Probels with my RPC build

TL;DR; - 10hp self starting RPC build is using 490uf run caps, still not starting well, and runs hot.

I'm upgrading my 3hp RPC due to a new VMC purchase, and am running into an issue and am trying to diagnose what might be the problem.

For reference, my existing 3hp motor is an old 1940-somthing motor from a retired Monarch lathe. I have 110uF worth of run caps (a 50uF and 60uF run in parallel) and it starts right up, no issues. Voltage is fairly well balanced at around 265v on my L3 generated leg and the L1 drive leg. L2 is my regular line voltage which here in central rural Texas always runs a little hot where my 110v is usually somewhere around 126v to 134v and my 220v tends to run around 250-270. Not knowing exactly how much in the way of capacitance I'd need for the new 10hp motor, I purchased a varying array of run capacitors so I could mix and match to find the value I'd need.

My new RPC motor is a used Baldor purchased from a shop that was shutting down. I was told the motor was taken out of service because they upgraded their dust collector, and needed a larger motor for it, so they retired the 10hp and put in a new 20hp motor for their collection system. I tested all wires to the chassis, and all looked good. I wasn't able to test the motor under power tho. I figured these things are damn hard to kill, so purchased it anyway. That was almost 3 years ago. We all know how it goes. Projects only advance when they have to. {shrug}

So, setup with my used Baldor 10hp motor, I started to bench setup the new RPC in a self starting configuration. My reference was my 3hp motor, and 110uF, so that was ~36uF per HP. So, I assembled a chain of run caps with a value of 360uF total, verified the capacitance with a multimeter, wired everything up, and threw the switch. All the motor did is buzz with the shaft moving very slightly back and forty. Shut it off, and added another capacitor. Lather, Rinse, Repeat as necessary. Eventually I had used up my full assembly of caps, so I stole the caps off my 3hp RPC, and added those to the chain as well. By this time, I was at 490uF, and the motor would start, tho not crisply. It would sometimes buzz, other times it would start to slowly rotate, and after 2-3 seconds it would then snap up to full speed. My generated leg is running just over 300v. This seems very high to me, and might be what's contributing to my motor getting as hot as it does as quickly as it is. To investigate, I pulled the back side cover of the housing, and the bearing didn't look great. Not horrible, but not great. So I decided, a bearing swap was in order. No time like now given it's going to get setup and (hopefully) ignored for who knows how long!

My problems:
1) I'm well north of the anticipated MFD - tho this might be expected given it's a newer motor and I just have to keep adding until it starts crisply.

2) When the motor DOES start, it's getting hot, and I mean HOT! Like, the case of the motor got almost too hot to touch in about 20 seconds of running hot. After I checked the voltages, not wanting to damage anything, I went ahead and just shut it all down, and figured I'd post on PM and see if I'm just being stupid.

3) I don't know how much more MFD I might need, or if I'm chasing things the wrong direction here and maybe have too MUCH capacitance maybe???

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Motor Specs:
10hp Baldor - Model M3313T - M3313T - Baldor.com
Wired for low voltage / 220v - This has been verified multiple times, both by myself and independently by a buddy who was giving me a hand.
Front Bearing - 6307ZC3
Rear Bearing - 6206Z
Rotor - It has a little surface rust on one side, likely just from humidity, but otherwise looks to be in good condition. There is no signs of it rubbing the windings or anything like that.
Stator - The windings have their share of dust and a few cobwebs, but otherwise everything looks pretty good in there. No signs of rust, rubbing, or any shorts / scorch marks.





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My thoughts for where to go next for further debugging: I've ordered some 100uF run caps to both consolidate the the number of caps being used down, as well as give me more range of MFD to play with. I've also ordered the new bearings, and will press those on before putting everything back together. While the rotor was out, I cleaned up the rotor shaft by putting it between centers on the lathe and just ran a (very used) foam sanding pad over it in the spots where there was surface corrosion and/or dried on grease. I'm considering doing the same to the rest of the armature, but the initial priority was cleaning up the shaft so I can get the new bearings on when they arrive later this week.

