Motor Rewind to upgrade HP 1

[rockman7892]

In another thread I talked about possible damage to a rotor on a re-wound motor. In discussing all of that I became interested in exactly how a motor is re-wound to increase its hp. I can only guess that larger and more stator copper is used to increase both the flux density and the anount of current the windings can withstand.

Can anyone explain, or point me to a good explanation of how a motor is re-wound to increase its hp. I'm looking for the theory and how its related to the motor equivelent circuit model.

 

[ankervik]

An increase in HP requires an increase in conductor size and a decrease in turns. Coil pitch, coil grouping & internal connections remain the same. Assuming the core will take the increased flux density the new rewind data (turns & conductor cross sectional area) would be:
T2 = T1 * HP1/HP2. Where T = turns, 1 = original value, 2 = new value.
New CSA = original CSA * HP2/HP1.

 

[waross]

The last time that was done successfully was the rerate from NEMA "U" frames to NEMA "T" frames.
Your HP is limited by the maximum torque which is limited by the saturation of the iron.
Generally more HP means more I²R losses. You may wind with a wire that will accept higher temperatures.
BUT the manufacturers have pretty well pushed the concept of rewinding for more HP to the limit already.
There are a couple of exceptions.
1> A very old motor with a NEMA frame or a NEMA "U" frame may possibly be rewound and rerated for a higher HP.
2> It may be possible to gain HP by rewinding for a higher speed.

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

 

[electricpete]

My two cents. Sort of a repeat of what I said in the other post.

1 - One thing that can be done is to change the insulation.

1A - If we upgrade the insulation temperautre rating, we can generally run to a higher curent loading on same conducutors resulting in higher temperautre (but within rating of new insulation) assuming the temperature does not become a concern for other components beside the insulation (bearings).

1B - Another insulation "improvement" which can sometimes be made is thinner insulation (generally involves higher voltage per mil). That gives two benefits:
1B1 - Allows you to put more copper in the same slot.
1B2 - Less thermal insulation effect created by electrical insulation = lower temperautre at the hottest location on the inside surface of insulation next to conductor.
I put "improvement" in quotes because you have to consider reliabity as well.

Above items would generally allow a somewhat higher rating with same winding configuration except for conductor size a little larger. This should not result in any significant change to equivalent circuit parameters. (except for stator resistance which is not very important to the torque speed curve or current speed curve)

2 - So far we have stretched the amount of current we can get from a given conductor configuraiton. The next thing to do is to increase the flux density (if we can do it while maintaining satisfactory margin to saturation) by decreasing the number of series turns. Flux is related to voltage per turn, so this will increase flux density. (As ankervik said "decrease turns"). There may be a variety of ways to do this- considering we may be able to change: number of turns per coil, number of parallel circuits (which affects number of series coil per phase assuming constant number of coils per phase). This would generally decrease the stator leakage reactance.

3 - Good point by Bill that increasing speed could sometimes be an option also which of course would change the number of poles and the equivalent circuit.

All of above would be done with same stator core and the same rotor.

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[electricpete]

Item #2 above would also decrease the magnetizing reactance of course.

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[edison123]

As Bill says, old motors of pre-70's are the ideal candidates for an uprate. By 80's, limits were already being pushed and now they are bursting at the seams. A 125 HP motor made in 80's are already being sold as 150 HP without any change in frame or winding design. They just upgrade the insulation system or downsize the insulation.

In the old days they designed the motors to last a life-time. Now-a-days, they design them to last the warranty period.

 

[electricpete]

Good points.

Certainly most of the uprate strategies discussed above rely on having some room for improvement in the desing. One area is newere insulating materials with higher temperature ratings. Another is the general aspect of margin in the design (margin to saturation, margin in volts per mil, perhaps general thermal margin). Older motors generally had more margin. Newer motors tend to be more closely engineered with less margin.

I think in the NEMA frame sizes medium size motors ~ 250hp and less, off-the shelf motors are very cheap and it would not make economic sense to do a redesign. I have not heard of anyone doing such a redesign.

In the large motor size (above roughly 500hp), each motor tends to become more of an engineered product to begin with (especially as the HP climbs into the thousands). Then it makes sense to look at uprate vs replace, especially if there is a rewind planned anyway.

Redesign among large motors is not unheard of. There is an entire EPRI document devoted to it - TR 1009699. It has three case studies - real world uprates which were done resulting in 15 - 25% increase in power at same speed using same rotor and same stator. They don't say how old the original motors are but I suspect you are right they are not new. There are after all plenty of old motors out there - we have around 120MW worth of large electrical motors running at any time... the vast majority approaching 40 years old. At some point in the future, our plant will consider an uprate of the entire power output which means increased flow through secondary systems of somewhere around 7% I think. We will be looking at whether we have enough margin in existing motors or need to uprate or replace... certainly uprating the motors will be considered.



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[electricpete]

One clarification in bold:

using same rotor and same stator core

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[ScottyUK]

At what point does the rotor itself become the limiting factor? The above discussion seems limited to the stator, which is reasonable as it is the easiest place to make changes to the design and also the easiest to monitor the effects of those changes. Can anyone offer some insight into rotor design for squirrel cage induction machines? I understand the principles well enough but not the design process.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

The rotor should be looked at to make sure the uprate doesn't create problems. Here's my two cents:

Starting is likely limiting for the rotor. Increasing steady state rotor current by 10 or 20% won't likely cause any oproblem.

