2 Wire Motor Control

Discussion in 'Homework Help' started by uradnky, Feb 6, 2008.

  1. uradnky

    Thread Starter New Member

    Feb 6, 2008
    Im currently taking Mechanical Eneering technology and I am required to take an electrical circuits lab and lecture class.
    I understand most of what is taught in the lecture as I had taken electricity in physics previously, but I am having trouble with this section of our labs.

    1.) In a 2 Wire motor control circuit; if the start button is pressed, the control transformer fuse blows. What is the most likely cause of this problem?
    - Could this have anything to do with the Load contacts?

    2.) What is the difference between load and axuiliary contacts?
    - Is "auxiliary" contacts just another term for more than one load contact in a contactor?
  2. uradnky

    Thread Starter New Member

    Feb 6, 2008
    Is there anything that I could add in order for someone to help me with this?

    I understand it could be difficult to answer the first question w/o a schematic of the circuit.

    How about the 2nd question? The difference between a load and axuiliary contact.

    Any help would be greatly appreciated, thanks.
  3. techroomt

    Senior Member

    May 19, 2004
    auxillary contacts are like a spare set of contacts, generally n.o. and n.c., not initially required for the motor circuit.

    as far as the fuse blowing, make sure you have a load in the control circuit, typically a motor control relay coil. otherwise just check your wiring. there is usually (in series) a n.c.stop push button, n.o. start push button with a set of n.o. latching contacts in parallel, the control transformer secondary windings, a fuse, motor overload contacts, and the motor control relay coil. there can be additional devices of course.
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    The load contacts are not in series with the fuse, are they? They are separately protected? Enough grime across the relay coil leads might cause over-current condition. I know of one case where the control transformer was mis-wired, putting 460V across the controls and blowing the fuse. "Most likely" cause is would depend on operating environment and equipment specifications. Shoot, if the wrong size (too small) fuse were put it, it would blow when the start button was pushed!