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RPC idler motor RPM

I've built several of them. Ive used 1185, 1785, and 3550 rpm motors. The lower the rpm the more iron in the motor. The more iron in the motor, the easier it comes up to speed. I'd prefer a 1750 over a 3450 for building an RPC myself.;)
 
AT 3 or 5 hp you can use either a 3450 or a 1750. I prefer an open drip proof enclosure as you do not have the fan noise from a TEFC fan. I have a 5 hp 3450 that will start before you can remove your finger from the start button.
 
motor speed

Was just on Phase-A-Matic's website. They recomend the higher speed motor when making your own rotary using one of their static converters. It seems that when you half the rpm's, you half the horsepower. So a 5hp. motor becomes a 2.5hp. motor ? Sorta makes sense, but raises other questions also.
They give info & instructions in PDF style for download. Nice info to have for a novice.
 
Makes not much sense to me....... a slower motor is made to BE that, and the HP is good.

From outside looking from the line, you couldn't easily tell what it is, a certain number of poles pass per second either way.......
 
The Phase-A-Matic *static* converters contain a motor starting circuit *only* i.e. potential relay and starting caps plus an indicator circuit. They have *NO* run capacitors at all, therefore the motor is running on single phase after it starts. That probably accounts for the reduction in HP rating. If I were Phase-A-Matic, I would be embarrassed to sell the static converters.

Their rotary converters work fine. I have a 5 HP model and can recommend them.

IMO, you would be money ahead with good performance to make your own converter using both start and run caps with appropriate controls. Someone posted a pdf with instructions on how to select run caps. Can't find it at the moment, do a forum search on the topic.

Regards,
Dennis
 
Was just on Phase-A-Matic's website. They recomend the higher speed motor when making your own rotary
This statement by phase-a-matic was based on if your load motor was a hard starting type load, they feel that 34XX rpm idler motor is better suited to handle high inrush current for those type of application's.
 
Dpasek is referring to the HP reduction due to NOT using some of the motor windings. The motor is running single-phase, and will have a heating-limited power that is lower than if running 3 phase. It will also tend to stall out easier.

Macplus
A faster converter motor will possibly store more energy in the rotating mass than a lower RPM motor. They are probably referring to that
 
I tried it both ways...

I built my converter for a 3750rpm 10hp TEFC motor I got for free (shaft was shot).
Worked great, but howl'd like an air-raid siren. Started within about 2 seconds.

Found a 7.5hp 1800rpm ODC motor with similarly-trashed shaft. Lopped off the bad part, and swapped 'em. Started in under a half-second, Very quiet.

The 10hp motor is a LITTLE stiffer... but the 7.5hp pulls the loads just fine, with less start current. If I need more, I've got a contactor wired to a socket to which I can plug in the 10hp, flip it, and they're BOTH on... but I've never HAD to use it.
 
I have built well over 50 static and rotary convertors.
I have heard from others about 3750 being better for convertors,
but in all my experience, that has NOT been the case.

Slow OLD motors have lots of iron and great eddy currents for regeneration.
Modern "efficient" motors are terrrible and over heat many times.
They use aluminum rotors, which heat up and will act like a disk break
in the windings when loads get heavy ( did I say catch fire !)
High speed motors seem to be better balanced and quieter IN SOME CASES.
In plain simple terms guys, when I found a overweight beast , it worked great
When I used a light weight new motor, they tended to burn out or just stall.
The criteria that CNC needs a special motor for RPC is on the edge of falsehood also .
They may have closer stator to field clearences (??? if at all), but the older motors
do a great job and I have made several CNC RPC's very sucessfully.

Forget the RPM.......weight is what you want,, and don't discount the Open winding
motors, which factories toss out($$). They are quieter than most fan cooled units

Rich

The claim that a 3750 motor is twice the HP of a 1750 is untrue
HP is a function of the KW rating and motor efficiency.
RPM is irellevent when they are the same
 
Speaking of "high efficiency" motor risks, I just picked up a Baldor 10hp cheap for a RPC. I'm hoping that being Baldor it's not going to have the aluminum rotors and related issues. Should I be concerned?
 
aluminum motors

Nothing at all wrong or questionable about aluminum motors. Most motors both cheap and very high quality use cast aluminum for rotor windings. There are a number of induction motors that are wound with aluminum wire in the stator as well, If they are designed right they perform quite well. The aluminum stators might not stand gross abuse from overloading as well as copper, but we aren't supposed to grossly overload them are we.
 
I know it is a typo but just in case there is someone that does not: There are no 3750 rpm motors--rather they are 3450 rpm.

I prefer cast iron motors for idlers. I tried to use a high efficiency Baldor but was not happy with it. It worked but not up to the expectations I have. The early and late NEMA motors before the high efficiency era are good for RPCs.

Just my 2 cents
 
"when I found a overweight beast , it worked great"
That is what I do. If I have an old elephant that is too big to mount on anything, that is used for an idler. I have seen all rpm motors used for idlers. I have never seen a difference.
 
Ok, next question...

You guys may have noticed my post on a dual stage RPC. I have an existing 3hp RPC based on an old GE 3phase. Not an ancient iron monster, but not "high efficiency". I am trying to power a 7.5hp lathe. For the second motor, I have a 10hp Baldor HiE or a 7.5hp "Standard Efficiency" "Inverter Duty" motor. Which would be best for my needs? Or should I try to arrange for passing along that 10hp and finding a "SE" 10hp motor...
 
Can anyone quantify the "standard is better than high efficiency" point? How much better? Enough so that a 7.5 hp Standard is better than a 10 HE? Enough better that it's worth me abandoning a 10 hp HE "in the hand" to go back to beating the bushes? Will it really matter running a 7.5hp lathe when I'll likely NEVER actually use more than 5 hp for a cut. I'm too timid to expect I'll ever push it that hard. Plus, while I'm a bit impatient, I'm not generally in THAT much of a hurry. I would love for someone with more knowledge and experience to at least give me a gut feeling on the matter.
 








 
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