Submersible pump current insreasing with time

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by jackmccarron, Mar 26, 2013.

  1. jackmccarron

    jackmccarron New Member

    Messages:
    3
    Location:
    AZ
    I have a 1 1/2 HP sub (3 wire, single phase) sitting at about 125 feet down powered by a 7500 watt generator (Troy-Built). Two weeks ago while pumping, my generator SUDDENLY groaned heavily, coming VERY close to a stall, then recovered to normal rpm. Inspection disclosed that the overcurrent sensor on the well controller box had tripped, removing the load and allowing the generator to recover.

    First thot was something jammed in the pump impellers. Waiting a couple hours before I had a chance to try again, I found that the pump ran normally and completed filling the tank. Crossed my fingers that the problem was gone. However, within a week, and several times since then, the generator has strained -but not stalled- before the overload sensor tripped.

    I went out and bought a clamp-on current meter to help me debug the problem.

    Today I was lucky enough to catch the problem in the act. When starting the pump was running normally and voltages were at a solid 240 and running current was: Yellow about 11.8 amps, Black a bit lower, Red = 1.3 amps. After about 3 minutes, I noticed the Yellow current start to increase. First, from 11.8 to 12.1 to 12.8, etc, and finally up to almost 16 amps when the breaker tripped. The generator was laboring at that point. During the time the current was increasing, the Red current remained at 1.3 amps. (I didn't get to check the Black current but assume it would have to be increasing with the Yellow.)

    Earlier I had checked the Start and Run capacitors (open circuit) and they behaved normally, with "resistance" increasing over time as they charged from the meter current. I assume they're probably ok, and not the cause of the problem. Also checked the resistance to ground of all three (Yellow, Black and Red) leads going into well. All in the megohm range, so ok also.

    My guess is that the motor is overheating and drawing excess current.

    Is this correct? Are there any other possible explanations?

    What are my options? What would cause this - the pump is about 10 years old.

    Thanks,
    Jack
  2. jackmccarron

    jackmccarron New Member

    Messages:
    3
    Location:
    AZ
    How about some responses? Is my pump overheating leading to increased current?

    Thanks,
    Jack

  3. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    If/when a sub overheats, the armature swells and closes the gap. The swelling never returns to the former state. If that is the case, it's days are numbered. IMHO, replacement is your only option.
  4. WorthFlorida

    WorthFlorida Member

    Messages:
    105
    Location:
    Lake Worth, Florida
    LLigetfa is probably right, the pump is very close to failure. You didn't mention how old the pump is and without a generator, what is your line voltage? The higher the voltage the less current a motor will draw but an old pump the winding insulation may be failing, or as LLigetfa suggest the winding is swelling where by it is starting to hit the stator. However, the way you see the current spike looks like an indication that the motor is binding (slowing down and the current will increase) on something. Could be sediment or the impeller has a small piece broken off and it occasionally jams the impeller.

    Those portable generators generally do not have a regulated output voltage and they are hard on AC motors. No guarantee that your running at 60HZ unless you have a scope or a frequency meter. During a 5 day power lost after a hurricane, my refrigerator was having a hard time keeping things cold and with a washing machine connected to a 5500 watt (Troy-Built generator (no other load), it could not draw enough current to get it up in the spin mode. I'm sure a 7500 watt unit would also have the same issues.

    http://en.wikipedia.org/wiki/Commutator_(electric)#Simplest_practical_commutator

    http://en.wikipedia.org/wiki/Electric_motor
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    Another thing to consider is the water level and recovery rate on the well. The water level above the pump may be dropping significantly when running if it is not recovering well. The pump has to work harder pushing water up if there isn't as much head above it helping.
  6. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    The water level may also be a factor in so far as were it enters the well. If it enters above the pump, there is a lack of cooling flow across the motor. Larger HP pumps that are in an open body of water like a lake or cistern, or top fed in a bore hole should have a sleeve installed to direct cooling flow across the motor.
  7. valveman

    valveman Moderator Staff Member

    Messages:
    4,381
    Location:
    Lubbock, Texas
    My guess is the thrust bearing in the motor is going south. When the well is full of water there is very little pressure on the thrust bearing. But as the water level drops the pressure on the thrust bearing increases. With part of the thrust bearing worn off, it lets the impellers drag and the amps will increase over time.
  8. WorthFlorida

    WorthFlorida Member

    Messages:
    105
    Location:
    Lake Worth, Florida
    A quick A/C motor lesson. Motors are an inductive load, not a resistive load such as a light bulb. That means that the magnetic field that crosses the commutator is what causes the resistance. The lower the field, the lower the resistance, the higher the current. Also, a low voltage will also increase the current. Since the commutator is cutting through the magnetic field there is a Back EMF (resistance). When the armature/shaft is put under an excessive load and causes the motor to slow up, the Back EMF is decreased, therefore, the current increases. It is the increase of current that causes the motor to overheat, The overheating it self does not cause a increase in current.

    Back in the 1960-70's when brown outs from the electric grid was a common place, many motors especially compressors on refrigerators were burning out everywhere. Hence lower voltage higher current, higher heat. The government force the manufacture to add thermo-protection on many motors, especially compressors. You may have heard it happen during a power interruption from your refrigerator. The thermo switch will open the circuit and may take 10-20 minutes for the motor to cool down before the switch resets itself.
  9. DonL

    DonL Jack of all trades

    Messages:
    3,812
    Location:
    Houston, TX
    It could be something as easy as a bad Start or Run Cap, or the generator is not running at 3600 RPM.

    3600 rpm = 60HZ out for most single phase generators.
    Last edited: Apr 2, 2013
  10. jackmccarron

    jackmccarron New Member

    Messages:
    3
    Location:
    AZ
    Thanks guys for all the suggestions, ideas, etc.

    General consensus is that the motor is failing. By running it in short (2-3 minute) bursts, the current doesn't get high enuf to trip the breaker. This allows me to get the tank up to pressure. But it may get worse as time marches on. Although I checked earlier that the capacitors are indeed acting like capacitors I didn't actually measure their capacitance values. I now have a meter that can do that, so I will do that before concluding that I need to replace the motor.

    Again, thanks for the help.
    Jack
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