OT: Electric Motor Rebuild Question

[bsmith]

I have a goulds centrifugal water pump close coupled to a Baldor single phase 10 hp 3450 rpm 215jm frame electric motor. This is used to provide high pressure water to a soccer field sprinkler system. After studying the pump curve, I custom cut an impeller such that the electric motor would provide maximum pressure and maximum water throw and yet not exceed the motor's full load amp rating of 46 amps.

As it turns out, the motor does run at full load amps for the entire 5 hour watering cycle. And for about 30 seconds between the transition from one sprinkler circuit to the next (there are 125 sprinkler heads spread over 14 circuits) the amp flow jumps to about 49 amps before the gpm flow and amps settle back down again. So the motor runs on the ragged edge of its limits, but I do have a supplementary box fan running to help cool it.

The original motor lasted three years in this application, but it burnt up last summer and I had it rebuilt (rewind and new bearings). The rebuilt motor lasted for one more year before burning up again this July.

To improve this situation, ideally, I would either trim the impeller or use a bigger motor. But if I trim the impeller, I would not get sufficient throw from the sprinkler heads as I am just barely getting the water throw I want right now. As for a bigger motor, single phase motors over 10 hp are rare and cannot be found with a 215jm fame which is needed to close couple to my pump.

As an aside, my backup system uses a 15 hp single phase 1740 rpm motor that I use a dual belt pulley drive to a pedestal hypro pump with a 3:1 driver to driven ratio. This setup is hard to keep tensioned and eats belts. And I can't find another motor like that as it was a rare ebay deal. But right now it is working for me.

With this background, I am now trying to decide what to do on my broken close coupled system. Buying a new 10 hp motor for about $950 seems destined to a 2-3 year life and a stock $525 rebuild may only give a one year life as before.

But what about a non-stock rebuild? Is it possible to rebuild the motor with more turns in the windings than stock and thus increase the torque and horsepower rating of the motor? Of course space limitations, limit how many extra wiring turns can be added. I don't care about any reduction in efficiency that might occur and I am willing to provide substantial extra fan cooling. I just want the motor to be more durable for the load I am giving it.

When I ran this idea past my local motor shop, the owner didn't know what to say or predict. He says he just always copies the stock number of wiring turns on the rotor for any motor rebuild he does. We both agreed to take about a week to research the situation before deciding to rebuild or not.

I have successfully "hopped up" many things in my life, but have never tried it with an electric motor. Maybe some of you guys can comment on what could be done in a rebuild to make an electric motor more durable than stock with the same or better power than stock.
Bill

[rhmalsch]

I don't know about the motor. Other things you could consider. Fewer heads per circuit. Change timing of circuits so that next circuit opens before last circuit closes. Install 3 or 4 groove sheaves on 15 hp pump, use grip notch belts, and install an electronic starter to ramp start the motor over a longer time period. Find a 10 hp with a higher service factor. Use and test an electronic overload relay to prevent motor failure. Driving a centrifical pump beyound its design speed can cause cavitation.

[Evan]

Have the motor rewound with VFD rated wire. It has thicker insulation and is good for higher temperature. There is also no reason that a proper rewind should last less time than the factory wind if the rewind is done correctly. Changing the number of turns doesn't work the way you think. To get more power you use heavier wire with less turns. More turns will require going up a gauge and that will mean less torque and higher flux in the core which may saturate it increasing losses. It will also result in lower current draw in part because of the higher resistance of the windings which will also increase losses.

[oldtiffie]

Put a new line between the inlet and outlet ports of the pump and install a shut-off valve and a relief valve in that line.

That "by-pass loop" will limit the pressure in the line. The fluid will circulate between the outlet and inlet ports when either valve is open.

The relief valve will control maximum pressure. The "over-flow" from the relief valve can be routed back to the inlet side of the pump as well.

The shut-off valve can be set to "full-open" when starting to minimise starting load/current and closed when the motor has run up to speed. Use a manual control. There is no loss of fluid.

In one job I had, this was standard practice to start very heavy duty hydraulic positive-displacement swash-plate pumps. I have used it on smaller set-ups as well.

[John Stevenson]

Replace it with a 3 phase and VFD drive.
This way you can get whatever power you need and also tune the VFD to protect the motor.

[Mark McGrath]

It would be difficult to rewind it with thicker insulation or more winding turns because there won`t be enough room in the stator slots.

[bsmith]

Rhmalsch, can't really do any of the things you suggest except to the backup system regarding the addition of more sheaves and better belts. Can't modify the sprinkler system because it's too much digging and turf damage. All sprinkler controllers providing 24 volts ac to pilot operated control flow valves always seem to have some overlap as one circuit closes and the other opens. It think it is more with how the pilot operated control valves respond in that they don't close off quickly as the next circuit opens.

Tiff, I always need the maximum possible water pressure from this close coupled centrifugal pump/motor and a bypass loop would just reduce the pressure available and cause too small of a water throw from the sprinkler heads.

John, water sprinkling systems perform best with the maximum pressure you can give them all the time. The more overlap on throw you can get from one sprinkler to another the better. If you ever have a circuit where you actually have what might look like excessive pressure (very rare), you just reduce the time that circuit is on. So being able to vary the power to a sprinkler pump using a VFD is not really a useful feature. I have thought about using a rotary phase converter and a bigger than 10 hp 3 phase motor, but I would need a new pump as well and all of that adds up to several thousand dollars.

