# Upgrading my simple RPC, questions



## Investigator (Aug 25, 2020)

I am very close to moving into my new shop space.  When I do, I would like to upgrade my RPC for the lathe.  First let me say that I don't find VFD's to be an option, call it bias, lack of experience, bad decisions or hard-headedness.  I like the RPC and the ability to use the machine as built.  For the record I am powering a single lathe, a Logan 12" with 2 speed motor and a really handy Reeves Drive, all this in a machine from the 1960's.

What I have now is a simple unbalanced RPC as detailed here MY RPC .  I am using a 7.5hp 3ph motor as an idler, I have no caps to balance anything out.  To start the RPC I wrap a small rope around the idler shaft and give it a pull to get it moving, then hit the switch and we're off.

Now, what I would like to have when I move is an upgraded control panel.  I would like to have a way to simply push a button and the panel start to motor and keep it going without me having to spin the motor manually.  My question is how hard would that be to build myself, and would it just be worth it to buy a panel to hook up to my idler motor?  If buying a panel, what brand(s) should I look for and which ones should I stay away from?  At one point I thought one of our members made panels for sale, is the correct? If so who?


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## Karl_T (Aug 25, 2020)

There are RPC kits without motor on ebay that's a good deal. Or build it all yourself. here's one fine tutorial.

<EDIT> here's the eBay offer for a 3hp. I think this vendor would have a 7.5 also.








						3 Hp phase converters control panel CONVERTER ROTARY made in USA RP03   | eBay
					

With this setup you will have a 3 hp rotary phase converter, your 3 hp motor should be rated at 230volt or 230/460volt and a rpm of 17XX or 34XX. Rotary Phase Converters. Rotary Phase Converter Panels.



					www.ebay.com


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## markba633csi (Aug 25, 2020)

I think all you need is a motor starting type capacitor and a push button to start the motor instead of pull-starting it.  You would connect the cap in between one of the incoming power legs and the 3rd generated leg of the RPC.  The button would be in series with one of the cap legs to connect it momentarily when you push it.
Not sure what value cap you need- Mike probably can give you a ball-park figure- I'm thinking about 200 microfarad at about 400 volts, oil-filled type
-Mark


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## Nutfarmer (Aug 26, 2020)

I ran into the same problem as you have, a two speed motor. I couldn't figure out away to run it off a VFD. So I put in a rotary phase converter. It works so I am happy. I didn't have to rewire the lathe ether. Have used a VFD  on my surface grinder and mill that worked out fine,but a two speed motor i had no clue.


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## cathead (Aug 26, 2020)

Adding a starting capacitor to one of the other legs of the three phase motor to spool the motor up
is quite easy to do.  I turn on the main power and push the momentary switch simultaneously and
hold it down until the motor is up to speed.  My motor has large cast iron pulley sheaves on it which
makes it very smooth running.  You need a hefty momentary switch or alternatively use a heavy duty
relay to power the capacitor.  Evening out the voltages can be accomplished by adding capacitors across
the legs but not necessary in my experience.


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## Ulma Doctor (Aug 26, 2020)

7.5 hp will need around a 500Uf capacitor to start quickly.
A 400Uf cap would be the very least I would attempt.
The panel is simple to build.
I would be happy to help out or wire it up or to make a panel for you.


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## Investigator (Aug 26, 2020)

Ulma Doctor said:


> 7.5 hp will need around a 500Uf capacitor to start quickly.
> A 400Uf cap would be the very least I would attempt.
> The panel is simple to build.
> I would be happy to help out or wire it up or to make a panel for you.



