# Rotary Phase Converter-Idler Moter RPM?



## Walt (Jun 2, 2013)

Apologies if this has been covered, I'm not able to find an answer by searching the forums.

Does the idler motor speed matter for constructing a rotary phase converter?

I have a newly purchased lathe that needs 3-phase power and from what I've been reading, a Monarch runs better with a RPC than a VFD so I'm looking for an idler motor. So far I've been seeing 1740 or 3540 rpm. Does it make any difference?

Walt


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## Old Iron (Jun 2, 2013)

Both of my 10 HP motors are 3450 and they do make a lot of noise your best be is to fine a 1750 lot better.

Paul


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## 8ntsane (Jun 2, 2013)

From the number of RPCs I have built, the only motors I will use is the 1750 rpm or 1200 rpm. 
If you can find a 1200 rpm motor, they are my favorite. Nice and quiet, but harder to find.
All depends on how much noise you can stand. :whistle:


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## JohnG (Jun 2, 2013)

I've run both 3450 and 1750 rpm, and my current convertor is a hybrid with one of each @ 7 1/2 hp.  I've noticed no difference in running motors with any of those setups, but it does seem that motors which start under load, like a hoist or air compressor, have more starting torque when the convertor motor has additional mass in the armature as is typical in a lower speed motor.  It's still not the instant on snap of true 3 phase, but the start up is less labored.


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## Walt (Jun 2, 2013)

JohnG said:


> I've run both 3450 and 1750 rpm, and my current convertor is a hybrid with one of each @ 7 1/2 hp.  I've noticed no difference in running motors with any of those setups, but it does seem that motors which start under load, like a hoist or air compressor, have more starting torque when the convertor motor has additional mass in the armature as is typical in a lower speed motor.  It's still not the instant on snap of true 3 phase, but the start up is less labored.



Thanks to everyone for confirming a 1750 rpm motor will work, this will be my first choice.

In this link:

http://www.electro-tech-online.com/re-projects/100563-3-phase-converter-schematic-miller-system.html

the author states:

"On a 230 volt  60Hz system a typical load number is around 4 amps per HP at full  running load. Starting load can easily be 5 times that though. A 40 amp  circuit can easily start and run a 10 HP motor provided that the actual  source can support the possible 200 amp start cycle load with out  excessive voltage drop at the motor itself. "

1) I have to assume that the "200 amp start cycle load" refers to the total power delivered by the live current plus the power delivered by the starting capacitors. Otherwise the 220 v single phase circuit supplying the RPC would trip the breaker every time. I think this is an issue of sizing the starter capacitors correctly rather than having massive line current supplying the RPC.

2) Is the "total load number" the power of the idler motor or the power of the working machine motor?

My 220 circuit has 2x110v 20 A breakers linked together. If the guideline refers to the idler I will need 4x7.5= 30 amp breakers. If the guideline is for the machine motor I need 4x3 hp =12 amps. The 20 amp breakers I already have in my panel will work fine.

 Walt


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## Ulma Doctor (Jun 3, 2013)

as a point of information, a balanced RPC will have less noise than an unbalanced unit.
3450 or 1750 rpm, it makes no difference whatever you can get your hands on.
 the older the motor the better, the old school motors have thicker windings and seem to put up with a lot of abuse.
i have ran 7.5 hp motors from an old 5hp RPC without consequence to either.


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## Old Iron (Jun 3, 2013)

Both of my 10 hp RPC motors run on 20 amp breakers.

Paul


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## Walt (Jun 3, 2013)

Old Iron said:


> Both of my 10 hp RPC motors run on 20 amp breakers.
> 
> Paul



Thanks!

What HP motors do your machines have that are running off the RPCs?

Walt

- - - Updated - - -



Ulma Doctor said:


> as a point of information, a balanced RPC will have less noise than an unbalanced unit.
> 3450 or 1750 rpm, it makes no difference whatever you can get your hands on.
> the older the motor the better, the old school motors have thicker windings and seem to put up with a lot of abuse.
> i have ran 7.5 hp motors from an old 5hp RPC without consequence to either.



