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Rotary phase converter rotor problems

Mook1004

Plastic
Joined
Mar 25, 2024
Location
Ks
Not a machinist but an electrician that works on agricultural irrigation systems. We work on several different brands of rotary phase converters and have one that a local shop makes that we are having issues with. Seems from reading through past blogs on here that we started to narrow down to a bad rotor. These phase converters are often in remote locations and controlled remotely so when we show up to work on one the generally look like a failed start board. But once in awhile after all components (start caps, multiple boards and contractor) still have failure to start. They just hum in a locked position. Finally we found that turning rotor 5 degrees Would make them start. We then tested the rotor according to our rewind shop and found high and low readings in the rotor. Roto wouldn’t start if we put it on a low reading but would on the high. So we’re guessing rotor is bad. The catch is the person who designed the rotophase has passed away and new shop owners seem to not have a lot of knowledge or unwillingness to share. When I approached them about a new rotor they had never heard of that and there best option was a total replacement. Which is great for me I’d gladly replace with two Arco’s but I don’t think customer is interested in that route yet. As best I can find they are a weg 324t frame size motor. Is there a common size rotor per frame size? Or are rotors different between brands. Can you buy a motor from grainger and cut the output shaft off? We are just at a loss with this outfit. We have 3 to 4 different customers with the same problem. These phase converters we most likely flooded in 2019 Missouri River flood. All were taken to rewind shop for rebearing and cleaning of windings. Included picture from grainger of same size and model of weg motor. Thanks for any info.
 

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I am far from an authority on all rotary phase converters but, I'm pretty sure that the basis of all of them is a plain ol' 3-phase motor. Yes, they saw off the output shaft and that's it. I believe any troubleshooting will be the same as if you were checking any other 3-phase motor.

Beyond that, there is a set of start capacitors, a contactor and a timer. The timer is used to drop the start caps out of the circuit once things are spinning. It might be that the start caps have died or the timer isn't staying engaged long enough to start the motor. The contactor obviously switches the converter on and off.

Once the motor is spinning, the run capacitors attempt to balance the voltages in the three phases while under the desired load.
 
So it got all new start caps and a timer. It doesn’t even try to rotate. I’m just curious if there is a difference between rotors from the same frame size or where do you find a replacement rotor.
 
You may be able to get a "phase converter motor" fom baldor, I have seen several phase converters using baldor "motors" and they all looked like they came from baldor as converter motors, per the tag. Granted all the ones I am talking about were in the 5-10 hp range, not the 40 hp you seem to need.
Otherwise almost any 3 phase motor can be used as a RPC motor.
 
Well we sell Arco phase converters that are made from a baldor motor. I’m really looking for just a rotor replacement. Also seeing if there is a common size to rotors and common size to motor frame size. Or if all rotors are built internally the same?
 
Yes, most 3 phase motors will "work" as an idler.

A really purpose built unit will work better. Some Arco, at least older ones, have a double cage rotor, for instance. The outer one is a thin wire high resistance set of conductors, for good starting. It reacts well when the slip frequency is high, as at start.

The inner cage is higher inductance, with larger conductors, and works better once the rotor is up to speed. It will give a stronger magnetic field, providing better performance during heavy loads like starting a load motor.

With that, you get the best of both. It starts well, and still has good output.

Using any old motor, you don't know what the rotor design is, and so you don't know what the performance will be like, even though it WILL "work", starting and generating a usable "wild leg" output. Most motors will be designed to a particular purpose. You may not know how it will perform as an idler.

That said, don't be afraid to replace the motor if it needs replacement. Most motors that have good running characteristics should majke decent idlers. You might have to make changes to the start capacitance in order to get better starting performance.

A motor that won't start unless its rotor is turned a few degrees sounds like a rotor bar problem. If you can get a replacement rotor, great.

The problem is that it really ought to be a real OEM replacement.

Putting in a different rotor that "seems to fit" may or may not work as expected, although the odds are that it will turn and drive a load to some degree as long as it basically fits the hole in the stator. Might not be great, or even as good as the original rotor. There are a large number of factors that change performance.