My current close coupled single phase 10 hp motor/pump does the job if it could just last long enough. If I could go five years between a rebuild that would be ok. I forgot to mention that the motor rebuild from a year ago did use the higher quality inverter rated wire as Evan suggested. But another detail left out was that I removed the old "heater" style Square D overload starter relay and just used a contactor. In the first month of usage the overload relay was tripping out too often and I thought it was just faulty nuisance tripping as the amp measurement under full load was normal. When the motor worked for many months (11 more to be exact) with no overload protection, I thought everything was ok.

My question for Evan and other motor experts now is would I see longevity improvement if the motor was rebuilt with fewer winding turns, but using bigger than stock gauge inverter duty rated wire. Along with this, also use an electronic overload relay that I can dial in specifically the amps and time range to cut out at if an overload does occur. What do you think?
Bill

[Evan]

The additional thickness of inverter duty wire insulation isn't enough to preclude it's use when rewinding most motors. Motors are never wound to maximum fill and the xtra thickness of the insulation isn't that much. It doesn't take much to greatly increase the insulation resistance and durability.

Rewinding with fewer turns of heavier wire will increase current draw but will decrease heating as the resistance losses go up/down as the square of the wire cross section. In a dc motor it would result in higher idle rpm and higher current but less heating. In an AC motor it will result in a lower slip angle at full load but with higher current that may exceed the nameplate maximum and therefore require a higher capacity circuit. It will not result in higher magnetizing forces as they are proportional to the number of turns.

In most cases it should be possible to rewind the motor to factory spec but with better insulation capability. It is the insulation that is failing so replacing the wire with VFD rated wire is what I would do.

Incidentally, I am not talking through my hat. I just purchased about 1000 feet of VFD duty wire to rewind some motors. The insulation is MUCH better than your regular duty magnet wire. There are very significant differences not only in the thickness of the insulation enamel but also the actual composition of the enamel and it's heat tolerance. One possibility it to have the motor rewound close to spec by going down a half gauge in wire size. Half gauge wire (odd numbers) is commonly available for just this purpose.

[Richard-TX]

Replace it with a 3 phase and VFD drive.
This way you can get whatever power you need and also tune the VFD to protect the motor.


That is exactly what I would do as well. VFDs are mandated by law in some areas for irrigation pump systems.

[lazlo]

Have the motor rewound with VFD rated wire.

It's not cost-effective in most cases to have an old motor re-wound -- it'll cost as much as a new motor, which will have F-class (what you're calling "VFD rated wire"). I'm surprised that the motor shop quoted the OP $525 for a 10 HP re-wind. Did that include varnishing? MikeyD got an estimate to have a 3 HP re-wound in Austin, and it was $400 -- about the same price as a new 3 HP motor.

By the way, extra wire insulation necessary on a VFD is highly related to the voltage of your drive/motor. What happens is that the dv/dt of the IGBT (the power transistor) on a PWM (VFD) drive, and the reflected wave when the PWM hits the motor, causes a lot of stress on the insulation.

So at 230V, you can use an ordinary Insulation Class B motor (i.e. an old stock common induction motor) as long as it's not hundreds of feet away from the drive.

At 460V, as a general rule Insulation Class B is not good enough. You need Class F Insulation.

At 575V, Class F is good enough only for the shortest of motor leads.

[bsmith]

Lazlo, it is true that for most 3 phase motors and smaller single phase motors it is not worth paying to have such motors rewound. But for large single phase 230v motors, it is a different story. The best price I can find on a new 10 hp 3450 rpm 215 jm frame single phase motor is nearly twice the cost of rewinding from my local motor shop (Frank's Electric Motor) in Bryan, TX

I don't know of the shop's price included "varnishing" as you mentioned, but he claims it does include using what he calls the better inverter rated wire. Since VFD's are basically inverters I assume that the terms VFD and inverter rated wire are synonymous.

I will look into the wiring type issue further as Evan points out that VFD rated wire is MUCH better and that the key to motor survival is the survival of the turning wire insulation.

As concerns improving on a stock ac electric motor design it sounds like there are quite a few trade-offs to consider. With gas engines there are many easy ways to improve them, but I guess not so with electric motors as their original factory design locks in so many limitations. This indicates that it may not be wise to deviate too far from the stock number of windings.

It is also great to know that there are odd gauge number sizes in VFD rated wire and its thicker insulation. If winding space is tight, drop a half gauge to maintain winding number and get the better insulation.

I will explore several approaches on this with the first approach being to go with the same gauge wire in the highest quality VFD rated insulation I can get and duplicate the factory number of turns.

But just from a theoretical perspective if I went with larger gauge VFD rated wire than stock and fewer turns than stock which causes higher amp draw but more power output than the normal name plate rating of the motor, would this be a problem if I use additional fan cooling, sufficient size motor leads, and an appropriately set electronic overload protection relay?
Bill

[lazlo]

I will look into the wiring type issue further as Evan points out that VFD rated wire is MUCH better and that the key to motor survival is the survival of the turning wire insulation.

Bill, that's my point -- the VFD wire itself is not any different, it's the insulation, which is more able to withstand the voltage spikes that you get when you drive a motor with a VFD. With old-school PVC wire insulation, the VFD spikes can degrade the insulation over time, it gets brittle, then cracks and shorts.

But more importantly, if you're driving the motor with 230V, you don't need VFD F-class insulation unless you have a very long run from the VFD to the motor.

