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  • Need a new AC motor for compressor

    This is my first post and I am very happy to have found you folks. I read the post by Cuemaker about his Worthington compressor while I was hunting around for info on Worthingtons. There appear to be some very knowledgeable people here.

    I just won a bid on a Worthington compressor model 3CH8. It has a two stage compressor run by a 3 hp 3 phase 220 volt motor. It was manufactured around 1967 and it is in very good shape. It spent its life in the basement of our Village Hall and was used to air the firetruck brake systems while they were parked and also to sound the annunciator that identified the zone where the fire was. It was well maintained by a fastidious fellow, so I am confident in its condition.

    What I need to do is replace the 3 phase motor with a single phase one. I wanted to use the 220 volt 4 hp motor from my rusted out compressor but it runs at twice the rpm as the stock motor. I'm trying to figure out if I can get away with just putting a smaller pulley on the single phase motor or what. Any ideas on what running the compressor too fast would do, aside from generate a lot more heat...

    Here's the data plate of the original:


    Here's the data plate of the one I want to use:


    Here's the compressor:


    Any help or advice is greatly appreciated.

    Kyle

  • #2
    Get a vfd to run the 3ph motor off single phase. That's how I'd do it anyway.

    The 3ph motor is more efficient than the single one, and the cost of the vfd is pretty reasonable.

    Comment


    • #3
      Harbor Freight has a converter for $199 I'd keep the original motor.

      Comment


      • #4
        Thanks for the input, guys.

        I did a little research on converters and it looks like the Harbor Freight one is a static type and statics aren't recommended for compressor use... Rotary ones start around $500 and head north quickly. Since I'm into this about $150 so far, that seems a bit steep when I can buy a new compressor for not much more than what I'd have into the whole kit with the converter.

        I read a little about making a converter, but it requires a second 3 phase motor of equivalent output to the one you want to power so the cost climbs quickly too.

        Since I have the 1 phase, 4 . SPL hp, motor already, I think I am going to try throwing a pulley that is half the diameter of the one on the original motor (to bring the rpms down to what the original put out) on it and see if it will work. The cost of the experiment will be nominal. I found a pulley at Ace that looks like it will work.

        Does anyone see any warning flags I'm missing? Thoughts before I go ahead?

        Thanks.

        Comment


        • #5
          Originally posted by Kyle
          Thanks for the input, guys.

          I did a little research on converters and it looks like the Harbor Freight one is a static type and statics aren't recommended for compressor use... Rotary ones start around $500 and head north quickly. Since I'm into this about $150 so far, that seems a bit steep when I can buy a new compressor for not much more than what I'd have into the whole kit with the converter.

          I read a little about making a converter, but it requires a second 3 phase motor of equivalent output to the one you want to power so the cost climbs quickly too.

          Since I have the 1 phase, 4 . SPL hp, motor already, I think I am going to try throwing a pulley that is half the diameter of the one on the original motor (to bring the rpms down to what the original put out) on it and see if it will work. The cost of the experiment will be nominal. I found a pulley at Ace that looks like it will work.

          Does anyone see any warning flags I'm missing? Thoughts before I go ahead?

          Thanks.

          Here's a 3 hp Rotary Phase Convertor on eBay for $344 with free shipping. VFD's are even less expensive. I'd keep the 3 phase motor if at all possible or at least get a motor that runs at 1,750 rpm.

          http://cgi.ebay.com/3-HP-ROTARY-PHAS...item56364a8fc1
          Last edited by gnm109; 11-17-2009, 10:02 PM.

          Comment


          • #6
            If you're trying to get away with using the motor you have, can you double up on the smaller pulley and run two belts? Of course you'd need two grooves on the large pulley-
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

            Comment


            • #7
              Using a RPC to run an air compressor is dumb. You lose the efficiency you gain from the 3ph motor when you have to use single phase to spin the RPC.

              A VFD is the only logical solution other than going to a single phase motor. IMO

              Getting a 5hp vfd shouldn't cost you more than 150 used on fleabay.

              Comment


              • #8
                With a half size motor pulley you should be good to go. There could be a problem with belt slippage or over-flexing if the pulley is too small for the belt size. Belt manufacturers generally specify minimum pulley sizes for their belts. Some belts are a lot tougher than others. Be sure you have the tension reasonably correct.
                Don Young

                Comment


                • #9
                  Bogus HP on that new motor..

                  See the Spl ? That stands for special HP rating (otherwise known as BS...)

                  15 amp Single phase 220 is probably really closer to 3 hp...

                  Sell it and put a VFD (which will do the 3 phase conversion) on...

                  Notice the 3 phase draws 11 amps for 3 hp it is a more efficient motor..
                  Last edited by Bguns; 11-17-2009, 10:16 PM.

                  Comment


                  • #10
                    Originally posted by Jim Shaper
                    Get a vfd to run the 3ph motor off single phase. That's how I'd do it anyway.

                    The 3ph motor is more efficient than the single one, and the cost of the vfd is pretty reasonable.
                    Can you give an example of a VFD that might suffice? They all seem very expensive to me, but I might be looking at the wrong thing..

                    Edit: Found 3hp http://dealerselectric.com/mfg-subca...cID=165&mID=-1 at a reasonable price compared to a new motor

                    Been looking for a reasonable one for 5hp.. but cant seem to find one...
                    Last edited by cuemaker; 11-17-2009, 10:20 PM.

                    Comment


                    • #11
                      Going bigger won't hurt anything, going too small might cause the drive to error on over-current protection. Generally speaking (unless the manufacturer states otherwise), you need double the rated vfd for single phase power input.

                      Check with the mfg for specs, but this one might be ideal:

                      http://cgi.ebay.com/FREQROL-A100-5-5...item1c0df48c32

                      Most listings will be more specific on input voltage.

                      Comment


                      • #12
                        would that be able to handle 220 1ph in and run a 5hp 3ph motor? If so show me more and I will buy...I have a very nice 5hp motor just sitting around that I want to use

                        Comment


                        • #13
                          I've got no stake in that listing - just pulled one up to show you they're out there cheap. You can do your own homework. The drive needs to be rated for the input voltage you'll be feeding it, then double the output you need it to produce.

                          Once you get a drive, they're childs play to get hooked up. Some can be harder than others to dial in, but none of them requires a ph d.

                          Comment


                          • #14
                            Thanks for all the input.

                            What I am going to try first, for the buy-in of less than $20, is putting the 1 Phase motor on and seeing what it will do. If it really is a 3hp motor(4. SPL), I would think it would work, since it is replacing a 3hp motor... I can get the pulley I need from Tractor Supply Co. and I have everything else here.

                            If the 1 phase isn't up to the job, I will look hard at converters, but I have a hard time with the idea that running 2 of three phases can be good or efficient for the old 3 phase motor. Because that is what a solid state converter does after it gets the motor going. The cost of a rotary converter would argue for scrapping the compressor and buying new... And, I question whether the losses of converting 1 phase to 3 phase really can give me an efficiency gain compared to just running a 1 phase motor. TANSTAAFL, after all...

                            I'll report back this week.

                            Comment


                            • #15
                              you need to calculate the speed the compressor pump runs on the existing motor. im guessing around 900 rpm. you probably cannot alter the pulley arrangment using the 3450 rpm motor to achieve the design rpm.

                              a new motor, 1750 rpm, at northern tool is about $250. the 1750 rpm is what makes it more expensive. . . . . but thats what you need. i am currently replacing one for a customer as we speak.

                              a phase converter, unless its a very expensive one, will not start a 3 phase motor as used on a compressor, possibly not even an expensive one would work. compressor motors are just that, compressor motors.

                              davidh

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