I would enjoy building it myself, I don't have a problem with things like that.  What I do need is schematic and parts list.  I did find this 'kit' on the 'bay and wonder if it is what I need and/or priced well 








						7.5HP Rotary Phase Converter kit   | eBay
					

With this setup you will have a 7.5 hp rotary phase converter, your 7.5 hp motor should be rated at 230volt or 230/460volt and a rpm of 17XX or 34XX. -Heavy duty run capacitors rated at 370vac to balance the generated leg from L1 and L2.



					www.ebay.com


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## benmychree (Aug 26, 2020)

I made my own RPC using a static phase converter for starting (I already had it) and used oil filled capacitors to balance the voltages; it works fairly well, but some machines spool up more slowly than they normally would on "real" 3 phase.  The RPC motor is 1750 RPM, and my lathe has a two speed motor, 1750 and 3450 RPM, it starts well on low speed, but is slow to spool up on high speed, which I rarely use.


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## mksj (Aug 26, 2020)

It may be worthwhile to purchase a 7.5Hp RPC control panel unless you happen to have parts readily available. Getting the right specs. and decent components, I often find I am not saving that much at the end of the day. They also have some safety detection circuits for phase loss.








						CP-7 Pro-Line 7.5HP Rotary Phase Converter Control Panel - Loaded with Features  | eBay
					

Find many great new & used options and get the best deals for CP-7 Pro-Line 7.5HP Rotary Phase Converter Control Panel - Loaded with Features at the best online prices at eBay! Free shipping for many products!



					www.ebay.com


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## Investigator (Aug 26, 2020)

Will this do it?










						7.5HP Rotary Phase Converter kit   | eBay
					

With this setup you will have a 7.5 hp rotary phase converter, your 7.5 hp motor should be rated at 230volt or 230/460volt and a rpm of 17XX or 34XX. -Heavy duty run capacitors rated at 370vac to balance the generated leg from L1 and L2.



					www.ebay.com


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## Ulma Doctor (Aug 26, 2020)

Investigator said:


> Will this do it?
> 
> 
> 
> ...


it will do but you can do it for a lot less money

there are many ways of doing it, but it doesn't get much cheaper than this...









						Rpc simple design unbalanced
					

Rpc simple design  Here's a simple plan for an unbalanced 220/240 v single to 3 phase converter. this design will start ANY 3 phase motor of ANY horsepower with a change of start capacitors! keep in mind,the contactor coil is 220v in this design, adjust coil voltage for high voltage use(480v)...




					www.hobby-machinist.com


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## Investigator (Aug 26, 2020)

Ulma Doctor said:


> it will do but you can do it for a lot less money
> 
> there are many ways of doing it, but ti doesn't get much cheaper than this...
> 
> ...



My question/concern is having to use 2 switches basically at the same time to get things started; 1 to power the assembly and 1 momentary to start the motor.  Is there a single switch that functions the key ignition on a car in that turning it 'on' is a half turn with a momentary on in position 2 that will automatically turn it back to position 1 when released?


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## Ulma Doctor (Aug 26, 2020)

the normally closed stop in combination with a normally open start is a common motor starting latching circuit.
the capacitor in my design is operated by the start switch, once the idler is started, the capacitor is out of the circuit.

there are ways of using a toggle switch or using a maintained contact switch, but those may require the use of a secondary relay.

the control system runs many types of equipment with electrical latching.
there is safety to consider, the latching circuit will not start without operator input in the event of a power outage.
a maintained circuit will not have the same safety without the use of other devises to otherwise prevent operation without operator input


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## Investigator (Aug 26, 2020)

Ulma Doctor said:


> the normally closed stop in combination with a normally open start is a common motor starting latching circuit.
> the capacitor in my design is operated by the start switch, once the idler is started, the capacitor is out of the circuit.
> 
> there are ways of using a toggle switch or using a maintained contact switch, but those may require the use of a secondary relay.
> ...



