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1.5 HP 1 PH vs 1.5 HP 3 PH Motor

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  • 1.5 HP 1 PH vs 1.5 HP 3 PH Motor

    Is the HP rated/measured the same? I am planning on converting my Griz 7X12 band saw to VFD for easier speed changes. I have read the associated posts regarding this endeavour. However, I assume there is no power diferences regarding HP ratings of 1 Ph and 3 Ph. There is also the rcommendations to double the HP rating of the VFD. I know that pully position changes may still be required to assure low speed power and that motor speed will have an effect on cooling.
    Here is the motor in mind. It is identical except for shaft diameter of the 1 Ph stock motor.
    https://www.surpluscenter.com/item.a...tname=electric
    I don't really need to do this but I would like to expose myself to the "variable speed" world and this seems to be a good place to start.

    Regards,
    HAP
    Who do I think you are...?

  • #2
    Horsepower is the same regardless of single or 3 phase.
    Full power is at rated RPM though so as you reduce speed the horsepower drops and the cooling fan becomes less effective.
    I'm going down this road too and I'm going to wait and see about the thermal issue.

    I had not heard of over-rating the VFD but I have read that over-rating a simple phase converter(no VFD) is a bad idea.
    Mike

    My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

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    • #3
      I am planning on converting my Griz 7X12 band saw to VFD for easier speed changes.
      A VFD should be OK over a 2 or 3:1 range (greater with a vector drive).

      Torque drops off rapidly as motor RPM drops (most VFDs have a boost function that just dumps more power & heat into the motor), so you might have problems if you are trying to run slowly (i.e. metal cutting speeds with wood speed gearing) and, as Mike says, the fan doesn't work so watch out for over heating.

      Bill
      Last edited by BillTodd; 12-22-2010, 10:47 AM.

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      • #4
        Originally posted by BillTodd
        Torque drops off rapidly as motor RPM drops (most VFDs have a boost function that just dumps more power & heat into the motor)
        Power drops off rapidly as motor RPM drops. Most induction motors are constant torque.

        Power = Torque X RPM.

        So when the VFD lowers the RPM, the torque stays the same, and you get less power: 1/2 the power at 1/2 the RPM, 1/4 the power at 1/4 the RPM.

        This is why VFD varispeed conversions usually double the motor size.
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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        • #5
          I don't think there's any difference in power between singe phase and three phase but the current draw is higher on a single phase.

          VFD's are nice but it seems like a bit much for a bandsaw. I have 4 speeds on my 7X12 Enco saw and I seldom change. It seems to work beautifully on the lowest speed using flood coolant. Even if I do change speeds, the motor is always running at constant speed with the belt and step pulley design so the fan is always doing its job.

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          • #6
            Yes,

            But the problem is that as the frequency of output of the VFD drops the stator reactance drops and the output current approaches the VFD's limit (i.e. the available torque drops off with the drive frequency)

            [edit]as my previous statement made little sense :LOL:

            Originally posted by lazlo
            Power drops off rapidly as motor RPM drops. Most induction motors are constant torque.

            Power = Torque X RPM.

            So when the VFD lowers the RPM, the torque stays the same, and you get less power: 1/2 the power at 1/2 the RPM, 1/4 the power at 1/4 the RPM.

            This is why VFD varispeed conversions usually double the motor size.
            Last edited by BillTodd; 12-22-2010, 11:19 AM.

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            • #7
              I have added VFD's to my Mill, Lathe, and Bandsaw. Best idea since slice bread - just turn the knob and the speed changes. I cut plenty of wood / metal in the band saw and the speed change with just a turn of the know is much simpler than trying to change all the belts.

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              • #8
                Another hint: If you haven't got your motor yet, think about getting a higher base speed, then using the greatest reduction set on your pulleys (or even going for a bigger spread than stock). I did that, and it allows the motor (fan) to run at a reasonable speed and still gives a great tool speed range.

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                • #9
                  Okay, this is where I get confused. I get what torque is, but not what exactly it means WRT power transmission and units of measurement used to quantify it. What units ultimately matter for a machine tool? HP, Torque rating? Here's why I ask: I just paid good money for a Marathon inverter-duty 1.5HP motor to run my lathe.

