Rotary Phase Converter

[theoldwizard1]

So I know enough about electrical wiring, transformers, etc to be dangerous !

Because I was curious, I did some research on rotary phase converters. I had always assumed it was a single phase motor connected to a 3 phase alternator, but this is not the case.

Most rotary phase converters are just a 230V 3 phase motor with two input leads hooked to both the single phase as well as 2 of the 3 phase output legs. The third leg of the motor generates the 3rd leg.

There are some starting capacitors required or some old timers use a rope starter !

So here is my question. Normal 3 phase, each leg is 120° "out of phase" with the other. Obviously this is not the case with the output of a rotary converter as T1 and T2 are 180° out of phase. Obviously this "close enough", but what is the real impact on 3 phase motors and on 3 phase welders ?

 

[lametec]

The phases are 120° out, even on an RPC.

T1 and T2 are not two different phases, they are one phase (i.e. single phase). The TWO other phases (T1 and T3, T2 and T3) are generated by the RPC.

 

[theoldwizard1]

From How to Build a Rotary Phase Converter

• The three legs of power that you are supplying are not balanced, because of their different sources.
• The phase separation which is supposed to be 120 degrees between all legs also won't be just right, because the single phase line voltage to L1 and L2 is 180 degrees apart.
• Because of the imperfect nature of the power supplied to both the equipment motor and the idler neither of them will run at full efficiency - they will be down on power, noisier, produce more vibration and heat, will consume more energy, and won't last as long as they would if supplied with proper three phase power.

All that being said, it will still work, and there are things that a determined diy (do-it-yourselfer) can do to make the system work better

• Add a magnetic motor starter switch
• Add a starting capacitor
• Add run capacitors

I'm looking for real world feed back on rotary phase converter "limitations".

 

[mmurphy]

Are they not 90 degrees instead of 120?

 

[rsanter]

There are also capacitors in there system that help with the difference in phase

Using a RPC in my opinion is the second best way to power a 3ph motor. To me the best is the VFDs. As far as powering a welder I don't know.
For the welder you may need to build one of the 3ph generators that have a single phase motor spinning a 3ph motor

Bob

 

[Rookie2]

I built a rope start system with an old iron 3hp motor to generate the 3rd leg for a millimg machine. no power problems with the mill. But I looked like i was doing that wedding chicken dance to get it started !

They make less expensive solid state converters now, see ebay !

 

[theoldwizard1]

So who is using an RPC with a welder ?

 

[wyliesdiesels]

Good question wizard. I've always wondered this myself!

Are they not 90 degrees instead of 120?

No because 90 x 3 does NOT equal 360!

 

[lametec]

I haven't read about anyone doing it on this forum, but I've seen it on practicalmachinist.com.

My welders are both single phase. I do run a lathe, mill, and press brake off an RPC, though.

 

[offroadsteve]

I'm no expert, nor do I have a phase converter, but rsanter has it the most right based on what I've read, your two input legs are 180 degrees apart, and the rotary converter generates the third leg somewhere in the middle. The capacitors help smoothing things out and balancing voltage on your three legs under load.

I definatly understand that technically 240V power is "single phase", hence talking about two input "legs". What is important is that the waveforms on those two legs are 180 degrees out of phase (when speaking from a purely waveform perspective). I'm sure running them into a 3-phase motor with windings 120 degrees apart does some intersting things, but i've not studied that in sufficent detail. However, I have read enough things on the internet to believe that it works in most cases. And you can belive everything you read on the internet

In all the reading I've done, it seems that running a welding machine on a rotary phase converter is cost-prohibitive. Since the RPC is not doing anything for you voltage-wise, you are still looking at the 208/240V amperage numbers. For an older, 250/300A output welding machine, you need on the order of 50-70+ amps of input current, even running on three-phase. Even at 50A, thats 12,000 watts, assuming unity power factor and no loss in the RPC, you need at least a 16 HP motor for your RPC, meaning to be sure you are OK at full load, you need a 25-30+ HP RPC. That's a big motor.

You would be better off buying a welding machine that can run on single phase, unless you can get large, 3 phase motors for free or nearly free.

 

[theoldwizard1]

... Even at 50A, thats 12,000 watts, assuming unity power factor and no loss in the RPC, you need at least a 16 HP motor for your RPC, meaning to be sure you are OK at full load, you need a 25-30+ HP RPC. That's a big motor.

Concur. You can occasionally find them cheap on CL.

