Consequences of increasing the length of the iron core in electric motor 7

[EngRepair]

Hypothetical, theoretical question about electric motors.
Let's say we have a fully functional three-phase squirrel-cage LV motor.
Let's imagine we made another one with exactly the same geometry of stator and rotor lamination, exactly the same winding (turns/coil, wire size, pitch, etc...).
The only difference should be the length of the stator and rotor cores.
Let's say the length is increased by 10%.
Also, the motor load will remain the same as before.
What changes will this cause in terms of hp, torque, rpm, FLA, NLA, efficiency, and power factor?
It would be greatly appreciated to hear some expert opinions.

 

[electricpete]

pete

Let me simplify this further for you.

Torque is proportional to the product of flux density and current in any dc/ac motor.

With more core length and with same no. of turns, flux density will be decreased and current cannot go up without exceeding original current density (Amp/sq mm). Hence, the torque will go down since the current is same and flux density is lower.

The length you keep talking about is already accounted for in the flux density/emf equation.

You're relying on a single relationship: torque proportional to B (for a given current).
It is indeed true, but B is not the only thing changing. L is also changing and L also affects torque in its own way independent of B (they both show up in the torque equation T = R N I L B costheta).

Let's contrast two different scenarios which both result in change in B, but in different ways:
[ul]
[li]Scenario 1 as proposed by op. Increase length by 10%. B goes down by roughly 10%. .[/li]
[li]Scenario 2. Reduce voltage by 10%. No change in length, but B still goes down by roughly 10%[/li]
[/ul]
Both scenarios have 10% lower B. Are you saying they give the same torque for the same current, even though the first scenario has a longer core? If that's what you're saying, then it would seem like you're saying the longer core doesn't contribute to torque production all other things being equal (same flux density and current etc) but it still contributes to losses (longer core has more I^2*R losses and more core losses). And if all of that were true, then it seems like someone could get rich quick designing really short cores since length doesn't contribute anything useful but costs money and causes losses. That seems to me like a contradiction with reality, which should steer us back towards figuring out what led to the contradiction. (I think you know my opinion on what led to the contradiction)

 

[electricpete]

The length you keep talking about is already accounted for in the flux density/emf equation.

I'm posting a figure to try to outline the relationships/variables that I see as relevant, because maybe it explains my point better than all my rambling words.
There are three variables (L, B, T). L is considered an independent variable (input). B and T have equations. Arrows are drawn to show where one variable affects the equation of other variables.
Of course the important part of the figure is there are 2 different arrows leaving L, one of them goes to change B but the other goes directly to T. So there are two ways that L affects T, but I think you are only considering one of them.

 

[electricpete]

1. Torque per unit length: Less force to generate torque per unit length, but a corresponding increase in length.

I like that explanation. You have a way of cutting to the heart of the matter.

number of fox viewers

That statement is suspect.
Did it come from a design engineer or from a sales engineer?
It may mean that the connections are identical.

Wait what? Fox viewers? Either you pasted the wrong thing from your clipboard, or else something went right over my head (again).

 

[waross]

WOW. Talk about a seniors moment and a cut and paste search gone completely sideways.
Correction made.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[IRstuff]

I thought maybe you were typing your response in your tractor at a traffic light during a long haul ;-)

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[waross]

No, I was earlier doing a search.
When I tried to copy the intended text, for some reason the old text in the cache was not replaced.
And, I always proofread my posts. (Except when I forget.) Grin

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

Funny. I have to resist the urge to comment on anything political

If we don't assume magnetic linearity, the product of L B is going to increase (rather than remaining constant) as we increase L... and that favors the lengthened core to perform even better.

That's an incorrect statement or at least needs clarification. In the sinusoidal world the product L B remains constant. If we consider non-linear effects it is a small benefit to the increased length core, but not through a product of L B. When you increase core length and decrease flux density, the magnetizing current will be more sinusoidal and have less harmonic currents and associated fluxes. Those harmonic currents and associated fluxes contribute nothing toward the fundamental current and fundamental flux and therefore nothing toward useful torque production. But they do contribute towards increase I^2*R losses and increased core losses. So the longer core has less of the useless harmonic components and less of the associated losses for the same useful fundamental I and B. So it's another small benefit on the side of increased length, but I shouldn't have called it an increase in product (L B).

