# 3 phase motor on single phase power



## John Valdes (May 17, 2007)

If you are only concerned about running 3 phase motors, maybe a drive would be cheaper and much easier to install?
All drives come with some I/O and some use onboard power to utlize the control scheme. Either way you could do lots of stuff using the inverter. To control the motor only.


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## Almost Retired (Sep 14, 2021)

John Valdes said:


> If you are only concerned about running 3 phase motors, maybe a drive would be cheaper and much easier to install?
> All drives come with some I/O and some use onboard power to utlize the control scheme. Either way you could do lots of stuff using the inverter. To control the motor only.


I spent probably a couple hundred dollars on that (not counting the 15 HP motor), and since it was my first one 3/4 a day tuning it. The next one could be half that time.

How much for a drive that would power several motors totaling 30 HP also a PLC and controls?


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## joe-nwt (Mar 28, 2019)

Almost Retired said:


> Now for the question: Anybody else seen or heard of anything using just caps and a Voltage sensing relay to run a 3 phase machine on single phase ?


These guys are very knowledgeable and easy to talk to:

Digital Phase Converters


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## Almost Retired (Sep 14, 2021)

joe-nwt said:


> These guys are very knowledgeable and easy to talk to:
> 
> Digital Phase Converters


any idea of the cost for a single converter for a total of 30HP at 230V ? I have been googling and cant seem to find any pricing. Would it be hundreds or thousands?


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## joe-nwt (Mar 28, 2019)

Almost Retired said:


> any idea of the cost for a single converter for a total of 30HP at 230V ? I have been googling and cant seem to find any pricing. Would it be hundreds or thousands?


I got a quote about 1-1/2 years ago for under $6000 for a 10HP unit.


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## 460 Delta (May 9, 2018)

Yeah AR I’ve heard of using a potential relay to switch in “booster” caps on varying loads and it works well I’ve been told.
I went with a slightly different approach at my last job running a R-mix plant off of a 60hp RPC. I took out 100 uf in the converter, and then put actually a little more than that total in the combo starter units to switch in and out with the motors. The voltage stayed pretty close to balance as the plant was running and conveyors and augers and pumps and compressors automatically switched in and out.
I feel for the poor soul who will need to figure out what I did someday.

I learned sooo much about this trade coming in behind old timers and brutal hacks drawing out what they had done and then devising a repair.


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## paulengr (Oct 8, 2017)

You can run a VFD but it’s going to cost you. First you need one that does single phase which would be unusual for anything over about 10 HP. Then roughly speaking double the HZp/amps for sizing. So that 30 HP motor requires a 60 HP VFD. I would just run it directly because you don’t even want to consider trying to supply 6x FLA for starting. So then look at the PLC side which is more likely around say 5 A which is very doable and stable. For that matter run the motor with the RPC which can handle the unstable voltage and only the controls on the VFD. The VFD price for 60 HP is going to be around $6,000 if you can find one that big…I’m having trouble finding one. Same price on the RPC according to Americanrotary.com


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## Almost Retired (Sep 14, 2021)

460 Delta said:


> Yeah AR I’ve heard of using a potential relay to switch in “booster” caps on varying loads and it works well I’ve been told.
> I went with a slightly different approach at my last job running a R-mix plant off of a 60hp RPC. I took out 100 uf in the converter, and then put actually a little more than that total in the combo starter units to switch in and out with the motors. The voltage stayed pretty close to balance as the plant was running and conveyors and augers and pumps and compressors automatically switched in and out.
> I feel for the poor soul who will need to figure out what I did someday.
> 
> I learned sooo much about this trade coming in behind old timers and brutal hacks drawing out what they had done and then devising a repair.


