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Re: [TowerTalk] Another Tailtwister question

To: Bill Aycock <>,,
Subject: Re: [TowerTalk] Another Tailtwister question
From: Jim Lux <>
Date: Mon, 29 Nov 2004 15:19:05 -0800
List-post: <>
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.

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
Barry W2UP--
Barry Kutner, W2UP              Internet:
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.


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