I will say all my earlier comments (roughly "it's a wash but we don't know for sure") were all based on a premise of being asked to pass judgement knowing only that core length increased but not anything else. Now I picked up on what waross already picked up on, the OEM told you the new motor has the same horsepower rating. So the null hypothesis is the replacement motor meets the original horsepower rating... that makes the case even stronger. What the OEM said matches what we suspected, and we certainly have not seen a single thing that would lead us to second-guess the OEM who knows more than all of us about this motor (setting aside the question of whether the turns did truly remain the same, which was a legitimate point of earlier discussion).

 

[Gr8blu]

Ahh. (Lightbulb goes off after OP's post of 09 March). We have a "generational" gap in design - from the same manufacturer - leading to some differences in observed geometry.

Here's a theory: OEM decided to minimize parts count across the pump motor HP range, leading to some "combining" of ratings within a certain casing length. I actually did this once as an optimization project for a major OEM, with the result that what used to be 50 unique designs became only 10. There was no major change in performance over a given segment of the population, but resulted in much simplified drafting, documenting, and equipment production. Sure, the new frame could run anywhere from X to Y hp and NN to MM speeds; the winding was sized for the highest power (current) and the mechanical construction sized for the highest torque. Yes, sometimes the performance was a bit different for a specific hp/speed combination - but still well within nominal tolerances. And since everything was sized for "worst case" conditions, a bit of overload was no big deal when looking at the larger picture. End users often did not even see the performance difference - even if they did notice the core length difference.

Is the new motor going to do what it needs to? If it fits (axial length) in the required position, sure thing. OP did not mention the original core length, but I suspect the difference will be something like going from 20 to 22 inches of active material (or maybe only 15 to 16.5).

Converting energy to motion for more than half a century

 

[edison123]

pete

T ∝ B I cosɸ

Eph ∝ B L D Tph N

Have fun with all the math and theoretical weeds. I am done.

@OP


Most likely the clueless sales guy. They should talk to the winding designer.

Muthu

http://www.edison.co.in

 

[electricpete]

T ∝ B I cosɸ

Eph ∝ B L D Tph N

Have fun with all the math and theoretical weeds. I am done.

What you wrote is correct. But the conclusions drawn from those relations in this thread would not be correct. Here's why:
[ul]
[li]∝ means "proportional to".[/li]
[li]That means there are factors omitted for the purposes of simplifying the relationship when we assume those factors are constant.[/li]
[li]T ∝ B I cosɸ would work just fine for options in rewinding an existing core where there might be considered change in E or N resulting in change in B....[/li]
[li]BUT the corresponding full equality for torque includes the three factors B I cosɸ AND some additional factors R N L Kdp:[/li]
[li]_____ T = B I cosɸ R N L Kdp[/li]
[/ul]
That full equality is needed when any of those additional factors R N L or Kdp changes. Those additional factors include L, which is most relevant to our discussion.

The full equality also has the benefit that you can actually calculate the torque by plugging in the numbers (the units work out correctly, and you get a correct answer for electrical torque). In contrast, try plugging units into the right side of T ∝ B I cosɸ and you will not get correct units to represent torque, much less the correct answer. That is another aspect of the difference between a proportionality and an equality.

In summary:
[ul]
[li]T ∝ B I cosɸ.... is a [highlight #EF2929]proportionality[/highlight][/li]
[li]T = B I cosɸ R N L Kdp... is an [highlight #EF2929]equality[/highlight][/li]
[li]Sometimes proportionalities are enough to answer a given question, and sometimes they aren't. This is one circumstance where the proportionality incorporates an assumption of a constant variable L, but that assumption is invalid for the current discussion where L changes. Therefore the assumption inherent in the inequality is invalid for this discussion and the full equality is needed.[/li]
[/ul]

Sorry for a post that was probably much longer than needed, but I wanted to make sure the point was clear.

 

[EngRepair]

The old pump has already been repaired and returned to the customer.
The spare pump will eventually be used in another place, with a lower HP requirement.
That's what the customer decided.
By the way, in brief, I had asked the same question elsewhere and was told:
Increasing the length of the stator package while keeping the same number of turns per phase will reduce both the torque and horsepower, just as increasing the number of turns per phase while maintaining the same package length would.

 

[electricpete]

By the way, in brief, I had asked the same question elsewhere and was told:
Increasing the length of the stator package while keeping the same number of turns per phase will reduce both the torque and horsepower, just as increasing the number of turns per phase while maintaining the same package length would.

"turns per phase"... I assume you mean number of turns per coil was not changed and accordingly series turns per circuit have not changed.

The claim is meaningless unless you include a source and/or proof. And I would question your source.

