Question about stator iron and saturation

[John in CR]

In the past I've been able to guesstimate how much current a motor can take by looking at the amount of copper on each turn....more copper per turn means less resistance and less turns means more current to reach saturation. That worked fine with motors with a modest amount of steel in the laminations. Some nearer motors I got have a lot more steel behind the stator teeth. Does that just mean they're capable of more magnetic flux, so I can look at the copper in roughly the same way, OR does it also mean that it take less current to make the same flux if the copper and turns are the same as a motor with less steel behind the teeth?

Is there a fairly way to estimate saturation?

John

 

[Miles]

Ok, I'll have a stab at this.....

The balance between the amount of space given to the iron and the amount of space left for copper will affect the point at which saturation occurs. More iron means higher saturation point.

Assuming that flux density and % copper fill is the same, more iron means greater flux in the magnetic circuit and therefore more torque per Amp (higher Kt). The penalty is greater iron losses and higher resistance.

 

[John in CR]

Ok, I'll have a stab at this.....

The balance between the amount of space given to the iron and the amount of space left for copper will affect the point at which saturation occurs. More iron means higher saturation point.

Assuming that flux density and % copper fill is the same, more iron means greater flux in the magnetic circuit and therefore more torque per Amp (higher Kt). The penalty is greater iron losses and higher resistance.

Thanks Miles, and I understand from the standpoint of either iron or steel, but hubbies have plenty of space on the side away from the magnets. The approach with these high efficiency hubbies seems to be much more iron than typical on the side away from the copper. All 3 motors out of this factory have similar approach to the stators, ie much less steel between the windings and mags than on the other side of the copper.

Here's a picture of one. That's a 50mm long 150mm diameter stator that weighs about 5.5kg, and as far as I can tell the part with the yellow covering is the lams. The center mostly open spoked hub is cast AL.

View attachment MidMonster lams.bmp

What I'm trying to make sure of is that all that iron doesn't significantly lower saturation. I have a pretty good handle on the limits of the typical type hubbies, whose stator iron is a relatively thin ring. They seem to have a common ratio of iron vs space for copper, and the thickness of the copper bundle guides me in the current limits. I haven't burned up a motor yet, so it works for me on the typical style.

These higher efficiency hubbies are quite different and I want to make sure a nice thick copper bundle means similar current handling as with the lesser hubbies with less iron. Plenty of high quality steel and plenty of copper is good, right, and a nice thick bundle of copper on each turn means I should be able to let the current fly. Look at what those RC motor take. Doesn't all that iron help lower the Kv, so it's not just obtained by less copper on more turns?

John

 

[Farfle]

As far as i understand, a given amount of iron can make a given amount of torque at saturation. More iron (assuming its in the correct places) Will give you more torque at saturation. The ratio between copper to iron kinda tells you peak torque vs continuous torque. More copper will give you more continuous torque, as the copper can flow more amperage, because there is more of it. (to a point, too little iron will be into saturation too quickly). On the flip side, more iron will yield higher peak torque, but lower continuous torque (as more iron means less copper, so more heat in the windings, cause there is less of them)

 

[Miles]

The approach with these high efficiency hubbies seems to be much more iron than typical on the side away from the copper. All 3 motors out of this factory have similar approach to the stators, ie much less steel between the windings and mags than on the other side of the copper.

You're referring to the "yoke" section of the laminations which completes the magnetic circuit by joining the teeth on the inside (in this case) of the motor. The relative width of this will vary with the number/length of the magnet poles. It is usually set at a lower flux density than the teeth themselves, so probably wouldn't determine the saturation point. If it did, more of it would raise the saturation point. Neither will it affect Kt/Kv unless it's close to saturation, in which case, Kt will drop.

 

[Miles]

More iron (assuming its in the correct places)

That's the key point.

 

[Miles]

If you give us Kv, Rm and at least 2 values for Io, we can get a handle on your motor. If you give us the measurements of the core, we can analyse that.

 

[John in CR]

These will be those 6 phase motors.....Actually two 3 phase motors wound on a 24 slot stator on alternating teeth, so each is a 12 slot motor with 20 magnets. How do we handle Rm in that case? Also, what is "Io", since you didn't list it in the motor terminology post and I can't tell if that's Lo or io. I'm sure it's something I should already know, so sorry about that. Thanks for the motor terminology link. I only knew the Alt code for degree 0176 = °

Now that I think about it Kv is probably affected by the 6 phase arrangement too. I do know it turns 16 rpm for each volt, and I'll work on the rest.

Regarding the back iron on the stator, there must be a reason for it. It's a Japanese motor made in a Chinese factory using a design and stator steel supplied by the Japanese, so the Chinese aren't very helpful about the technical aspects. Could it be that the magnetic circuit includes the magnet and backing ring at some points of the rotation? I've always wondered if the flux on the outside of the can of an RC outrunner changes from the static state to what is is during operation.

 

[Miles]

These will be those 6 phase motors.....Actually two 3 phase motors wound on a 24 slot stator on alternating teeth, so each is a 12 slot motor with 20 magnets. How do we handle Rm in that case? Also, what is "Io", since you didn't list it in the motor terminology post and I can't tell if that's Lo or io.

If there are two sets of terminations, add both resistances together.

Sorry, Io is no load current. I'll add it to the list.

 

[Miles]

Could it be that the magnetic circuit includes the magnet and backing ring at some points of the rotation? I've always wondered if the flux on the outside of the can of an RC outrunner changes from the static state to what is is during operation.

It always does but neither the flux density or its "direction" changes with rotation.