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Old 03-04-2012, 01:11 PM   #1
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Default Question on energized contactor

I'm really having a hard time getting my head around how this contactor thing works. I'm in the process of wiring up a system that will use 2 - 5500W heating elements with 2 pumps. It'll be HERMS based. In the picture below I can't see how the heating element wouldn't have 120V going through it at all times as long as switch 7 is on. This certainly wouldn't be good. Can someone take a look at this and let me know if I'm just going crazy here? Maybe explain how it's supposed to work?

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Old 03-04-2012, 02:19 PM   #2
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I see that, but what about sw 5. It looks to me that when sw 7 is in the other position it may be using the thermal to control it and the other position may be a manual over ride to turn the heater on manually. Since only part of the drawing is shown I can't tell.

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Old 03-04-2012, 02:22 PM   #3
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I wondered about that too but regardless of what position #5 is in, the one leg is always going to be energizing the coil as long as #7 is in the 1-2 position. Below is the complete schematic. It's one of P-J's drawings.

Sigh... the picture is hard to read but the original is here http://www.pjmuth.org/beerstuff/images/BCS-460-wiring-2-4-a.jpg

bcs-460-wiring-2-4-.jpg  
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Old 03-04-2012, 02:33 PM   #4
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I'm really having a hard time getting my head around how this contactor thing works. I'm in the process of wiring up a system that will use 2 - 5500W heating elements with 2 pumps. It'll be HERMS based. In the picture below I can't see how the heating element wouldn't have 120V going through it at all times as long as switch 7 is on. This certainly wouldn't be good. Can someone take a look at this and let me know if I'm just going crazy here? Maybe explain how it's supposed to work?
The heating element is a 240V element. It has one side of the 240V connected (blue wire) when the contactor is energized. The other side of the 240V is connected through the contactor and then through the SSR (red wire). The element will have power applied only when the SSR is allowing current flow. When the SSR is blocking current flow it is effectively disconnected. With only one side connected there is no path for current to flow.
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Old 03-04-2012, 02:38 PM   #5
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The heating element is a 240V element. It has one side of the 240V connected (blue wire) when the contactor is energized. The other side of the 240V is connected through the contactor and then through the SSR (red wire). The element will have power applied only when the SSR is allowing current flow. When the SSR is blocking current flow it is effectively disconnected. With only one side connected there is no path for current to flow.
Hmmmm..... I'm still not getting it. Isn't the contactor energized at all times, meaning the blue is providing 1 leg of current to the heating element? And, when the SSR allows, the 2nd leg is providing current to the heating element? I guess I'm not seeing how the SSR is prohibiting current from reaching the element when it doesn't appear to be controlling the contactor.
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Old 03-04-2012, 02:43 PM   #6
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Hmmmm..... I'm still not getting it. Isn't the contactor energized at all times, meaning the blue is providing 1 leg of current to the heating element? And, when the SSR allows, the 2nd leg is providing current to the heating element? I guess I'm not seeing how the SSR is prohibiting current from reaching the element when it doesn't appear to be controlling the contactor.
Ok... Question for you. What is the path for current to flow through the element when the SSR is set to not conduct current?
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Old 03-04-2012, 02:50 PM   #7
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The SSR acts as a very fast digital light switch, controlled by the PID. The one leg that might always be on still won't power the element because the other leg is open.

Remember old christmas lights, where if one of the last bulbs went out, the entire string went out? There was still always power there, even to the first bulbs, but the entire circuit was open because one of the bulbs had burnt out. If you pulled out the neutral leg on a light bulb, and powered the hot leg, the light bulb wouldn't turn on. Circuit needs to be complete for any power to move.

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Old 03-04-2012, 02:54 PM   #8
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This is what I'm envisioning the current of the blue (line 1) to be doing when #7 is switched to 1-2. Am I wrong?

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Old 03-04-2012, 02:55 PM   #9
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I see, the contactor stays on but the controller cycles power to the element.

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Old 03-04-2012, 03:01 PM   #10
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This is what I'm envisioning the current of the blue (line 1) to be doing when #7 is switched to 1-2. Am I wrong?
You are showing a power path through the equipment ground connection at the element (green wire). There is no electrical path from the element shell to the power input terminals.
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