Motor Theory...... 2

[Wedoca]

question.

lets say I have a 5 hp motor , but I really need a 8 hp ... can I just swap out the stator with a bigger one???? ( assuming the two stators are swapatable)

would that cause any problem???

 

[rhatcher]

I would say the answer is no. Even if you had an 8hp stator of the same frame size as a 5hp, it is most likely that the rotors would be different. By the way, 8hp is not a standard size for a motor. The next larger standard sizes above 5hp are 7.5hp and 10hp.

 

[Wedoca]

well...Rhatcher , what kind of problem do you foreseen???

 

[edison123]

Rotor.

Muthu

http://www.edison.co.in

 

[waross]

That's an interesting idea. I don't think that it has ever been done before. Let us know how it works out.
BTW. The proper motor may be cheaper than the wrong stator for a 5Hp rotor.

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

 

[rhatcher]

waross, you are probably right. If you could find someone to sell just a stator, it may cost as much as a complete motor. If so, it will certainly be sold with no warranty, especially once the supplier knows that it is to be matched with a different rotor.

 

[edison123]

The stator just transfers power to the rotor a la transformer. I don't see how a 5 HP rotor would pull a 8 HP load. (assuming of course the 8 HP stator is one-to-one replacement not just mechanically but also magnetically).

Muthu

http://www.edison.co.in

 

[electricpete]

Other problems:
1 - are they mechanically interchangeable? (outer diameter, bearing seats).
2 - does the rotor / stator slot combination cause problems
3 - is the magnetic design compatible?
4 - As above thermal design for rotor (I^2*R losses)
5 - Characteristics of rotor fan - provides enough cooling for the stator?
6 - Torque speed characteristic is determiend by the manufacturer based on motor and stator together. Are you going to figure out the new torque speed characteristic?
This is the tip of the iceberg. I wouldn't do it in a plant because odds are stacked agasint you. Maybe just for fun but not if you need it to work.


=====================================
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[FeX32]

The OP has an attitude problem.


Fe

 

[rhatcher]

electricpete, a star for you, old friend, for a good job explaining examples of why the wrong stator-rotor combination may be a problem.

For FeX32, I did not take the OP's response, "well...Rhatcher , what kind of problem do you foreseen" to be bad attitude. It seems natural that if you tell someone not to do something that they will ask why. My boss does it to me all of the time.

And, we should always remember that many of the people who post here speak English as a second language and may respond in a way that seems to show attitude when it is not meant that way.

That being said, I have seen many many examples of regular contributors to this forum showing 'attitude'. For example, a response that begins with 'Fine, whatever...'

 

[FeX32]

Very true.


Fe

 

[waross]

The basic problem;

( assuming the two stators are swapatable)

Not a good assumption. Do a little research on some motor manufacturer's sites. Look at Hps, frame sizes and dimensions.
You may find a 7.5 Hp or 10 Hp, 3575 RPM stator in the same frame size as a 5 Hp, 1775 RPM motor. If it is the same frame size, it may be swappable. But the cost of a replacement rotor at spare parts pricing will probably be much more than a competitively priced complete motor.
I have seen pricing ratios of 4:1 and 5:1 comparing replacement MCC sections with the same equipment priced as a competitive bid.
A $20,000 bid for an MCC section under replacement pricing dropped to $5000 as a competitive bid.
Probably the same situation for motor parts.

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

 

[Wedoca]

Thanks for all the response guy, and Rhatcher like you said , I did not mean to offend you in anyway...with my previous statement "well...Rhatcher , what kind of problem do you foreseen???"" I had a question, and thats the way it came out... anyway ....

also, Thanks to electricpete

those are exactly the type of answer that I was looking for, a star for you ...

 

[jraef]

It should be known, people do this sort of thing quite often, but not in an off-hand way. Many custom centrifuge motor are done this way because of the high inertia and long spin-up time. But usually they go the other way: a (magnetically) larger rotor for a smaller stator. You get less shaft power out of the combo as compared to what the rotor would have been capable of, but the long acceleration time doesn't damage the rotor. Most motor's thermal design limits are based on the rotor damage curve, not the stator.

But the stator windings are what determines the power consumption and power output. So putting in a larger rotor in a 5HP motor would not increase the HP capacity of the motor anyway.


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[rhatcher]

Wedoca, it is all good.