3ph motor lost power over time
I have a friend that has a 1950's vintage Oliver 270 direct drive saw. It has a 5HP 220V motor, swings a 14" blade and used to cut through 2" oak as fast as he could push it through when it was new to him about 20 yrs. ago. He has moved his shop a couple of times since then, now the saw is powered by a 10hp rotary converter. He says the saw now will not cut 2" oak without bogging down losing power. He is a master door maker and he knows what all his machines are capable of when they are working right. All other equipment in the shop works fine. Apparently the power has declined over time gradually and he has become concerned, and asked for my help.
Rotary converter output is 256V 256V 265V . About a 10V drop each leg under heavy load @10ft. of 10gauge wire to box and only .4 volt differential in each supply leg between converter and motor. I checked the current under load, pulls about 22A at startup and spins up easy. Motor 6.4 ohms between all fields. Motor has no bearing axial or endplay and turns freely. No unusual vibrations and fence is aligned with blade, saw has a feeder that has been slowed down to allow for loss of power, blade is sharp. I cannot find any mechanical problems, can a 3ph motor become less powerful just by getting older? I have not heard of such a thing but I cannot find anything wrong. Any suggestions of what could be wrong or what else to check?
Last edited by Porschefix; 03-27-2012 at 08:02 PM.
Reason: added info
I don't have an answer to your question! I have worked on an identical saw.....a electrical device is dumb..it works as is or doesn't..electricity doesn't wear out! I would look for a external cause..a bad saw grind, poor incoming power or a bad batch of oak! You have analyzed the motor and have not found fault...look at other things.
I'm sure others here will have better ideas and you likely have considered or done the following.
1. If the belts are worn they can slip slightly even with the right tension without you realizing it and your saw starts to bog down forcing slower feed rates when you're near the limits of the machine. I'd check the belt tension, that they are matched, fitting in the pulley grooves correctly/that the pulley groves aren't worn and everything is still aligned. New belts often produce surprising results.
2. I've had some motors where they spun fine and without load everything seemed OK but under load the shaft would shift slightly causing greater friction in the bearings or elsewhere in the motor. If this was the case you should notice parts of the motor running hot.
If I read your response correctly, since all the legs have the same resistance etc. this should eliminate the chance of a partially shorted winding. Since its likely not feasible to move the saw to try in on line 3 phase versus the converter, in the long run it might be simpler to have a motor shop test it. If the belts etc. are OK I would suspect the converter or maybe just the combination of that converter with that motor.
A properly powered table saw is a joy to use, underpowered ones are a pain and dangerous. I will also be interested in the responses of others. I once picked up a free 2 HP motor (single phase) that seemed to produce about 3/4 of a HP going downhill. I checked everything I could, changed the capacitor and never did find the reason. I ended up doing what the previous owner had done, passed it on for free but on a machine where all it needed was 3/4 HP.
All the 3 phases I've picked up so far, some from scrap piles may have looked ugly but have produced at least if not greater than the rated HP as far as I can tell. I think its far more likely that the motor isn't the issue.
Rule out the drive but if your saw motor is really slipping the belts they should smoke. But you never know.
I suspect the problem is the phase converter. If the generated leg is weak, that is its balance capacitors have pooped out the load motor will develop only 1/2 to 2/3 its rated mechanical power. If full rated motor power is important and no real three phase service is available, I strongly suggest a VFD or Phase Perfect. This may seem expensive but it will allow your motor to develop full rated HP wheras even the best factory phase converter will allow about 85 - 90% and a better home made will deliver about 75%. There is no escaping the weak leg. If you absolutely positively have to have full rated three phase motor HP you will not get it from a single phase line via a phase converter - unless the idler motor has a separately excited rotor and the other required electrical embellishments are in place. A phase converter like this would be prohibitively expensive.
Suggest to your friend he acquire a 7 1/2 to 10 HP 230 volt rated variable freqiency drive and install it following the manufacturer's reccommendations, ie: no electrical contacts between the VFD load terminals and the motor leads.
This VFD is to be dedicated to the Oliver saw. If three phase is required for other shop equipment that is a separate problem possibly solvable with a rotary phase converter or more VFD's. A colplete solution will not be cheap unless someong involved is an inspired and dedicated scrounger.
I see one thing that might cause trouble: there is about a 4% voltage imbalance (256/256/265) between the various phase connections. It's not well known that imbalances cause Negative Phase Sequences that cause a reverse torque to be generated which is subracted from the net mechanical output. Try Googling "negative phase sequence affect on power"
Here's a link: http://www.larsentoubro.com/lntcorpo...e_sequence.htm According to this, a 5% imbalance can cause a 25% power output reduction.