You would be better off buying a welding machine that can run on single phase, unless you can get large, 3 phase motors for free or nearly free.

Don't forget the cost of a new welder to replace a proven good, well maintained, 3 phase welder.

I also agree that doing any kind of "heavy" (thick material) "production" work with a welder running on an RPC is not really practical. Backyard shop, intermittent use, probably less than 1/4".

 

[210Hardtop]

I run a 10HP vacuum pump and a 10Hp spindle on my CNC router, no problems using a RPC.
We see swings in voltage in my rural setting, especially during A/C season, Because of this I have wired additional capacitors, that are manually switched to balance the voltages on the generated leg.
Usually a quick check in the morning and were good to go for the day.

You should make sure that the 230V controls are not using the generated third leg.

 

[lametec]

I definatly understand that technically 240V power is "single phase", hence talking about two input "legs". What is important is that the waveforms on those two legs are 180 degrees out of phase (when speaking from a purely waveform perspective).

The two legs are one phase, and are only 180º apart with reference to neutral/ground, since it's split at the transformer on the pole. So there are not two waveforms on 240V, just one. N/G are not used in powering the RPC idler

 

[wyliesdiesels]

I definatly understand that technically 240V power is "single phase", hence talking about two input "legs"....

240v comes in several flavors of 3 phase as well...

 

[Grumpy365]

So the question is what the impact to the motor and the equipment????

The motor is just an idler, it has no load, it would be hard to quantify a number on how it impacts the motor. I would bet you will see the bearings go, before anything happens to the electrical aspects of the motor.

A far as the welder.
You don't get perfect voltage between legs, so you blance it out with "RUN Capacitors" to get equal voltage between legs.
I think you are over thinking it.

 

[jw_]

so i want to hook up a really simple rotary phase converter using a 5hp idler motor to generate 3ph power for a big lathe (also 5hp)

some questions: does it matter which wire from the 3 phase motor goes where? in my feeble understanding it wouldn't, except that the motor might run backwards in 50% of the possible wirings?

also: do i need to use capacitors, or does that just help the efficiency? i doubt if i'll ever load this monster very heavily according to its design, so some loss of power wouldn't be missed.

breakers: do the breakers on the single phase panel need to be on opposite sides of the bus panel? I'm seeing 250v across a pair of 20a breakers on the same side. so am i ok?

breakers again... i assume i need breakers that will handle the combined amp draw of both the idler motor and the tool motor. is that right?

thanks for any help. i'm obviously poking around in some unknown territory here.

 

[Grumpy365]

so i want to hook up a really simple rotary phase converter using a 5hp idler motor to generate 3ph power for a big lathe (also 5hp)

some questions: does it matter which wire from the 3 phase motor goes where? in my feeble understanding it wouldn't, except that the motor might run backwards in 50% of the possible wirings?
It may run backwards, if so Switch any 2 legs

also: do i need to use capacitors, or does that just help the efficiency? i doubt if i'll ever load this monster very heavily according to its design, so some loss of power wouldn't be missed.
The Start capacitors are needed to start thge motor, the RUN CAPACITORS Balance voltage between legs. Some people say they don't need them,

breakers: do the breakers on the single phase panel need to be on opposite sides of the bus panel? I'm seeing 250v across a pair of 20a breakers on the same side. so am i ok?
Not sure what you are asking. You need the single phase to come off of a single phase DOUBLE POLE breaker.

breakers again... i assume i need breakers that will handle the combined amp draw of both the idler motor and the tool motor. is that right?
The Idler motor will not be loaded up. It is basically free spinning. It wont pull face plate amps

thanks for any help. i'm obviously poking around in some unknown territory here.

The answers above are how I understand things to be.

 

[jw_]

ok, another stupid question... how does a double pole breaker differ from a pair of single pole breakers?

i get that both legs break on overload, but other than that it would be essentially the same as a pair of adjacent single breakers in the panel, right?

 

[Norcal]

ok, another stupid question... how does a double pole breaker differ from a pair of single pole breakers?

Handle tie and internally linked trip mechanism, & some rare cases a 240 volt rating instead of the more common 120/240 volt rating

 

[jw_]

thanks.

 

[wyliesdiesels]

...
breakers: do the breakers on the single phase panel need to be on opposite sides of the bus panel? I'm seeing 250v across a pair of 20a breakers on the same side....

thanks for any help. i'm obviously poking around in some unknown territory here.