All arguments made in this thread that torque and horsepower go down by 10% have been addressed. I've been expecting someone to chime in to admit their mistake, but that doesn't always happen.

Proof was provided above that full load torque capability and full load horsepower capability are expected to stay roughly constant (it certainly wouldn't go down 10% and there's certainly no reason to question OEM's assertion, given you have assured us about turns staying constant.). I think it's safe to say waross, Gr8bLu and myself all agree on that. It shouldn't really be controversial among knowledgeable folks if they have sat down and thought carefully about it.

 

[waross]

There is a significant difference between:
"Increasing the length of the stator package while keeping the same number of turns per phase will reduce both the torque and horsepower."
AND
"Increasing the length of the stator package while keeping the same number of turns per phase will reduce both the torque and horsepower, PER UNIT LENGTH."
And therein lies he confusion.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[EngRepair]

I will try to add a couple of questions (instead of answers) that simulate this situation and maybe make this puzzle easier to solve.
Let's consider an original submersible pump motor with a power rating of 60 HP, where we will replace only the winding according to the original winding data of a 50 HP motor. The dimensions of the lamination will remain the same as well as the core length.
What will be the resulting power and torque? Will the new motor have a power rating of 60, 50, or 40 HP?

Let's take a step further: what will be the resulting power and torque if we replace the winding with one that has a power rating of 40 or 30 HP?

 

[electricpete]

Are you still trying to answer the old question and posing this new scenario for sake of argument to try to prove some point related to it?
[ul]
[li]If you have a point to make you should first come out and say your point plainly.[/li]
[li]I will say my own point plainly since politely tip-toeing around seems to have gotten us nowhere:[/li]
[li]___1 - Anyone who advises you that increasing length by 10% will cause full load horsepower capability and full load torque capability is going to go down by 10% based on a proportionality T ~ I B is clearly mistaken. The error in their logic can be seen by comparing the proprotionality to the full equality from which it came.(see my post 11 Mar 23 15:49). [/li]
[li]___2 - Proof that factors are acting in both directions to support "it's a wash" (approximately) are shown in my figure 9 Mar 23 18:57. If you disagree with "it's a wash" I'd invite you to identify which equation you disagree with or what is missing from this analysis[/li]
[li]I'll be glad to try to decipher your scenario for sake of argument if that's your purpose, but I want to understand where you're coming from first[/li]
[/ul]
Or is this a brand new and unrelated question?
[ul]
[li]If it's a new question I'd suggest a new thread.[/li]
[/ul]

 

[electricpete]

Let's consider an original submersible pump motor with a power rating of 60 HP, where we will replace only the winding according to the original winding data of a 50 HP motor. The dimensions of the lamination will remain the same as well as the core length.

I think it would be helpful to focus on my previous comments rather than wandering into something new for reasons stated above.

If you prefer this line of discussion instead then I don’t want to stand in your way. BUT I think clarification is in order. Prior to replacing anything (let’s call that the initial condition) you are presupposing we know the relationship between two motors, one of which is 50hp and one of which is 60hp motor. I can’t figure out what we are supposed to assume about these two initial motors other than the hp ratings:
[ol 1]
[li]Do these two motors initially have the same dimensions? In that case the hp rating difference would presumably be associated with some winding differences. If we then make the windings the same, we end up with two motors that have the same dimensions and same winding configuration. I don’t understand the purpose of that excercize.[/li]
[li]Are we supposed to assume the initial 60hp is 10% longer and everything else equal. That would match the same horsepower rating according to our previous comments, but you didn’t believe our previous comments, and more importantly there is no associated winding difference (only the length changes), so it’s not clear what changing the winding would involve.[/li]
[li]Are we supposed to assume the initial 50 and 60hp motors are independently optimized? That's a lot of variables. It's not clear what you're looking to compare (and why) nor whether you can keep the laminations the same given different radiuses (radii)[/li]
[/ol]

 

[EngRepair]

Hi, Electricpete,
I'm quite surprised if you're not recognizing that this is the same question, not a new one.
I have some experience as a winder, but I'm not particularly fond of theory.
Additionally, I didn't see any specific answers to my last, very straightforward questions.
So, could you clarify if it's possible to use a 50 HP motor winding in a 60 HP pump motor?

 

[edison123]

pete


That's not the torque equation, that's emf equation sans the PF & I.

So, your whole argument(s) in this thread is/are based on a false equation and hence not worthy of consideration.

EngRepair's post you called irrelevant is actually a different and correct way of his framing his original question.

