# Idler motor, does it matter which one?



## Investigator (Mar 8, 2017)

I have made the decision to power some equipment with a RPC.  I will use a 5hp RPC.  I am buying a panel and adding an idler motor to it.  To that end, does it matter which 5hp 3ph motor I use?  I have found one semi-local that I can pick up without shipping for $100 bucks.  It is a Dayton 200/230/460  at 1750 rpm.  It was taken off of an air compressor so the gentleman could re-motor it to single phase.  Tag says it is continuous duty.

As I understand it, any 5hp 3ph will work, and I _think_ they all work the same.  I just wanted to make sure.

Thanks in advance.


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## gr8legs (Mar 9, 2017)

When I tried an RPC the biggest difference for me was 1800 RPM vs 3600 RPM. Much quieter / nicer to be around with the 1800 RPM. But, that said, when I converted to VFDs it was heavenly and getting variable speed was a big bonus. YMMV.
Stu


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## Firestopper (Mar 9, 2017)

Like Stu said 1700-1800 RPM will run quieter but equally important if not more is choosing a idler motor that is 1.5  X or double the HP of your largest machine. You lost me on "the gentleman could re-motor it to single phase". An idler is a 3 phase motor that generates a third leg (wild leg) when running, adding to the other two legs of single phase that is powering the motor.  Back in 2001 very little information was available but I did find enough to build my first RPC and still use the same motor today. The motor parameters for building a RPC called out specific numbers  for building a balanced system to include capacitors. The best bang for you buck would be to have Phase-craft (Jim) build the controller using your motor specification. Very affordable and balanced.5 HP ROTARY PHASE CONVERTER CONTROL PANEL make your own true 3-phase power!
I couldn't purchase the parts for less. The first system I built cost me 3X in parts (not including the motor). 
Paco


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## Investigator (Mar 9, 2017)

firestopper said:


> You lost me on "the gentleman could re-motor it to single phase".
> Paco



Sorry, the source of the motor.  A gentleman bought a used air compressor with a 3ph motor.  He decided to replace the 3ph motor with a single phase so he could use it at  his residence.  The 3ph 'take off' motor is now for sale.

And although I was unaware of his membership here, I have already contacted Phase Craft through Ebay.  I was looking at the panels he has so I could add my locally obtained motor.  At this time, I have 2 lathes, a Southbend with a 2hp, and a Logan with a 1.5hp motor.  



gr8legs said:


> But, that said, when I converted to VFDs it was heavenly and getting variable speed was a big bonus. YMMV.
> Stu



I have considered the VFD's.  My thinking goes like this:  I have old machines, and I am but a newbie.  It might be better to learn within the original limitations of my machines before going off the reservation.  Also in relation to cost, at the present time the cost comparison between setting up an RPC for the entire shop and buying 2 VFD's for my immediate need is about a wash.  If I head down the VFD route now, I will have to purchase more VFD's as I buy equipment (I am already on the hunt for a mill). If I go the RPC route, a reasonable sized mill should be plug and play. 

So, back to the original question...... Given the same hp size and voltage (230 3ph) does it make a difference on which particular motor I use for my RPC?  In other words, is there a reason to stay away from any particular brand/style?


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## Firestopper (Mar 9, 2017)

Investigator said:


> Sorry, the source of the motor.  A gentleman bought a used air compressor with a 3ph motor.  He decided to replace the 3ph motor with a single phase so he could use it at  his residence.  The 3ph 'take off' motor is now for sale.
> 
> And although I was unaware of his membership here, I have already contacted Phase Craft through Ebay.  I was looking at the panels he has so I could add my locally obtained motor.  At this time, I have 2 lathes, a Southbend with a 2hp, and a Logan with a 1.5hp motor.
> 
> ...



Good thinking on RPC to run multiple machines. I run nine machines from a 7.5 hp RPC and have added two VFD's for fine control (lathe and plate roller).  Just remember if and when the time comes for  adding  a VFD, purchase a 3 phase unit.
As far as motor brand (idler) I would use a quality motor with continuous run rating like you mentioned. I'm running a vintage Howell that was rebuilt over 15 years ago. Cast iron construction and very smooth and quiet. You might reach out to Jim (Phase craft) and get his input as he's seen way more motors than me.


You might consider mounting the motor on isolators.


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## Ulma Doctor (Mar 9, 2017)

here is some additional information...
http://www.hobby-machinist.com/threads/rpc-simple-design-unbalanced.12712/


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## whitmore (Mar 9, 2017)

There's a theoretical reason to prefer a 3450 rpm motor for your idler; the motor's rotor is the
flywheel (and an energy storage element) for the converter output.   If a 3450 rpm motor
is used, the flywheel has (for a given mass) four times the stored energy.   The motor
mount will vibrate less, and the expected output imbalance might be somewhat improved (but
users don't report that to be an issue).    A flywheel could be added to any idler motor, if it
could be secured safely and balanced, of course.

A continuous motor rating sounds good, but the idler doesn't have much shaft torque,
so even a lower rating (or worse cooling) motor ought not to overheat.


