Zero Crossing Switch for Central AC Compressor?

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Old 07-21-18, 05:41 PM
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Zero Crossing Switch for Central AC Compressor?

Has anyone had any success with installing a Zero Crossing Switch in a Central AC Compressor?

I'm asking because, when ours turns on, it throws a huge power spike throughout the house and flickers the lights, and that cant be good for things on the line, like electronics.

I'm a former electronics R&D technician from years ago.
 
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Old 07-22-18, 03:46 AM
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Most central AC units use a simple run capacitor as a starting means.
Adding a starting relay/capacitor unit will give the compressor a much quicker and more positive start which could reduce light dimming.

You could also check that your circuits are evenly balanced in your electrical panel and that the on/off differential in your thermostat is not cycling the AC too many times/hr.
 
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Old 07-22-18, 05:17 AM
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You may also want to have the power company check your connections on your electrical service.
 
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Old 07-22-18, 09:01 AM
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Undersized wiring from the panel to the outside condenser is also a cause of lights dimming as the compressor is not given enough current to start properly.

A zero crossing switch would mean semiconductor control of the compressor starting. Although a great idea I'm not sure how a generic compressor would respond to that.
 
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Old 07-22-18, 06:49 PM
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PJMAX, I'm thinking it's a great idea. What could work better for inductive loads like a compressor motor, than a zero-crossing switch? No spikes!
 
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Old 07-22-18, 07:28 PM
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How do you know you have a power spike? Most of the time a compressor starting will generate a sag, not a surge that covers many cycles. Any spike might be measurable but is usually momentary and inconsequential.
 
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Old 07-23-18, 07:12 PM
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The link "starting relay/capacitor unit" is to a hard-start kit. A well defined label for a specialized unit to facilitate AC compressor motor starting. It senses lower voltage on capacitor during startup and then switches a second capacitor in parallel. Google "hard-start kit" and see descriptions of them.

To call it a starting relay/capacitor is missleading, non specific and can be confusing. There are many kinds of starting relays which are not hard starts.

Hard-starts are a major help in getting some AC compressors motors started, especially older ones. To confuse their properties is undermining the use of a very helpful product.
 

Last edited by doughess; 07-23-18 at 07:46 PM.
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Old 07-23-18, 09:08 PM
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A "hard start kit" is precisely a start capacitor and a solid state relay built into the cap.
A run capacitor is simply connected to provide voltage to the start winding........ the "hard start" kit is connected in parallel across the the run capacitor terminals.
When energized the start capacitor applies higher voltage to the start winding to provide a quicker more positive start.
Once the compressor starts the solid state relay on the "hard start kit" opens to allow the run capacitor to pass lower voltage to the start winding for increased efficiency.

Here is one of several sites Google will bring up on this.
 
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Old 07-24-18, 09:45 AM
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This is getting off into other areas that weren't even a concern when I posted. All I know is that we get flickering lights when the AC starts, but not when the wind blows, so it's not loose connections outside the house. I was wondering if a zero crossing switch might work well in this application. Was thinking it might even make compressors last longer. Now I'm thinking that the starter cap might have to be removed to use one? ( we've had to have some of those replaced over the years - for some reason they don't last ) But anyway, it looks like there's no interest in this.
 
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Old 07-24-18, 10:27 AM
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A start cap. and a potential relay would be a "hard start." Best to replace both at once, if this is what you have.
 
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Old 07-24-18, 10:34 AM
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We try to respond to questions as accurately as possible but are not shy when it comes to the possibility that what is being asked is not the proper approach.

It is highly possible that your compressor is taking longer to start than it should which would cause the symptoms you are experiencing.
Some are of the belief that a "hard start kit" is only for when something is broken but in actual fact they are a benefit to any compressor that does not use a starting relay which includes pretty much every household a/c unit made today .

IOW you are really overthinking this.
You need to look into the suggestions offered here that do not involve what you are asking about.
 
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