When buying switches or the 3 pole contactor coil, is there any problem buying oversized?  For instance this one:






						50 Amp Definite Purpose Contactor 3 Pole, 120VAC Coil 40A, 50A, Lighting, Heating, Refrigeration: Amazon.com: Industrial & Scientific
					

50 Amp Definite Purpose Contactor 3 Pole, 120VAC Coil 40A, 50A, Lighting, Heating, Refrigeration: Amazon.com: Industrial & Scientific



					www.amazon.com


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## Ulma Doctor (Aug 26, 2020)

i always oversize, when in doubt!
it would not be overkill to use a 50 amp contactor.

i might be tempted to seek 60 amp gear like this...









						Fuji SC35AA Magnetic CONTACOTR 3 Pole 600 Volts 220 V Coil W/ Overload Realy for sale online | eBay
					

Find many great new & used options and get the best deals for Fuji SC35AA Magnetic CONTACOTR 3 Pole 600 Volts 220 V Coil W/ Overload Realy at the best online prices at eBay! Free shipping for many products!



					www.ebay.com
				





note that it has a 220v coil.
( if you were to follow my design, you'll need 220v coil)

if you were to use a 120v coil, you would need to add a step down transformer


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## Ulma Doctor (Aug 26, 2020)

here is a great candidate too









						C25FNF360B Eaton / Cutler Hammer DP Contactor - 60 Amp • 3 Pole • 220-240V Coil   | eBay
					

C25FNF360B  /  60 Amp / 3 Pole / 208-240V. The universal mounting plate eliminates changing hole patterns when replacing competitive devices. Airstar is quickly becoming a Leader in HVAC & Indoor Air Quality Solutions.



					www.ebay.com


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## Reddinr (Aug 27, 2020)

I understand not wanting the complexity of a VFD but, for the record, it is possible to use one with a two speed motor and use the lathe controls normally.  It requires an LC filter (Sine wave filter) on the VFD output and running the VFD at a constant output voltage and frequency by twiddling with the settings.  Some VFD's allow this.  That is my present setup with a 2 speed motorized 16" lathe.  It doesn't take advantage of speed control of the VFD at all but for me I stubbed my toe by buying an RPC that was too small for the lathe and rigged up the VFD instead.  Moral of the story, don't cheap out on an RPC that is just barely big enough to deal with the inertia of the lathe because you will tire of the loooong spin up times.  My VFD setup works great though.  At some point I will likely replace the motor with a larger HP single speed and take advantage of the speed control.  Have not missed that capability yet though.


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## mksj (Aug 28, 2020)

Some VFD's have setting for 2 separate motors, so you can setup the VFD to operate at both motor speed settings. One can use 3 way changeover switch 1 0 2 to switch the motor wiring and activate a VFD input that changes the motor parameters. The flip side, is in most cases no functionality of using this approach (I have tried it several times). Two speed motors come in a number of different configurations, but typically you loose Hp on the lower speed setting. In this case you use only the high speed wiring configuration directly wired to the VFD. If you you have a constant Hp motor (typically 4P and 2P), then you wire the motor for 4P (low speed) and run the VFD to 120 Hz which double the torque on the 4P setting and Hp is constant from 60-120Hz). Use the same parameters for both low speed and high speed motor setting, I would not recommend it. The VFD settings such as amps, RPM, poles, and the autotune data is very different and likely to result in sub optimal performance. 

As a general note, I have had a number of poor performance issues of matching a VFD to multiple speed motors, the exception being constant Hp motors.  I have done several VFD conversions with constant torque (variable Hp) motors and in all the cases I ended up replacing the motor with a single speed. The motors would run rough and we saw cogging at the lower speeds. I have also seen this infrequently on a few single speed 3 phase motors.


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## Investigator (Sep 3, 2020)

Ulma Doctor said:


> the normally closed stop in combination with a normally open start is a common motor starting latching circuit.
> the capacitor in my design is operated by the start switch, once the idler is started, the capacitor is out of the circuit.
> 
> there are ways of using a toggle switch or using a maintained contact switch, but those may require the use of a secondary relay.
> ...