[Evan]

It's not cost-effective in most cases to have an old motor re-wound

For some odd reason the rewind shop I bought my wire at is doing very well. The sawmills and mines here have their motors rewound. The rewind shop has a bridge crane and a 12 foot tall varnish tank. That should tell you a bit about what they handle.

Robert, I will point out that his motor is failing because of insulation breakdown. Anything that will prevent that is worth it.

[lazlo]

For some odd reason the rewind shop I bought my wire at is doing very well. The sawmills and mines here have their motors rewound.

No doubt, but I'd gather those are gigantic motors, yes?


Robert, I will point out that his motor is failing because of insulation breakdown. Anything that will prevent that is worth it.

It doesn't sound like his motors are failing from insulation breakdown. It sounds like he's pushing to motors past their normal operating conditions, and they're just flat-out burning out:


the motor does run at full load amps for the entire 5 hour watering cycle. And for about 30 seconds between the transition from one sprinkler circuit to the next (there are 125 sprinkler heads spread over 14 circuits) the amp flow jumps to about 49 amps before the gpm flow and amps settle back down again. So the motor runs on the ragged edge of its limits

He's talking about rewinding with larger gage wire to "hot-rod" the motor, which is completely different than the insulation classes for VFD wire, where the wire gage is the same whether it's Class B insulation or Class F "VFD" insulation.

I think he might be better off just stepping up to a bigger motor and running it at a lower service factor.

[Evan]

It doesn't sound like his motors are failing from insulation breakdown. It sounds like he's pushing to motors past their normal operating conditions, and they're just flat-out burning out:


I don't see a difference between "burning out" and insulation failure. From the info given the motor is being operated at maximum, not over it. If it can't take it then better quality insulation should help. To step up to a 3 phase motor and inverter will cost quite a bit more than a rewind. I agree it is the best option but it seems cost is a major consideration. Are single to 3 phase inverters even available in a 15 hp size?

[edit] 49 amps intermittent isn't over spec for a 10 hp motor.

[jcarter]

I am a mechanic at a golf course. While I do not typically work on our irrigation system I am called on from time to time to help diagnose problems with the irrigation system. The system we use now consists of a 15hp pressure maintenance pump and two 25hp main pumps, all three phase variable frequency drive submersibles. The last course I worked at had a 10hp pressure maintenance pump and two 25hp three phase vertical turbine pumps with a pressure tank and Clayton valve to regulate flow. Was your system designed and built by a company that does irrigation systems? It sounds like your pump is not matched to the rest of your system. I am just thinking out loud but is it possible to run another pump parallel to the existing one to help carry some of the load? This is standard for larger systems such as golf courses. Is there a pressure tank or Clay valve in this system? I think to reach an accurate conclusion more info about the entire system may be needed.

[bsmith]

Every burnt up electric motor I have seen had burnt insulation and shorted out wire turnings. I guess there are other things that could fail on a motor that leads to burnt insulation, but I think in my case it just comes from continued degradation of the insulation. If the load duty was extremely severe, the motor couldn't last a year or up to three years as the original did.

Unless I was misunderstanding something, I was thinking that the main advantage of the VFD rated wire was it's better insulating and heat resistance capabilities. I think using the best wire possible in that regard with a good electronic relay overload protection would get me back to 3 plus years longevity. This would be to use the same gauge wire and same number of turns but have the thicker insulation.

Concerning my hot rod question, I am still wondering if I went with larger gauge heavy insulated VFD rated wire than stock and fewer turns than stock which causes higher amp draw, would this help the motor withstand the load easier if I use additional fan cooling, sufficient size motor leads, and an appropriately set electronic overload protection relay?

Concerning jcarter's comments, this sprinkler system is for two lighted private soccer fields arranged end to end. The water is pumped from a 30 acre lake 800' away. This is for a youth competitive soccer club and there is no public money involved. It is funded mostly with my money plus some contribution from the club. So economic engineering is required, and in general, I would say that we have been very successful with all our electrical and mechanical systems compared to what the city spends on similar public services.

I designed the sprinkler system myself and knew that I was pushing limits on what a 10 hp motor could do long term. But 10 hp single phase on a close coupled pump is the biggest you can go single phase.

Utility 3ph power is prohibitive in cost to this location. I use a 10 rpc in my shop which is located a long ways from the pump. Guess I could buy another RPC, and put it by the pump. But again, that approach needs a new 3 ph motor and pump that will mate with it.

I might try one more rebuild of the 10 hp motor, but with greater protection using premium wire and a good electronic overload relay. And if I can get my belt situation to be more reliable on my rare 15 hp single phase backup system, I could make that the primary system and the 10 hp close coupled system the backup system. That approach for sure would let the close coupled unit survive many more years than it has so far.

One way or another, I'll keep water flying in the air and that fine GN1 turf grass growing green.
Bill

[Scishopguy]

bsmith...It sounds like you are running on the hairy edge of the capacity of the pump and motor to irrigate the field. As J Carter suggested, you might need to redesign the system and break it up into a minimum of at least 2 zones. This should give you better performance from each half and not overload the motor. It should not hurt to water one half and then the other. You can use solenoid valves and a timer to take care of it. It would not even mean replumbing much of the system, just split it and feed one side and then the other.

[lazlo]

Every burnt up electric motor I have seen had burnt insulation and shorted out wire turnings.

Right, but most of the time it's because you've exceeded the current limits of the wire (often from a short), so the wire is melted from the inside.

