220V motor hum

Discussion in 'General Electronics Chat' started by rickfisher, Jul 17, 2008.

  1. rickfisher

    Thread Starter New Member

    Jul 17, 2008
    4
    0
    Hello, I am new here and looking for some advice.

    I have a brand new 220V x 5hp (Cap start/cap run) induction motor. The motor is rated for 23 amps and is attached to a large peice of woodworking equipment.

    As a brand new motor and machine, I plugged it into a 220V x 30 amp receptacle with a 30 amp breaker. This motor is operated by a mag switch with the overload built into the switch.

    When I hit the power button, the motor just hum's and does not spin. The motor tries to spin, a turn of about 1/10th of a rotation is noticable. There is nothing blocking the equipment from spinning.

    The solution the Mfg. offered was a fautly starting capacitor, they sent one to me and I got it today.

    This Start Cap is rated as 400 MFD and has two screws which the wires must be attached to.


    My question is, is it dangerous for me to change this capacitor? I supplied as much information as I have.

    My natural inclination is to grab a screwdriver, undo the screws and re-attach the new capacitor. I am a bit worried about touching a screwdriver to the already installed capacitor.

    The machine is of course unplugged :)

    Thank you.
     
  2. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    You could discharge it thru a resistor.... or perhaps a lightbulb.

    Perhaps the motor was humming because it didn't know the words..?
     
  3. rickfisher

    Thread Starter New Member

    Jul 17, 2008
    4
    0
    I really appreciate the response. I am a woodworker, not an electrical person.

    Could I ask, how long does this capacitor hold a charge? Its been unplugged for about 15 hours.

    Is the charge inside the Capacitor 220V? Will a light bulb handle that type of charge?

    I hear about cap's being dangerous but dont know if this one is even conisidered big or small. Another woodworker suggested rubber gloves, a cheap screwdriver and welding goggles. :)
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    If the motor hasn't been plugged in for a day it is safe, if it for a few minutes the cap can hold a charge which could be uncomfortable at best, dangerous at worst. If in doubt discharge the cap with a screwdriver, holding said screwdriver by its plastic handle (make sure everything is unplugged!). It will be safe to handle then.

    No, it will not light a bulb, there is just enough potential juice to light you up.
     
  5. rickfisher

    Thread Starter New Member

    Jul 17, 2008
    4
    0
    So how would this capacitor compare in size to the capacitors used in older televisions ?
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Not sure, but not very big. I'll let someone else field this one.

    Thing about the TV's, they were for power supplies, so I would be willing to be they were a lot bigger.
     
  7. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    400 uF at 220v seems pretty big to me. I think that you could use a 220v light bulb and 2 crocodile clips attached to the capacitor terminals. It should keep the capacitor discharged and safe to handle. If in doubt, it's better to ask an expert to do it. 220v is no joke.

    What catches my attention is the 30 amp breaker. If the motor uses 23 amp normally, how much does it use when starting? If i'm not mistaken, slow-blow fuses are used to protect motors, because a fast fuse would blow when the motor is starting.
     
  8. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    That size of capacitor will definitely light up your life ! beentheredonethat

    Main caps in photoflash gear -- Most are 350v, and anywhere from 200mfd in one-time cameras, to 20 K+ mfd in studio equipment, bite really hard !! I do not recommend the screwdriver method.

    A priceless tool to discharge capacitors safely, is a series circuit consisting of : two respectable shielded alligator clips, a 100 ohm 10 watt [ minimum ] ceramic wirewound power resistor, a SPMC pushbutton switch rated at least 20 amps, and #18 hookup wire as long as you like. Solder all connections, and pot the whole arrangement into an adequate length of pvc tubing.
    If you routinely deal with the monster capacitors of a whole Farad or more in these magnum-watt car stereo systems, you will want to upgrade components considerably.

    Norfindel.........it is true. Single, and I believe 3 phase motors as well, draw up to 5X running amperage to start, albeit this is a quick pulse. Our man could wind up installing a 40 amp breaker to avoid nuisance trips.
     
    Last edited: Jul 17, 2008
  9. rickfisher

    Thread Starter New Member

    Jul 17, 2008
    4
    0
    Really appreciate the information. This Capacitor is about as big around as a Red Bull can and nearly as tall.

    The rule of thumb for dual cap motors is 80%. The motor draw should be no greater than 80% of the size of the breaker. It is this cap that allows it to start. It gives the motor a burst of power big enough to get it running on a breaker of that size.

    I made some calls this morning and found out that they use something similar to a Jumper cable to short these out prior to removing them. On that note, I will call my electrician :)

    Thanks for your help.
     
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