[TowerTalk] Another Tailtwister question

[Jim Lux]

At 04:31 PM 11/29/2004 -0600, Bill Aycock wrote:

>Barry- I think the Cap is really a phase control in the reversing circuit, 
>not a starting Cap.. The value may be more critical than for starting. 
>Check the manual.
>Bill
>
>At 06:49 PM 11/29/2004 +0000, Barry wrote:
>
>>Tnx for all the responses about my stuck rotor.  Of course, today, it
>>is working just fine! However, I thought I'd replace the motor
>>starting cap anyway, as it is 14 years old.
>>
>>The original cap is rated 130-156 mfd.  Is the value critical?  I
>>have a motor starting cap here that's 88-108 mfd.  Is that close
>>emough?
>>Tnx,
>>Barry W2UP--
>>Barry Kutner, W2UP              Internet: w2up at mindspring.com
>>Newtown, PA                     Frankford Radio Club
>>

Those kinds of values (especially as a range) usually indicate a motor 
start capacitor.    How big is the motor?

A motor run capacitor (used in things like PSC motors or reversible AC 
motors) will typically be somewhat smaller, and a single discrete value.

As to the effect of the wrong value..

Mostly, it changes the starting or running torque.  A smaller capacitor has 
a larger impedance, so less current flows through the winding connected to 
the capacitor. Less current means less torque.


For a motor start application, it's probably not a big deal, since 
capacitor start motors are usually used in applications where huge starting 
torques aren't required (think about running a fan or a washing 
machine).  All the capacitor (and the start winding) have to do is get the 
rotor started turning the right direction, and then, the usual induction 
motor mechanisms take effect.  Induction motors are designed with various 
torque speed curves (identified by letters like A or D), but basically, 
once it's turning at all, the primary (non starting) winding will dominate 
the effects, since the current is MUCH higher in it.  Consider that at 60 
Hz, 100 uF is about 26 ohms reactance (about 4-5 amps)

If it's a run capacitor, a too small capacitor has two effects: It reduces 
the current through the winding with the cap; it makes the phase shift 
between the two windings different thatn what it's designed to be.  Both 
effects generally cause a reduction in the torque (at a given speed).



The real difference between run and start capacitors is the duty cycle.  A 
start cap is designed for a low duty cycle (current flows through it for a 
few seconds, then it's disconnected).  A run cap is designed for current 
always flowing through.  The loss on the run cap must be much lower or it 
will overheat.  Run caps are usually much larger than start caps(for a 
given voltage and capacitance) and more expensive for this reason.