LOL @ poor soul. I know thats right ! 
So you were running a 3 phase plant with a single phase supply ? I wanted to do the same thing for the mill with the RPC, but they didnt even have starters for their motors, just a non fused disconnect. And it was more than they wanted anyway, they were happy with run to failure the way it was. And yes i have learned a lot also, working on these small shoestring mills.

since the time i did the RPC job, people started asking me to work on deep well pump starters: 240V 1 phase. They are similar to motors with the centrifugal switch on the back of the motor. They have a start winding and run winding, they also use a VSR/potential relay to switch out the start cap and winding. The run winding also has caps that stay in the circuit all the time. The deluxe ones also have an O/L of sorts for the start and run circuits each. They are a single line "push to reset" button like you find on the backs of some 110 motors It didnt take me long to buy a cap meter. cheap one to start, about $15 maybe. But boy oh boy was that better than just replacing caps and it still didnt work. lol


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## Almost Retired (Sep 14, 2021)

Almost Retired said:


> LOL @ poor soul. I know thats right !
> So you were running a 3 phase plant with a single phase supply ? I wanted to do the same thing for the mill with the RPC, but they didnt even have starters for their motors, just a non fused disconnect. And it was more than they wanted anyway, they were happy with run to failure the way it was. And yes i have learned a lot also, working on these small shoestring mills.
> 
> since the time i did the RPC job, people started asking me to work on deep well pump starters: 240V 1 phase. They are similar to motors with the centrifugal switch on the back of the motor. They have a start winding and run winding, they also use a VSR/potential relay to switch out the start cap and winding. The run winding also has caps that stay in the circuit all the time. The deluxe ones also have an O/L of sorts for the start and run circuits each. They are a single line "push to reset" button like you find on the backs of some 110 motors It didnt take me long to buy a cap meter. cheap one to start, about $15 maybe. But boy oh boy was that better than just replacing caps and it still didnt work. lol




And i hope you are feeling better too !


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## frankendodge (Oct 25, 2019)

A few years ago I was sent to a small woodworking shop to service a rpc that had let the smoke out. replaced the blown cap and all was well.. for 2 days. They had some german made table saw that wouldn't play nice anymore. when they ran the saw, the VSR would cause the start contactor to chatter and let the smoke out again. Main contactor coil inside the saw would burn up.. The caps in the rpc kept blowing. I got ahold of a tech from another RPC they had and after some chatting he suggested to replace the VSR with a timer.
Converter ran like a champ after that.. the main started coil in the saw would still melt.
Disassembled that coil and found someone had tampered with the return spring so that it wouldnt let the core completely clamp together. Replaced the coil and the spring. Never heard of any problems since.
European equipment is a pain in the ass.


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## 460 Delta (May 9, 2018)

frankendodge said:


> A few years ago I was sent to a small woodworking shop to service a rpc that had let the smoke out. replaced the blown cap and all was well.. for 2 days. They had some german made table saw that wouldn't play nice anymore. when they ran the saw, the VSR would cause the start contactor to chatter and let the smoke out again. Main contactor coil inside the saw would burn up.. The caps in the rpc kept blowing. I got ahold of a tech from another RPC they had and after some chatting he suggested to replace the VSR with a timer.
> Converter ran like a champ after that.. the main started coil in the saw would still melt.
> Disassembled that coil and found someone had tampered with the return spring so that it wouldnt let the core completely clamp together. Replaced the coil and the spring. Never heard of any problems since.
> European equipment is a pain in the ass.


Of course @frankendodge the real repair is always a NEMA replacement starter. Lol


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## AK_sparky (Aug 13, 2013)

I worked at a place that had a similar situation. They ended up buying a diesel generator. You'd have run some numbers to see if it's worth it or not, but it is an option.


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## CMP (Oct 30, 2019)

460 Delta said:


> Yeah AR I’ve heard of using a potential relay to switch in “booster” caps on varying loads and it works well I’ve been told.
> I went with a slightly different approach at my last job running a R-mix plant off of a 60hp RPC. *I took out 100 uf in the converter, and then put actually a little more than that total in the combo starter units to switch in and out with the motors.* The voltage stayed pretty close to balance as the plant was running and conveyors and augers and pumps and compressors automatically switched in and out.
> *I feel for the poor soul who will need to figure out what I did someday.*
> 
> I learned sooo much about this trade coming in behind old timers and brutal hacks drawing out what they had done and then devising a repair.