Heating of an unloaded rotor during starting is equal to the final kinetic energy of the whole rotating inertia (including load inertia). Adding load torque doesn't change this much unless at some speed the load torque is a significant fraction of motor torque. (I have provided the analytical expression somewhere in the old threads).

So, if we go with option 1 (increased current but no change in flux densit) - then no change in starting characteristics. No problem as long as we don't increase the load inertia and as long as load torque remains reasonably far below motor torque during start.

If we go with option 2 (increased flux density) - we do have a change in starting characteristics of the motor: We have higher starting torque and higher starting current. These two effects tend to act in opposite direction and in fact they cancel out.... As we said before, the total heating of the rotor remains the same if we don't change the inertia and don't significantly change the load torque during start.

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[waross]

Hi Scotty. I wondered that myself.
My answer to myself was that increasing the horsepower means increasing the current. I expect that the current in both the stator and the rotor will increase in approximate proportion to the increase in horsepower. From that I would expect the I²R in both the rotor and the stator to increase proportionally. The rotor may not reject the extra heat as well, but the rotor is not as sensitive to high temperatures as the insulation on the stator windings. Other comments welcome.

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

 

[waross]

As I understand it, the torque curve is a function of the design of the rotor and the stator (regardless of windings) and rewinding and/or rerating just moves the operating point on the torque curve. For instance the maximum torque does not change but expressed as a percentage of an increased rated horsepower the percentage will be a lower figure. Given the small drop in speed required to develop more HP the changes will be slight.
Example, if a motor is rerated from 10 HP to 15 HP, the drop in speed is slight and the motor still develops the same pull out torque at the same RPM. Either recalculate the pullout torque as a percentage of the 15 HP torque or remember that it is a percentage of the 10 HP torque.

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

 

[electricpete]

I think your first comment said what I was trying to say Bill. The thermal limits for the rotor occur during starting. Steady state heating does not pose a challenge for the rotor.... the reason being there is no insulation. The rest of my comments addressed starting.

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[rockman7892]

Pete

So If I understand what you are explaining correctly you are saying there are basically two ways to increase the hp one being increasing the stator winding insulation to be able to handle more current, and two use less turns in the stator in order to increase the flux density.

I'm trying to figure out for each of these how the motor would draw more steady state current?

By the increasing insulation option I'm guessing that the motor could just be used on a larger load for example and would draw more current as demenaded by that load, and because the insulation is rated highher the motor can now withstand this current increase required by the load. The rotor remains unaffected.

By decreasing the stator turns option and thus decreasing flux density I'm guessing that there is more current due to the fact there is a lesser overall impedance in the stator. Less impedance will equal more current, and it is this current that will be used to provide more torque and hp.

 

[electricpete]

By the increasing insulation option I'm guessing that the motor could just be used on a larger load for example and would draw more current as demenaded by that load, and because the insulation is rated highher the motor can now withstand this current increase required by the load. The rotor remains unaffected.

Correct. No change in torque speed characteristics with the option where we use improved insulation (thinner or higher temperature rating). If you didn't change the characteristics of the fan or pump, you'd be at the same operating point. Presumably there must be a new load connected to draw increased load at same speed. Otherwise there is no change (unless we increase motor speed.... a different option).

By decreasing the stator turns option and thus decreasing flux density I'm guessing that there is more current due to the fact there is a lesser overall impedance in the stator. Less impedance will equal more current, and it is this current that will be used to provide more torque and hp.

For one thing, decreasing turns actually increases the flux density (flux depends on volts per turn) which does increase the torque speed curve. We will operate at lower slip for a given load As far as the rest I have to step back and think. This has a variety of effects.

Bad effects:
* It costs more magnetizing current and more core loss, both of which hurt the thermal performance of the stator.


Good effects:
* Since rotor power factor angle doesn't change, we can extract a little more torque per rotor current (torque would be proportional to rotor current times flux density times a power factor angle related to power factor of the rotor). So we reduce I^2*R heating in the rotor for a given power levle. But we have altered the "turns" ratio between rotor current and stator current, so we don't save anything on stator load component current. But reduced heat production in rotor can help the stator thermal performance.
*If we have eliminated 10% of series stator turns, then we can afford to make stator conductors 10% larger areawise which will reduce I^2*R losses

Both of the above good thermal effects (to the extent they exceed the bad thermal effects) recduce heating for a given load level and allow us to push a little more without exceeding thermal limits.

Also increasing the torque speed curve can be a good thing in terms of less heating during start for both rotor and stator.


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[rockman7892]

Is there any advantage/disadvantage with using aluminum windings for the stator windings? Or replacing copper windings with aluminum?

 

[waross]

No. That's a throwback to the days when the aluminum lobby was trying to have aluminum replace copper everywhere. It performs almost as well as just rewinding the same configuration with copper two AWG gages smaller. (Smaller copper, two gage numbers higher.)

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