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DIY Central Vacuum

  1. Sep 22, 2016 #1
    So, I know this isn't entirely an Engineering question, but I am an EE and I wanted to get a few opinions on my planned project.

    I have had a central vacuum in the past three homes I have owned. I have always installed the unit and all plumbing myself, but the units are still $500-$1000 just for the power unit in the garage. The units are EXTREMELY simple, essentially a metal drum (like a cylindrical trashcan) with an off-the-shelf vacuum motor attached to it, typically made by Ammetek Lamb. The motors run anywhere from $50 for a chinese knock-off, to $85-$200 for genuine Ammetek.

    One of my units that I owned was a dual-motor 240V unit, each motor was 240V.

    I am looking at building my own unit, utilizing a stainless steel cylindrical trashcan (parallel sides), and buying the Gore fabric filter intended as a replacement for the off the shelf units. All said and done, I think I can build a pretty good dual-motor unit for about $300 or less.

    Here is my question that I'd appreciate any feedback on... Would there be an issue running 120V motors but running a single 12/3 w/ground Romex line from a 20A 240V dual-pole CB to a 240V outlet (looks similar to a regular outlet, but with the blades sideways), then inside the unit I would run each motor off a single leg. The alternative is a single pole 30A breaker in the panel and run a 10/2 w/ground romex and run the motors in parallel, but I could still put a 10-15A breaker on the unit for each motor (many of the central vac units have built-in CB(s) to help reduce risk of fire or damaging motor).

    The other thing I'd like an opinion on is regarding soft-start. The motors are universal type motors, like a circular saw or router. Some of the more modern/$$$ units have a circuit that soft-starts the motor to help motor life and start-up current. Second to building a soft-start circuit using a triac or something, I was thinking about just using a time delay relay that will have the motors in series off one leg, then one motor switch to the other leg and each getting full 120V after 2 seconds.

    Any thoughts/opinions would be appreciated!
     
    Last edited: Sep 22, 2016
  2. jcsd
  3. Sep 22, 2016 #2

    anorlunda

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    Those sound like questions for a licensed electrician. There are codes that must be followed for legal and safety reasons, which makes it more than a technical question.
     
  4. Sep 22, 2016 #3
    Codes really apply more to facility wiring rather than things you plug in to an outlet. For example, you can't put a 14g wire in the wall on a 20A CB, but you can plug a device with even something as small as an 18g wire into a 20A circuit, maybe even 20g.

    In my case, the circuit breaker in the panel would be sized to match the wire in the walls, which would follow code. I'm sure there are "guidelines" to the construction and wiring of internal devices, but electricians likely don't have a clue about that, and would fall more in line with UL best practices and things of that sort. I am not really asking from a facility wiring perspective as I am from a unit design perspective... I know it is common for 240VAC devices to pull 120V off one of the legs, while using both legs for other parts of the device (e.g. an oven uses 240VAC for elements, but only pulls off one leg for the electronics), so I wondered what anyone's thoughts are on pulling a single 120V motor from each leg of 240V. I guess I'm perhaps answering my own question by pointing out the example of an oven or dryer using just one leg internally to the device to power a 120V circuit, but am still open to opinions, and also regarding running them in series for a few seconds as sort of a simple soft-start method...
     
    Last edited: Sep 22, 2016
  5. Sep 22, 2016 #4

    Averagesupernova

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    What you described with running a 2 pole breaker should be permitted from any code perspective I know of. You will of course be using a 4 prong plug if you want just one plug. Why not split a standard duplex receptacle up and feed it with the romex you described and a double pole breaker? If the breaker trips both halves of the outlet will be de-energized.
     
  6. Sep 22, 2016 #5
    Thanks for the input... So you're suggesting wiring a two-pole breaker and break the tab to separate the duplex outlets of a normal duplex recepticle, and then have two power cords coming out of the unit? That would work too, if that's allowed per code (splitting a duplex outlet such that the hots are from opposite phases), I am not entirely sure it is allowed, however, from a safety stand point, I don't see a problem with doing this... Most the time I see outlets split is to make one of them a switched outlet, but it uses the same leg of the 240, so hot of one outlet to hot of the other is 0VAC even with switch turned on.

    However, I just found 240VAC motors for only $60 each, 95CFM (2" open orifice) with 137" water lift (sealed)... I think I'll pick two of them up! The dual motor unit I used to have had both motors mounted to the chamber in "parallel" (I mean airflow wise), so I think that gives around double the CFM and about the same suction strength...

    Any thoughts on running it on 120VAC from one leg just for a second or two, then switching to 240VAC? I am assuming 120VAC will be enough to still make it spin... If not, then I'll have to get creative if I want a soft-start...
     