This is the correct approach for multi motor equipment on one rotary phase converter (RPC). It's roots goes back to the 40's. An individual RPC can only be balanced at one load point. Normally units are balanced at no load, with the generated leg slightly higher than the single phase legs. This allows for some voltage drop on the generated leg when the load comes on. If the generated leg is boosted too much without load, then you can have problems with control and motor circuits being too high at startup or unbalanced at lighter load.

By placing run caps at each motor and switched with the motor, will retain the best balance with varying loads. Just enough compensation for the individual loads that are currently connected and no more, and switched out of circuit along with the load. It takes more effort and time to do it this way, hence it's not very common or popular but produces a much better end result.

Most users focus on voltage balance of the system, likely due to the use of potential (voltage) sensing starting relays. But for 3Φ motors the current balance is more important. It is the current that produces torque. Thats why overload relays measure current and not voltage. For IEC overloads, they contain a current imbalance sensing tie bar mechanism, and will nuisance trip with unbalanced currents, even if the current is below the nameplate rating.

It's also best if all 1Φ control circuits and 1Φ other ancillary loads are connected to the utility supplied 1Φ legs for stability, under varying loads. For critical 1Φ loads such as PLC's, HMI's and switch mode power supplies, these loads can be separated out and placed on their own 1Φ circuit for better reliability and longevity, but provide a separate labeled disconnect adjacent to the main 3Φ disconnect.

One last thing is most purchasers and users of RPC do not know or understand that the output is a center tapped delta output, with the generated leg producing an output of greater than 208V to ground. This is a direct result of using center tapped single phase supply that is bonded to ground. Even though most motors used for RPC's are dual voltage, wound in a Wye configuration. The high leg output is detrimental to VFD inputs and many other electronic loads that use the generated leg.

As for the newbies that come behind you, sorting out what you did? Like others that pound you for it, wire tags and a hand drawn print will go a long ways to giving them a clue.


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## 460 Delta (May 9, 2018)

CMP said:


> As for the newbies that come behind you, sorting out what you did? Like others that pound you for it, wire tags and a hand drawn print will go a long ways to giving them a clue.


More likely the disconnected caps in the converter and the caps sitting in the combo cabinets connected to the starter lugs are what’s going to loop the guy that has to come behind me. I’m pretty much the only one around who does it the way I do it around here. A sharpie note on the door would have been a nice touch I suppose but I didn’t think to do that.


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## wcord (Jan 23, 2011)

Almost Retired said:


> Now for the question: Anybody else seen or heard of anything using just caps and a Voltage sensing relay to run a 3 phase machine on single phase ?


Willet Manufacturing out of Saskatchewan builds a model that is just caps and relays


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## CMP (Oct 30, 2019)

Those are termed static phase converters,
This company has been selling them for decades. They give reduced HP output compared to a RPC or utility power.









Static Converters On Phase-A-Matic, Inc.


Browse Static Converters in the Phase-A-Matic, Inc. catalog including Item Name,Description,




phaseconverters.phase-a-matic.com


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## 460 Delta (May 9, 2018)

CMP said:


> Those are termed static phase converters,
> This company has been selling them for decades. They give reduced HP output compared to a RPC or utility power.
> 
> 
> ...


There are models that sorta “splits the middle” and use a potential relay to start, and leaves some oil caps in the circuit to force the motor to try and generate its own missing phase. They work at best as meh, the current on the made leg is really too low to be very useful, but like everything, they do have a place. Maybe something like a tablesaw or drill press or something like that as a one off in someone’s hobby workshop.


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## CMP (Oct 30, 2019)

I've heard of guys using them to start an idler motor, as the basis of building an RPC, and then adding some additional tuning caps.


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