The rotor-to-stator radial gap is a biggie in performance, for instance, and if wrong, power, torque or current draw may be inferior to the original, which will affect performance as a idler also. Same with number of bars, how they are placed in the rotor, size of the bars, the end connections, etc, etc.

Of course, one might be able to find a rotor with characteristics making the result an even better idler. But you need to know what you want, and what affects that.

I'm not a motor designer. Someone who is can give much more detail.
 
And that’s were I think I’m at. The company that made these were specifically made for phase conversion. Im not trying create my own version or reinvent the wheel. Would be hard to put a number of how many of these where made but there were a lot. The motor tags little information and like I said the new owners seem to just not know. Is there a way to distinguish a rotor by characteristics to find a replacement. This particular one we think we have narrowed down to a weg just by frame shape. But it’s also not the only style the used as I’ve seen others that have fins on the motor frame like many other motors and not a smooth frame like the pictured weg. This particular set up is parallel 40hp phase converters and going to be very costly to replace.
 
They just hum in a locked position. Finally we found that turning rotor 5 degrees Would make them start. We then tested the rotor according to our rewind shop and found high and low readings in the rotor. Roto wouldn’t start if we put it on a low reading but would on the high. So we’re guessing rotor is bad.
I have been hum-bugged before. What does your starting circuit look like?

Did you know Phase-a-matic convertors are made with Baldor motors with lightened rotors and no shaft.

Cutting a motor shaft off is something only a heathen would do. I like the shaft but don't like being given one.

Your question about replacing a rotor. Do you imply that you would press the shaft out of a bad rotor and press it into a replacement rotor?
 
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FWIW, we had the same failure on a unit that had ran for years. It was a home made unit (by me). New 3 phase motor fixed it.
 
I have been hum-bugged before. What does your starting circuit look like?

Did you know Phase-a-matic convertors are made with Baldor motors with lightened rotors and no shaft.

Cutting a motor shaft off is something only a heathen would do. I like the shaft but don't like being given one.

Your question about replacing a rotor. Do you imply that you would press the shaft out of a bad rotor and press it into a replacement rotor?

I’m not sure you could press the shaft out and that’s not really where I was going. Im more thinking that if I could find a replacement rotor that had an output I could cut it to size. This picture is the start circuit drawn from the the company that makes them. At the top of picture is two banks of 300-360mfd start caps. There are four hooked in series and the the banks are hooked parallel to each other to the windings. Bottom left is contractor for starts, middle object is the start board and bottom right is run caps. Still confused on why there are so many start caps in series as series connection of capacitors diminishes over all capacitance but raises voltage. That’s a question for a whole different time. But I’ve also seen up to then start caps and they usually add 2 more to one bank. I’ve experimented with up to 12 caps and changed different configurations and series and parallel to avail. I know replacement is an easy solution and I know a new would would 100% solve the problem. But I also feel like this is a problem that can be solved.
 

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On this forum I have seen posts about pressing shafts out of rotors. Need more than one of those bench presses for that.

Are you thinking about trimming the rotor diameter to fit? What about the location and size of the two bearings? The shaft?
I guess you could make a new shaft.

I would dig in there and make a accurate schematic drawing of all those capacitors.
 
Trimming the rotor will change characteristics, and in some cases, may loosen the bars. Depends on how much you have to do.

Typical clearance radially is around 10 thou. Not a lot, but details are important.
 
Yes I understand the drawing is very vague and I understand how they work. Just trying to give you guys and idea if that helps. I’ve talked to a few different motor shops and most are like they are trash and you need new. They also said getting parts from a motor manufacturer is a joke as they have a hard time figuring which fan goes to which motor. It also appears that rebuilding a rotor of the hp just isn’t cost effective either. Arco’s of same size that we sell retail at 11k apiece and this site has two. Just seeing if there are availability of non OEM rotors or is that not even a thing? This phase converters work it just seems that now we are seeing parts start to fail.
 