I haven't ever built a RPC of this size tho, so I'm open to ideas and explanations as to what I'm dong wrong.

Das_Wookie said:

I haven't ever built a RPC of this size tho, so I'm open to ideas and explanations as to what I'm dong wrong.

Click to expand...

10 HP is not large as RPC go. Sounds as if you have too much "run" capacitance and not enough "start" capacitance - nor the momentary pushbutton, time-delay relay, or potential-sensitive relay to switch-over.

I have. .wot.. over 60 years of pushing electrons, enough NEW caps and relays to have built three or four RPC.

I also have other demands on my time that have greater time-value if only 'coz I cannot hire them done.

I cheated. An RPC was an item I COULD "hire done", and could expect to last my remaining years as well.

Bought one of Jim's "Phase Craft" turnkey boxes for a 1750 RPM motor (he optimizes for RPM) for less than the time-value of f*****g around.

"Your mission, if you choose to accept it".. is to invest the time in learning what you need to change.. or buy what is a proven solution and go do something more useful.

If it heats that fast...... you have a problem.

1) Are the start caps being disconnected after start?

2) have you TRIED less capacitor?

3) if you disconnect ALL capacitors, and ROPE START the thing, does it STILL heat up?

4) Are you CERTAIN you do not have any windings reversed or other easy-to-make mistake?

Das_Wookie said:

2) When the motor DOES start, it's getting hot, and I mean HOT! Like, the case of the motor got almost too hot to touch in about 20 seconds of running hot. After I checked the voltages, not wanting to damage anything, I went ahead and just shut it all down, and figured I'd post on PM and see if I'm just being stupid.

Click to expand...

OK, so on a "couldn't hurt" line of thought, I picked up the phone and called Baldor this morning, trying to get an idea on the motor and what their take on it would be with running with 300v, and the temperature that we were seeing. She told me that the NORMAL running temperature for the motor is 167 degrees, and that anything up to 200 is OK. After that, things start to get worrying. She said "too hot to touch" however is normal for this motor. She didn't have an idea of how long it should take to get up to temp, but that if I was shutting it off because it was "getting hot fast" that it MAY not be a bad sign given the expected temperature for the motor. I agreed, and that while "to touch" wasn't a good measurement of temperature, clearly I may just be being a Nervous Nelly and shutting it off while it's just doing what it would normally be expected to do, given that "Holy hell that's HOT" typically starts at ~140 degrees. So I'm going to have to resort to temperature testing other than touching... so I'll break out the IR temp sensor for further temperature monitoring, and may stick a thermocoouple on it and put a remote LCD display next to the remote on/off switch to monitor the motor temp once it's placed into service up in the rafters and out of sight.

So, the temperature issue may not be an issue after all... I guess for THAT part of things, I won't know until I get the new bearings on it, get the motor reassembled, have the additional run capacitors in hand to try and get it reliably started it quickly, and then see if I can do something about balancing that 300v voltage.

I still have no idea of what amount of caps I DO need to get this motor started, but hoping the five 100uf caps I've ordered will be enough to add to the ones I have in hand already.

Have a look at this document for building and balancing a 10HP RPC.

http://www.practicalmachinist.com/FitchWConverter.pdf

SAF Ω

JST said:

If it heats that fast...... you have a problem.

1) Are the start caps being disconnected after start?

2) have you TRIED less capacitor?

3) if you disconnect ALL capacitors, and ROPE START the thing, does it STILL heat up?

4) Are you CERTAIN you do not have any windings reversed or other easy-to-make mistake?

Click to expand...

1) There are no start caps, only run. They act as both start and run caps, and stay connected. This is a self starting configuration for the RPC. The other variant being a momentary version where a start cap is used to get it up and running, and then removed from the circuit. I may yet go to that style of configuration, but my preference is to have a self starting config so I can more easily remote start it, or have someone who's helping me in the shop who may not know squat about starting an RPC and I can just tell them "Can you hit that green button over there by the light switch" if I need the RPC turned on. Explaining to somebody how to do the momentary switch is a pain, and I'd rather not complicate the setup with additional things like an automatic timer or something.

