# Static phase converters vs. vfd's



## Rangerjoe2 (Mar 29, 2013)

Can anyone explain the differences between the two?  I know vfd's allow variable speed control, but why would you use one instead of the other?   Looking for the best way to power my new south bend lathe.


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## Rangerjoe2 (Mar 29, 2013)

Well, it pays to read a little....just read the sticky about powering up 3 phase on single phase power.


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## twstoerzinger (Mar 29, 2013)

There is pretty extensive discussion on this topic here in the electrical part of the forum, but here is the quick summary.
Both systems are used when you need to power a 3 phase motor from single phase. If you have a single phase motor, you don't need one of these.

A static phase converter essentially gives the motor a short dual-phase burst to get the motor running. The the converter drops off-line and the motor continues to run in "single-phase" mode at reduced power, about 60% power if I remember correctly. A number of members have used these successfully for many years, but my personal preference is not to single phase a 3 phase motor. The motor is prone to heating in this mode, and you have to be careful not to overload the motor. The motor also tends to hum a lot.

A VFD (variable frequency drive) uses single phase power to generate 3 phase power (with an approximation of a 3 phase sine wave). With the VFD you get full motor power available. And, as you point out, you have the opportunity to run the motor at reduced speed (also with reduced power). Single phase input VFDs are available up to 3 HP output. I have had good experiences with a 3 hp VFD on my Clausing lathe. You can purchase one of these for under $200.

A third option is a rotary phase converter. It is an old design, and produces a good sine wave for the motors when properly set up. The device is essentially a 3 phase electric motor (called the idler) with  a starting system similar to static phase converter to get the idler started. Once running, the motor acts as a rotating transformer to generate 3 phase output power. It is possible to build one of these using a standard 3 phase motor with some added controls. There are also several manufactures who sell specially designed systems. For 3 HP, I think a new, factory designed system is under $500.

Terry S.


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## Rangerjoe2 (Mar 31, 2013)

Thanks for all the info.  When I get my lathe next week I'll post the motor info and the vfd I'm looking at...


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## Ulma Doctor (Mar 31, 2013)

my 2 cents..
a Vfd would be a good idea if you have no other machinery that requires three phase power. they get expensive above 3 hp.
if you have multi 3 phase motors to start or run, a Rotary Phase Converter is most times the best solution. you can build you own fairly inexpensively from scratch, even cheaper if you have another spare 3 phase motor of equal or greater hp value.
I have made static converters for 10hp motors in functioning experimentation ... it is possible. and pretty cheap
maybe this may help... i made a 2hp static converter to start the 3 phase motor on single phase for Tamper 84 to illustrate the principle.

http://www.youtube.com/watch?v=xD8xwh_P8mI

i hope the info helps...


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## martik777 (Apr 2, 2013)

Does the idler on a rotary need to be greater than the sum of all 3 phase motors connected to it or just greater than the LARGEST motor in the chain?

Just connect a 3 phase to single phase power and start it with a rope - Instant static "converter"


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## stupoty (Apr 2, 2013)

Hi,

how smooth do you find the rotation (on load) with a static convertors?

if you have multle 3 phase machines but only have one opperator as most home hobby machinists are have you considered a power distribution system that allows you to run any machine from the same vfd.

if you have a shop where their is more than 1 person working it's slightly out of order to ask somone other than yourself to start a machine with a rope. (Unless its a brigs and straton)

A new single phase motor is about the same cost as a vfd of the same size.

put a smaller motor on the machine, why have a 4hp motor at 30-50% efficiency insted of a 1.5hp motor at 100% efficiency ?

Stuart


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## Ulma Doctor (Apr 2, 2013)

martik777 said:


> Does the idler on a rotary need to be greater than the sum of all 3 phase motors connected to it or just greater than the LARGEST motor in the chain?
> 
> the idler should be as big or bigger than the largest motor to be started.


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## toag (Apr 11, 2013)

I have used all three at one time or another so my 2 cents are...

a static phase is a nice quick and easy set up.  If you want to start throwing oil and chips, a static phase can be set up in 10 minutes.
yes you will get less power, and i agree the make the motor sound "funny", and mine was running hot after 10 minutes or so of use (which doesnt make sense to me, usiong only 2 wirings instead of all three at one time.) they are also cheap,  a 1 1/2 hp converter for $60 on ebay.

a vfd is also a nice step up.  you have to do your homework but you can find a VFD that willl convert single phase to 3 phase on ebay for 100 or so.  But you need to look at the tag and make sure it is for 220V and that it will convert single phase to 3 phase, and is rated for your motor (it is usually in kW).  If your VFD is three phase in and three phase out, you can (on some models) put single phase in and get 3 phase out, but you need to derate it by 50%.  I used a VFD for 3 machines, putting a pigtail on the VFD and plugged the machine in i needed.  This worked well for me.  since i can only use one machine at a time, it was i nice solution.  The motor was much smoother, and much much more power.  and you can vary the speed of the motor/cut.  I still use it on a small horizontal, and being able to tune the speed is nice.