That's a very different failure mode than VFD breakdown, where high frequency voltage transients on motors running 460V or higher will degrade class B insulation (typically PVC) to the point where it disintegrates and falls of the wire and shorts.

In other words, it doesn't sound like your failure mode is related to VFD transients -- I bet it would fail the same if you were on native 3 phase power.


Unless I was misunderstanding something, I was thinking that the main advantage of the VFD rated wire was it's better insulating and heat resistance capabilities.

That's true -- the higher the NEMA class, the higher the temperature the insulation is rated. There are several levels beyond the F-class insulation that's normally referred to as "VFD Wire":

http://www.engineeringtoolbox.com/nema-insulation-classes-d_734.html

But the wire gage is exactly the same, so if you're exceeding the service factor of the motor, I don't think the insulation class is going to help much.

[lazlo]

49 amps intermittent isn't over spec for a 10 hp motor.

A 10 HP motor on 230V power has a Full Load Current of 28A. So he's almost 100% over his service factor. That's really pushing it.

[Evan]

Robert,

You can't just crank out the numbers with Ohm's law to figure out the current rating of a motor. A very similar 10hp motor from Baldor is rated at 42 full load amps at 10 hp. An intermittent 49 amp load is well within the specification.

http://vts.bc.ca/pics5/baldor.jpg

[ammcoman2]

Another thing to check, if you haven't already done so, is to verify that there is not a voltage drop between the supply and the motor. If there is, then the motor will draw more than full load amps to maintain HP, therefore running hotter.

Look into Class H wire as I believe inverter duty wire has a higher die-electric withstand but not higher temp limits.

Geoff

[lazlo]

You can't just crank out the numbers with Ohm's law to figure out the current rating of a motor.

Evan, I'm quoting the NEMA/NEC standards for Full Load Ratings. The NEMA standards specify 28A @ 230V, and the Baldor nameplate says 31A. The 3A difference is due to the motor efficiency -- that's a high-efficiency motor.

The 42A is at it's minimum voltage.

http://i164.photobucket.com/albums/u15/rtgeorge_album/FLC.gif

[bsmith]

Jim, my sprinkler system already has 14 circuits or zones as you call them. The control valves are basically solenoid valves. I cannot separate them easily into any more divisions.

Lazlo, I think you have missed the part about my motor being single phase. Your amp rating was for a typical 10 hp 3 ph motor. My motor is this motor http://www.electricmotorwholesale.com/index.cfm?fuseaction=catalog.prodInfo&productID=128&categoryID=103 except that I have the 215jm frame for mating with a close coupled pump. Note that the full load amps rating is 46. Note also the hefty $1018 price.

I guess I need to study the wire classes more because it is not VFD effects that I need protection from it is heat. And like Evan has been advising, what we are calling VFD wire has good thermal protection. I just need to make the best selection I can for new turning wire. Like Geoff said, that could be class H wire which has higher heat resistance than some of the basic VFD wire.

Voltage drop is not a problem for this motor. I am 20' from the panel using #4 size conductors so there is very little voltage drop even at full load.

Anyone want to venture a guess on what would happen if I went with larger than stock gauge premium VFD rated wire and fewer turns than stock? Or is there just not really a good way to "hop up" an electric motor to get more power and maintain stock reliability?
Bill

[lazlo]

Actually Evan, you also picked a weird "Air Over" motor: one of those odd HVAC motors that is totally enclosed, but doesn't have fan, so it has to be installed in an air stream.

Here's Baldor's normal 10 HP motor:

http://i164.photobucket.com/albums/u15/rtgeorge_album/Baldor10HP.gif

[lazlo]

Lazlo, I think you have missed the part about my motor being single phase. Your amp rating was for a typical 10 hp 3 ph motor. My motor is this motor http://www.electricmotorwholesale.com/index.cfm?fuseaction=catalog.prodInfo&productID=128&categoryID=103 except that I have the 215jm frame for mating with a close coupled pump. Note that the full load amps rating is 46. Note also the hefty $1018 price.

Ah, I didn't notice that. But like I mentioned earlier, most modern motors, including that one, already has F-class insulation, which is the "VFD rated" insulation that you're referring to.

Like I said, there are several NEMA grades of insulation beyond F-class (H and I-class insulation, which I think are for the totally enclosed explosion-proof motors). But If you keep burning out a good Baldor motor, especially since it's single-phase, you really want/need to move up to a bigger motor.

[fasto]

I'd measure the motor's temperature rise. Most are rated no more than xx degC, usually 40 or less degC. A box fan isn't going to cut it. I'd get a BIG fan and really move some air over the motor. At least 200+ LFM, maybe 400.

Check motor vibration, as that can eat up bearings, leading to more resistance turning, therefore more amps. If the impeller has been in use for some time (I believe you said 6 years) it may be slightly out of balance.

I think Tiffie's suggestion of a bypass was to temporailly unload the motor for a few seconds while the zones were switching to eliminate that overcurrent spike. Most 1ph motors that big have a service factor of 1.0. That means if you overload it it burns out, almost immediately. The unloading idea has merit, open the bypass, switch zones, close bypass.

