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Milling Machine Horsepower Question

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  • Milling Machine Horsepower Question

    I have two questions for the milling machine/electric motor experts.

    Currently I am re-powering my old Index Model 40 mill with a DC motor to obtain variable speed. While researching motors and milling machines I noted that horsepower rating on various machines seems to vary widely. For instance, some Bridgeport mills came with 1 horsepower motors, other with 2 HP, my Index originally came equipped with only 1/2 HP and some were 1/2 HP two speed 3 Phase motors which I assume means that at the lower speeds horsepower was less than 1/2. Some of the Mill/Drills come with motors rated at 2 1/2 HP. I mean no offense to anyone who has a Mill/Drill but it seems unlikely that a round column Mill/Drill could possibly be as rigid as a Bridgeport therefore it is unlikely it could require more power. So, why are the motors installed on these machines so varied in output? The only reason I can come up with is that carbide tooling may not have been available when my Index and early Bridgeports were being produced.

    While on the subject of power, Mr. Richard Kinch's web site has some interesting information about re-powering a Bridgeport with a DC motor. On that page he states that replacing the original 1 HP 3 Phase AC motor with a 3/4 HP DC motor can replace an AC motor of 1 to 2 HP. How can this be so?

    Thanks in advance to any who respond.

  • #2
    The man is full of baloney. HP is HP. Motors are limited by their magnetic circuits and windings how much HP they can develop at a given RPM.

    That said most shunt and PM DC motors have considerable overload torque for a short period of time. So if you are speaking of DC developing more HP than an induction motor then you have to set a time limit on it. Overload any motor for too long and the smoke gets out.

    Also most induction motors can be overloaded for a time dependent of the insulation class, the motor start temp, and the amount of over load but 150% of FLA ratings is about the reasonable limit. DC can run 250% FLA but for a shorter time.

    Manual home shop machine tools are seldom run to motor HP or cutter capacity. Most home shop people are concerned about overloading or breaking something expensive when the chips really fly. Experienced production mechanics keep an ear tuned to the sounds emitted by the machine and back off when they hear the motor bog down a bit or the tooling gives signs of reaching limitations.

    Honestly rated motors run to capacity over a period of time are equal more or less. DC has some advantages and AC others. The advantage falls to AC when motor failure occurs. A DC motor is several times more expensive to replace than an induction motor other things being equal.

    However I think perhaps your motor guy is wedded to his notions about the superiority of DC over AC. Don't disillusion him but the facts show little to choose from in these days of flexible reliable solid state controls for both AC and DC motors.

    My preference is based entierly on cost. My experience has shown OTC complete AC drive/motor combos are roughly half the installed cost of equivalent DC.
    Last edited by Forrest Addy; 09-24-2011, 03:35 AM.


    • #3
      Different types of motors have different properties. One strength of the DC motor is great torque at extremely low speeds. A larger motor is often used with a VFD to provide sufficient power at lower speeds.

      As for the disparity in sizes... there are so many reasons for this. Sometimes the decision is arbitrary. Sometimes the HP is matched to the limits of the hardware. An over-sized motor can cause serious damage to the machine when there is a crash. A plastic or aluminum part like a shear pin is often included in the power train to act as a sacrificial part.

      There are also the power requirements to take into account. The bigger the motor the more it taxes the power system. A 5HP motor will not work on a 15 amp 120V circuit.

      At the end of the project, there is a profound difference between spare parts and left over parts.

      Location: SF East Bay.


      • #4
        Surely variable speed DC motors provide increased torque at reduced rpm whereas variable speed AC motors provide only constant torque at reduced rpm. This means the HP output of DC motors will be higher than a comparable AC motor at reduced speeds. At east in the context of a low cost hobby mill.

        Or did I missunderstand something?



