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Continuous duty & Continuous Load Motor

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    Continuous duty & Continuous Load Motor

    Quick question...

    Say you have a continuous duty motor that is also a continuous load. Do you multiply the FLA by 125% for being a continuous duty motor per NEC 420.33 and then again by 125% for being a continuous load per NEC 210.19?

    #2
    No.

    "Continuous," in the NEC, refers to anything that is 'on' for three hours or longer.

    "Continuous Duty" is a design specification used by motor makers. You need not multiply your figures unless the motor actually is operating for more than three hours at a stretch. A 'continuous duty' motor will usually have larger bearings and a better cooling fan than an 'ordinary' motor.

    BTW, 'ordinary' motors are designed to operate with no more than six on/off cycles per hour. Motor makers also make motors that are designed to 'short cycle' as well.

    Comment


      #3
      Yeah, I was thinking of a conveyor motor that would be running indefinitely. Continuous duty is defined as "Operation at a substantially constant load for an indefinitely long time". Whereas, continuous load is max current for 3 hours or more. So the motor would be continuous duty, but would it be considered "max current"?

      Comment


        #4
        Originally posted by MyBeardAndMe View Post
        Yeah, I was thinking of a conveyor motor that would be running indefinitely. Continuous duty is defined as "Operation at a substantially constant load for an indefinitely long time". Whereas, continuous load is max current for 3 hours or more. So the motor would be continuous duty, but would it be considered "max current"?
        The only difference for continuous-duty motors is the overload protection.
        [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

        Comment


          #5
          No.

          The motor draws according to the load on it. That is, a 10-hp motor draws pretty much the same running amps as a 1-hp motor, when there's only a 1/2hp. load applied.

          Yes, there are differences in the starting amps and how quickly the motor can respond to new loads.

          Comment


            #6
            Originally posted by renosteinke View Post
            No.

            The motor draws according to the load on it. That is, a 10-hp motor draws pretty much the same running amps as a 1-hp motor, when there's only a 1/2hp. load applied.

            Yes, there are differences in the starting amps and how quickly the motor can respond to new loads.
            You replied "no" to my post. Please cite references if my post was incorrect.

            I realize there are differences, but the question here is in what ways are continuous-duty motors treated differently under NEC requirements.
            [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

            Comment


              #7
              Originally posted by renosteinke View Post
              That is, a 10-hp motor draws pretty much the same running amps as a 1-hp motor, when there's only a 1/2hp. load applied.
              I disagree with that. Both only deliver 1/2 HP to the load, but they will not have same characteristics that result in same input current. The mass of the rotor on the 10Hp motor will likely require more input power just to keep it at speed than the rotor on the 1 hp motor for a place to start with differences.
              I live for today, I'm just a day behind.

              Comment


                #8
                Originally posted by kwired View Post
                I disagree with that. Both only deliver 1/2 HP to the load, but they will not have same characteristics that result in same input current. The mass of the rotor on the 10Hp motor will likely require more input power just to keep it at speed than the rotor on the 1 hp motor for a place to start with differences.
                The larger motor will draw less current given the same load. Kind of like a car with a big motor putts up the hill while the guy in the little car has to put the pedal down.

                Comment


                  #9
                  Originally posted by Aleman View Post
                  The larger motor will draw less current given the same load. Kind of like a car with a big motor putts up the hill while the guy in the little car has to put the pedal down.
                  And the car with the big motor used more fuel than the little one to accomplish the same task. It is not that simple.

                  The larger motor will not draw less power. In the example given, a load requiring 1/2 hp is driven. Both the 10 hp motor and the 1 hp motor will deliver the needed 1/2 hp to the load. That is the only thing that is the same in both situations. The 1 hp motor likely does so more efficiently, as both motors not only need the 1/2 hp of input energy that gets transferred to the load, they also need whatever energy necessary to spin its own rotor, overcome its own friction, and other losses. Because of physical differences, those other losses are going to be higher in the 10 hp motor than in the 1 hp motor, meaning it will take more input energy to do the same work with the 10 hp motor.

                  A 10 hp motor with that small of a load on it probably has a terrible power factor, which is not a direct loss but it does contribute to what is needed for input.
                  I live for today, I'm just a day behind.

                  Comment


                    #10
                    Originally posted by kwired View Post
                    And the car with the big motor used more fuel than the little one to accomplish the same task. It is not that simple.

                    The larger motor will not draw less power. In the example given, a load requiring 1/2 hp is driven. Both the 10 hp motor and the 1 hp motor will deliver the needed 1/2 hp to the load. That is the only thing that is the same in both situations. The 1 hp motor likely does so more efficiently, as both motors not only need the 1/2 hp of input energy that gets transferred to the load, they also need whatever energy necessary to spin its own rotor, overcome its own friction, and other losses. Because of physical differences, those other losses are going to be higher in the 10 hp motor than in the 1 hp motor, meaning it will take more input energy to do the same work with the 10 hp motor.

