is usually economical to avoid excessive saturation as less magnets are needed.

 

[waross]

I don't think that saturation decreases flux density, it just doesn't increase very much with increasing magnet strength.
(Just doesn't increase as much:> at complete saturation the increased flux in that part of the magnetic circuit may only increase by a factor of 1/10,000 or 1/20,000 compared to increases below saturation.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jpts]

Hi
What I meant, was that if you have iron parts in heavy saturation, you will waste big part of the magnet's magneto-motive force in the iron-part, which means that you have less magneto-motive force left in the air-gap. Less magneto-motive force in the air gap means less air gap flux-density, and less induced voltage.

If we assume that magnets produce 100 units of magneto-motive force into flux-path, without saturation maybe 5 units is consumed in iron parts (yoke, teeth) and 95 units is left in the air gap to produce air gap flux. If we allow the iron parts to saturate heavily, we might waste e.g. 15 units in the iron (due to its poorer magnetic conductivity when saturated) and we have only 85 units left in the air gap. Less mmf in air gap means less flux density.

In practice saturating iron parts increase the effective air gap on flux path, which decreases induced voltage. Saturation decreases the magnetic conductivity (i.e. permeability) of iron heavily, which means that there is a bigger magnetic resistance in the flux path due to saturation. With constant magneto-motive force we can produce less flux due to higher magnetic resistance. This is fully analogous to Ohm's law U=RI (which is now: magneto-motive force=Reluctance*Flux). If we increase the resistance of electric circuit (reluctance with magnetic circuit), we have smaller current (flux with magnetic circuit) with constant voltage (mmf with magnetic circuit).

I think we are talking about the same thing with Waross though, but from different perspectives. But the message should be the same: if you allow the iron to saturate, you will get much less flux (voltage) than without saturation. This means that you will need more magnets to produce enough flux to get enough voltage.

I hope I was not too confusing

 

[waross]

Yes saturation is a waste of magnetic strength. No, saturation doesn't reduce the flux density, it doesn't increase it much. Saturation is just a waste of magnet strength.
Put it another way, reducing the amount of iron in a circuit will reduce flux density and reduce induced voltages. Increasing the magnetic strength does not reduce flux density or induced EMF. It has a similar effect as adding an air gap in parallel with the iron as part of the magnetic circuit but the effect is caused by an increase in magneto motive force (MMF) so there is no reduction of flux.
Your statement may be correct if iron was removed from an existing design. Once the iron is saturated increasing the MMF will not cause a reduction in flux density.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jpts]

That is what I was trying to say; if you take existing design and push the core into saturation by reducing iron, you will get reduced flux due to saturation. I don't think I said (at least didn't mean to) that for saturated core increasing mmf will reduce the flux. Like you said, it will then increase proportionally only to vacuum premeability.

 

[waross]

OK. I guess we are in agreement.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sreid]

If your motor design is 4 pole, try making it an 8 pole design. I think you'll like the results.