Most every panel has breakers arranged in a different manner. Just the reason u need to make sure each breaker pole is pulling power from the opposite bus bar, making it so there's 240v potential between the 2 breaker poles!

ok, another stupid question... how does a double pole breaker differ from a pair of single pole breakers?

i get that both legs break on overload, but other than that it would be essentially the same as a pair of adjacent single breakers in the panel, right?

A double pole breaker has a handle tie and may have common trip!

 

[jw_]

ok, so if the rated amperage at 240v 3ph is 18, what size breaker do i need to power it from a single phase source through an idler motor.... approximately?

 

[jw_]

i have it connected to a 240V 30A circuit and it will now start and run in both directions, but the breaker is tripping after about 10 seconds. how do i calculate what size breaker i need?

 

[Grumpy365]

i have it connected to a 240V 30A circuit and it will now start and run in both directions, but the breaker is tripping after about 10 seconds. how do i calculate what size breaker i need?

Are you using start capacitors? If so, they need to be on a momentary push button or a timer so they drop out after the motor starts.

You shouldn't be popping a breaker with just the idler motor running.

Do you have a picture?

 

[jw_]

no start capacitors... i'm starting the idler motor with a rope.

 

[lametec]

FLA of 18A on 240V 3 phase equals about 31A on single phase. You shouldn't be tripping the 30A breaker assuming the motor is actually up to speed before it trips.

If the motor is still gaining speed, then it might not be getting enough power. What size wires are you using to feed the RPC and lathe?

 

[nine4gmc]

in for info

 

[Grumpy365]

If you don't have the lathe wired up and you are just running the RPC, do you pop the breaker?

 

[jw_]

no, i can run the rpc all day no problem. the lathe starts and runs but soon trips the breaker. also i can't tell if it's getting up to full speed or not.. kinda seems slow to me.

it is a squirrel cage induction motor. it can be started in either direction by a set of triple-pole air break contactors in the lathe controls. (i got that from the docs)

 

[jw_]

FLA of 18A on 240V 3 phase equals about 31A on single phase. You shouldn't be tripping the 30A breaker assuming the motor is actually up to speed before it trips.

If the motor is still gaining speed, then it might not be getting enough power. What size wires are you using to feed the RPC and lathe?

i'm using 12ga wire to the RPC, and the existing cables that were on the lathe when i bought it, which also appear to be 12ga.

 

[jw_]

it's starting to sound like there is a problem with the lathe motor, no? it's definitely not picking up speed like it should.

i'm measuring 1.2-1.3 across each of the windings on a 200 ohm scale. i don't know what i should be getting, but the fact that they all look about the same makes me think that's not my problem?

what else can i be missing?

 

[jw_]

i just tried switching the 3 leads going into the lathe 2 at a time. switched 1,2, no change, switched 1/3, no change... interesting thing when i switched 2/3 it caused the contactor to arc and jump.

voltages are like 249/208/212 is that bad enough to keep the motor from starting?

really, it's just chugging along like it were single phased. i'm perplexed.

 

[Grumpy365]

i just tried switching the 3 leads going into the lathe 2 at a time. switched 1,2, no change, switched 1/3, no change... interesting thing when i switched 2/3 it caused the contactor to arc and jump.

voltages are like 249/208/212 is that bad enough to keep the motor from starting?

really, it's just chugging along like it were single phased. i'm perplexed.

You were asking about run capacitors.

I wonder if that is your problem.

It doesn't sound like it isn't running long enough to check your voltage between legs, UNDER LOAD, but you may look at it.



Also, is the 3 phase Idler motor wired to 208/230 as opposed to 460? for that matter is the lathe motor wired to 208/230 as opposed to 460?

 

[jw_]

You were asking about run capacitors.

I wonder if that is your problem.

It doesn't sound like it isn't running long enough to check your voltage between legs, UNDER LOAD, but you may look at it.



Also, is the 3 phase Idler motor wired to 208/230 as opposed to 460? for that matter is the lathe motor wired to 208/230 as opposed to 460?

BANG! I had checked the voltage wiring on the lathe motor, but ASSumed that since the idler motor was running on 240 ok that it was hooked up correctly... wrong. there was a jumper block that you reverse for low/high and i'll be damned if it wasn't set to hi! fixed that and she hopped right up and ran!

I'm grateful for the assistance, y'all... now i'm gonna go cut a couple of spacers for this bike