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Soft Starter for single phase electric motor

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  • Soft Starter for single phase electric motor

    I have a Miller Blue StarDX which puts out 5.5kVa of 60Hz single phase power..I would like it to run an electric air compressor for intermittent jobs away from the shed...From what I have learnt the generator will not start an electric motor over about 1hp in size but can run an electric motor upwards of 5hp as starting current is about 8 times running current..I have tried to get it to run a 1 3/4hp compressor but it cannot start the motor

    I am wanting to try to get it to run a 3hp air compressor so was wondering if a soft starter would allow it to start a 3hp motor under load..

    Another thing to consider is that power here is 240V 50Hz so any motor I run off this generator will be running a bit faster than nameplate ratings as the generator will be putting out 60Hz..

    Any ideas?? Is it feasible??
    Precision takes time.

  • #2
    Ringer, the only way you're gonna get an induction motor like that started, is if you can bring it up to speed by another means.
    Something like a 12v sytem running a starter motor.
    On a cyclic duty, like a compressor, I don't like your chances.
    That means your starter system will have to kick the compressor motor into life every time the compressor cycles.
    You'd be better off with a petrol powered compressor.
    Just got my head together
    now my body's falling apart

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    • #3
      Just thinking out loud... makes me think about those high-end building contractor compressors that run the engine all of the time and freewheel the compressor after pressure is achieved. Doubt if you could buy/build that part of it for the cost of a small gas engine though.

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      • #4
        Ringer,
        I wonder if you could start your motor unloaded, then 'clutch' in your compressor using a jockey pulley arrangement to tighten the belts, maybe with an unloader on the compressor to help as well. You could unbelt the motor to test this theory, i.e. to see if the motor will start unloaded. This is only a wild idea, I am not confident it will help as the starting current may still be a problem regardless of load. Would be a dumb idea if the compressor doesn't have belt drive...

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        • #5
          OK, so soft starters cannot start anything that has a load on it from the start..Unlike say a lathe spindle which has no load at startup...
          Precision takes time.

          Comment


          • #6
            A magnetic coupling will do the job. A plain magnetic coupling provides an automatic soft start with delayed torque transfer. As it comes up to speed efficiency improves to about 95%. They are best for constant load applications such as pumps and compressors.

            You could also use an air conditioner clutch from a vehicle.
            Last edited by Evan; 06-16-2008, 09:09 AM.
            Free software for calculating bolt circles and similar: Click Here

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            • #7
              Any decent compressor should have an "unloader" on it, which allows the motor to start with no load.

              You can identify ones that do by the hiss after they stop as pressure in the delivery pipe is relieved.

              With an unloader, the motor should start OK, either with or possibly without a reduced voltage starter.
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #8
                You are missing the point.
                It takes a large current just to start a motor even if it is not loaded.
                The only reliable way is an IC motor with clutch, a la auto AC.
                Just got my head together
                now my body's falling apart

                Comment


                • #9
                  It takes a large current just to start a motor even if it is not loaded.
                  That varies a great deal depending on the type of motor used. The best motor for efficiency and low starting current is the capacitor start-capacitor run single phase motor. It takes less than 200% starting current, still develops good starting torque and is well suited to compressor duty.

                  Your average capacitor start-induction run motor takes about 500% current to start and a split phase motor, the kind with no capacitor but just a centrifugal switch for a start winding can take up to 1000% starting current.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    Yeah Evan, I've also "zip-started" an induction motor with a length of string, but I wouldn't wanna do it on a cyclic load
                    Just got my head together
                    now my body's falling apart

                    Comment


                    • #11
                      I made a soft-start for the 1HP capacitor-start motor on my lathe by installing a push-putton switch in parallel with the regular switch. Then I added about twenty-four feet of #18 wire in series with the PB switch. I hit the PB to soft-start the motor, and flip the regular switch on as soon as it's up to speed. It works really well.

                      I used eight feet of 18-3 extension cord with the wires connected together so it's acting like 24 feet. It'll get slightly warm if I leave the motor running on the PB switch for a minute or so, but there's no reason to do that. The soft-start circuit is energized for two seconds or less.

                      Roger
                      Last edited by winchman; 06-16-2008, 01:29 PM.
                      Any products mentioned in my posts have been endorsed by their manufacturer.

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                      • #12
                        Originally posted by Swarf&Sparks
                        You are missing the point.
                        It takes a large current just to start a motor even if it is not loaded.
                        The only reliable way is an IC motor with clutch, a la auto AC.
                        Nope.............

                        it does only if you either

                        a) have it loaded, inertially or actually

                        b) want it started in a big hurry

                        If you start at reduced voltage, one way or another, resistor or auto-transformer, it can take much less current. Little enough that the generator should easily handle it.

                        There is a reason why the power companies require reduced voltage starting for large motors.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          Originally posted by J Tiers
                          Nope.............



                          If you start at reduced voltage, one way or another, resistor or auto-transformer, it can take much less current. Little enough that the generator should easily handle it.

                          There is a reason why the power companies require reduced voltage starting for large motors.
                          I thought that if you reduce the voltage the amperage goes up to do the same work.
                          Steve

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                          • #14
                            Other means??

                            Why not leave a large-ish single-phase power-driven compressor in the shed and run a large (1") hose (to reduce pressure drop) a smaller remote compressor - preferably with a large receiver - or just a large receiver. Get a large receiver, charge it from the mains compressor in the shed and take the receiver to the job site. If your are driving nailing guns and the like - get the "airless" type that run off a fuel cell and need no air at all. If you are using an impact gun - get a 12v DC one and run it off your vehicle battery. Small AC stuff can be run from the vehicle battery via an inverter.

                            Other-wise why not buy an old "Lincoln" trailer-mounted generator that puts out a sizable AC supply in addition to the welding DC supply?

                            I have a close-coupled 2 HP compressor with an unloading valve and it works fine. That compressor runs at 10 bar (145psi). Evan is correct as that compressor will not start if the receiver pressure is below the "switch on" setting on the controller (7 bar - 115psi) as it just baulks at it. I have to switch the mains supply off and the regulator off and then the mains "on" and then the compressor switch "on" and all is fine - providing the "unloader valve" is working. I've never had a problem with it.

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                            • #15
                              Why not try a hard start kit? These are popular in the RV crowd where they have large AC systems running on a limited generator system. The hard start kit is essentially a large capacitor to provide some start torque and a relay that will disable this cap after the initial kick. Take a look here and see if this may work for you:

                              http://www.grainger.com/Grainger/wwg...+Start+Kits%2C

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