Question about Potential and current relays

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Old 08-04-08, 06:31 AM
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Question about Potential and current relays

I have been retired for a long time.
I do not remember hearing of PT's ANd CT's in single phase compressor starting circuits.
Question: Are they used in residential hvac units, or are they used mostly in household refrigerators?
I still enjoy staying up with things of this nature. Tom
 
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Old 08-04-08, 08:18 PM
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On refrigerators current relays have been gone a long time.
They were the original single phase relay and were normally rectangular boxes often mounted on the frame or back of the refrigerator.
They stopped using these in the late sixties.

Residential a/c normally comes with psc starting which is just a run capacitor feeding the start winding

Potential relays are commonly used with solid state relays creeping in.
Solid state relays are a service person's dream because they are pretty much universal.
Six solid state relays will fit 120 and 220 volt compressors in most common sizes for coolers freezers and a/c!
 
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Old 08-05-08, 05:43 AM
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Thank you for the update. Tom
 
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Old 08-05-08, 08:16 AM
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In some places they still refer to fractional compressor start relays as current relays. The little plug on start relay that plugs onto the start and run windings energizes the start windings and sometimes a start capacitor. The high starting amperage energizes this relay hence some call it a current relay. These are easy to spot due to the heavy copper coil which you can sometime see.
The potential relay does the same thing but uses Back EMF or countervoltage to energize the relay. Remember that a compressor also acts like a generator and produce its own higher than line voltage which is used to operate the potential relay.
During the starting cycle the compressor windings produce lots more back EMF as well as inductive reactance (impedance) which makes the compressor highly inefficient so the introduction of a capacitor into the circuit counteracts that and makes the compressor windings more efficient.
 
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Old 08-05-08, 08:15 PM
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Originally Posted by barkleydoggy View Post
In some places they still refer to fractional compressor start relays as current relays. The little plug on start relay that plugs onto the start and run windings energizes the start windings and sometimes a start capacitor. The high starting amperage energizes this relay hence some call it a current relay. These are easy to spot due to the heavy copper coil which you can sometime see.
The potential relay does the same thing but uses Back EMF or countervoltage to energize the relay. Remember that a compressor also acts like a generator and produce its own higher than line voltage which is used to operate the potential relay.
During the starting cycle the compressor windings produce lots more back EMF as well as inductive reactance (impedance) which makes the compressor highly inefficient so the introduction of a capacitor into the circuit counteracts that and makes the compressor windings more efficient.

Actually, It doesnt work that way at all,... in Theory or Practice. Good colorful story though...

*Id explain how it really works ( and trust me, I do know - its not that complicated) but...Ive been informed that "Theory" is not allowed on this site. Nor grandstanding, ect... In order to not violate these rules (or being viewed as a violation), I will just say...Nuts to this post concerning Potential/current relays, capacitors, and how they interact with motors - Yes.. A compressor is a motor!

*I have found that "Theory" type posts are not welcomed here and get deleted, and... as now with this mis-informed post I have quoted, Frankly I see why! - why waste the typing or time
 

Last edited by The Real Deal; 08-05-08 at 09:20 PM.
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Old 08-06-08, 06:45 AM
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Originally Posted by The Real Deal View Post
Actually, It doesnt work that way at all,... in Theory or Practice. Good colorful story though...

*Id explain how it really works ( and trust me, I do know - its not that complicated) but...Ive been informed that "Theory" is not allowed on this site. Nor grandstanding, ect... In order to not violate these rules (or being viewed as a violation), I will just say...Nuts to this post concerning Potential/current relays, capacitors, and how they interact with motors - Yes.. A compressor is a motor!

*I have found that "Theory" type posts are not welcomed here and get deleted, and... as now with this mis-informed post I have quoted, Frankly I see why! - why waste the typing or time
http://www.achrnews.com/CDA/Archives...00f932a8c0____
http://64.226.146.90/Tutorials/Motor...Capacitors.htm

Is your understanding different than this? You must have had too much coffee!
As for theory, some is necessary to fully understand the workings of mechanical systems either for the professional or do-it-yourself person. The difference between parts changer jacklegs and professionals is theory and understanding.
As for your vitriolic allegation of "grandstanding," I have given truthful and accurate information which folks can either take or ignore.
If you disagree with my previous post feel free to PM me and I can explain it further. Also, please refer to the above links to validate my post. I can also recommend some publications on AC current and phenomena as well as AC motor operations if you are interested in furthering your knowledge.
 

Last edited by barkleydoggy; 08-06-08 at 08:40 AM.
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Old 08-06-08, 04:22 PM
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Gee Tom, you can stir a nest up.

As far as current relays go pertaining to your question, I'd lean towards Greg.

I look for a current relay to do two things for me. One is to prove that something (compressor) is or is not running. Carrier uses these as lockout boards. Basically is the comp should be running and is drawing no current, something is wrong and the circuit is locked out.

Others I use are for over current. Actually, looking more into that as we have a job with a faulty Micro Control on a 60 ton Trane chiller. The Micro is bad and is $7000 our cost. We are putting together a way to do everything except comp run time averaging to replace the Micro. But, these types will monitor comp current draw. Over current will lock out the circuit.

Potential relays are only for start capacitors in general use. I've used them for other stuff, but that would be theory and here I deal in facts only (even when I'm incorrect and am corrected).