        • #5
          Forest Addy

          I have to disagree with your comment HP is HP

          back in the days of yore when i was an apprentice doing armature winding it was pointed out that many of the old same frame size motors ( with differing HP ) were in fact the same winding details i.e. same HP . it was the practice in the stores fore the store keeper to just stamp on the HP order by the purchaser up to the max design HP

          so a good number of years pass and the drawing office decided to replace the motors of they when and took the name plate details and ordered up the new stuff goes what the lathes would not evan turn over

          these motors had been stamped 5 hp and were in fact 25 hp .

          so the moral is beware of old motor name plates the can and do mislead

          One final point chinese hp is not the same as UK or USA hp its much smaller and is not cont rated somewhat like 15 mins at FL

          so my point is that all motors are not made to the same specs i.e. 1 HP = 746 watts at 100% efficiency



          • #6

            I can't dispute your anecdotes and I have no doubt that many makers even reputable ones were casual in their design approach to HP in the early days. Fact is, "HP" is an international standard unit of mechanical power having legal definition and conversion factors. That's where I was coming from.

            I spent a few weeks once with little to do and unsupervised access to a wide selection of induction motors and an absorbtion dyno. Most motors I tested were very close to their HP rating judging by temp rise under nameplate FLA and rated Volts. There were exceptions high and low. I tested no Chinese motors but I did test a few Hitachi made to NEMA frames. IIRC their actual outputs were indistinguishable from their US built counterparts.

            I'm vaguely aware large special purpose DC motors were often hard to rate for HP as they served as traction motors, elevators, marine propulsion, machine tool drives where they were subject to wide variations of Armature Volts and Amps, and often operated under field weakening and high momentary overloads as part of their duty cycle. Thus I'm not surprosed that a nominal 5 HP DC motor could deliver 25 HP under some conditions. Traction motors rated at 150 HP run 12 ton electric trolleys with a passenger overload up steep hills stopping and starting from primitive 4 point starters. Work the math and you find "150 HP" is a mere number and actual power delivered was double or more than name plate ratings. Acceration was dragster-like. If the bus was near empty and the driver up to the challenge he could smoke the tires from a standing stop. I ran a Niles planer whose table weighed 54 tons. The 125 HP motor pegged a 750 Amp meter every table reversal.

            Addressing Chinese motors, time is passing and with every flip of the calander page the Chinese seem to be converging on their goals as a world class manufacturer. They still put put out cheap crap for the cheap crap market. OTH they compete as vigorously for the high end industrial systems market (industrial processing systems, OTR and rail transportation, ships, bulk food storage and processing etc) as their Korean, US and German counterparts. They are not yet fully successful but they soon will be. They have the talent and drive, and are quickly acquiring the experience. No amount of Jingoistic denial will offset the eventual Chinese success. Only vigor and hard work keep the US in the game but the financial, corporate, and government stuffed shirts would rather play money games than face responsibility and enthusiastically meet challenges.

            That's a long way from the original topic of motors and HP ratings. In these matters you cannot allow anecdotes to govern engineering decisions. If you have an older motor you suspect is capable of more than the nameplate ratings would indicate, simple tests using improvised apparatus and a couple hours of experimenting will determine numbers safe to use in actual service.

            Otherwise go by nameplate ratings. You are in fact legally required to if you want your insurance carrier to honor a claim initiated by that specific motor's failure.
            Last edited by Forrest Addy; 09-24-2011, 10:17 AM.


            • #7
              Honestly rated motors should put out rated HP out the shaft. Not sure how lower priced imported motors are rated. Electrical input HP? Fantasy?

              My shop vac say 6.0 peak HP. I'm certain my shop vac motor is not putting out 6 shaft HP.


              • #8
                Originally posted by Blackadder
                One final point chinese hp is not the same as UK or USA hp its much smaller and is not cont rated somewhat like 15 mins at FL

                The problem with Chinese motors is they measure them with ponies not horses.
                The shortest distance between two points is a circle of infinite diameter.

                Bluewater Model Engineering Society at

                Southwestern Ontario. Canada