I see too that the phase to phase voltage is about 256 VAC. Assuming this motor is wound for 208 VAC, then it's being operated at 23% above nominal. Even if it's wound for 220V, it's higher than nominal. I wouldn't be surprised if the Negative Phase Sequence currents referred to in my first paragraph are made worse because the iron in the stator is closer to saturation. ie., closer to its absolute limit.
What to do? See what the voltage imbalance is when the saw is heavily loaded and if more than say a couple of percent, try to improve the balance, perhaps by changing the run capacitance (if any) in the RPC. I don't know if reducing the voltage is an option, but if it is it might be worth a try.
Best o' luck,
Electrical motor rookie question - I recently replaced the carbon brushes and polished the armature contacts in a small chop saw with a single phase motor. Is it possible there are brushes in a motor like the Oliver's, or are all 3 phase motors brushless?
Last edited by SteveF; 03-28-2012 at 07:13 AM.
No brushes in an AC motor. Sort of one of their "low maintenance" characteristics is they run for a long time unattended so to speak. Bearings may need a shot of grease every 5-10 years depending on service.
Not true. My poor old Wells bandsaw has an AC motor with brushes. It may be safe to say that no new AC motor has brushes, although absolute statements can (almost) always be disproven.
No brushes in an AC motor.
Originally Posted by Jim_Lou
Was that bandsaw motor single phase or three phase?
Matt - As I pointed out I just replaced brushes in single phase AC motor so those can have them.
You said it draws 22A on start up. What we want to know is what are the voltages and amp draw of each of the legs while the motor is operated under an almost stall load. This is where the system needs to be balanced.
Just for kicks, tighten the belts before starting this test. I have seen belts that were properly tensioned slip without creating much heat and no noise. Some belts can get old and glazed where they won't pull a proper load.
Anecdotal report: when I began acquiring machines many years ago, and got started with a rotary phase converter, I began with a 10 hp rated unit. The biggest motor I had at the time was a 5 hp, running a lathe with a 12" chuck on the spindle. It had no jam at all to get going in any of the higher speeds above 450 rpm.
When we finally got real 3 phase from the power company a few years later, the difference was like night and day in motor performance.
Heavy loads require something like 200% nameplate amperage draw for a few seconds and drawing from the power company allows the motor to dip into this reserve. That, and the generated phase is not spaced 120° in time from the other phases, so I've been told, and this has some detrimental effect on the power you get out of the motors, like Duak explained above (thanks for that explanation).
The phases are essentially PERFECTLY aligned at 120 deg, regardless of what someone told you..... otherwise the motor itself was badly made.
But, this applies at a no load condition.
The problem with a rotary phase converter is NOT any of the odd things you probably have been told by someone on the internet....
The basic problem with an RPC is that there is a different SERIES IMPEDANCE from the line to the generated leg than there is on the other two lines, which are direct connected to the incoming power. This impedance is made up of a resistive component, which you can't do muuch about, and an inductive component, which causes "lagging power".
What the so-called balance capacitors do (or should be chosen to do), is to correct the lagging power AT THE HEAVIEST LOAD you will use.
Correcting the inductive lag by adding a capacitive "lead" will raise the load voltage under load and cancel much of the inductive impedance. The resistive part is still there. And you do not want to correct to perfection, since at lighter loads that may cause a higher voltage.
If the RPC has higher voltage at the generated output under no load, that will probably correct itself under load, as long as it is no more than 5 to 10 percent high at no load.
Using a much larger RPC can work without balance caps simply because the large motor has a lower series impedance (less inductance and resistance), and thus causes a lower voltage drop. A problem load needs a higher rated RPC than usual, for this reason.... all the "legitimate" RPC companies explain that on their websites.
BTW, generators have the exact same issue..... but they have it on all three leads, so you do not see it as an imbalance, but rather as an overall voltage drop.
What this load may need is a considerably larger RPC. The 10 HP is within the "normal" range, and a 15 or 20 HP may be the ticket for a problem load like this.
We had an unusual failure of a 3 phase motor. It had ingested a lot of fine metal dust that shorted the magnetic field poles in the stator. After a good cleaning the motor worked again at full power.
Measuring inrush amps when motor starts is difficult because instantaneous value is so high,and drops so fast.Unless the problem is slow coming up to speed, it doesn't mean much anyway.
Measure amps in each leg under load. Full-load amps should be around 7 - 8 each leg. Measure voltage at the same time. You may need at least two meters and helpers to load saw as somebody reads meters. Measure voltages at the motor terminals, and at the converter terminals (both converter 3ph output and 1 ph input).
With the info from these readings, it should become obvious if the problem is in converter, in wiring, in connections, or in motor windings.
Jerry - Since you're one of our resident electronics experts, can you comment about my question on the brushes. Have 3 phase AC motors always been brushless or is it possible that his 60 year old motor might have them?
My oldest motor is from the same time period so I'm kinda' curious.