Does "balanced" mean "even weight distribution"? Or something else, like having capacitors in the run circuit to equalize the voltage/current produced by the wild leg?

After reading a little on motors and 3-phase electricity I'm starting to realize my knowledge is small on this topic.

Walt


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## Ulma Doctor (Jun 4, 2013)

Does "balanced" mean "even weight distribution"? Or something else, like having capacitors in the run circuit to equalize the voltage/current produced by the wild leg?

After reading a little on motors and 3-phase electricity I'm starting to realize my knowledge is small on this topic.

Walt[/QUOTE]

You are correct the generated leg's voltage is lower than the other two legs.
 Walt, we add run capacitors between phases to get the voltage as close to equal as possible. 
a 10% difference is considered acceptable, i have been able to achieve 3% with a lot of experimentation.

if i can be of assistance feel free to message me with any questions i might just be able to help out.
mike)


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## JohnG (Jun 6, 2013)

For a while I ran a 3 man shop off a phase convertor, and I became interested in how it behaved when several motors were run at once.  We might be running 3 machines with an aggregate 10-12 HP off a 7 1/2 HP convertor.  I noticed:
1)  If you just look at a schematic, the convertor looks exactly like the motors it is running.  
2)  When several motors are running, they all behave like auxiliary convertors depending on how heavily they are loaded mechanically, just as a literal interpretation of the schematic would tell you.  The voltage variation of the generated leg that Mike mentions diminishes with each added motor.
3) When an additional motor starts up, it draws power from all the other motors already running in addition to the convertor, which again is literally what the schematic would indicate.  The convertor has to be big enough to supply the full load run amperage of the running motors plus the start amperage of the new motor, or they all bog down trying to start up the newcomer.

In my experience "balanced" motors have a dynamically balanced armature to spin without vibration.  It wouldn't matter in a washing machine with a belt drive; but woodworking machines especially mount cutter heads on direct driven spindle motors to spin very smoothly, especially when driven by a high frequency convertor.


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## twstoerzinger (Jun 6, 2013)

A "balanced" 3 phase motor has all 3 phases pulling nearly the same amps. The standard quick and dirty field test is to use an Amprobe successively on each of the 3 motor  leads to check the current load per lead. A balanced motor runs smoothly and delivers full rated HP without cooling issues. An unbalance motor heats the heavily loaded windings more than the other(s), and in extreme cases cannot deliver rated HP.

Out in industry, large plants with 3 phase power systems can experience imbalance if there are a lot of connected single phase loads such as resistive heaters or lighting. If a plant system becomes too unbalanced, the motors experience "unbalanced" amp loads. The designers and electricians always try to arrange and connect the single phase loads such that the system, as a whole, remains mostly balanced.

For RPC users, with only 1 or a few motors on an RPC, the unbalance is due the capacitors (or lack of) in the system. Many users run RPCs successfully with little or no balancing. From what I have seen, you only need to have a well balanced system if one or more of the motors routinely run near rated HP, or if you have 3 phase electronics such as CNC drives and the like. With an RPC system, you can use a voltmeter to estimate the balance by comparing the voltage between each of the 3 phases.

Terry S.

edited to correct spelling


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## Restorer (Jun 16, 2013)

When I started out with a home shop I acquired industrial equipment from a high school shop.
They had 1/2 to 1 HP three phase motors.

The factory where I worked was required to convert motors to TEFC by an Insurance Co.
The motors removed were in a pile in the corner of a storage room.  I received permission to obtain a 5 HP
motor to build a RPC.  It was a huge heavy motor twice as large as a modern 5 HP with a 1.25 dia. shaft weighing over 100 lbs.

A welded frame was made and the 5 HP was bolted down.  A 1/3 was bolted next to it and they were belted together.

A stand off was added and an electrical enclosure was bolted on.  Incoming 220 v single phase was fused and a
 2 pole switch added for On/Off.  120 volts were tapped off and fused and a toggle switch added for the 1/3 hp motor.
for the output power a 3 fuse block was installed.  I know lots of fuses but my Dad didn't want me to burn down his building.