You might look into power factor correcting caps, too. Not starting or running caps you've already got those. Power factor correction will give a bit more "apparent voltage" with the motor running, as it cancels out the low PF of induction motors (fully loaded, anout 0.82 to 0.84 PF).
--
Aaron

[bsmith]

One of my first lathe projects was turning down an impeller to just the size that the charts predicted for the hp available, the pressure and flow I wanted, and for the pump style I was using. The impeller diameter I needed was not a stock factory size. I was so pleased with myself when I turned down an oversize impeller to my custom size, installed it in the motor, ran the pump, and measured 46 amps.

But in retrospect, I should have probably factored in other situations such as what happens when there is a pipe break and the pump might run against little resistance and want to flow way more gpm and thus draw more amps than full load amps. Another situation occurs when the system is drained dry and 1200' of 3" diameter pipe has to be filled and pressurized before water flows from the sprinklers. Yet another situation where the pump could run for some minutes with little resistance and thus draw too many amps.

I don't have those situations very often, but that's when your overload protection should kick in. And it did for the first three years. Just started tripping too much in the first month of using the rebuilt motor and then I foolishly disabled it, but still it ran for a year.

Unfortunately, they just don't make a bigger single phase motor with a 215jm frame to fit my pump. I will rewind this motor with the best wire and turn it into a backup system with good overload protection reenabled.
Bill

[Evan]

Actually Evan, you also picked a weird "Air Over" motor: one of those odd HVAC motors that is totally enclosed, but doesn't have fan, so it has to be installed in an air stream.


No it isn't. Air over simply means it needs a fan to provide circulation. Look in the specs where it says "Enclosure". It's open frame.

[Evan]

I think Tiffie's suggestion of a bypass was to temporailly unload the motor for a few seconds while the zones were switching to eliminate that overcurrent spike. Most 1ph motors that big have a service factor of 1.0. That means if you overload it it burns out, almost immediately. The unloading idea has merit, open the bypass, switch zones, close bypass.

You might look into power factor correcting caps, too. Not starting or running caps you've already got those. Power factor correction will give a bit more "apparent voltage" with the motor running, as it cancels out the low PF of induction motors (fully loaded, anout 0.82 to 0.84 PF).
--


The service factor on Bill's motor is 1.15 so 49 amps is well within the rated 53 amp limit. The power factor is .86 so that could be improved some with a capacitor.

[rdfeil]

Lazlo and Evan,

You are both right, The OP specified a single phase 10 Hp motor. Lazlo, your specs are for a 3 phase motor. One of my mentors has been in the motor business for 60 years and he has stated over and over again that motors will withstand service factor applications for an indefinite time. So, going with a 46 amp full load current and even a 1.1 service factor the motor will run 24/7 at 50.6 amps as long as it is cooled properly. In my business I regularly get involved with large compressor motors (100 Hp plus) which are loaded to rated full load current 24/7 and into the service factor some of the time. Motor failure is VERY rare.
Now for the OP's original question... I would expect that the rewind was sub-standard or there is more going on than you realize. A proper rewind, with the proper wire, slot insulation, packing and varnish will last as long as the original windings. I would look over the windings closely to determine the mode of failure. Are the windings evenly baked? Are there point failures near the slot edges? Did the start switch fail and leave the start winding engaged or fail open and with no overload did the run winding fry because of locked rotor? There are a lot of reasons why a motor can fail but, the operating conditions described are not one of them.

Edit: Thoughts on the rewind price $525.00, way to cheap for a 10 Hp rewind in todays economy. I would be suspicious of quality.

Robin

[gearedloco]

[ ... ]

But in retrospect, I should have probably factored in other situations such as what happens when there is a pipe break and the pump might run against little resistance and want to flow way more gpm and thus draw more amps than full load amps. Another situation occurs when the system is drained dry and 1200' of 3" diameter pipe has to be filled and pressurized before water flows from the sprinklers. Yet another situation where the pump could run for some minutes with little resistance and thus draw too many amps.

[ ... ]
Bill

It doesn't work that way. With less of a load, the rotor will turn _slightly_ faster causing the slip-frequency to decrease, the back-EMF will rise, and the motor will draw _less_ current.

Since air goes through the nozzles easier than water, when the water gets to the nozzle there will be a pressure transient and the load on the pump will increase, the rotor will slow down _slightly, the slip-frequency will increase slightly, and the motor will start to draw more current.

I don't know how close you are to saturating the iron in both the stator and the rotor. Economics being what it is, I doubt that there's a whole lot of reserve at full load. But if the iron starts to saturate, the current will rise drastically, the windings will get really hot, and if the over-current doesn't trip real quick it's off to the winder's shop again. And it probably happen very quickly.

By the way - the data sheets I've seen have all been for 3-phase motors. The current ratings shown have little to do with the current ratings for a single-phase motor. I used to know how to make the conversion, but it doesn't come to mind at the moment.

OTOH - without being able to see and "lay hands on" the actual equipment in operation, my advice is certainly worth no more than you paid for it.

-bill

[Evan]

By the way - the data sheets I've seen have all been for 3-phase motors

The one I posted (the first one) is for a "farm duty" single phase 230vac 10hp motor.

[ACF]

If your motor is burning up due to heat, have it rewound using MW-16 magnet wire, as this has the highest thermal stability of all the wire enamel coatings. It is a polyimide and is the top of the coating for magnet wire.

Lot's of motor people have cut costs to the point their motors are not what they should be. When VFD's first came to the market many motors failed, however some makers such as Lincoln, that were doing their motors properly had a lot less number of failures. These were the days before inverter duty rated wire and Lincoln would guarantee their motors for 5 years in inverter use.