To those of you who advise checking the belts, read the OP - the Oliver is a direct drive saw with the blade mounted directly to the motor shaft and so has no belts. If indeed there is a loose belt situation, the operator's pants will fall down. The OP states that the saw's owner is an experienced woodworker. This eliminates (or should eliminate) many potential problems,such as a stone dull blade, and the fence has been checked for parallelism to the blade. Does this machine have a brake? Some Olivers have a manual brake and that could be interfering when the saw is in use. You'd think this would be apparent when the saw is off, but we already know this is not an easy, normal problem, so something odd like this can be the culprit. Does it have a short stop electronic brake? That could be an issue, too. Check to see if the motor tie bolts are tight. If not, it's possible the end bells could be shifting under load, causing binding. If the motor has inspection covers, remove one or two to see what (if anything) is inside the motor. I once took off the cover on a presumably sealed Unisaw motor and to my amazement the darn thing was 3/4 full of sawdust! Didn't seem to do any harm, but I blew/vacuumed all of it out, anyway. Highly unlikely to yield results, but check the contactor to see if everything is as it should be.
If all of the other machines in the shop are running properly, then the supply should be OK. He may want to try plugging the machine into another machine's feed to see if that makes any difference. Some sort of problem should become evident. A previous respondent has recommended trying to get a handle on the voltage/current profile under load. This is a great idea and should yield useful results. I hope you have three voltmeters and three ammeters, because that's pretty much what will be needed. Check your motor nameplate to be sure, but I would expect FLA at 230V to be in the range of 15 amps. Your 22 A starting current tends to support this. If this thing is eating 5HP worth of electrons, and you aren't seeing them being put into the work, those electrons are being spent somewhere, and that implies heat being generated. Make some cuts then turn the thing well and truly OFF and start feeling around for the heat.
Just thought of something else. Are any other 3 phase machines running when the saw is on, such as a dust collector? If so, do they bog down, too? If you do not have anything else running, try running some other machine when the saw is in use and see what happens. If the other machine bogs, you do have a supply issue. If nothing changes, then the problem almost certainly lies within the saw, or its unique power supply components. In the highly unlikely event that the saw runs BETTER when something else is going, then that other motor is helping improve the 3 PH balance and again this points to some inadequacy in the RPC.
btw, I have a similarly sized Greenlee saw with a 5HP, 440-only dual speed motor. I have run it with a very basic 3HP RPC and a three phase step up transformer. With a sharp 14" blade, the thing is ferocious, even in 2 1/2" white oak. With a dull blade, it's a friction saw and smoke generator. Either way, it doesn't back away from a load. IMO, your 10HP RPC should be fine, assuming everything is correct and working properly.
on edit, this has nothing to do with the OP, but I will weigh in on the 3 phase brushes issue. I have seen any number of three phase motors with brushes. These have usually been in the 50 to 75 HP range and were used for large HVAC blowers. If the motor has a wound rotor, then it will have brushes. These are usually used in conjunction with some way to vary the power going to the rotor, to control starting rate and motor speed. I have never seen any small motors built like this, and certainly none were fitted to the table saws in question. It's a completely moot point in this context.
I had a conversation a couple years ago with a EE who'd retired after 40 years with Onan. He has built a number of RPC's and said that, for a given idler HP rating, the older and heavier motors will outperform newer and lighter motors, and that the current crop of high efficiency motors make the absolute worst idlers in terms of performance. He said he looked for motors no newer than the early 60's for use as idlers, and mentioned terms such as impedance and saturation as the reasons why the older and larger motors work better.
Originally Posted by JST
The technical explanation largely flew over my head, as most electrical things more complicated than I squared R do, but what you wrote about the need to use a larger converter made me wonder if these old big motors act as if they're somewhat equivalent to one of the new and smaller but higher hp motors when used as an idler.
At the time of our conversation, he demonstrated a large Lempco head and block surfacing grinder for diesel engines that he had running on an RPC he'd built. This grinder used a vertically oriented motor driving a large plate mounted rock, and the combination of rock and plate had significant rotational inertia. Even so, the grinder would come to full speed as fast as I'd expect it to if connected to our utility 3 phase at the shop, and his big old idler carried the same hp rating as the grinder motor.
I thought the same thing I'm checking everything else I know. Thanks for your reply.
I would note that this always direct drive there are no belts. The blade is connected directly to the motor output shaft. I've checked the bearings. I can't find any axial play or end play in the motor that might cause a bind but I suppose that's the next step is taken motor Park and inspected physically. I suppose there's a possibility of moving the sought to direct three-phase line somewhere but we live in a small town Thank you for your information.
again it's a direct drive saw. There's no belts. I also suspect there's some kind of problem between this and the converter but I don't know what it is. As far as changing supply to the VFD. I understand the advantages to that system. However, I believe he's probably financially limited. I really Just want to find out what's wrong with the set up now so we can figure out how to go forward from here. Thanks for your input. I'm working on all of the stuff.
