[TheForge] Fwd: Shop tools & layount (3-phase converter)

Sun Jan 18 21:39:00 2004

A couple of points.  

First,  regarding pulling extra wire for three phase, not a bad idea if 
you really think you will use it, but it will be more expensive.  Also, 
if you are building under a permit, the electrical inspector can get a 
little excited about extra unconnected wires in your boxes.  If you are 
not an electrician and you explain it is for possible three phase later 
they will likely get even more excited.  All depends on the inspector 
and the code you are operating under.    I would suggest if this option 
interests you, wire your shop for 220, two hots, neutral and a ground 
for a select number of outlets.   This is not the standard 20amp 220 
plug though.  This will meet code and can be used later for three phase.

Regarding three phase, the first thing you need to look at is the 
voltage for the three phase motors you are looking at.  They typically 
are for 208 or 480.  You will not have much luck with 480 volt motors on 
220, you want to pick up only 208 volt motors.    Next thing to look at 
is the winding type.  There are Y wound and Delta wound.    Delta wound 
are purported to work better as rotary phase convertors and to run 
better on "converted" single phase power.     

Next you have a variety of ways to "convert" single phase to three 
phase, all of them using inductors to shift current.  None of them ( 
with the exception of VFD) will run a three phase motor at its full 
efficiency, and generally they will be less efficient that a comparable 
quality single phase motor.   Three phase motors are much more efficient 
than single phase motors, but only when powered by real three phase 
power.    I know someone here has stated that a three phase motor 
running off of a  rotary converter drew less current than the single 
phase that it replaced, but that is probably due to the fact that since 
three phase motors are almost always intended for industry, they tend to 
be better made and more efficient to begin with and you probably aren't 
pulling anywhere near nameplate hp.   A comparable quality single phase 
motor will use less power running on single phase for the same torque/hp 
produced.   The reason they are not going to run as well is because the 
power factor and phase separation is messed up.   There are a lot of 
disadvantages to running three phase motors on single phase.  The only 
advantages are:  You can bump stop three phase motors, and often 
inductrial equipment comes with integral three phase motors that can't 
be easily replaced with standard single phase. 

Here are the ways of getting three phase motors to work.

1) Static Converter, which is made by capacitors to shift the incoming 
current and create an additional phase.   This is is essentially the 
same method used for starting a typical single phase induction motor.   
Some single phase motors also use a "run" capacitor, which exactly 
mimics a static three phase converter.    These are not as popular 
because they are a little more difficult to get set up, and it takes 
time to balance the size of capacitors to the load, particularily if the 
load varies. Finally, they won't allow you to pull full load 
continously, genrally recommending maximum of 75% of nameplate.  I 
expect that this is the result of the capacitors screwing up the power 
factor, resulting in higher current draw.   The advantage of them is 
they are compact and can be wired into the machine switch, so using the 
machine is as if it were ordinary single phase.   This is better for air 
compressors and other equipment that runs automatically.   I have seen a 
lot of different recommendations for sizing capacitors, but the first 
book I pulled down recommends 28mfd per HP for the start capacitor and 
60 mfd per HP for the run capacitor.    

2) Autotransformer.  Not very popular outside of industry, but does a 
great job of simulating three phase.  It basically uses capacitors to 
shift current ahead for one leg and an autoformer to shift it back for 
another leg. This gives three phases unlike the static converter and 
rotary converter that only create two phases.  

3) Rotary Converter.  This is the easiest.   Basically, you use a three 
phase motor to generate a second phase.  The generated second phase is 
generally weaker, and power factor is an issue, but it is usually 
sufficient and if you have multiple motors on the circuit, they pwer 
factor will be improved.   You will still not get nameplate HP from the 
motors.  To build a rotary phase convertor, you start with a delta wound 
three phase motor to act as the converter.  Wire two of its motor leads 
to your 220 in.  and run wires from these to the motor you want to 
run.   The third power lead is run directly to the load motor.   At this 
point, if you were to apply power to the converter it would just hum and 
eventually burn out, nuch like a single phase with a bad start capacitor 
or centrifigal switch.  There are a couple of ways to fix address 
this.    One, you can belt up a single phase motor to spin the converter 
on start up.  This is my favorite way, since it results in the lowest 
starting current.    I have actually used the start switch in the single 
phase motor to hold open a TPDT relay for the converter.  When the start 
switch on the pony motor kicks out, the power kicks in on the converter 
motor and off for the pony motor.     The second way to start it is with 
a rope, or if it is near the floor, spinning it with your foot.   
Remember to always spin it the same way, or the controls on your 
machines hooked to it will be reversed.    Finally, you can put a start 
capacitor on it to start it that gets switched out when it is up to 
speed.  This will have the highest starting current of the three.    
Once it is running, it works by inducing a current in the third winding 
using the residual magnetism in the rotor from the the other two 
windings.   This necessarily means that the current and voltage in the 
third winding will be less than the other two.  You can improve things 
by monitoring it with an amp meter, and adding a capacitor or two to 
balance the currents.   This does not produce true three phase, it 
produces quasi unbalanced two phase with a hinky power factor.   

4) VFD  - Variable Frequency Device.  This is a solid state device that 
uses high power semiconducters to create a very accurate three phase.   
It is also capable of controlling the shape of the waveform created as 
well as the frequency.  These two capabilities combined allows it to 
control the speed of the motor exceptionally well.  Once very expensive, 
they have considerably dropped in price.   They generally are installed 
on a single machine due to the speed control capability.   These make 
three phase motors an absolute joy.   It is probable that they will 
result in a shift of many products to three phase motors under the 
covers.  For example, using a VFD you could build a gearless washing 
machine. 

I have written up quite a bit on this and intend to publish it someday 
in my Blacksmiths Omnibus, otherwise known as "What you need to know now 
that you've made your hook in your barbecue forge" along with sections 
on pneumatics, hydraulics and basic stress analysis among other things.....

Charles

Kenny O. wrote:

>
>
>
>
>  is there a way to change a three phase motor to be used with 220 ...
>  I see them much cheaper at the tool zoo?  I also saw this thing
>  called a phase-a-matic which cost $150.00 .  Its claim is to be a
>  "static phase converter".  Has any one used these things or
>  attempted to make there own?
>  Thanks
>  Kenny o
>
>> On Friday, January 9, 2004, at 05:23  AM, Chuck Robinson wrote:
>
>
>  Bettina:
>  I would suggest that when you wire your shop , you pull an extra
>  wire for 3
>  phase power and just leave it un-connected in your electrical
>  boxes....
>
>
>
>
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