I am not a fan of Baldor motors because they have cut costs to the point their motor reliability has suffered. If I were buying a motor I would try a Reliance, Lincoln, or a Marathon motor assuming you can get the ones made in America.

I was in the electrical insulation (varnish) industry for 25 years and watched as the motor industry degraded their products to cut costs and watched the quality of their products go down hill. They would blame the varnish for not doing it's job, but they didn't apply the proper amount of varnish and they didn't bake it thoroughly and then wondered why they had bunchs of failures. It was sad to watch.

Sorry to be long-winded, just my 2 cents worth.

Chris

[bsmith]

It doesn't work that way. With less of a load, the rotor will turn _slightly_ faster causing the slip-frequency to decrease, the back-EMF will rise, and the motor will draw _less_ current.

Since air goes through the nozzles easier than water, when the water gets to the nozzle there will be a pressure transient and the load on the pump will increase, the rotor will slow down _slightly, the slip-frequency will increase slightly, and the motor will start to draw more current.

I don't know how close you are to saturating the iron in both the stator and the rotor. Economics being what it is, I doubt that there's a whole lot of reserve at full load. But if the iron starts to saturate, the current will rise drastically, the windings will get really hot, and if the over-current doesn't trip real quick it's off to the winder's shop again. And it probably happen very quickly.

By the way - the data sheets I've seen have all been for 3-phase motors. The current ratings shown have little to do with the current ratings for a single-phase motor. I used to know how to make the conversion, but it doesn't come to mind at the moment.

OTOH - without being able to see and "lay hands on" the actual equipment in operation, my advice is certainly worth no more than you paid for it.

-bill

My ranch workers were always amazed when I would turn the pump discharge ball valve to the sprinkler system towards being closed and show them how the amp draw on the meter decreased. When I opened the valve, (making it easier to flow water) the amp draw increased to full load.

It seems counterintuitive at first, but when you restrict output flow the impeller slips against the water and thus less water is pumped and less work is being done and thus less amps are required. When you make flow easier to occur, the impeller takes bigger bites of the water (less slippage occurs) and does more work. That electric motor is trying to maintain a constant rpm and making it flow more water makes it hard to hold rpm.
Bill

[bsmith]

Lazlo and Evan,

You are both right, The OP specified a single phase 10 Hp motor. Lazlo, your specs are for a 3 phase motor. One of my mentors has been in the motor business for 60 years and he has stated over and over again that motors will withstand service factor applications for an indefinite time. So, going with a 46 amp full load current and even a 1.1 service factor the motor will run 24/7 at 50.6 amps as long as it is cooled properly. In my business I regularly get involved with large compressor motors (100 Hp plus) which are loaded to rated full load current 24/7 and into the service factor some of the time. Motor failure is VERY rare.
Now for the OP's original question... I would expect that the rewind was sub-standard or there is more going on than you realize. A proper rewind, with the proper wire, slot insulation, packing and varnish will last as long as the original windings. I would look over the windings closely to determine the mode of failure. Are the windings evenly baked? Are there point failures near the slot edges? Did the start switch fail and leave the start winding engaged or fail open and with no overload did the run winding fry because of locked rotor? There are a lot of reasons why a motor can fail but, the operating conditions described are not one of them.

Edit: Thoughts on the rewind price $525.00, way to cheap for a 10 Hp rewind in todays economy. I would be suspicious of quality.

Robin

Robin, thanks for all the good info. I have the motor sitting at the shop right now awaiting a decision. $525 was last year's price, don't really know what it is this year. I am going to look over the motor myself for the details you mentioned. But it is great to hear that in your experience running motors at the limit of their ratings is generally not a disaster waiting to happen.

What do you think about using MW-16 magnet wire as ACF suggested for the rewind? What do you think is the best wire for a rewind on my 10 hp 1 ph motor?
Bill

[Evan]

It seems counterintuitive at first, but when you restrict output flow the impeller slips against the water and thus less water is pumped and less work is being done and thus less amps are required

It's called "churning". When a pump is churning it isn't doing any work other than heating the water a bit. Naturally the current goes down.

[J Tiers]

Have the motor rewound with VFD rated wire. It has thicker insulation and is good for higher temperature. There is also no reason that a proper rewind should last less time than the factory wind if the rewind is done correctly. Changing the number of turns doesn't work the way you think. To get more power you use heavier wire with less turns. More turns will require going up a gauge and that will mean less torque and higher flux in the core which may saturate it increasing losses. It will also result in lower current draw in part because of the higher resistance of the windings which will also increase losses.


The additional thickness of inverter duty wire insulation isn't enough to preclude it's use when rewinding most motors. Motors are never wound to maximum fill and the xtra thickness of the insulation isn't that much. It doesn't take much to greatly increase the insulation resistance and durability.

Rewinding with fewer turns of heavier wire will increase current draw but will decrease heating as the resistance losses go up/down as the square of the wire cross section. In a dc motor it would result in higher idle rpm and higher current but less heating. In an AC motor it will result in a lower slip angle at full load but with higher current that may exceed the nameplate maximum and therefore require a higher capacity circuit. It will not result in higher magnetizing forces as they are proportional to the number of turns.

In most cases it should be possible to rewind the motor to factory spec but with better insulation capability. It is the insulation that is failing so replacing the wire with VFD rated wire is what I would do.


I work for a motor designer, we do motor drive consulting and design, as well as some motor and related device design.

