Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Induction motor load estimation from a variable resistive load

  1. Dec 3, 2012 #1

    I'm trying to find out how much energy an induction motor pulls over a 24 hour period when connected to a resistive load. The only data I have are the specifcations of the motor, so I can estimate how much energy it will draw based on what capacity it might be running at.

    Really what I need is someone with experience/knowledge on the behavior of induction motors connected to oil and gas pumpjack units. I am trying to design a power system to provide energy for the pumpjack to operate over a 24 hour period. I understand that the only way to accurately do this is to take tests on existing pumpjacks to determine the energy consumption, but for now all I can do is speculate.

    Anybody have any ideas/insight on this matter?


  2. jcsd
  3. Dec 6, 2012 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    To electrical types, the phrase "Resistive Load" infers an electrical resistor,
    which doesn't seem logical as a motor load.
    Do you mean the mechanical load on the motor is "resistive", ie mechanical work or friction,
    or "inertial, ie resisting acceleration", ie flywheel?

    What's a pumpjack?

    If you have motor specs you can estimate the motor's losses from full load efficiency,
    and add to those your mechanical work drawn by the pumpjack,
    the result of that wil be conservative(a bit high).

    Pay attention to the "KVA code" of the motor - that tells you its starting current which can be several times full load and highly inductive. It'd be embarassing to have a power system that won't start your motor. (A similar thing happened to me once).

    old jim
  4. Dec 6, 2012 #3
    What's your design goal for the power supply? Is it there as a backup system in case grid power fails? If so, the worst case scenario (maximum power draw when not locked) is probably the one you want to design around anyway.
  5. Dec 6, 2012 #4

    When I said resistive load I indeed meant load mechanical load is resistive (friction and mechanical work).

    A pumpjack is synonymous with a nodding donkey if you've heard of that. When you think of a land oil rig you should think of a pumpjack. Its the rotating device that pumps oil out of a well. The induction motor powers this unit. When the rod inside the well moves up and down because of the pumpjack, it experiences resistance due to material inside the well, causing the motor to run at inconsistent load capacities. Over a period of time, it could be running from 50 to 100 percent load capacity.

    So far, I've calculated the energy the motor will draw if its running at 50, 75, or 100 percent load (using provided power factor numbers from the spec sheet). The problem is the design and cost of my system varies greatly depending on which load capacity it's running at. If I speculate that it's running at 50 percent, the cost of my system will be much less, but I risk under-engineering the power requirements. Designing my system under the assumption that the motor is running at 100 percent capacity would be very expensive and potentially means that I am over engineering the system.

    I have taken into account the starting current (in-rush current) and is overcome by soft start functionality.


    The design goal is to be able to run the induction motor for a 24 hour period. The system is OFF GRID (meaning many, many batteries will be used). There is a small backup propane generator only meant for supplementary power when needed, but in no way can run the system for an extended period of time.

    Again, designing the system as if it were running at 100 percent capacity 100 percent of the time makes the design solution not realistic. I believe it to be unlikely that the mechanical load will require the induction motor to run at 100 percent capacity for 24 hours. I would expect the number to be closer to 75 percent and hopefully even lower.

    Do you know of any known systems that require the induction motor to run at 100 percent load for the duration of it's run time? That may help me at least make some comparisons.

    Thanks for your input gentlemen

  6. Dec 6, 2012 #5
    I suppose I need to ask the question a different way. Are you saying that 100% of the time, the system will be off-grid? How will you be replenishing the power supply in that case, hauling in freshly charged batteries every day and swapping them or something along those lines?

    I'm asking because if you design the system to be able to run the system at full swing for 24h, if you only run it at half speed, then it should be able to run another 24h without a 'resupply' and it sounds like resupply may be part of the cost you are factoring in?

    Overall this sounds more like a business question than an engineering question. Do you spend the money to ensure the system can run at full capacity when it needs to, or do you spend less and live with a limit on how hard you can run the pumps for how long?

    In most of the systems I work with like this (network systems, not mechanical ones), our target is the 95th percentile. We measure the system (as you initially mentioned) for some interval, and then design (or bill) based on the level that the system operates at or under 95% of the time.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook