Help with AC project

Discussion in 'The Projects Forum' started by Mklangelo, May 21, 2014.

  1. Mklangelo

    Thread Starter New Member

    May 21, 2014
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    Hello to all!

    I'm building a home HEPA filtration unit and have a blower fan that produces more CFM that required, so I bought this: http://www.ebay.com/itm/161296624597?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

    The transformer uses Banana Plugs. Since the unit is rated at 20 amps and will be using household current, I was hoping someone here could point me to somewhere I could purchase the properly rated banana plugs. I'm needing 3 sets of males and one female panel (with Pos and Neg) to mount on the outside of the fan box itself.

    One set of males for the AC input and the other two for running from the Variac to the side of the cabinet which lead to the blower fan itself. I want to mount the plate on the outside of the cabinet which will be wired to the fan.

    Any help would be greatly appreciated!

    MK
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Sorry, can't help re the banana plugs, but do you realise that a variac transformer does not provide mains isolation? Both input and output will have a hazardous LIVE terminal. I'm surprised that banana plug and sockets are allowed.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    They appear to be standard Banana plug size, where are you located, in N.A. DigiKey is one source.
    Max.
     
  4. #12

    Expert

    Nov 30, 2010
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    I'm having some worry about this. First, lowering the voltage is not how the motors like to have their speed controlled. Second, I can't find a banana jack that's good for more than 15 amps. Third, even if I could, the National Electrical Code would skin me alive for using them in a machinery control panel.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it is a shaded pole motor you most likely could use a common ceiling fan triac controller?
    Is it the motor that draws the 20amp?
    I would imagine it would be something else drawing that current, if at all?
    If the motor current is much smaller than 20a, mount the Triac controller on the unit using a standard outlet box and just control the fan itself..
    Max.
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Also, those 20A "variacs" will trip 30A circuit breakers when you flip them on...even with no load. Not every time...but a good 50%. We got four for one of our labs, but had to send them back. Even a guy I called at STACO (real Variacs) said that's normal. Depending on the phase at turn on, the big ones look like a very low impedance.

    Ken
     
  7. Mklangelo

    Thread Starter New Member

    May 21, 2014
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    It's a Permanent Split Capacitor motor, single speed.
     
  8. Mklangelo

    Thread Starter New Member

    May 21, 2014
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    The motor draws 4.9 amps at full load but as I understand it, it can easily exceed that at at start up. It's a Permanant Split Capacitor motor.

    Funny thing is, the 20 Amp Variac was cheaper than the 10 amp one. So...
     
  9. Mklangelo

    Thread Starter New Member

    May 21, 2014
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  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As #12 observed, a split phase cap motor may object to lowering the voltage, they are prone to drop out of run when powered too low, but if any motor is started less than full voltage you will not experience the same inrush as full line start current.
    Max.
     
  11. inwo

    Well-Known Member

    Nov 7, 2013
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    IMO, it would have been better to ask before buying.:)

    Not the best solution.
    Not even one I'd consider, for reasons above and more.

    Some fans can be pinched off to get lower cfm.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    He says he is building it? If so you can set the design criteria I would have thought?
    Max.
     
  13. inwo

    Well-Known Member

    Nov 7, 2013
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    I'm sorry.:p
    Talking about buying the variac. A great item for testing. Use in the final design? Not so much.
     
  14. #12

    Expert

    Nov 30, 2010
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    The usual way to throttle a squirrel cage fan is to restrict the intake air.
    The fan blade loses its grip on the air and the motor uses less power to run.
    Much better than trying to reduce the output of the fan because the motor will fight the obstruction.
     
  15. Mklangelo

    Thread Starter New Member

    May 21, 2014
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    That's what I'm thinking. Since this blower produces 1014 CFM at the Static pressure I've figured for my system, I might be OK...

    I need only 600 cfm so this Blower will only be started at perhaps ~60% capacity. So that roughly equals 70 volts.

    Based upon 115v household current.

    So I could introduce more filtration (Static Pressure) in order to reduce my cfm. Safer and lets the blower do it's task.

    Perhaps I've been coming at this problem from the wrong angle.
     
  16. Mklangelo

    Thread Starter New Member

    May 21, 2014
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    I do have plenty of cfm to restrict the airflow. Perhaps that is an option to consider...
     
  17. Mklangelo

    Thread Starter New Member

    May 21, 2014
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    I'm seeing that now. I think I'll pinch it which serves my purposes I have a ton of headroom to add more filtration.
     
  18. #12

    Expert

    Nov 30, 2010
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    There is no relationship between ultimate air flow and start voltage because there is no air flow during start. The motor will start as hard as it can just because the rotor isn't turning. You're also missing the basic fan laws.
    CFM2/CFM1 = RPM2/RPM1
    Static pressure2/static pressure1 = (RPM2/RPM1)squared
    HP2/HP1 = (RPM2/RPM1) cubed!
     
    Last edited: May 21, 2014
  19. Mklangelo

    Thread Starter New Member

    May 21, 2014
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    The cabinet is built and I have the Mother of All HEPA filters. It weighs 80 pounds & is rated @ 99.996 efficiency @ 3μm.

    These considerations are all new to me. I have a bit of money to experiment with the final design. Results are all I care about.
     
    Last edited: May 21, 2014
  20. inwo

    Well-Known Member

    Nov 7, 2013
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    Pinch may have been a poor choice of words.
    Restriction on the intake is better description.

    Also, my criticism of buying a variac might have come off harsh. :(
    You won't be sorry owning it. It's one of my most versatile test tools.
    It just seems not suited for use in the final design.
     
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