I happen, along with my boss, to have recently studied motor winding overvoltage failures.

1) VFD rated wire is a straw man. The insulation of each wire, turn-to-turn, is NOT what typically fails from overvoltage spikes. It makes great ad copy, but VFD rated wire makes very little difference actually IN a motor.

2) The usual failure is from phase-to-phase, where there is supposed to be thick insulation separate from the wire.

When that insulation is shifted due to the hammering that is used to form the coils after the winding inserter is done, there may be a close spacing from phase-to-phase. That will fail with sufficient voltage.

If proper phse-to-phase insulation is in place, normal wire insulation is normally perfectly adequate for higher voltages. I have seen a motor core fail at 800V, and then pass at 1800V+, merely by a small piece of insulation being moved or inserted. These are non-destructive tests, the nature of which I am not at liberty to discuss.

3) as mentioned, altering the turns will alter the current, and may change the rated voltage of the motor, or delete a 50Hz rating. It obviously changes the back EMF, the locked rotor current class, and also a number of other things, not all of which are good, especially since you will NOT be changing the alloy or thickness of the rotor end rings etc, and are stuck with a given rotor resistance.

4) the good news is that most motors are designed on standard punchings, and so are not necessarily fill-restricted.

5) the bad news is that most motors ARE designed to have the optimal stack, by changing the number of standard punchings used to form the stack. So they probably ARE saturation-restricted as far as number of turns etc. Without knowing details, it would be difficult to determine if any advantage is possible for a different number of turns.


The best possible suggestion with most motors is to put them back as they are, or merely adjust turns for voltage differences when desired.

If the motor is wrong for the application, power-wise, it will probably still be wrong no matter what you do to it turns-wise.

With knowledge and care one CAN rewind for a different speed, etc, although going lower often will result in a power problem.

[bsmith]

The consensus appears to be that there is little to be gained as well as some risk in trying to second guess the original number of winding turns on stock ac motors. So I will have my local shop rewind my motor with the stock number of turns.

And although VFD rated wire may not be that much protection for voltage spikes, I will still try to use the best wire I can get for resistance to thermal breakdown as heat in my application is probably the biggest factor to address for longevity. Maybe the best wire is the so called MW-16 magnet wire.

Jtiers, if the separate non-wire insulation is so critical for phase to phase protection, can you recommend any particular practice or procedure to use on placing additional insulation when rewinding a 230v 10 hp single phase motor? Or maybe that separate special insulation placement isn't an issue for single phase motors which can't be driven from a VFD?
Bill

[John Stevenson]

Get them to put a thermal in the windings and bring the wires out to your contactor thru a pair of aux contacts.
This way if it does get too hot it will trip.

There should be no reasons nowadays for modern motors to burn out with what we have on hand to monitor them.

That is one reason I suggested a VFD in that they can be programmed to look after the motor and not to imply you wanted variable speed.

I must have fitted as many VFD's to machines over the last two years just for operation on single phase and monitoring as the ones that need variable speed.

I have one on my 14 x 40 lathe just for the soft start as it's a direct on line start and it makes it easier on the transmission, the pot isn't even wired.

.

[J Tiers]

Jtiers, if the separate non-wire insulation is so critical for phase to phase protection, can you recommend any particular practice or procedure to use on placing additional insulation when rewinding a 230v 10 hp single phase motor? Or maybe that separate special insulation placement isn't an issue for single phase motors which can't be driven from a VFD?
Bill

The "standard" insulation practices will work fine for non-VFD uses. Most motor manufacturers actually test their motors for breakdown and mark the higher voltage ones as "VFD rated". The rest go for normal use.

Single-phase will be similar to "normal use".

BTW, single-phase motors are not ALL non-vfd.

The relevant type for actual power use is the "PSC" type, which has a run cap and no starting switch. They can be speed adjusted by various means, as can the shaded pole type at lower powers.

That isn't very relevant to your use if you will use a standard single-phase motor, especially a larger one.

I will say that 10HP or 15HP single-phase motors are not common items. If you are getting poor life, something is wrong. if the windings "burnt up", it is a fair bet that you had excessive current, or possibly poor cooling air flow, low voltage, or other bad condition.

Since you say the current is OK, cooling is a possible problem.

OTOH, if the motor runs at FLA, AND a bit more for a while, is it's service factor 1.1? That would be wanted to cover variations such as voltage, load, etc.

it is not uncommon to run a motor at full load and use its overload capability to handle peaks. You often get optimum power factor etc that way.

But, you need to be careful to use your overload capability "only once".... I.e. don't count it for covering low voltage, and then AGAIN for overloads, and yet AGAIN to cover less than optimum cooling.

Since this is a pump, the load should be predictable. because of that, one of the transformer-type static phase converters might allow use of a cheaper and larger 3 phase motor instead.

I do NOT mean the usual "Phase-A-matic" type static converter, but instead the phase-shifting type which power all 3 wires, using a network of parts to shift the phase for the 3rd wire. They work well for a constant, or nearly constant load, and will probably allow you to use a lower cost motor of larger size, which is much more easily replaced (and less likely to need it).

There are some patented static systems, advertised as really good for pumps, and some fairly well-known types which are NOT patented. They work somewhat similarly to a motor of the PSC type, but may be of MUCH higher power.

I have not used one, but they are supposed to actually work rather well.

