Re: (ET) motor run caps

["Konstanty, Walter \(GE Indust, ConsInd\)" <Walter Konstanty ge com>]

>From the Standard Handbook of Electrical Engineers:

- Starting capacitors
     A centrifugal switch is used to disconnect it from being in parallel 
with the motor winding.  Typically, low voltage electrolytic capacitors 
are used.  Oil ones are used for more severe starting conditions or higher 
voltage.  The value of the capacitor is calculated to provide maximum 
starting torque while limiting thermal stress.  Basically they limit the 
voltage to the winding during accleration to full speed.
- Run Capacitors
     Used in some small motors where starting torque is quite low.  With 
the capacitor in the circuit, the motor losses increase.  Larger capacitor 
run motors have an auxiliary winding in series with the capacitor to 
improve operating characteristics.  Fan motors are a common run capacitor 
motor.

So Wombat, you're pretty much right.  Start cap doesn't boost, it limits 
inruch current by restricting the voltage as the cap absorbs it.  
Capacitors don't like dv/dt (changes in voltage per time) while inductors 
don't like di/dt (changes in current per time)....that's why designers use 
one or both depending upon need.

...Walt

-----Original Message-----
From: elec-trak-bounces cosmos phy tufts edu
[mailto:elec-trak-bounces cosmos phy tufts edu]On Behalf Of Neil Dennis
Sent: Thursday, July 27, 2006 5:01 PM
To: elec-trak cosmos phy tufts edu
Subject: (ET) motor run caps


Mike, if you go to an electrical supply house rather than an electronics 
house, they can tell you the difference.  I think the difference is that 
the start cap just is to give a boost to the motor at startup while the 
run cap actualy "tunes" the circuit.

wombat - somebody tell me if I'm wrong, pse

_______________________________________________
Elec-trak mailing list
Elec-trak cosmos phy tufts edu
https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak