contactor 24v-220v

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can anyone explain this to me theoretically.

situation: 220 v electric furnace. two 120v legs off a double pole 60 amp breaker. leg A connects to a fuse and then to one end of metal heating coil. Leg B connects to a contactor(24v-220v). the other side of the contactor connects to the other end of the same heating coil (ime leaving 2nd coil out of equation for explanation purposes)

i understand how the contactor works and that when the coil is closed, the heating element heats up and everything works and there is no bucking of phases or anything.

My question: if leg A is connected to one side of coil and Leg B(when contactor is energized) connects to other end of same coil, wouldnt the two phases theoretically buck being that they are connected together through the heating element?

thanks in advance
 

under8ed

Senior Member
The fact that you found this forum and seek answers is assuring as your posts indicate "on the job" training only. This field almost always requires some formal training to be accepted as a competent electrician. The rare exceptions are the ones that; learn some basic electrical theory & formulas, and have a strong mechanical & science aptitude. I'm, (I am), also in the south, and I believe the acceptance of those without classroom training occurs more often here than in the rest of the nation.
A good understanding of ohm's law is critical. With this simple formula the electrician will not only know why the breaker doesn't trip; they will know, (with a resistance reading or the nameplate wattage), exactly how many amps the circuit will draw.
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Begin with the simple E=I x R chart. Simply cover the unknown to reveal the math to solve it. Learn the variations of the formula like you know when to eat oysters and then move on to the other chart for a deeper understanding. Keep asking questions. As long as you remain truthful about what you do and do not know you can expect to be treated with some respect here.
 
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My question: if leg A is connected to one side of coil and Leg B(when contactor is energized) connects to other end of same coil, wouldnt the two phases theoretically buck being that they are connected together through the heating element?

Voltage is measured between only two points, and here the neutral is confusing you, so forget about it for it. Now, measure voltage between the two hot leads, you should see around 240v. That's what the heater element will see.

To be a little more specific, when you compare the 60 cycle AC waves between neutral and hot A, and between neutral and hot B, you'll see that the instantaneous voltage of both rise from zero at the same time but one moves positive and the other negative. Now again, forget the neutral and look at only what's between A and B. Conceptually, you now turn "B" upside-down (reverse the polarity) since you're using it as one measurement point, and now B's 120v + A's 120v gives you 240v.

240120SinglePhaseWaveformTransparente0e210f2a73b4e2aa95274447e1b5892.gif

Make sense?

Ignore the peak voltage of "170" for the time, we refer to AC voltages by their RMS value, not the peak.

Also take a look at http://www.electricianshangout.com/basic-ac-theory/, it looks pretty good.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Circuit Paths and Loads

Circuit Paths and Loads

" My question: if leg A is connected to one side of coil and Leg B(when contactor is energized) connects to other end of same coil, wouldnt the two phases theoretically buck being that they are connected together through the heating element? "

thanks in advance[/QUOTE]

A " coil " is a LOAD.
The way you wrote it, most likely on a 24VAC control circuit whic is common for HVACR systems.
Control coils can however be any voltage they are designed to be.
Just like the heating element the coil is a load subject to the electrical laws stated.
Understanding loads was what helped me instantly one day after which electricialy was a completely different matter.
 
The circuit is being completed phase to phase through the heating element. Magically take out the resistance out of the heating element and there would be a phase to phase fault, leave the resistance in and you get heat.

thnks dave u are the only one smart enough to see where i was going with this. I was just hoping for a diagram or animation of why the two sine waves dont cross.
 
Voltage is measured between only two points, and here the neutral is confusing you, so forget about it for it. Now, measure voltage between the two hot leads, you should see around 240v. That's what the heater element will see.

To be a little more specific, when you compare the 60 cycle AC waves between neutral and hot A, and between neutral and hot B, you'll see that the instantaneous voltage of both rise from zero at the same time but one moves positive and the other negative. Now again, forget the neutral and look at only what's between A and B. Conceptually, you now turn "B" upside-down (reverse the polarity) since you're using it as one measurement point, and now B's 120v + A's 120v gives you 240v.

View attachment 12346

Make sense?

Ignore the peak voltage of "170" for the time, we refer to AC voltages by their RMS value, not the peak.

Also take a look at http://www.electricianshangout.com/basic-ac-theory/, it looks pretty good.

there is no neutral. its a 240 heating element. phase A connects to one side of heating element. phase B connects to other side of same element. its a 5 kw resistance element (ni-cr wire coil)
 
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kwired

Electron manager
Location
NE Nebraska
situation: 220 v electric furnace. two 120v legs off a double pole 60 amp breaker. leg A connects to a fuse and then to one end of metal heating coil. Leg B connects to a contactor(24v-220v). the other side of the contactor connects to the other end of the same heating coil (ime leaving 2nd coil out of equation for explanation purposes)

When you said 24-220 volt contactor - it raises questions as to whether or not you actually know what you are talking about. Most likely the 24 is the contactor coil voltage and 220 (likely is actually 250 or even 600 volt rated) is the contact rating. The mentioning of the 24 volts is irrelevant as it has nothing to do with the question. In fact the contactor doesn't really have anything to do with the question that was asked, but the way it was asked had some wondering if you even knew what you were asking.

Sorry, but that is how I saw it when I first read the OP, and likely others did as well.

Add: if English is not your first language we may cut you some slack, but your English looks better then what some people use - so still not quite certain what to think.
 
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augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Welcome to the Forum , but please note: The attitude you displayed in Post #7 will not bode well with the folks on the Forum and with us Moderators. The answer given by under8ted for the most part seemed an attempt to be helpful.
Please refrain from being condescending to other members.
 

charlie b

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Location
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Occupation
Retired Electrical Engineer
The attitude you displayed in Post #7 will not bode well with the folks on the Forum and with us Moderators.
That post #7 has now been deleted and no longer has a post number. The subsequent posts have been renumbered.

 
Welcome to the Forum , but please note: The attitude you displayed in Post #7 will not bode well with the folks on the Forum and with us Moderators. The answer given by under8ted for the most part seemed an attempt to be helpful.
Please refrain from being condescending to other members.

augie 87, I apologize for the reply to undr8ted. I felt he was being very condescending especially to us folks in the south. I really appreciate this forum and the members who take time to help others. It has helped me on countless occasions and I hope to one day help others also. Thanks
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
One of the limitations of the forum for really basic conceptual problems is that you cannot point and wave your hands at each other. This often leads to either not enough or too much detail in the attempted text explanation, either of which can sometimes come across as condescension.

Back in the non-electronic book days, the author of an engineering book on the mechanics of ocean waves and their forces on structures commented in the Forward that he really wished that he could have moving illustrations.
And he referred to a booklet put out by the NY Transit Authority describing changes in bus schedules. The cover had an illustration of a particular bus not stopping at 32nd street any more. :)
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
augie 87, I apologize for the reply to undr8ted. I felt he was being very condescending especially to us folks in the south. I really appreciate this forum and the members who take time to help others. It has helped me on countless occasions and I hope to one day help others also. Thanks

Thank You.
Most of us step on toes occasionally but you will find for the most part folks here are acting in good faith to assist each other.
 
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