Thread: 3phase on single phase plus a capacitor ?

Quote snipped from this site http://www.ibuildit.ca/Reader%20Proj...rojects-6.html

The motors are all 3-phase, and as I do not have 3-phase in my shop I run these on single phase and a capacitor for the third phase. This reduces the real output power, but the drive motor is rated 550w and the geared "stroke" motor is labeled 90w, and both are more than powerful enough. The motor has never stalled.

Am curious, the way this is casually discussed makes me think that for this chap its common practice to do this, and also that its quite easy.

Do many people use 3 phase motors like this ?

Gotta be some traps ?

ie Find a competent sparky to install capacitor.

Not sure why anybody would do this unless they had a 3 phase motor lying around spare and couldnt be bothered to buy/find/swap a single phase motor at the time.

Bill

Not all that uncommon, if you google "static phase converter" you should find some circuits that show how to do it... the rotary phase converter is more common, and uses a static phase converter type circuit to drive the idler motor..

But with VFD's being as cheap as they are these days, the VFD is a much better choice, no phase distortion, and variable speed.

Ray

PS... Circuit below is from here... http://www.waterfront-woods.com/Arti...econverter.htm

Is the output of the cheaper VFD's a square wave rather than a sine wave ? If so does this have any effects on the motor such as running hotter or any other nasties ? Mike

Originally Posted by RayG

Not all that uncommon, if you google "static phase converter" you should find some circuits that show how to do it... the rotary phase converter is more common, and uses a static phase converter type circuit to drive the idler motor..

But with VFD's being as cheap as they are these days, the VFD is a much better choice, no phase distortion, and variable speed.

Ray

PS... Circuit below is from here... http://www.waterfront-woods.com/Arti...econverter.htm

Originally Posted by morrisman

Is the output of the cheaper VFD's a square wave rather than a sine wave ? If so does this have any effects on the motor such as running hotter or any other nasties ? Mike

Here is the output of a cheap VFD. that I obtained during some motor/VFD experiments. The wave form is indeed made up of little short time segments of approx square waves but the overall output is a reasonable approximation of a sine wave. The sorts of time periods that involve the little segments have a minimal impact on motors that run at the lower frequency end of their outputs.

Originally Posted by BobL

Here is the output of a cheap VFD. that I obtained during some motor/VFD experiments. The wave form is indeed made up of little short time segments of approx square waves but the overall output is a reasonable approximation of a sine wave. The sorts of time periods that involve the little segments have a minimal impact on motors that run at the lower frequency end of their outputs.

That's essentially the output of a D/A converter. Nothing wrong with that. If you need a closer sinewave approximation then it's just a matter of how many bits the D/A conversion is. Simon

I have an RPC that uses that sort of capacitor configuration to run all my 3 phase gear. It includes extra bits like a contactor, breaker, RCD, etc, but that is the basic config.

I also run everything from 480V rather than 415 or 240 (note the motors are actually 415V motors).

VFD's have some advantages, however if I was to but one VFD for every machine (5) and a couple that are around 6 - 7HP, it would get very expensive. The RPC cost me less than $100 to set up.

Id be keen to see how you set up your RPC. Id be guessing a 3 ph motor and an old multi voltage welder?

Be keen to know the exact details...

Simon

Simon

Originally Posted by simonl

Id be keen to see how you set up your RPC. Id be guessing a 3 ph motor and an old multi voltage welder?

Be keen to know the exact details...

It is basically as per the drawing above, except 480V line to line, rather than 240V. I have 480V split phase connection that goes through a RCD/CB to a contactor that is used for on/off control. This then goes to a 5hp 3 phase (415V) motor ... NO transformer to bring the 480V down to 415V. The capacitors are setup to give me the best overall voltage balance. There is also a switch that connects in some additional capacitors at start up to allow for a quick start up, they are then switched out.

Very basic, but very effective system. It is used to run a panel saw, dusty, milling machine, lathe and spindle moulder.

Thanks. I didnt see that diagram. So how do you get 480V if you only have single phase? Is there some sort of voltage doubler going on there?

Maybe I missed something. Ill read again.

Simon

Originally Posted by simonl

So how do you get 480V if you only have single phase? Is there some sort of voltage doubler going on there?

I am on what they call a split phase supply. The transformer on a pole across the road gives me 2 x 240 phases onto the property. These phases are 180º apart (out of phase), so when you measure across the phases you get 480V.

With normal 3 phase, where the phases are 120º apart, you get 415V across the phases.

I hope that makes sense.

Originally Posted by simonl

Thanks. I didnt see that diagram. So how do you get 480V if you only have single phase? Is there some sort of voltage doubler going on there?

Maybe I missed something. Ill read again.

Simon

Hi Simon, If you want 415 3 phase, just get a 240 to 415v single phase transformer. ( of appropriate rating ) and use that to power your RPC, but unless you happen across one at auction, they are pretty expensive ( and heavy)

Incidentally, that's what the phase changer has inside it. with the addition of a processor that automatically switches capacitors in and out as required to balance the phases as the load changes.

Ray

I run my 1.5 hp three-phase mill motor on single phase by the capacitor method.
The 415V motor was reconnected as delta (was Y), 230V on two 'corners' with capacitor between one line and third 'corner'.
Starting capacitor for 1.5 hp is quite large - that is the biggest expense. Switching out the starting capacitor at the right time/speed automatically can be fun. I do it manually quite easily.

All info contained in two Workshop Practice series of books:
No 16 - Electric Motors by Jim Cox
No 47 - Three Phase Conversion by G. Astbury.

Originally Posted by Vernonv

I am on what they call a split phase supply.

Sounds like you are on a SWER supply?

The split phase thing is what Yanks have in their homes, except in their case it's 110/220V

Originally Posted by RustyArc

Sounds like you are on a SWER supply?

Technically no. With SWER the supply to the transformer is a little different, however the output to the "customer" is the same (480V). SWER uses a single supply line to the transformer and split phase has 2.

Originally Posted by RayG

Hi Simon, If you want 415 3 phase, just get a 240 to 415v single phase transformer. ( of appropriate rating ) and use that to power your RPC, but unless you happen across one at auction, they are pretty expensive ( and heavy)

Incidentally, that's what the phase changer has inside it. with the addition of a processor that automatically switches capacitors in and out as required to balance the phases as the load changes.

Ray

Thanks Ray. Jayson (think it was) showed me his home made RPC which used an old multi input welder to up the voltage. I have an old Goodwell welder that has provision to run on 240 & 415. I was wondering whether I could use that run get 415V out.

Only an idea. I had seriously looked into getting 3 phase to the shed, it was doable for probably $3K but I really can't see myself needing 3 phase now. The grinder was probably my last "big" machine purchase.

Simon