2) I started with 360uF. By all accounts, that would most likely be "not enough" and worked up. Adding 30-50uF at a time. At one point one of the wires in the cap chain came off, I think after the 3rd cap, which would have been 210uF, and it made a hell of a racket. I flipped the power back off, and that's when we found the problem with the disconnected wire, so... sorta.

3) Didn't try, no. I will once I get the new bearings later this week.

4) Yes. 100% certain. I double checked myself multiple times, and then had my buddy check everything with my not providing any info just to make SURE. My memory can't be trusted (long story) so I never trust myself without double or triple checking everything. Anything important is written down, not held in my head, and I make sure I (almost) always verify against reference material... be it blueprints/drawings or researched material. Whenever possible, I have someone else double check me. The buddy I had helping me is an electrical engineer who's far more detail oriented than I am, and knows his stuff about RPCs. That's why I asked him to come lend a hand while I built this one. I had made a few of those easy to make mistakes which he caught prior to my doing my double check pass. So, yeah. I know this isn't a wiring problem.

Das_Wookie said:

I still have no idea of what amount of caps I DO need to get this motor started, but hoping the five 100uf caps I've ordered will be enough to add to the ones I have in hand already.

Click to expand...

Well... if you won't read JST's advice, I guess you could start a business selling slightly used surplus capacitors?

Put the entire TRIBE of the silly things in some other room. Out of sight. Beyond reach.

Now... rope-start that 10 HP motor, no caps at all. Let it run for an hour. No waste. Its wintertime. Even in Texas. Any heat thrown-off reduces the load on whatever else you are using to keep warm.

Presuming it stinketh not, the Baldor is good to go.

Then go READ online or right here on PM until you find a diagram and parts values you feel comfortable duplicating. Exactly.

No need for YOU to experiment until you re-discover workable component values or how to wire them.

There are multiple thousands of 10 HP RPC out there running.

Some do not own even ONE capacitor, nor any other component AT ALL, beyond the idler motor itself.

SAF said:

Have a look at this document for building and balancing a 10HP RPC.

http://www.practicalmachinist.com/FitchWConverter.pdf

SAF Ω

Click to expand...

Thaks SAF. I've sent this doc to my buddy who helped me with my build. The RPCs he's built in the past have always used a momentary switch, where mine have always been self starting. In this instance, I may need to change my mind about going that route. I have an aversion to the momentary switch start method, but I might just have to suck it up and go that route after all. I'll let me EE buddy look this over and see what his thoughts are. The linked doc you forwarded to me looks easy enough to understand tho. The prior reference material I'd found for doing a balanced RPC were always for smaller motors with 7.5hp being the upper end of things, so this doc gives a good plan of action for how to build one and balance it.

Thanks!

another possibility is to use a centrifugal clutch from a small motor, as the switch to switch out the capacitors for your rpc. you should use a relay rather than subjecting the contacts to such abuse.

Monarchist said:

Well... if you won't read JST's advice, I guess you could start a business selling slightly used surplus capacitors?

Put the entire TRIBE of the silly things in some other room. Out of sight. Beyond reach.

Now... rope-start that 10 HP motor, no caps at all. Let it run for an hour. No waste. Its wintertime. Even in Texas. Any heat thrown-off reduces the load on whatever else you are using to keep warm.

Presuming it stinketh not, the Baldor is good to go.

Then go READ online or right here on PM until you find a diagram and parts values you feel comfortable duplicating. Exactly.

No need for YOU to experiment until you re-discover workable component values or how to wire them.

There are multiple thousands of 10 HP RPC out there running.

Some do not own even ONE capacitor, nor any other component AT ALL, beyond the idler motor itself.

Click to expand...