A rotory phase converter is nice if as before you have several machines that have feed stops (mill or shaper) or can run on their own (CNC).  It can run 3 phase power to all, and can be tuned using caps to make nice clean balanced three phase.  you can buy one (i didnt) or make one for silly cheap (150 or so), and plans are everywhere on the internetz.

SO if its for kicks get a static, if it is just a 1 or 2 machines, get a VFD, if you plan on throwing your cars to the weather and filling your garage with van norman 16, KT 3ck, 2 bridgeports, and a 20" G&E, make a nice 30 hp converter)


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## stupoty (Apr 11, 2013)

Hi,

The single coil set up gets hot partly due to massive shock loading on coils that are acting as a genorator eachtime they pass through the magnetic field.  Its more complex than that but i would have to look it all up to explain.

the problem with rotory convertors is the inificiency,
i have simplified numbers significantly hear.

1000w input motor used to run system

1000w , run a single phase motor, lose 20 % of input power
800w , transfer kinetic power to a genorator, lose 5% of power
760w , generate 3 phase from kinetic energy, lose 20% of power
680w , run a 3 phase motor from generated electricity, lose 20% power
544w , output kinetic energy from motor spindle.

you are losing a large percentage of your input power to the system.

I'm working on a fixed frequency 3 hase solid state drice circuit curently to provide cheep 3 phase from single phase, should be in the range of 20-60 dollars worth of parts when its done.

Don't expect it to be finished tomorow ;-) but hopefully within the next month or two.

Stuart

as a foot note i would use rotory over static personaly as you risk burning out your motor with a static convertor, and a new motor expense kind of defeats the point of saveing 50-200 dollars on a vfd.


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## Ray C (Apr 11, 2013)

Given a choice between a static converter and a VFD... If I were not allowed to have a VFD, I'd convert my equipment back to single phase. Static converters product "dirty" power and virtually all modern motor manufacturers will not warranty the product if it fails while on static power. I just purchased 3 new Leeson 3Ph motors and all clearly state "Warranty VOID if used with Static Converters". They are at best 66% efficient and much of the loss is taken-up as heat in the motor because the "phases" are not 120[SUP]o[/SUP] apart and that causes the motor to fight against itself.


EDIT: 

Rotary converters:  They're pretty good -provided you like having another set of motors to maintain and a couple hundred pounds of additional stuff in your shop.  At least they produce good power.  They can be made cheaply -if you happen to know someone giving away a 2, 3, 5+ HP 3-phase motor.


VFD:  They work perfectly, cost $125-$150 bucks.  Only downside is a little learning curve and reading their poorly translated user manuals.  A friend of mine owns a food processing company and they must have 100-200 VFD in operation at any given moment.  About once a year, one of them fails -usually due to external damage.


Ray


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## stupoty (Apr 11, 2013)

Ray C said:


> Given a choice between a static converter and a VFD...  If I were not allowed to have a VFD, I'd convert my equipment back to single phase.  Static converters product "dirty" power and virtually all modern motor manufacturers will not warranty the product if it fails while on static power.  I just purchased 3 new Leeson 3Ph motors and all clearly state "Warranty VOID if used with Static Converters".  They are at best 66% efficient and much of the loss is taken-up as heat in the motor because the "phases" are not 120[SUP]o[/SUP] apart and that causes the motor to fight against itself.
> 
> 
> Ray



Hello, totally agree,

i stupidly left swapping the motor out for a single phase of my list.:whistle:

whoops.

Stuart


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## itsme_Bernie (Apr 11, 2013)

I have used VFD and rotary converter (only last week, found one cheap).

I was perfectly happy with VFD on multiple 3/4hp machines, swapping the connection via TwistLock connectors.  FANTASTIC to be able to dial the RPM!  

I only got the rotary converter to run my horizontal Hardinge TM mill table powerfeed while using my VFD to cut with Rusnok vertical head..

VFD is quieter than my rotary as well I guess.  Used Kay Phasemaster 5hp unit.


Bernie


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## MrWhoopee (Apr 30, 2019)

I ran a static converter before I got the rotary last year. It worked fine. When running my lathe, I would turn on the mill, letting it function as an idler/rotary converter. I now have a 7.5 hp rotary converter. I hate the continuous whine and it draws a lot of power, even when the machines are off. Seriously thinking about going back to the static.


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## NortonDommi (Apr 30, 2019)

Some good information on the pros and cons of Transformers, Rotary Converters and VFD's here:  https://unique3phase.com/


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