AC motors in external DC field 2

[mac2]

Is there information available that defines the limit of an external magnetic field that an AC motor can tollerate without adverse effects? This situation occures when motors are operated near other magnetic equipment (i.e. magnetic drums).

 

[rhatcher]

Before now I have not considered the effect of DC fields on induction motors and have never been involved in a case where this was considered as a potential issue. I would think that the DC fields encountered in most industries would be relatively weak in terms of their ability to affect the flux in the air gap of an induction motor. However, I love the fact that our chosen profession is so deep that no amount of experience will allow that all may be known. That being said, I wish to know if your question is hypothetical or based on an actual case.

In either case I may be able to offer some input as I do some work for an Alcoa plant that produces Al from bauxite. The process (I am not an expert!) requires DC and I am told that the bus voltage and current for their process is in the range (range means I do not specifically remember but can obtain the specific answer if needed) of 125-250VDC and 150-200kA. If no others post a good response for you I can pursue this by inquiring whether they take any precautions with respect to placement of AC motors near the DC bus and, if so, why.

 

[mac2]

In this case, the AC Motor will be located in the magnetostatic field produced by a large suspended electromagnet. The magnet is used to separate "tramp iron" from the flow of ore that is being "cleaned up" before it goes to a crusher. Things like digger teeth and re-bar are considered "tramp iron". The field will be in the range of 500 gauss (0.05T). My concern is whether the DC field will saturate the motor in some way that will cause the AC current to exceed the motor's rating.

 

[electricpete]

Like rhatcher I've never heard of this being a problem. I tried to think it through and came up with some related questions. I put my best guess at the answers after each question.

Q - What is the normal magnetic flux density in the core of a motor?
A - I'm not sure, but EE Handbook shows saturation for silicon steel approx 15,000 gauss, so I suspect typical order of magnitude is 10,000 gauss for operating flux density in motor core iron.

Q - Does 500 gauss measured cause 500 gauss in the core?
A - I think the 500G corresponds to the B-field measured in air. (Actually corresponds to an applied H amp-turns of 500gauss/mu0) If core iron were inserted in the same field, perhaps the B-field would be much higher due to high relative permeability (500G*MU/M0)?

Q - Does the stator frame provide shielding for the core?
A - No - It would provide some shielding against external AC magnetic field, but none for a dc magnetic field.

Q - Is it likely that the orientation of applied magnetic field would be coincident with orientation of internal magnetic field
A - It seems possible but not likely.

Q - If the dc magnetic field did reach the stator and/or rotor cores with the proper orientation and magnitude, would there be any adverse effects?
A - Theoretically as I think you mentioned the core might be forced into saturation once per ac cycle (either positive or negative peaks, depending on dc polarity). That's what happened in 1989 when solar geomagnetically-induced currents in the soil caused dc voltage gradients in earth's surface and led to overexcitation of some generator stepup transformers (those connected to long transmission lines) with hi-side connected in grounded wye. So core overheating of the motor might be a possibility. Also harmonics in excitation current to the motor might conceivably cause slight additional heating in motor power supply equipment (cables and transformers). Doesn't seem likely you could experience any other ill effects (no likely performance problems other than remote possibility of core overheating)

By the way rhatcher, do you work at Point Comfort?

 

[electricpete]

I think I was a little off-base when I said that the 500 gauss in air could be increased by the ratio of mu/m0 when core iron was put into the path. As we insert iron in place of air, there might be some increase in flux due to reluctance/fringing effects, but not of the magnitude of mu/m0.

Where's that "delete message" button when you need it?

 

[rhatcher]

Very thoughtful post electricpete. Stars for you are becoming a habit for me.
mac2, I have done some checking with my friend at Alcoa. The nominal bus voltage is 700VDC, 5V per pot with 140 pots in series according to him. The current is 215kA. I haven't done any EM theory since college, so I will leave it to you guys to figure the field strength as I would hate to have to pull that book off of the shelf...that class was difficult enough then when it was fresh in my mind. Anyway, he does not know of any specific problems attributed to induction motors in a DC field. However, they did replace some 400hp units with new 500hp motors and have experienced problems with shaft currents on the new motors. The manufacturer is telling him that it is caused by the DC fields, but these are only 3 motors out of hundreds without problems (including the original 3), so he is skeptical and I had to agree. However, upon reading your post I thought of another customer with an application specifically like that which you describe. He is a at a steel mill where they use large DC electromagnets to handle the scrap steel that is their raw material. At one place, boom cranes use the magnets to to load scap onto conveyors operated by AC motors and therefore the magnets and motors can be within a few feet of each other. As well, inside the plant overhead cranes are equipped with DC magnets that operate in the vicinity of AC motors on floor mounted equipment such as conveyors, etc. He does not know of any problems like you describe.

That being said, I do still think it is important to have a basis for understanding a problem before accepting an answer, even from trusted sources. That takes me back to electricpete's post.

"Q - What is the normal magnetic flux density in the core of a motor?"
- My EASA "AC Motor Redesign" manual gives the following for 60hz Class B motors:
air gap flux: 46000-54000 lines per inch (another source gives 65,000 as the max, perhaps for Class F or H?)
tooth flux: 120,000 lines per inch
back iron: 115,000 lines per inch
where 1 Tesla = 64,500 lines per inch = 10,000 gauss = 1 weber per square meter = 10,000 lines per square centimeter. If I am reading the numbers right it would appear that the DC field is fractional compared to that found within the stator of an AC motor.

As well, another thought I would not have had without electricpete:
"Q - Is it likely that the orientation of applied magnetic field would be coincident with orientation of internal magnetic field"
- electricpetes answer was a good one, but I want to add that there are two types of "iron" in an AC motor. (I will say that I am not an expert in electromagnetic theory). The first type is the frame which is usually cast iron or rolled steel and the second type is the active iron of the core and the rotor that is laminated steel. I would think that the frame may act to concentrate any flux passing through it regardless of orientation, but I would also think that the permeability of the laminated steel core and rotor would be relative to the orientation. Specifically, a flux oriented with the shaft of the motor would be perpendicular to the plane of the laminations and would see them as a stack of thin plates separated by insulation that would have a low permeability. On the other hand, a flux at a right angle to the shaft would see the laminations end on and would have a path of low permeability. I imagine that for angle between those, permeability would vary with the angle and some field distortion would occur.

Anyway, based on the relative field strengths shown above, I guess that the orientation of the motor is unimportant, but it was (for me at least) an interesting question to think about.

electricpete, here is a link to the company I work for:

http://www.excelapparatus.com

(I am not recruiting or selling!) It is not much of a site right now but we have someone working to improve it. Anyway, since you asked I thought you might be interested.

 

[peterb]

Just in passing, Electricpete, do you have connections with Pt. Comfort? I was there recently on an audit & wondered if we are in fact in the same line of work.

 

[electricpete]

I am geographically not too far from Point Comfort on the Golden Gulf Coast (Texas, that is!).

I work at a 2500MW power generating station nearby.

 

[jbartos]

Suggestions:
1. EM modeling may pay off.
2. There are various DCs. Some of them may be variable DC that are very detrimental
3. Active Shaft Grounding system may be applied if justified.
4. Visit

http://surfacewarfare.nswc.navy.mil/magazine/so22_quietconfidence.html

http://www.fnc.co.uk/prodserv/services/mag.htm

http://www.thomasregister.com

type Monitors: Shaft current & grounding
which will return two companies to choose from
etc. for more info