How about these:






						Motor Start Capacitor 430-516 uf MFD 220-250 Volt VAC MARS2 11990: Amazon.com: Industrial & Scientific
					

Motor Start Capacitor 430-516 uf MFD 220-250 Volt VAC MARS2 11990: Amazon.com: Industrial & Scientific



					www.amazon.com
				










						TWTADE / 2pcs 22mm Panel Mount 10A 440V 1NO 1NC DPST Red Green Momentary Push Button Switch Pushbutton Switches (Quality Assurance for 3 Years) LA38-11BN/GR: Amazon.com: Industrial & Scientific
					

TWTADE / 2pcs 22mm Panel Mount 10A 440V 1NO 1NC DPST Red Green Momentary Push Button Switch Pushbutton Switches (Quality Assurance for 3 Years) LA38-11BN/GR: Amazon.com: Industrial & Scientific



					www.amazon.com
				










						50 Amp Definite Purpose Contactor 3 Pole, 120VAC Coil 40A, 50A, Lighting, Heating, Refrigeration: Amazon.com: Industrial & Scientific
					

50 Amp Definite Purpose Contactor 3 Pole, 120VAC Coil 40A, 50A, Lighting, Heating, Refrigeration: Amazon.com: Industrial & Scientific



					www.amazon.com


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## Ulma Doctor (Sep 3, 2020)

the cap and switches are fine, the contactor needs to have a 220v coil , unless you plan on utilizing a step down transformer.



			Amazon.com
		


the 40 amp would suffice, if a 50 amp can't be easily located


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## MikeInOr (Sep 4, 2020)

I built my rotary phase converter 25 years ago using a 10hp cast iron framed motor.  I could never figure out the specs on the voltage controlled relay so I used a timing relay instead.  It took some experimenting of adding and removing the run capacitors to balance the three legs.  I have several VFD's now but I still use my RPC every time I go to the shop.

https://www.ebay.com/itm/7-5HP-Rota...134475?hash=item264360300b:g:rzQAAOSwo4pYNIuX
$119.50 doesn't sound like a bad price to me for a 7.5hp RPC kit.  It does have a voltage sensing relay so it would be a single button start.  What the kit is missing is the start / stop buttons but you can grab those from any motor contactor box. Capacitors aren't cheap if you order new ones and the caps in this kit are rated for 370vac.  I blew several cheap 220v caps trying to save a buck when I was building my RPC.  It didn't take too long before I just paid the premium for higher voltage capacitors.

For a first time RPC builder I think that kit sounds like a great deal!  I would have bought one if there was one available 25 years ago!


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## Investigator (Sep 4, 2020)

Ulma Doctor said:


> the cap and switches are fine, the contactor needs to have a 220v coil , unless you plan on utilizing a step down transformer.





Ulma Doctor said:


> the cap and switches are fine, the contactor needs to have a 220v coil , unless you plan on utilizing a step down transformer.
> 
> 
> 
> ...



my plan is to use a single hot leg from the contactor to power the coil.   

Now, I just need an enclosure. I'm not sure if the one I currently have is big enough.  On that note, is there any requirements that the enclosure be metal?  what about plastic or even wood?


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## Ulma Doctor (Sep 4, 2020)

Investigator said:


> my plan is to use a single hot leg from the contactor to power the coil.
> 
> Now, I just need an enclosure. I'm not sure if the one I currently have is big enough.  On that note, is there any requirements that the enclosure be metal?  what about plastic or even wood?


NOTE:
The single hot leg through coil would need to meet up with the neutral wire to get 120v.
you can't just ground one side of the coil and put 1 leg of the 220v supply circuit to it, and have it function correctly.
This would require a 4 wire 240v supply to get usable 120v from 240v single phase

Plastic Nema enclosures are relatively cheap on amazon.


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## Barncat (Sep 4, 2020)

Surplus center also usually has cheap capacitors, usually some in the 370v range, if you want some extra protection instead of running close to the upper limit of a 250v capacitor


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