motor pulls too many amps

Discussion in 'General Electronics Chat' started by Seldomseen, May 24, 2007.

  1. Seldomseen

    Thread Starter New Member

    May 24, 2007
    I'm about to rewire my pool pump to 240v, and just for S&G I threw an amp meter on the hotwire within my breaker panel and it said the motor was pulling 24-25 amps at 120v when it should be only a max of 18.6!. Its a 1.5 hp motor. I know voltage drops with distance, are amps inversely related? or is something else going on? Its about a 100' run from the panel to the pump. Any help would be appreciated.
  2. recca02

    Senior Member

    Apr 2, 2007
    when are you taking these readings?
    the starting current of motors is generally high.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    If your motor wiring is buried, you may have a break in the insulation and some leakage. Conversely, the wiring may not be heavy enough, so there is an excessive voltage drop. That could cause the motor to pull too much current. It wants to use so much power. If the voltage is low, then it will draw extra current to compensate (power is volts times amps).

    If there is no current flowing with the motor off, it might be interesting to see what the voltage was on the motor terminals while it was running. If it's low, the only cure would be to replace the wiring with a size or two larger. If that's the case, it's amazing that the motor hasn't fried.

    Things will be easier going to 240 VAC, as the current will be lower for the same power. You'll still need a larger wire gauge than ampacity charts might suggest, as buried wire can heat significantly, causing a big voltage drop.
  4. Gadget

    Distinguished Member

    Jan 10, 2006
    Also try disconnecting the pump section. It may have partially siezed.
    On the same note, check the motor rotates freely when you manually turn the shaft.... the bearings might also be toast.
  5. cumesoftware

    Senior Member

    Apr 27, 2007
    This doesn't has to do with distance. In a series circuit, the current is equal at any point.
    An 1.5hp motor should have 1125W of output or shaft power, but depending on motor efficiency, it will consume more than that (more than 9.375Amps). If you read the 18.6A from the plate (seems to be a nominal value to me), then your motor is probably with too much load, that is, you are demanding too much from the motor in terms of load.

    One question, what kind of motor it is? Is it from a centrifugal pump? Or form a piston pump? (should be centrifugal).
    You should check for the outflow load and for incrustations in the pipes. Believe me, I'm graduating as a chemical engineer and have some experience. Also, tell me the flow you have in the pump (litres per second).

    Also, as sugested before, this could be due to having the motor running below the nominal voltage, and this situation might overheat the motor as the motor at lower voltages will have much less speed, thus drawing more current (has the same effects of applying too much load). If the sum of the windings has a resistance of, say, 6 Ohm, the wire should have a resistance smaller than 0.6 Ohm. Using heavier gauge might not be enought, if you are looking for the amps. When calculating the wire gauge, you have to take into accout the length that you use.

    First thing, measure the voltage at the motor terminals with the motor running. Also, measure the voltage at the other end of the wire (away from the motor). Use protective wear like isolating gloves and boots, so you won't sue me in case of accident.
    If the voltage differ substantially, they you might have the wrong kind of wire. If not, then the problem is due to excessive load.
  6. PackratKing

    Well-Known Member

    Jul 13, 2008
    A pool pump motor.........[ having rebuilt more than I care to remember ] is likely a 3450rpm single phase

    The most common problem with them, is grounded stator windings due to a leaky pump seal letting water into the motor via the output shaft. Water w/ pool chemicals, is very corrosive.

    Secondly, is the start switch / capacitor circuit water damage for the same reason. causing the start switch to hang up, allowing your motor to sit and burn. OTOH it should start with a snap.

    You mentioned a 100ft run. What size wire are you hooking this up with ? It may be inadequate for the draw.

    Noisy running otherwise, is the 6203 bearings.
    Last edited: Jul 23, 2008
  7. subtech

    Senior Member

    Nov 21, 2006
    I'm going to lean the other way...
    What type of meter and/or current ring are you using to take your measurements? Is it a true rms type?
    Some are averaging etc.