🚀 go-pugleaf

jvincent@bnr.ca (John Vincent) writes:

>In article <10932@rama.UUCP> jdc@rama.UUCP (James D. Cronin) writes:
>>Every now and then I see ads for electric motor "power factor
>>controllers."  They claim to reduce the amount of power used
>>by electric motors.  Are they for real?  How do they work?
>>I'm sure this should be more interesting than the ongoing
>>"solar cell" flame wars :-)

>It's been a while since I last took a motors/power course but I believe
>the device you are referring to regulates the reactance of the
>load presented by the motor.  In a "perfect" scenario the motor would be
>a purely resisitive load under all conditions. This is not usually the 
>case.  The load will usually have a capacitive (rarely) or inductive (typical)
>component as well.  This will increase the power drawn by the motor.  
>The job of the controller is to maintain the load as near to resistive as 
>possible, usually by switching in capacitors to reduce the inductance.  


You are correct in describing how conventional power factor correction
works but that is now what James is asking about.  The device James is
asking about was developed and patented by NASA who licensed it to a
number of companies to manufacture.  You can find a writeup in an old
Nasa Tech Brief and can order a TSP from NTIS if you are interested.  The
device works by measuring the power factor and then reducing the voltage
to the motor until the power factor improves no more.  It works because
an inductive motor's power factor increases as the mechanical load
approaches its rating. Lightly loaded motors have terrible power factors.

When I was a process control engineer with M&M Mars in the late 70s, we
evaluated these units and ultimately rejected them.  Here are some of the
reasons.  First, they are very expensive.  We found that we could save as
much by carefully sizing motors to the load.  Since the majority of the
motors in the candy factory were under 5 hp, a new motor was about as
cheap as the controller.  A simple power factor meter indicates how well
the load is matched.   

We also found that these units cause power factor problems UPSTREAM of
the controller.  Since they reduce voltage by phase angle controlling an
SCR pack, the INPUT power factor is very low.  This causes heating and
losses on the substation equipment  and transformers.  In our test setup
using a small 480 volt to 120 volt transformer as our simulated "unit
sub", we could measure a temperature increase in the transformer when the
controller was turned on.  

Lastly, these things are/were fairly complicated which raised a serious
maintenance concern.  WE decided that we'd save as much power by using
better engineering and in some cases changing to DC motors and static
controllers (which also have terrible input power factor.) 

John

-- 
John De Armond, WD4OQC        | "Purveyors of speed to the Trade"  (tm)
Rapid Deployment System, Inc. |  Home of the Nidgets (tm)
Marietta, Ga                  | "To be engaged in opposing wrong offers but 
{emory,uunet}!rsiatl!jgd      |  a slender guarantee of being right."