                    A 10 hp motor with that small of a load on it probably has a terrible power factor, which is not a direct loss but it does contribute to what is needed for input.

                    The current draw on a motor is load dependent. You can see this on a fan with adjustable pulleys. Lower the rpm of the fan and the motor current will drop. There are other factors of course, mass and inertia etc. Starting current will be higher for the larger motor. Perhaps the comparison of a 1/2 to a 10hp is not a good one. There's a curve on a chart there somewhere but that is beyond me. But given the same load, the smaller motor will have to work harder to move it. So, to run the same load, if the smaller motor uses more current then it is also using more power.

                    I do understand your point and you are right it isn't so simple.

                    Comment


                      #11
                      Originally posted by Aleman View Post
                      The current draw on a motor is load dependent. You can see this on a fan with adjustable pulleys. Lower the rpm of the fan and the motor current will drop. There are other factors of course, mass and inertia etc. Starting current will be higher for the larger motor. Perhaps the comparison of a 1/2 to a 10hp is not a good one. There's a curve on a chart there somewhere but that is beyond me. But given the same load, the smaller motor will have to work harder to move it. So, to run the same load, if the smaller motor uses more current then it is also using more power.

                      I do understand your point and you are right it isn't so simple.
                      You ever clamped an meter on a lead supplying a (running)motor with no connected load?

                      Input energy is always higher than output energy, until someone designs a motor that is 100% efficient.

                      Most motors are much more efficient at/near full load current than they are at only 5% load (like the 10 hp driving a 1/2 hp load would be).

                      I do understand what you are saying, but you have to remember it takes a certain amount of energy to spin the 10 hp motor whether there is a load connected to it or not. Same with the 1 hp motor, but that energy is likely less than for the 10 hp. The additional 1/2 hp load is simply added to the "no load" input energy.
                      I live for today, I'm just a day behind.

                      Comment


                        #12
                        Smart $, we may be having a problem communicating here ....

                        The NEC has no provisions for "contunuous duty motors.' The NEC does not recognize the term. There are motors, period. Then there are loads, which may be continuous or not.

                        The NEC defines a continuous load as one that runs for three hours or more. The load can be a light bulb, or an 'ordinary' motor. In this situation - where the load is present for mor than three hours - you apply the 125% multiplier.

                        This has nothing to do with 'cintinuous duty' motors. A 'continuous duty' motor is one that says 'continuous duty' on the nameplate. This means that the manufacturer has optomised the design for long run times. That's all that means. The motor need not be actually run for long periods. You wire this motor as you would an ordinary motor - using the 125% multiplier only if the motor will actually be run for more than three hours at a time.

                        That multiplier has nothing to do with the motor; it is there for the wire.

                        Comment


                          #13
                          Originally posted by renosteinke View Post
                          Smart $, we may be having a problem communicating here ....

                          The NEC has no provisions for "contunuous duty motors.' The NEC does not recognize the term. There are motors, period. Then there are loads, which may be continuous or not.

                          The NEC defines a continuous load as one that runs for three hours or more. The load can be a light bulb, or an 'ordinary' motor. In this situation - where the load is present for mor than three hours - you apply the 125% multiplier.

                          This has nothing to do with 'cintinuous duty' motors. A 'continuous duty' motor is one that says 'continuous duty' on the nameplate. This means that the manufacturer has optomised the design for long run times. That's all that means. The motor need not be actually run for long periods. You wire this motor as you would an ordinary motor - using the 125% multiplier only if the motor will actually be run for more than three hours at a time.

                          That multiplier has nothing to do with the motor; it is there for the wire.
                          For a single motor, you multiply by 125% regardless. If it is run continuously, you do not compound that multiplying by 125% twice. As for continuous-duty motors, see 430.32
                          [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

                          Comment


                            #14
                            121211-1037 EST

                            Alerman:

                            The load on the motor is 1/2 HP or 746/2 = 373 W. That is exactly the same for both motors.

                            Both motors run at essentially the same speed. The large motor has larger bearings, and more force load on the bearings than the smaller motor. Thus, more frictional loss from the larger motor bearings.

                            The larger motor has more rotor surface area and likely a larger diameter, thus much higher windage loss than the smaller motor (this is also friction) .

                            The magnetic core in the larger motor is larger, thus more core losses (again essentially friction).

                            All of these factors tell you that the larger motor will require more power input than the smaller motor for the same output mechanical load.

                            The electrical power input that supplies the mechanical power output is exactly the same for both motors.

                            You need to study conservation of energy, and techniques of using energy and power in the analysis of problems.

                            The smaller motor will be running neared its full load capability than the large motor. Thus, at a more efficient operating point. Really that is what the above loss analysis tells you.

                            .

                            Comment


                              #15
                              gar,

                              You just explained all of the losses in the 10hp motor compared to the 1hp motor, and then said the input power is exactly the same?

                              Wouldn't the output be the same and the 10hp motor would draw more current compared to the 1hp motor?

                              Comment

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