With all due respect, you are not a winding designer or a rewinder.


Muthu

http://www.edison.co.in

 

[waross]

edison 123
Surely the length enters into the torque equation.
Extending the length of the winding will reduce the torque per unit length. Accepted.
But,that lower torque is now acting over a greater length.

So, could you clarify if it's possible to use a 50 HP motor winding in a 60 HP pump motor?

It depends.
If the core has been designed as a 50 HP motor, the core may saturate if wound to 60 HP.
On the other hand, if the manufacturer has decided that consolidation of motor cores is more economical than close optimization, and uses a 60 HP stator in both 50 HP and 60 HP motors, the 50 HP motor may be rewound as a 60 HP.
The standard shaft size is different between a NEMA frame 50 HP and a NEMA frame 60 HP.
Another issue is space. The 50 HP core must have enough space in the slots for the 60 HP winding.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

I'm quite surprised if you're not recognizing that this is the same question, not a new one.
Additionally, I didn't see any specific answers to my last, very straightforward questions.
So, could you clarify if it's possible to use a 50 HP motor winding in a 60 HP pump motor?

I already asked for clarification on your "very straightforward questions" in my post 15 Mar 23 13:10.

I have some experience as a winder, but I'm not particularly fond of theory.

Take the theory at whatever level you are comfortable. Ask questions. If there is a disagreement, that's a time to be even more skeptical and ask more questions.

If you reject any reference to theory then what's left... intuition? Does it pass your intuition test when someone claims to you that T ∝ B I independent of length? So if we have a loop carrying current I in a field B, the torque is the same REGARDLESS of the length? A one meter long loop sees the same torque as a 10 meter long loop? I'd think that would set off some intuitive alarm bells for you.

Quote (T = B I cosɸ R N L Kdp... is an equality)

That's not the torque equation, that's emf equation sans the PF & I.

So, your whole argument(s) in this thread is/are based on a false equation and hence not worthy of consideration.

It is the torque equation. There's nothing to do with emf anywhere in sight there. I'm not sure what you're looking at.

The full derivation of the torque equation quoted above (excluding Kdp) is given in the attachment to my post dated 7 Mar 23 14:21, starting from first principles F= q*velocity x B =L I x B (with suitable background to explain why force on conductor equation gives the correct result).

And it matches a textbook equation that I also referenced in that attachment except the textbook equation adds the Kdp)

The equation is important. It's at the center of our disagreement.

I have already invited you to provide an alternate equation but you have not. I assume you understand the difference between an equation and a proportionality.

Let me ask the same question in a different way. if I told you the air gap flux, the current in the slot conductors, and the total number of conductors, how would you quantitatively estimate the numerical value of the torque? feel free to identify additional parameters needed to support the calculation. I claim the additional parameters needed are the ones that I already said in the equation you quoted.
This is an important question and I would like for you to not ignore it. you can look it up in a textbook.

if you don't have a textbook, there's a shortcut that will get you there which is to look at the units of the variables.
B has units of Tesla = volt*sec / m^2
B x I has units of (volt*sec / m^2) *A = (A*volt)*sec/m^2 = (watt*sec)/m^2 = Joule/m^2 = N-m / m^2 = N/m
Newton/m... that sounds like force per length.
(And indeed that's what we already knew since F = qVx B = I L x B, so F / L = I x B.... so I x B gives force per length. Yup.)
If I had a loop I might be interested in the torque. So I'd multiply by the radius.
R*F / L = T / L = R * I B
The units are now newton-meter per meter. Torque per length on a loop. Sounds kind of like what Bill was talking about.
I wonder how we would convert FROM torque per length TO torque. Multiply by length maybe? Apparently you don't think so. I'll be interested to see how you make the units work out when you answer the question of how to quantitatively estimate torque.

To recap I am waiting for one or both of the following:
[ul]
[li]1. What equation would you propose to use for torque instead of the equation that I provided?[/li]
[li]2. If you had to quantitatively estimate torque developed by a given motor given B and I, tell me how you would do it, including what other parameters would you need. (Are you really sure you don't need L?)[/li]
[li]3. EDIT - BONUS QUESTION. Let's say I have 2 loops carrying the same current in the same field, each one similar to this. The only difference between the two loops is that one is 1m long and the other is 10m long. Does the 10m loop experience the same torque as the 1m loop? (PS - if you are looking for R and L within the link I just posted earlier in this bullet, they are hidden in area A = 2 R L)[/li]
[/ul]