Prior to rewinding a 167 HP stator, we performed a coreloss test (with a Lexseco Coreloss Tester) and obtained a result of 8.233 watts/lb loss (out of 10 - considered "marginal" by Lexseco). Once assembled, the pump drew high amps. We re-tested the wound stator in the housing and this time obtained a result of 22 watts/lb loss indicating that the loss was "too high". Any ideas?

I would have to start by asking your oven temp when you burned the motor out. Also Easa recommends not winding a stator with a core loss above 6.0 I believe. I know we won't risk them above 5.0 it is to risky. How long was the motor running at that highs amps. If there was alot of heat that would effect it as well. I know the quality of new laminations has decreased over time as well. They are finding they don't hold up to the burn out as well as the older ones. Type of metal they are using now. Just another way to cut cost. Hope this help a little., thanks.

I think we would all need some answers before being able to offer any explanation.
Did you to a core loss test before the burn out?
If so what were the results?
Was the old winding processed in a burn off oven?
If so what was the temperature setting?

I wouldn't have rewound the stator at 8 watts per pound loss, You should try to improve it and if you can't improve it you should give your customer the option of a core rebuild.

Simple steps that sometimes improve the situation: Try "WATTKNOCKING". This is a non scientific process where you stand the stator on its end, lay a piece of two inch lumber over the back iron and the teeth and beat the heck out the core with a big hammer. Do this all the way round the core at both ends and re-test. If the core improves, which it quite often does, then give it a good soaking with water and leave it to oxidize for a day or two. re-test.

I think your core may have lost its permeability. If it is an older motor and been rewound an excessive number of times this is a distinct possibility. This would mean that more energy would be required to magnetize the core iron and therefore increasing the no load and consequently, the full load current. Your description of what has happened would indicate that this is a possibility.
Hope this helps

Any hidden copper spots or stuck iron? I know that's the bare basics any good winder should know, but you'd be amazed. Also I've had iron packages that have blown in the slots and been repaired successfully cause a high core loss. I agree totally with Tim if it was over 5 after burn out it should have been inspected further and resolved before winding. Different steel material also core loss at different rates, but I would stick with the basics. Stator iron is stupid it only reacts to predictable causes... unfortunately, another burn out is probably in your future and a possible restack. I don't personally believe oven temp has anything to with bad cores, I've yet to see anyone melt down a core due to oven temps. The Wattknocking method Graeme suggested is the normal way of dealing with stuck iron after a good soaking in vinegar (promotes rusting which is the best insulate for burned out iron), and now I have learned a new name... wattknocking. Hope that helps too.

That's certainly a perplexing problem. 8 watts before the rewind and 22 afterwards with the stator in the housing. Back when these core loss testers first came out there was a problem with repeatability. Naturally that's when we bought one of the dang things. Almost sounds like this could be an issue here. If this were my problem I would remove the stator from the housing and test it again and see if it returned to somewhere around the original 8 watt reading. 8 watts core loss shouldn't affect the current draw all that much in my opinion. If it does recover I think that I would look closely at the winding data that was used to rewind it. I've got a hunch that you might find your problem there.

In the past, I have seen motors that have been rewound several times. Each time a stator is burned out the insulation coating of the lamination looses some of its integrity.This loss of integrity could very well be the culprit in the marginal core loss test. Watt-knocking might help a little if you can see where the rotor might have dragged the stator due to a possible bearing failure. In some cases. watt-knocking is very effective. If that doesn't help, you might consider replacing the iron.

American MTS is developing an oven control system call OvenDaq (Oven Data Acquisition) that I think is one of those "Its about time someone thought of that" ideas. Some ovens have a simple on/off timer with a thermostat that only tell how hot the oven is. The OvenDaq gives you the actual temperature that the stator reached before the varnish and insulation begins to burn. at that point, there is no sense in letting the oven temp to continue to rise. After all. after 700 degrees the laminations start to loose that insulation coating. If it has been rewound before, there is no telling how high the temp actually got.
The OvenDaq sends you a message via phone app or computer if the oven temp is out of control or when the stator is completely burned off. I am still learning about it my self. but here is the website

OvenDaq.com

The core loss test is best used only for before/after comparison purposes. When a core-loss test is done **in the housing**, the loop must not encircle the housing. It must be passed between the OD of the core and ID of the housing. Encircling the solid metal housing, too, will induce excessive losses, and therefore is not a valid coreloss test. When " pump drew high amps", if this was no load and at the right voltage and balance supply, then most likely culprit is coreloss--which can be verified using power analyzer to separate real power from reactive power. High reactive current can be a saturation problem (i.e., wrong connection for the voltage ?).

Exactly...Keith hit the nail on the head. That's why I believe the problem is the winding data/connection. 8 watts per lb shouldn't be a stator killer.

In today's electric motor repair business it is all about efficiency. Our industry continually gets accused of depleting efficiency during the rewind process and here we are advocating rewinding a core with high losses. When we adopt this attitude we (1) increase the customers operating costs, (2) increase the operating temperature of the motor and therefore shorten the motors life and (3) increase the customers maintenance costs. It was the introduction of High Efficiency Motors that led to the increased use of core testers and 8 watts per pound loss is anything but high efficient.
I for one pay attention to what my core loss tester tells me.
Responsible repair facilities will advice a customer what the options are and what the best option is, even if that advice means a replacement instead of a repair. when it comes to motors that are not so easily replaced, a core rebuild is an option hat the customer should be offered.
In some cases the customer may tell you just to go ahead. Fix it without the core rebuild but then, If you have advised the customer properly, you can't be blamed for a pre-mature failure.