[bsmith]

John, good idea on putting a thermal sensor in series with the circuit that energizes the motor contactor switch. I have heard that you can epoxy something like a Klixon bimetallic disc somewhere near the windings inside the motor that will open the contactor circuit if the operating temp gets too high. This web site describes that kind of product http://www.sensata.com/products/controls/lighting.htm

Although I can't use a VFD with my single phase motor, I am thinking now that if I had to do it over again I would go three phase. I didn't realize as JTiers points out that there are good static phase converters now available that offer the quality of a full featured VFD but at lower cost because the variable frequency option isn't included because many constant load applications don't need that. That option would probably be as low cost as an rpc and more reliable since there is no rotating machinery involved.

As I mentioned earlier, I also have a 15 hp single phase pumping rig that I am now going to make into the main pumping unit. With new individual belt tensioners, I think I can keep it reliable. For my existing 10 hp 1ph motor, I will rewind it with good wire, install a Klixon disc, use an eletctronic overload protection relay and use this system only as the backup system.

I am thinking that if I had a data log monitoring amperage that the 10 hp sprinkler motor has seen too much overcurrent use. If a worker also decides to open up an additional hose flow while the sprinkler system is flowing full that too would cause additional amp draw. When the lake is completely full, and NPSH goes up (usually a good thing), that too allows for easier flow and more amps drawn.

When I first measured the magic 46 full load amps, the lake was down 2 vertical feet, and I was running the sprinkler zone of maximum flow and it took a little while to settle at that number. I think that I have too many variables that can cause overcurrent conditions for the 10 hp motor and in the past year, I just didn't have enough protection from those situations.

But based on all the great info from you guys, I have much more confidence I can create a reliable system for the soccer fields and I am also better armed for my next electric motor challenges where I will attempt to irrigate some large pasture areas.
Thanks again,
Bill

[John Stevenson]

John, good idea on putting a thermal sensor in series with the circuit that energizes the motor contactor switch. I have heard that you can epoxy something like a Klixon bimetallic disc somewhere near the windings inside the motor that will open the contactor circuit if the operating temp gets too high. This web site describes that kind of product http://www.sensata.com/products/controls/lighting.htm


Thanks again,
Bill

Bill,
The ones I am familiar with are very small, like a reed switch with 2 wires coming off and you bury them in the windings.
There are only rated at very small power hence having to use the aux contacts on the contactor.

Burying them in the windings puts them directly where the trouble is going to start. You motor people should have more details.
On a single phase you can put one in the start windings and one in the run windings and wire them in series. This way it monitors both sets and if the start winding ever get left in as sometimes happens it should save the motor.

.

[J Tiers]

The internal thermals usually are rated for no more than 250V and about 15 or 20A. At my prior employer, we put resetting versions in all transformers over a certain size, and "one-shot" types below that size.

But they are not very suited to motor use. The places they fit are not usually the hottest places, and often they are not rated for the current due to size. Then you have to use them in series with the contactor coil.

The most usual place for a direct-wired type is on the frame adjacent to the stator core. This will detect long-term heating, and also will have some effect on short term current, since it causes heating of the disc. The frame mounted type can be rated for full current up into the small HP range. Larger motors may need to use the external contactor.

Electrically, it is put in series. For instance a "Y" motor, where the switch has 3 contacts and is put in the "Y" point, or a single phase where the switch is in series with one mains line, with a tie point after it to which the start winding and other windings are connected as if it were the mains line.

They are typically useful primarily for longer term overloads, and generally won't operate quickly except in cases of severe overload where disc heating assists the heat sensing.

[ammcoman2]

I used to work for Texas Instuments in the Controls Division (now Sensata). For this size of motor the 50AA (if it is still sold) would be ideal as the sensor is put on the winding (up to 3) and the controller hooks up to the contactor. Due to its low mass, the sensor can track an incredible rate of rise in temperature, so locked rotor conditions are not an issue. There are other options such as the 9700 or 7AM devices. Their mass is low enough that they will track temp rises on this size of motor but locked rotor will have to be handled by the start switch. We often sold these devices to rewind shops - I vaguely remember that they were sold in kits of 3 already wired up in series.

Geoff

[John Stevenson]

Those big thermals that wire into the main windings are old hat as they are not sensitive enough to work whilst having to handle motor loads of up to 20 amps.

As Geoff says the modern tiny senors can react very fast to differences in temperature and using a contactor, VFD or other form of monitoring device they can control the motor as opposed to just switching it off.

Most modern metric framed motors over here above about 5 Hp have these already fitted from new. Whether you connect then is up to the guy in charge.

.

[bsmith]

Great info guys. Fast responding tiny sensors buried right in the windings and wired in series with the low amp starter circuit sounds like just the ticket for great motor protection. I very likely would have missed the concept of using the tiny sensors buried right in the windings without John's and Geoff's clarification on these thermal devices.
Thanks,
Bill

[Don Young]

If I were in this situation I would monitor the motor temperature very carefully. I would be certain that I had adequate (probably more than before) cooling air flow not subject to interruptions and that the ambient temperature was not excessive under some conditions. I would certainly use overcurrent protection but it seems that you have had two motor failures without firm evidence that they were due to overcurrent.

As a comment on the current versus flow situation, I once tried running an HVAC squirrel cage blower out in the free air. It would overheat and trip the overtemp protection. I found that restricting the outlet would cause the motor to speed up, draw less current, and run cooler. So some centrifugal pumps definitely operate normally with some slippage and reducing the back-pressure will will increase the load.

Don Young