I noticed as you did the difference in the voltage between the legs as well as the high-voltage of 256 V versus the motors originally wound on the nameplate, it says 220. I know that even when I had regular 220 from the power company that I had what they called highleg and that the hi leg was 15 to 20 V higher than the other legs. I had to be careful of hooking up things like 110 light bulbs to a single hi leg to ground because it would blow out the bulb. I want to learn more about negative phase sequence currents and I wouldn't rule that out as being a problem at this point so I'm going to do some checking the that. This afternoon I will do a full load voltage and current check on the saw and I'll report back with the figures that I have. Thank you for your observation.
this afternoon I'm going to do a full load test to see what the motor is the drawing voltage and current under an almost stall load again no belts direct drive.
I have only had Rotary converters for about the last 10 years or so before that I always had three-phase from the power company by being in in a small town the power company wants $25,000 to run three-phase 75 feet from the church next door to my shop. Fortunately all the equipment that I run that's three-phase has no problems with the Rotary power but I believe there's a difference between the converter and line three-phase.
I've always heard is that with the Rotary converter if you have a number of pieces of equipment connected to the same converter that starting the pieces of equipment one at a time will actually create a larger capacity load for other three-phase equipment you wish to bring online. Which brings up a point that I neglected to mention in my first post. The reason I gave the voltage drop in the line between the box and saw is because there is an intermediate piece of equipment connected between the saw and the box. In other words, a 10gauge wire runs from the box to 3 hp planer and then another 10gauge wire connected directly to the input terminals of the planer runs from the planer to the saw a total of about 10 feet of wire combined. I checked the drop in the line to verify that there was not a voltage drop in the connection at the planer. In fact, I did not notice anything that would cause a problem. Thank you for your observations. They reinforce what I had already thought.
that's the sort of thing. I was trying to figure out what could be a problem along the lines of mechanical inside the motor. I'll be looking in there when I take this motor apart if I can't find any other problem outside. Thanks for your reply.
The DVOM I have is a high dollar fluke meter. It has the capacity of measuring maximin of whatever you're measuring.
I do have a fluke, two channel oscilloscope which would allow me to connect to all three phases at the same time and I will report back with what I have. I'll also have to remember to measure the load voltage at the single phase input. It should become obvious. Maybe to you but I'm not so much a genius when it comes to this sort of thing. So I'll get back with you and maybe you can offer me some more advice. Thanks for the detailed input.
thanks for reading the post and you're right his pants falling down would be the only problem caused by a loose belt. And you write about the shop owner is a master at what he does and has done for the past 40 years. The Oliver does have a manual brake and I have operated it you really have to push it hard and it's not that great a brake anyway. But you can hear it when it's engaged so I don't believe the brake is a problem as far is the bolts, well that's a possibility and I'll have to check that. I certainly wouldn't be surprised if the motor was loose or full of crap. Thus again I'll check that when I take apart. The saw's direct wired, but like I said it's wired through another machines and I don't believe this is a problem, but it's certainly worth noting. I have looked for heat and I guess I haven't put enough time into it the saw running under load to see if that's a real problem. I will do the voltage current profile check this afternoon. Sounds like fun. No other equipment in the shop seems to have a problem with power rpm operation etc. and I was already under the assumption that other motors running on converter will improve the three three-phase balance. For everyone's benefit I'll report back and let know whether this is brushed or brushless motor.
those are some interesting observations and along the lines of what I heard, as far as the converter needs to be of least is big as what you're using. However, converter manufacturers usually ask you to provide 50 to 75% more capacity and that's along the lines of what I have and what I have suggested to my friend. I have also heard that too about the older motors. Thanks for the comments.
Thanks again to everybody for your responses. I sure do appreciate it I'll be back in touch as soon as I get results.
Last edited by Porschefix; 03-28-2012 at 01:29 PM.
Reason: Corrected my Dragons mistakes
there are plenty of motors that run on AC and have brushes. AC UNIVERSAL type. Almost all corded power tools have them or vacuum cleaners etc... they are speed controlled by voltage. (and they would be happy running on DC also! - I just learned this)
then there are the typical stationary AC INDUCTION motors like a furnace blower motor or a bench grinder. They have no brushes and can only be speed controlled by variable frequencey.
I imagine there are some old AC motors that had brushes but looked like your typical stationary induction motors hence the confusion.
Single phase, 120 volt. Here's a picture of the brushes. When it runs it makes a high pitched whine that sounds like a very fine-toothed gearbox. The nameplate is pretty battered, but it is a Peerless and has a strange mount that may be proprietary for the Wells.
Was that bandsaw motor single phase or three phase?