                  I was going off of the "1000:1 Constant Torque" rating. This led me to believe that with a proper sensorless vector drive controlling it, I would have an effective speed range of essentially 0-base speed of 1750. What does it matter if you have constant torque if you have significantly derated power? See, that is where i get confused. Torque is an angular motion applied on a lever of specific length. If you derate the horsepower applied to the lever by reducing RPM---but the effective lever length is inversely proportionate to that, don't you end up with the same practical power output? Isn't this relationship what "constant torque" refers to? Or do I have my synapses all in a ninny twist...
                  Last edited by Arthur.Marks; 12-22-2010, 01:25 PM.

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                  • #10
                    FYI, the spec I mention above refers to the "Black Max" and "Micro Max" line by Marathon: http://www.marathonelectric.com/moto...uals/SB548.pdf

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                    • #11
                      Originally posted by chipmaker4130
                      Another hint: If you haven't got your motor yet, think about getting a higher base speed, then using the greatest reduction set on your pulleys (or even going for a bigger spread than stock). I did that, and it allows the motor (fan) to run at a reasonable speed and still gives a great tool speed range.
                      Chipmaker, that is a great idea! I'll check to see if there is one rated at a higher RPM.
                      Thanks to all replies.
                      HAP
                      Who do I think you are...?

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                      • #12
                        Suggest you keep thinking before you jump. 3ph/VFD has the drawback that horsepower drops along with speed. Most guys double the size of the motor for this reason. If you bump your motor up to 3hp you can still get a pretty cheap VFD (I know I have one) and when you slow the motor to 1/4 speed you still have 3/4hp available. Of course, you will still have your step pulleys and belts, although you will have to fit a step pulley to your new motor which may not have the same size shaft, so there will be some machining involved.

                        Just for the record, there's a real good reason they also make/sell gearhead saws - you change the speeds with a simple lever very quickly and easily, plus they run a lot quieter. They do cost about 25% more, but IMO they are well worth it.

                        metalmagpie

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                        • #13
                          Originally posted by Arthur.Marks
                          What units ultimately matter for a machine tool? HP, Torque rating?

                          What does it matter if you have constant torque if you have significantly derated power? See, that is where i get confused. Torque is an angular motion applied on a lever of specific length. If you derate the horsepower applied to the lever by reducing RPM---but the effective lever length is inversely proportionate to that, don't you end up with the same practical power output? Isn't this relationship what "constant torque" refers to? Or do I have my synapses all in a ninny twist...
                          Whew, about a dozen questions in that short paragraph!

                          It takes some fermentation (grain or brain or both??) to get used to the difference between torque and horsepower. I don't know if I can make it any easier but I will try.

                          Your concept of torque is about right, I would change it to read force applied to a lever of specific length. It only becomes angular force when we are talking about rotary motion.

                          Now imagine that force applied to the handle of a jack. Lets say you can apply enough force to the handle to raise the load, and lets also say that you were able to do this 6 times in one minute with the existing load. And lets say that these 6 strokes lifted the given load far enough to qualify as generating 1 horsepower. (which would be reeeeeaaallly hard for any single existing human, but bear with me)

                          Ok, now lets say we ate our Wheaties for breakfast, and we were able to apply that given force on the handle 12 times in one minute. With twice as many strokes in the given time period, it might be obvious that something doubled? The force on the handle is the same, the load is the same, the time is the same, the only thing left to change is the power or horsepower.

                          You should be able to apply this logic to see that a motor with a given horsepower rating at 1800 rpm will have less powerat 900 rpm because the amount of torque the motor can generate is the same (well, not exactly, but close) but it would be doing it at a rate of 900 strokes per minute as opposed to 1800 strokes per minute.

                          Does any of this make sense? To anybody??

                          Dave

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                          • #14
                            Sounds about right to me Dave

                            Bill

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                            • #15
                              Very clear explanation; I agree. Let me attempt to apply your analogy to machine tools. I would think what is ultimately of concern is the load. So, yes, you have less power at 900 strokes per minute, but the load is still moved. To further the analogy, the "load" could be, for example, the force exerted on a twist drill by the workpiece while the former is boring a hole. If the "load" still moves in your analogy, the twist drill is still turning. Right?

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