I'm taking the advise to heart. I'd already ordered the additional caps. My experience with RPCs is 5hp and less, and I've always used self starting configurations. My intent to use a self starter is more one of what I'm used to, and that I frequently have folks start it other than myself as I've typically shut the RPC off if it was not going to be needed for a while because 1) they aren't silent and 2) why have a motor spinning and adding to the electric bill if it doesn't have to be.

My asking for feedback was one genuinely seeking the input from others who are more experienced in this regard, and I am taking the advise to heart. I do plan to rope start it without any caps connected and check my voltages and the heat the motor generates. I also called and talked to Baldor themselves to inquire as to the temperatures this specific motor runs at. I was surprised to hear 167 degrees, but if that's the case, my nervousness at the motor heating up could very well be one that is negated because it's EXPECTED to run at a 'too hot to touch" temperature. Once I've got the motor re-assembled, I'll rope start it and plot the temperature it hits over X period of time. I was thinking something like 10 minutes, but if 1 hour is the recommended timeframe, then I'll do that instead.

I swear, I'm listening, and will follow recommendations. I'm -=NOT=- a know it all. The opposite in fact, which is why I'm here. I'm not looking for validation. I'm looking for "Don't be an idiot. Do this." type feedback. I've got a thick skin, and am used to making simple mistakes. I have brain damage (auto accident) which basically has destroyed my working memory. The result of that is I try VERY hard to take what some consider to be an insane amount of notes and rely on double or triple checking myself on near everything I do in the shop.

The doc that SAF forwarded above will be my plan of action once I have the new bearings in hand and get the motor reassembled.

I'll update this with how things go. For now, I'm somewhat in a holding pattern until parts arrive. I suppose I COULD press the old bearings back on, but at this point, I'm thinking the better plan of action is to just sit tight and start again, from scratch, once I have the new parts in hand.

johansen said:

another possibility is to use a centrifugal clutch from a small motor, as the switch to switch out the capacitors for your rpc. you should use a relay rather than subjecting the contacts to such abuse.

Click to expand...

There are a few means of doing an "automatic disconnect" for the starting caps. I hand't considered basically building/hacking on a centrifugal clutch, but that idea does have a certain attraction in my head. I'm not sure how I would DO it, but I do like the idea of a mechanical disconnect over a timer or other means of disconnect. I'll have to let that idea stew in the noggin a bit, but it's an interesting idea to be sure. Thanks!

How about some measurements of current?

Have you done any and compared to the full load amps as labeled? Motors work by current. Checking current is like taking a person's pulse, a pretty basic check on health and status.

Das_Wookie said:

I have brain damage (auto accident) which basically has destroyed my working memory. The result of that is I try VERY hard to take what some consider to be an insane amount of notes and rely on double or triple checking myself on near everything I do in the shop.

Click to expand...

Didn't know that. Thought it was just overheat from the damned fake-fur wookie suit.


But it is all the more reason you should build an RPC by EXACTLY copying one that works AND NOT try to re-invent or even modify.

Or buy a Phase-Craft 10 HP @ 1750 RPM rated box and follow the diagram for connecting it EXACTLY as well.

Lots and lots of other things in any life that are not so easily handed-off.

Best to reserve your resource set for dealing with those instead.

Das_Wookie said:

There are a few means of doing an "automatic disconnect" for the starting caps. I hand't considered basically building/hacking on a centrifugal clutch, but that idea does have a certain attraction in my head. I'm not sure how I would DO it, but I do like the idea of a mechanical disconnect over a timer or other means of disconnect. I'll have to let that idea stew in the noggin a bit, but it's an interesting idea to be sure. Thanks!

Click to expand...

If you like mechanical switches, have a look at these.

Rotary Phase Converters On TORQ Corporation

SAF

Das_Wookie said:

There are a few means of doing an "automatic disconnect" for the starting caps. I hand't considered basically building/hacking on a centrifugal clutch, but that idea does have a certain attraction in my head. I'm not sure how I would DO it, but I do like the idea of a mechanical disconnect over a timer or other means of disconnect.

Click to expand...

Really all you need is the momentary switch. Far fewer failure modes. NO "engineering".