  7. Sep 23, 2016 #6
    Don't do that! New codes require that every circuit has it's own neutral return line to the panel. This was allowed in the past but there are some circumstances where this can be a problem. What if one of your motors had a short fault for example. Now you would be putting 240 volts across the working 120V motor. Could we be sure that this would trip the breaker? What if the neutral line broke? The circuit would still run but the motor voltages would be out of balance. It's just better as a general practice to have fault currents handled exclusively inside the panel and the wires of the failing circuit but not in the wires of neighboring circuits.

    You could put the two motors in parallel on the same circuit assuming that the wire and breaker is properly sized. You could run 4 wires and a ground and put the motors on two breakers. That's all fine.

    Soft-starter are generally a good idea. Chinese knock-offs are generally worth less than what you paid.
     
  8. Sep 23, 2016 #7
    In my original example, I intended to convey that these two 120V motors would be internal to one "device" that is plugged into one 240V, with each motor pulling 120 from its own individual leg, similar to my example of a 240V appliance that will pull 120V from one of the legs to operate part of the device on 120V. When you said, "what if one of your motors had a short fault, now I'd be putting 240V across the 120V motor. Wouldnt that be if I was running the two motors in series? If I had one of the motors shorted internally, shorting line to neutral, it would either trip the panel CB, or the CB in the unit for that motor, rather than put 240 to one motor, right??? If I had a shorted motor, and had a failed neutral, then I'd put 240 to the other motor, as I think @Averagesupernova is saying below (this is an edit, obviously).

    That being said, as I mentioned, I ended up going with two 240V Ametek Lamb motors, $60 each, so I'll use a 2-pole CB (probably just 20A). I am thinking a 10A CB in the unit, one for each motor, just as my old one had a CB for each motor, I was recalling 10A, but I just found the control board for the unit online and looks like they may have been 8A CB's. What do you think about running each motor on 120V independently for 2 seconds, e.g. motor 1 across L1-N and motor 2 across L2-N, then switching both motors in parallel across L1-L2, as a simple method to soft-start them?

    The Chinese knock-off I saw was $45, also on ebay I think. I figured 'maybe' I'd have better motor life out of the Ametek motors for the extra $15 each, but maybe not. Alibaba has some cheaper ones, but many of them have dubious claims, such as claiming they are 1400W.
     
    Last edited: Sep 23, 2016
  9. Sep 23, 2016 #8

    Averagesupernova

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    How so? You have to assume a poor connection on the neutral before voltages move around. We are not talking about simply putting two motors in series. With the 240 volt 4 prong plug option the code has NO jurisdiction over this scheme.
     
  10. Sep 23, 2016 #9
    Real wires have some resistance so there is a voltage drop across them. The resistance of old connecting lugs in a panel can be pretty high. I did a stint as an electrician's apprentice. I encountered a handful of energized neutrals from various kinds of faults. I saw it myself with a meter. Trust me, you can have some pretty poor connections. I was told that this was the reason for circuit isolation codes. Btw, it wasn't a common problem just an occasional problem. The idea is that certain combinations of faults without circuit isolation can cause the motors to work in series which is not what we want.
     
  11. Sep 23, 2016 #10

    Averagesupernova

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    Then can you explain why we allow sub-panels with shared neutrals and 120 volt loads? It is the same thing. Technically the OP could install a sub-panel at the central vac unit and do what has been discussed. I don't know what you exactly mean by circuit isolation codes. I know they want to keep the wiring in the form of a transmission line so as to cancel EMFs. Current in all wires within a cable or conduit have to be equal and opposite but that has little to do with this. You may have been told it was one thing but that does not mean it was true.
    -
    BTW, what do you think mobile homes are fed with? A four prong plug with a shared neutral.
     
  12. Sep 23, 2016 #11

    wirenut

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    Are you referring to the part of the NEC where a set of wires that share a neutral must be put on a multipole breaker? and must identify all phase conductors associated with that neutral?
    This was implemented because on a shared neutral if you unknowingly remove the neutral without de-energizing all the phases could cause you to become part of the circuit by contact with the neutral and a grounded conductor, raceway, etc.
    These days most electricians run separate neutrals due to the use of AFCI/GFI breakers which cannot be used on shared neutral circuits.
     
  13. Sep 23, 2016 #12

    wirenut

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    This is allowed as per the NEC as long as both phase conductors are on a common trip multipole breaker
     
  14. Sep 23, 2016 #13

    berkeman

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    Thread closed for Moderation...
     
  15. Sep 25, 2016 #14

    berkeman

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    Thread is re-opened after some searching through applicable codes and practices. Please keep the discussion about how to accomplish this task while being sure to meet code so it can be inspected at the conclusion of the project. There are some good points being made, IMO.
     
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