No announcement yet.

3 Phase Motor with no plate what is the most accurate method of establishing HP?

  • Filter
  • Time
  • Show
Clear All
new posts

  • 3 Phase Motor with no plate what is the most accurate method of establishing HP?

    I have a 3 phase motor looks like an older squirrel cage Brooks/Crompton sadly it's missing it's information plate.

    What is the most accurate method of calculating it's KW or HP?

    The motor is wired Delta and has an across the terminals resistance of 8.2 Ω dividing this by 1.5x gives a single winding resistance of 12.3 Ω.

    Using Ohms law my calculations have given me this:

    V/R = I. therefore 230v ÷ 12.3 Ω gives 18.6992 Amps (I)

    230v x 18.6992 Amps = 4.3 Kw ÷746 = 5.765 HP

    Looking at the size of the motor it dosen't seem big enough to be this powerful, but that said it won't run for long on my 2HP RPC before the switch trips out.

    Any & all help appreciated.


  • #2
    Paul, Ohms law would be the stalled current if it was fed with DC! Hopefully, when running the current would be much less.

    I presume the only way to establish horsepower is to measure current under load or use a de Prony brake test.
    Last edited by The Artful Bodger; 09-11-2012, 06:26 AM.


    • #3
      If your not afraid to run it ... throw a clamp on amp gauge around it .. (they'r cheap and handy)
      make it load up with some resistance ... push a piece of wood against it ... then use this formula ..

      the horse power of an 230 V electrical motor with 85% efficiency pulling 10 amps can be calculated as:

      hp = 0.85 (230 V) (10 amps) / 746 = 2.62 hp
      John Titor, when are you.


      • #4
        What is the diameter of the shaft and the overall size of the motor?


        • #5
          Approximation method #1..... NEMA frame size.... per Macona.

          approximation method #2.... run it on nominal voltage and see what current is. Assume that to be 40-50% of FLA and look at a table of current draw.

          Approximation method #3... Run it on nominal voltage, load it until you get maximum torque. reduce load by about 10% and check current draw against tables

          Approximation method #4.... Do as #3 but calculate HP from RPM and Torque.

          Approximation method #5.... Do as #4, but leave the load on, adjusting load to get a temperature rise appropriate for the class of motor you think it is, likely class B. Then at that load calculate power from the rpm and torque.

          Your choice which is best for you..........

          Keep eye on ball.
          Hashim Khan


          • #6
            #6...... Take it to a motor shop and have them connect it up on their motor dyno.


            • #7
              An old Brooks/compton motor is unlikely to compy with Nema standards. As Jerry says in #2, The "idle" current is probably 40-50%% of the full power current. Measure idle, extrapolate and give yourself a safety margin. If you need more accuracy, a dyno while measuring current etc is a good way way.


              • #8
                I was GUESSING 3hp from amps. I figure a known motor pull X amps then if this motor pulls the same as X then i would guess its that motor.Confused? lol


                • #9
                  Thank you gentlemen some very good food for thought.

                  Some more information:


                  Length bell to bell 10"

                  Diameter 6-3/8"

                  Shaft Diameter 5/8"

                  Running (unloaded) RPM 2970 (difficult to load up as it's a T&C grinder)


                  L1 - L2 = 250v

                  L2 - L3 = 230v

                  L1 - L3 = 240v

                  Voltage mismatch is down to the homebrew 2HP RPC it is running from.


                  L1 = 1.5

                  L2 = 1.1

                  L3 = 0.7

                  Above measurements taken with motor running @ 28 degrees C

                  And as they say a picture paints a thousand words....

                  Thats a 6" wheel in the pics.



                  • #10
                    Anything from 1/2 to 1 HP, possibly 3/4, or about 570W.

                    230V 3/4 HP is 2.8A , while 1/2 HP is 2A (both FLA, at 40% 1.12 and 0.8 respectively)

                    Keep eye on ball.
                    Hashim Khan


                    • #11
                      Gentlemen again thank you for some very positive advice.

                      It seems I have by popular consensus a 1HP (or thereabouts) motor.

                      I can appreciate the difficulties of having a mixed bag of parameters to work from with differing motor types/sizes but is there no definitive equation?, how do the motor repair workshops establish a "rogue" motors HP or do they rely on the use of some kind of Dyno?




                      • #12
                        There IS no real definite maximum HP for an electric motor... unless you count the "breakdown torque". That is, the actual rating is NOT the point where the motor simply cannot mechanically produce more power.... it is virtually always considerably lower.

                        Most electrical devices are "heat limited", meaning that they will continue to produce more output past the point where losses would limit them to shorter term use to avoid overheating. Losses due to resistance of the wires, and to some extent losses in the "iron" heat up the motor, and cooling air takes away heat. At some point the cooling cannot do the job, and the motor temperature continues to go up.

                        So the "real" rating is the maximum output that can be produced without overheating beyond the temperature that the insulation can stand without being damaged during a "normal motor lifetime".

                        The motor may easily produce half again more power, but is in danger of overheating if that power level is continued.

                        Keep eye on ball.
                        Hashim Khan


                        • #13
                          You did look inside that junction box with the kidney-shaped lid, right? Many older motors had info in there glued to the back of the lid.

                          I vote for 1/2 horsepower.



                          • #14
                            Wire the motor to delta put capacitor between 2 windings and run it if its noisy or runs poorly change capacitor size until it runs smoothly. In the UK supply voltage is 240v and a 1 hp motor requires 40mfd a 1/2 hp 20mfd pro rata, you may be able to work out hp using this metthod.
                            Last edited by malbenbut; 09-13-2012, 03:22 PM.