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## Keith Foor (Mar 9, 2017)

When you are talking about a 3 to 5 HP RPC idler motor it's sort of  a wash.  The start current on the 3450 motor will be  abit higher and for a longer length of time because it's got to get accelerated up to a higher speed than the 1725 motor.  

Now when you get into a larger unit.  These higher currents and longer start times can have a more pronounced effect and the other issue is manual starting or pony motor starting.  I start a LARGE 15 HP (physical motor size would compare to a 75 HP open frame motor) with a 3/8 drill.  Now finding a 3/8 drill that will spin 3450 is gonna be impossible.  So the larger units should have a lower RPM motor in MY PERSONAL opinion.  But you know what his said about opinions.


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## cathead (Mar 10, 2017)

Whitmore is right about the stored energy with the flywheel effect.  When I set up my RPC, I picked a 1725 rpm motor at the 
scrap yard that had a massive two sheave flywheel on it about twelve inches in diameter.  That way you get a quiet running
RPC which you barely notice purring away below the lathe.  It's an old heavy motor with oil reservoirs and a ring around the 
motor shaft that dips into the reservoir to lubricate the bearings.  They don't make motors like that any more, like most Old
School stuff, better than the new stuff!


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## Firestopper (Mar 10, 2017)

whitmore said:


> There's a theoretical reason to prefer a 3450 rpm motor for your idler; the motor's rotor is the
> flywheel (and an energy storage element) for the converter output.   If a 3450 rpm motor
> is used, the flywheel has (for a given mass) four times the stored energy.   The motor
> mount will vibrate less, and the expected output imbalance might be somewhat improved (but
> ...



If we did everything according to theory, we would be disappointed often. One of the frequent complaints are how noisy or loud these converters are during operation. I can't emphasize enough how quiet my system is. In fact, It could use a pilot to prevent leaving it energized.  I contribute quick start, smooth operation, longevity and quiet operation to a few things to include balancing, but if you don't start off with a quality idler and lower RPM's you will be another guy claiming ALL RPC's are loud and noisy.  As Keith mentioned, a motor this size has plenty of rotating mass.  Stu, Ulma, Keith and now myself (again sorry) are speaking from experience  not theory alone. Investigator (IN) is trying to gather information to build his RPC system, we're hoping to assist him in acheving his goal, that said, why not build  a quiet system that will last a very long long time . My opinion as well as Im no expert in anything, but I can say the idler motor parameters I have shared, have been working quietly in the background since 2001. I have a backup Phoenix Phase converter (2006 model) that drove me nuts it was so freaking noisy, guess what PRM it is.....

cathead, your using a 1725 RPM motor, as well. I'm curious, if you removed the sheave it would still be quiet and easier to start. I'm thinking yes from the quality of the motor you discribed.


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## cathead (Mar 10, 2017)

firestopper said:


> If we did everything according to theory, we would be disappointed often. One of the frequent complaints are how noisy or loud these converters are during operation. I can't emphasize enough how quiet my system is. In fact, It could use a pilot to prevent leaving it energized.  I contribute quick start, smooth operation, longevity and quiet operation to a few things to include balancing, but if you don't start off with a quality idler and lower RPM's you will be another guy claiming ALL RPC's are loud and noisy.  As Keith mentioned, a motor this size has plenty of rotating mass.  Stu, Ulma, Keith and now myself (again sorry) are speaking from experience  not theory alone. Investigator (IN) is trying to gather information to build his RPC system, we're hoping to assist him in acheving his goal, that said, why not build  a quiet system that will last a very long long time . My opinion as well as Im no expert in anything, but I can say the idler motor parameters I have shared, have been working quietly in the background since 2001. I have a backup Phoenix Phase converter (2006 model) that drove me nuts it was so freaking noisy, guess what PRM it is.....
> 
> cathead, your using a 1725 RPM motor, as well. I'm curious, if you removed the sheave it would still be quiet and easier to start. I'm thinking yes from the quality of the motor you discribed.



It would spool up faster on start up but you wouldn't have the steady RPM  that the flywheel provides.  I think it would
still run quietly without the flywheel but lose some smoothness in operation and load control.


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## Investigator (Mar 10, 2017)

interesting.  So would adding a flywheel to an idler motor help with smoothness?  afterall, there is that unused shaft....


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## Ulma Doctor (Mar 10, 2017)

the flywheel increases inertia


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## Scruffy (Mar 18, 2017)

Knowing  absolutely nothing about electricity, when I needed 3 phases I contacted Jim at phase craft . I already had a 15 hp motor for the idler. He had me give him all of the information on the motor tag so he could build the control panel.
Once I had it installed the electrican checked the three legs and said he eas amazed in how close they matched..
I having nothing but praise for Jim and doubt I could have purchased anything better for less.
Thanks ron
   Ps my idler motor is made by allis- charmers and probably weighs 300 lbs?


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## markba633csi (Mar 18, 2017)

I've read that the motor should have cast iron end bells, not stamped steel ones.  You need the rigidity for long life and smooth running. 
Mark S.


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## cathead (Jan 5, 2021)

It would start faster, can't imagine it would run any louder although probably not as smooth running.

(Reply to Firestopper)


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