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Adding a known load to a DC motor

  1. Dec 9, 2014 #1
    Hi everyone,

    I have implemented PI control of speed of a PMDC motor under no load condition. I now intend to implement the same with a load added to it.
    I want to compare the results with simulation as well. Is there any way of adding a known load to a DC motor. In my set up the dc motor is mounted on a metallic box.

  2. jcsd
  3. Dec 9, 2014 #2
  4. Dec 12, 2014 #3
    If you have another motor - you can use as a generator and connect a resistor as load. Then as you change resistances you can change the load on the motor.
  5. Dec 12, 2014 #4
    Good idea if he knows well enough characteristics of another motor
  6. Dec 12, 2014 #5
    I was just thinking to use the same type motor - by monitoring the input and output electrical power - you can then "see" the characteristics of what is between the measurements and divide by 2 - roughly speaking a poor man's dyno.
  7. Dec 13, 2014 #6
    "Roughly" is a good word. If you have two identical machines operating like this, total loses (Pin-Pout) won't be shared in proportion 1:1 between them.To illustrate suppose measured input power is Pin=10 kW, and let the efficiency of the first machine η1=0.9. Loss of the machine is
    P1=(1-η1)Pin=1000 W, and power throughput P'=η1⋅Pin=9 kW. Suppose now that efficency of the second machine is also η2=0.9. Then, the loss of the second machine is P2=(1-η2)⋅P'= 900 W, and Pout2⋅P'=8.1 kW.Thus, one makes about 5% error assuming that loss of the motor 1900/2 = 950 W. The error increases in the case of small power machines or/and if the operating point is far from nominal.
  8. Dec 13, 2014 #7
    Hmmm.. somehow I think that 5% error for the OP would be pretty good..... esp if he does a few boundary cases : no load on the motor and not linked to the load Generator, not linked to the Generator and the motor in stall (full torque) for a second, linked to the load Generator w/no electrical load - etc.....lastly he can swap the two motors and compare the datapoints. I'll bet with some "good practices" he can determine the characteristics to less than 1%. In the real world - that is actually very good.
  9. Dec 13, 2014 #8


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    What is the watt rating of your motor?
  10. Dec 13, 2014 #9
    In the real world in many cases 1% accuracy is acceptable even for methods with professional dyno equipment.
    Simple Pout/Pin = ηtot = η1⋅η2 depends on assumption η12.
    This may be preceise enough for a bulky machine operating near rated load, but for "toys" with Pnom<1 kW operating far from their nominal point, 5% preceison would be quite success.
  11. Dec 14, 2014 #10
    Z85 -- I think we are on the same page -- but then in my "real world" there are kind of 2 real worlds... academic and industry... in an academic (lab) setting 1% is a good goal for real systems, in industry 5%... the challenge I face, that drives me crazy is academics (without much real lab experience ~1%) come into industry so used to "perfect" simulations and can not accept the 5% of industry ( paper engineers)....so to me this case is actually a very good exercise. However the question needs to asked at the outset - what are you trying to learn or achieve, including how accurate do you want to be.
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