A compressor may need a 35MFD cap to run but needs a kick in the butt to get going. Enter the start cap and pot relay. When the contactor is closed the start and run caps are in the start circuit to give a huge Paul Bunyan kick in the butt to get everything moving. When the rotor gets up to speed and creates enough EMF it The common winding) energizes the pot relay taking the start cap out of the circuit. Comp runs on run cap. Start cap usually has a resistor to bleed charge for next start.

barkleydoggy good links was looking for the achr one in a cap discussing (Real).

Dudes, don't fight. 'Splain Lucy.

Generally we deal with homeowners who can barely find the filter in some cases. I've talked with Tom in PM's about a Trane RTU issue (wonder whats up with that fan switch). I suspect, given his Amonnia back ground, Tom is sort of like Ed. If we could only know 1/2 of what they have forgotten we could demand any pay rate we like.

2 cents
 
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Old 08-06-08, 05:49 PM
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Originally Posted by barkleydoggy View Post
http://www.achrnews.com/CDA/Archives...00f932a8c0____
http://64.226.146.90/Tutorials/Motor...Capacitors.htm

Is your understanding different than this? You must have had too much coffee!
As for theory, some is necessary to fully understand the workings of mechanical systems either for the professional or do-it-yourself person. The difference between parts changer jacklegs and professionals is theory and understanding.
As for your vitriolic allegation of "grandstanding," I have given truthful and accurate information which folks can either take or ignore.
If you disagree with my previous post feel free to PM me and I can explain it further. Also, please refer to the above links to validate my post. I can also recommend some publications on AC current and phenomena as well as AC motor operations if you are interested in furthering your knowledge.
Reading the post and reading your link, is to different things. Even the link is somewhat vague in explaining this process in its most logical form. I assure you I understand Theory as well as anyone, including the one who wrote your link. Maybe more...

I do disagree with most of your post for many reasons - The way one says something is everything friend.

When I find some time, I will write you a synopsis of what it is you are trying to say in your post - Well enough.

Good evening.
 
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Old 08-06-08, 07:10 PM
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Originally Posted by gardener321 View Post
I have been retired for a long time.
I do not remember hearing of PT's ANd CT's in single phase compressor starting circuits.
Question: Are they used in residential hvac units, or are they used mostly in household refrigerators?
I still enjoy staying up with things of this nature. Tom

Hey! This is the start of the thread. This is why we all stopped by!

Continue any disagreement in PM!

Which part do you not understand?
 
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Old 08-06-08, 07:26 PM
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Just a friendly reminder that in this forum, the bottom post is the first post made in the thread.
 
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Old 08-06-08, 08:57 PM
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If you will notice, my original post title mentioned PT and CTs.. That was a mistake on my part, because earlier in my career I worked with watt hour metering. a PT is a transformer that reduces a high voltage down to 120 vac so that it can be used in watthour meters. Current transformers have an input of medium to very high values of current. but the output is always 5amps when used in metering consumption. I apologize if that caused confusion, but by the same token it does not appear that it caused any confusion at all in GreggH's post.
Thanks for all of the help. A great forum.
I learned what I wanted to know, and no more needs to be said. Thank each one of you for your contribution. Tom
 
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Old 08-07-08, 04:22 PM
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Whatever, as long as your happy Tom

Ecman, all forums have a display option.

Mine is set to oldest first. This way I can read the posters question first and then all the answers/suggestions after.
 
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Old 08-07-08, 05:41 PM
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Originally Posted by Jarredsdad View Post
Ecman, all forums have a display option.
Just fixed mine. Thanks for the tip.

But that still does not explain post #7, unless something flew over my head.
 
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Old 08-07-08, 05:47 PM
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Which part?

Potential relay is a potential relay.

Don't b***h at each other. Answer the question.
 
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Old 08-07-08, 07:25 PM
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I think there is a learning opportunity here for more than Tom.

I have the strong suggestion that if anyone wishes to offer any information to a member that they try to gauge the depth of their answer to the question.

If we need a lengthy technical dissertation it would better to Google a reference site dedicated to the topic at hand.
People generally come here for basic but accurate information.
 
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Old 08-07-08, 07:48 PM
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I agree. I thought about giving you Kudos for answering Tom's question but honestly don't know how you did it.

While I admit that my posts are sometimes long, I'm trying to answer the question at hand.

Posted links that support, prove, what you are saying are great.

We should not be bickering with each other in the post where a novice (what word) or homeowner askes a question.

I would think the original poster would/could think we are just a bunch of old guys argueing with each other.

Remember the BASIC reason for this site to be here in the first place.
 
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Old 08-08-08, 06:21 PM
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What I have found in these forums is that generally people want short simple answers.
If I try to read an answer and begin to nod off and I work in the industry, I can imagine that many novices would long asleep.

It's great that y'all know this stuff but make sure the information is necessary and wanted.
 
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Old 08-08-08, 07:43 PM
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Greg some questions are not short answers.

Some posts are not simple, and with questions answered become more involved. Which may require a long answer to educate the poster as to just what maybe going on.

You are correct that most poster want a short answer. Unfortunately, many of them start and end with "My A/C isn't working, what's wrong?". And there are those people who get advice and turn around to tell you how wrong you must be.

You like I can nod off but I think (hope) the poster is reading info that is foreign and totally new to the thinking proccess in correcting the current problem. Hence, I think they would be more attentive.

What I have a problem with in these forums is that it seems that not many read the question asked. So many times a question is asked and the first responce goes way off into left field.

Anyway, what I think.
 
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