[TheForge] Phase Converters

Thu Mar 27 09:36:00 2003

It is so simple and cheap to build your own rotary converter ( which 
does a better job) that unless you have a good reason, that is usually 
the best way to go.  All it  takes is a large three phase motor.   The 
disadvantage of this approach is it doesn't work well for equipment on 
standby type duty, i.e. compressors and such, since the rotary converter 
must be running first and continuosly.   This is an area where the 
static ( really just big capacitors in a box) are better.  Assuming you 
don't have a standby situation, the best approach is to build a rotary 
converter and add capacitors to balance the phases.   I pick up three 
phase motors for free all the time since most people can't use them.   
Your experience may differ of course.    

The ultimate way to go is with a variable frequency drive or VFD.  A 
little pricey, but they are convenient like a static converter, have 
better efficiency than a rotary and offer the ability to dynamically 
vary the motor speed( they will also operate with single phase 220 
motors for speed control).   Another poster the other day I believe 
mistakenly called this a variac. A variac alters the voltage, not the 
frequency and while it will sometimes slow a motor down, it does it at 
the cost of the windings.  An AC split phase  induction motor( the most 
common type) derives its speed from two things, the number of poles and 
the frequency of the input (60 cycles in the US of course)  This is why 
most motors have a speed of about 1800 or 3600 rpm, multiples of 60 
(actually 3450 and 1750, but the horsepower produced by a motor is a 
product of the "slip").   It is calculated by 120 * frequency/number of 
poles.   So a two pole motor gives  you 3600 and a four pole motor gives 
you 1800.   The common exception to this is the "universal motor" , 
popular in hand tools, and easily identified by its carbon brushes. 
 These will respond to voltage differences by altering their speed.

I have done a lot of repairs to small motors, but  am currently trying 
to learn how to rewind them as well.  This is difficult nowdays since 
current practice considers it un-economic to rewind anything under 10-20 
hp.   I, on the other hand, am  brain diseased enough to spend the time 
putting disposable utility knife blades to a whetstone, so I have much 
more liberal definations of economy.  Of course, as said before, my 
theory of recycling is to not throw it away in the first place.   I 
figure if we keep going the way we are, my theory will come to the fore 
eventually.

Charles

Larry and Pat Brown wrote:

>
> The manufacturers site;
> http://www.phase-a-matic.com/
> LBrown
>
> At 07:31 PM 3/26/2003 -0600, you wrote:
>
>> Found some interesting converters...  any comments on the 
>> "non-rotary" types ???
>>
>> http://www.penntoolco.com/catalog/manufacturers/PhaseAMatic.cfm
>>
>>
>> btw... this dealer has a lot of decent stuff at decent prices
>>
>> mike
>>
>>
>>
>>
>>         Michael Linn
>>       Artist Blacksmith
>>      McCalla, Alabama
>>       AFC Webmaster
>> http://afc.abana-chapter.com
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