To start the converter, switch on the 1/3 to get things turning .  When up to speed engage 220 v and turn off the 1/3 hp motor.


Later some running capacitors were added to help bring up the voltage of the induced leg.

I have since sold the 3 phase machines but my brother still regularly uses the converter with his home shop equipment.
Its still running 40 years later.


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## stuarth44 (Dec 29, 2022)

INNERESTIN' About to start a RPC  build, in Au one is limited to how many amps one can legally draw off of the single ph. supply, 40 amps max
I  have a hyd power pack 3 hp, which will not produce enough grunt to split wood, thru a vdd rated at 10hp, I  had to run it back on 3ph. from my Lincoln  welder/genset  and with fuel 10 dollars gallon here  makes it expensive
I  have a 12 hp lathe and a bandasw, the latter on vhd but not a lot of boogie, so IMO  VHD  are a bloody lie
I  will endeavor 15 to 20 hp RPC 
saw this on youtube




any other advice would be welcomed here


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## pontiac428 (Dec 29, 2022)

Stuart, on 40 amps per WIRE you can easily drive a 20 hp rotary converter that should handle the inrush from any 12 hp motor out there.  40a is the upper limit for US 10 gauge wire, so for household installations the Aus code makes sense.  It's not even that restrictive, tbh.  I have the option to run big wire to big motors as long as my panel and service matches, but most of us are working with residential power as well.

Oh, I see... I just tripped for a sec working the numbers, but it comes out the same.  On 230v EU/Oceania power, you are on a delta tapped system.  In the US, we are on a wye-tapped system, so we can draw 40 amps off each split phase, but that's 110v from each line to neutral, so 2(40a*110v)=80a*230v, so it's the same in the end, but your breaker will need to be 80A (line to neutral).  My breaker is 2x40a line to neutral.  Both are 230v.  The difference is you have thinner wire for the 230v mains than I do for 110v, so my ampacity is higher.  Hmm.  Are your breakers single pole, or two pole?  If you're single pole, you might not get the capacity you want without running heavier wire.  You're going to need to check with an Australian electrician, probably.  Do your RPC generator motors have to run backwards because of the inverse Coriolis effect?  I'd better figure that out, too, there are more layers to your question that I initially thought.

My houses in Germany all had 230 3-phase service, and the house I just sold there had 440v/230v 3-phase in the shop/garage...  I guess the Americans would have to charge extra for that.


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## stuarth44 (Dec 30, 2022)

pontiac428 said:


> Stuart, on 40 amps per WIRE you can easily drive a 20 hp rotary converter that should handle the inrush from any 12 hp motor out there.  40a is the upper limit for US 10 gauge wire, so for household installations the Aus code makes sense.  It's not even that restrictive, tbh.  I have the option to run big wire to big motors as long as my panel and service matches, but most of us are working with residential power as well.
> 
> Oh, I see... I just tripped for a sec working the numbers, but it comes out the same.  On 230v EU/Oceania power, you are on a delta tapped system.  In the US, we are on a wye-tapped system, so we can draw 40 amps off each split phase, but that's 110v from each line to neutral, so 2(40a*110v)=80a*230v, so it's the same in the end, but your breaker will need to be 80A (line to neutral).  My breaker is 2x40a line to neutral.  Both are 230v.  The difference is you have thinner wire for the 230v mains than I do for 110v, so my ampacity is higher.  Hmm.  Are your breakers single pole, or two pole?  If you're single pole, you might not get the capacity you want without running heavier wire.  You're going to need to check with an Australian electrician, probably.  Do your RPC generator motors have to run backwards because of the inverse Coriolis effect?  I'd better figure that out, too, there are more layers to your question that I initially thought.
> 
> My houses in Germany all had 230 3-phase service, and the house I just sold there had 440v/230v 3-phase in the shop/garage...  I guess the Americans would have to charge extra for that.


thanks so much, will answer later today


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