Small one controlling a larger "contactor" class of relay. Easily remoted clear t'other side of the building if need be.

Keep It Simple (and) Stupid. It is only held-in for a few seconds, becomes a familiar part of starting the RPC much as one starts a motorcar's engine or pauses on "glow plug" to start many a Diesel.

You were leaving the 500 or so mfd of capacitance in place after startup?

From 20 feet away, that's why the motor was getting hot. Remove the capacitance as suggested.
Start it with a pony motor, and measure the voltage balance with your load in place. Tune as
per the Fitch Williams instructions.

Then, and only then, figure out how to start it with start capacitors, which are switched out via potential
relay or other means, once it's up to speed.

There ARE idlers that use a start/run cap. I have a smaller ARCO that does that. Works fine, but it is a special design low start current motor.

BUT, any 10 HP size motor that heats up THAT fast is an issue.....has an issue.... CHECK THE FREAKING CURRENT AND DO IT NOW. Seriously, if the current is not something around 40% of full load current (or less) when under no load, there is something wrong for sure.

Rope or pony start it with no caps, see if it runs hot or stays cool. At a normal current it will get warm, maybe hotter than what you'd want a hot shower in, but motors can get up near boiling water temp, at least in the hot spots, with no issue, even with ordinary insulation types.

Since the motor was a "take-out", you cannot be SURE the guy you asked actually knew why it was taken out.

jim rozen said:

You were leaving the 500 or so mfd of capacitance in place after startup?

From 20 feet away, that's why the motor was getting hot. Remove the capacitance as suggested.
Start it with a pony motor, and measure the voltage balance with your load in place. Tune as
per the Fitch Williams instructions.

Then, and only then, figure out how to start it with start capacitors, which are switched out via potential
relay or other means, once it's up to speed.

Click to expand...

this was bench build for testing components as I didn't want to be doing all that with the motor in the rafters. The caps would still be right next to the motor in the final build tho. Only the switch for the contactor would be at a distance. Well that and the voltage panel meter and eStop button.

Once the parts arrive for putting the motor back together I will then do further testing. For now, it's idle until I get the parts.

JST said:

There ARE idlers that use a start/run cap. I have a smaller ARCO that does that. Works fine, but it is a special design low start current motor.

BUT, any 10 HP size motor that heats up THAT fast is an issue.....has an issue.... CHECK THE FREAKING CURRENT AND DO IT NOW. Seriously, if the current is not something around 40% of full load current (or less) when under no load, there is something wrong for sure.

Rope or pony start it with no caps, see if it runs hot or stays cool. At a normal current it will get warm, maybe hotter than what you'd want a hot shower in, but motors can get up near boiling water temp, at least in the hot spots, with no issue, even with ordinary insulation types.

Since the motor was a "take-out", you cannot be SURE the guy you asked actually knew why it was taken out.

Click to expand...

Can't check current until it's re-assembled. I will rope start it check voltages and currents with no caps inline after it's started. I'm thinking a self starting configuration will not be viable tho and I'm going to end up having to use the caps only to start it and then be pulled out of the circuit with only small amounts left to balance the voltages.

I like the FW schematic design that SAF forwarded here. I'm going to almost certainly end up going to that configuration at this point I think... once the motor is reassembled.

jim rozen said:

You were leaving the 500 or so mfd of capacitance in place after startup?

From 20 feet away, that's why the motor was getting hot. Remove the capacitance as suggested.
Start it with a pony motor, and measure the voltage balance with your load in place. Tune as
per the Fitch Williams instructions.

Then, and only then, figure out how to start it with start capacitors, which are switched out via potential
relay or other means, once it's up to speed.

Click to expand...

Yep, I agree. I never heard of leaving the start caps in place after startup. That's a bad plan.

I have two RPC's, a 5 and a 7.5 hp that I alternate with just for fun. Both use a potential relay and are 100% reliable for more than 7 years now.

I even like the rope start, button start and pony start concepts - whatever works, but you can't leave the caps in the circuit.