Finally we found that turning rotor 5 degrees Would make them start. We then tested the rotor according to our rewind shop and found high and low readings in the rotor. Roto wouldn’t start if we put it on a low reading but would on the high. So we’re guessing rotor is bad. The catch is the person who designed the rotophase has passed away and new shop owners seem to not have a lot of knowledge or unwillingness to share. When I approached them about a new rotor they had never heard of that and there best option was a total replacement. Which is great for me I’d gladly replace with two Arco’s but I don’t think customer is interested in that route yet. As best I can find they are a weg 324t frame size motor. Is there a common size rotor per frame size? Or are rotors different between brands. Can you buy a motor from grainger and cut the output shaft off? We are just at a loss with this outfit. We have 3 to 4 different customers with the same problem. These phase converters we most likely flooded in 2019 Missouri River flood. All were taken to rewind shop for rebearing and cleaning of windings. Included picture from grainger of same size and model of weg motor. Thanks for any info.
Couple of questions for you.

How did your motor shop advise you to field test a 40HP Rotor?

How did you get high and low readings on the rotor?

Without a manufacturer tag of the motor, how would you be able to tell what model motor it is, or even if you could get a correct rotor from the manufacturer?
Seems to me that the motor should be tested on a utility 3Φ power supply, at the voltage your phase converter is set to run at, to see if it starts and runs on the normal 3Φ supply connection. Then a load test if it appears to run correctly. To me if you can get it to start at all, seems unlikely that the rotor could be bad. Is the lamination's rusty or the squirrel cage rotor bars cracked?

If nothing else have you switched the two motors between control panels?, seems that would be a more productive way to confirm that the motor is good or bad or is there an issue with the controls, rather than hunting for an unobtanium rotor. With some testing on site, or at a different site and voltage supply you could prove to yourself and the customer where the actual problem is.Telling him he needs two matching 40HP phase converters, is a good way to make him someone else's customer.
 
Well the way our rewind shop say to test a rotor is take 120v ac with a Resistor to limit current to l1 of motor and neutral to l2 and take a millivolt reading from l3 to neutral. This is will all caps in hooked of course. Rotate rotor 360 degrees and not voltages. And swap legs and repeat 3 times. We found a range from 240mv to 207mv. Put rotor to a reading of 207mv hook all caps up and try starting and it’s a no go. Move rotor to reading of 240v and it starts perfect. Granted this Rotophase starts fine 95% of the time and doesn’t start 5% of the time. But this is remote located. Like 2 plus hours driver for customer so it needs to start every time. I supplied a picture of the motor I found online which fits the limited info on the motor tag. Not sure how many motors you have seen that look like it but if says it’s very unique. And by the way the company that still makes these says they would junk them and start over with new. They hadn’t ever even heard of a bad rotor ever. A competitive phase converter shop was the one that told me to rotate rotor and try starting as they have had brand new motors with bad rotors. This isn’t something I dreamed up myself and I feel like it’s well tested that this is the issue.
 
And im not telling him he needs two new matching ones. But he also wants something that works. I’m on here asking for help trying to save my customer money. But this is the 4th one that’s started doing this and the shop that’s making them can’t figure it out. Their response is to just sell them a brand new one.
 
Not exactly sure if you have a 40 Hp. But don't people use pony motors to start large Hp motors.
I would think the breaking line is 20 Hp.
 
I’ve personally never seen one with a pony motor and we work with rotary phase converters up to 50 hp started on caps
 
You can probably also test rotors on a "growler", although I have not done it myself.

The indication would likely be opposite to the normal one, since rotor bars are pretty much a shorted turn.

But, you would be looking for a difference in one area vs all the others.
 
I don't think rotors are standardised between manufacturers. Frame sizes are, but rotors could easily be (and probably are) manufacturer-specific.

It certainly sounds like a rotor fault. I've never heard of anyone trying to actually fix one. It might be possible to visually inspect the rotor after removing it? They're pretty simple (just bars, no windings).
 








 
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