Re: (ET) motor run caps

[David C Robie <mycroftxx1 juno com>]

A note aabout starting caps;

If the start cap is open on an AC motor, the motor won't start when you
turn it on.  However, if it's a motor with a shaft (like a grinding wheel
or belt drive power tool) you can strt it by turning the power on, then
giving it a spin by hand (be careful).

Dave
Weymouth MA


On Fri, 28 Jul 2006 09:04:23 -0400 "Konstanty, Walter \(GE Indust,
ConsInd\)" <Walter Konstanty ge com> writes:
> >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
> 
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