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Electric motor torque and HP,,,compressors

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  • Electric motor torque and HP,,,compressors

    Do electric motors have similar HP/torque numbers as petrol engines?
    For gas/diesel there is the accepted formula:
    HP x 5252 / RPM = torque
    With that, the lower the rpm the higher the torque. (for the same given HP)

    Does this hold true on electric motors??
    Does a 1725 motor make more tq than a 3450 motor? (for the same given HP)

    does the same rules apply to electric HP as does petrol HP?

  • #2
    Originally posted by Ringo View Post
    With that, the lower the rpm the higher the torque. (for the same given HP)
    I think with DC motors the torque is high at stall. Opposite with AC, I think. JR


    • #3
      You need to look at the torque at different RPM... for the different motors/engines... then it all becomes clear how that relates to hp.

      yes, a 1750 is 2x the torque of a 3450... for the same hp. But that's at the motor. For a compressor the measurement is at the head - and both generally spin that at the same speed with different pulley sizes. No free lunch.
      Last edited by lakeside53; 02-03-2021, 10:00 PM.


      • #4
        Power has no relation to what produces it.... the same power can come from an electric motor, a steam engine, a "petrol" engine, or a few donkeys turning a vertical shaft.

        One can argue about the "effectiveness" of power in different forms, but that is a different matter.
        CNC machines only go through the motions


        • #5
          I though this had been thrashed out before?


          • #6
            The main difference between electric motors and IC engines is that an IC engine puts out its design hp and that is it. Overload it and just slows down and may stall. An electric motor overloaded can put out maybe 6 times more hp for a short period of time. Thus an electric car with a 100 hp continuous rated motor can also put out 600 hp during acceleration from a standing start.


            • #7
              That depends entirely on the design of motor. Some , such as EV motors, are designed to have reserve, others are not and what you see is what you get.

              The winners usually are old DC traction motors, which can put out 300% or more of rated torque (not power) at low speed for a limited time. But they are series motors, and series motors have special characteristics, that vary widely with current, because the current also provides the field. That is true of series AC (universal) motors as well, and is why those are used on things like electric drill motors, etc.

              Motors used in CNC machines may be designed similarly, with built-in reserve, because they normally do not have variable transmissions.

              The limit is resistance in DC motors, and total impedance in AC motors. That is why the DC motors get non-linear at low voltage, because the resistance eats up a fairly large proportion of the available voltage. At normal operating voltage, the back EMF is the major limitation on current.
              CNC machines only go through the motions


              • #8
                Another example of the series motor is the older automotive starter, extremely high torque at zero RPM, but as it it a series motor, risky to operate with no load, as the field weakens the motor accelerates, sometimes to destruction, this is why larger wound shunt field DC motors have a field loss protection built in as a safeguard.
                Also the reason your Universal vacuum motor accelerates if you block the inlet, hence unloading it.