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Running big motors in home shops

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  • Running big motors in home shops

    Basically I've got 60amp 220v service to the shop in the garage and I'd like to know how big a motor I can run. I'm thinking of this to power a compressor, so a rotory phase converter makes sense, right (I think a static wouldn't be able to start unless I had some kind of clutch). So if I understand this correctly if a 3ph 5.0 horse 220v motor is draws 15amps that means it will draw 45amps on single phase 220v (assuming no other losses)? With all the NEC deratings and such how big an RPC can I run on 60amps and how big a final motor can I run off of that?

  • #2
    According to an Allen-Bradley booklet I have a 5hp 220v 3ph motor will draw full load 15.2 amp. A single phase 5hp 220v 3ph motor will draw at full load 28 amp.

    If you use a RPC with the three phase then you will have added current draw from the RPC depending on the draw of the size RPC you use. You will also have the RPC running even when the compressor is not.

    If you use a single phase motor on the comp you will not have a parasidic load running, just the compressor motor when it is needed.

    I think over time it will be cheaper to run a single phase motor for an air compressor.

    The only reason I use an RPC for my lathe and mill is they are 3ph and it would be expensive to change them to single phase if it's even possible. There is nothing to gain using a RPC and 3ph motors, it is more expensive to do so. Remember that an RPC is ghost 3 ph even with balanced legs and a static converter is still generating the third leg and is less efficient than single phase because of losses.
    Last edited by Carld; 05-26-2008, 10:37 AM.
    It's only ink and paper


    • #3
      A real 5 HP single phase motor draws 21 to 24 Amps (more if high service factor rated) but you should be able to start and run such a motor at full load on your 60 Amp service so long as heater or something doesn't kick in.

      Phase converters don't seem to have as drastic a start current as a similar sized single phase motor but three phase motors connected to them start briskly. Watch how you load your service or you will get some nuisance trips. You may get some nuisance trips anyway. Residential curcuit breakers have a reputation for more frequent nuisance trips after a few hundred motor starts. If your panel will accept them, move up to motor rated breakers.

      I submit running a three phase compressor or other fixed load equipment from an RPC is not a good idea because of bum efficiency and some times balked starts. I strongly suggest you re-power it with a single phase motor.
      Last edited by Forrest Addy; 05-26-2008, 10:40 AM.


      • #4
        Carl - that is what I just don't understand - when they say a 3ph 220v 5hp motor draws 15A that is 15A from each phase, right? and a 1ph 220v 5hp draws 28A that is 6kw which makes sense. There must be something going on with the phase angles as 15A * 220V * 3ph = 10kw and that doesn't sound right at all.

        Forest - This is powering a compressor that will hopefully fill scuba tanks. It won't be running very often and won't be running on/off like a shop compressor. I thought it would be cheaper to find some surplus 3ph motors than to find a 1ph 5hp-7hp motor. The compressor really should have a 10hp motor on it but I'm pretty sure it will work running half speed with a 5.

        I guess the next thing I need to do is visit some surplus shops and see what I can find for motors.


        • #5
          Only advice I would give is if you go for the 5HP single phase then also get some power factor correction fitted it may help with the bills!!
          I have tools I don't know how to use!!


          • #6
            Compressor in a small shop.

            I agree with Forrest Addy on this.

            Is it really necessary to have such a big motor? Why not use a smaller motor on a large receiver and make use of the non-demand time for the compressor to re-charge the receiver/s?. Or even have several receivers connected in "manifold" configuration?

            I have a single phase 230V 2HP motor driving my 10 bar (~ 145 psi) compressor (cuts in at 7 bar ~ 105 psi) which can handle my 40A plasma cutter - both of which will need to be running at the same time. The last thing you need if using a Plasma cutter is a power failure as the cutter head needs air to cool the cutter and the plasma power unit needs the fan to cool it - even after cutting has stopped.

            Also, if you are running a sizable MIG machine which also needs electrical power to cool the power unit - even after welding has ceased, the last thing you need is for a compressor to trip the board.

            A bigger compressor with a small receiver or storage capacity is not always better than a smaller compressor with larger receiver capacity.

            My compressor has an "un-load" circuit in it so that the motor starts under minimum instead of maximum load. The manual for it advises that the pressure switch on the compressor must be switched "off" when main power to it is switched "on". The pressure switch is then switched "on" to ensure minimum starting load/current.

            All of my machines are single-phase with induction motors with capacitor starts. These are more expensive to run than 3-phase but for the amount I use, converting to 3-phase is not an issue. In the rare event that I burn-out a capacitor start motor I either buy a new one or have the burnt-out one re-wound. It is how it is where I live in OZ as spares are no-where near as available or as cheap as they seem to be in many parts of the US.

            My MIG welder is wired for 50A on its own 60A circuit which is also used by my "stick" ("chokie") welder. As I have 2 60A phases the MIG welder uses 1 phase and the "Stick" welder is across 2 phases.


            • #7
              dockterj, scuba gear, ok, you may find it better to use the available 3ph comp. motor and use a static converter. I see now it is not a shop comp. and it would have helped if you had said that at the start. I have to go along with your way to do it. I would not change to a single phase motor. Is it fair to assume that there is no storage tank or at least a very small storage tank on the compressor?

              I am not an engineer and can't explain the full load amps but I am going to assume that is the sum of the three legs, not each leg. They would not put a full load rating and not say it's each leg other wise how could you compair each motor?
              It's only ink and paper


              • #8
                Originally posted by dockterj
                Carl - that is what I just don't understand - when they say a 3ph 220v 5hp motor draws 15A that is 15A from each phase, right? and a 1ph 220v 5hp draws 28A that is 6kw which makes sense. There must be something going on with the phase angles as 15A * 220V * 3ph = 10kw and that doesn't sound right at all.

                Yes, three phase can be tricky. It is done with vectors and vector products. A single phase motor draws only a single current with a single Voltage and the equations are straight forward. A three phase motor is drawing three currents but they are phased 120 degrees from each other and the same three lines also form the return path.

                So it is not just the sum of all three lines drawing maximun current. That never happens in any given fraction of a second. So if at a given instant, one line is drawing the full rated current, that current will return via the other two lines. And at that exact instant, each of the other two lines will only carry one half of that maximun current. At this instant, the total is not three times the rated current, but only two.

                As I said, it also involves vectors and vector products so the actual math is more complicated to account for all possible phase angles that the three lines will assume during a single cycle of current. To actually calculate it from scratch, you must use calculus. The short form is Power = Volts X Amps X 1.73 X Power Factor X Efficiency%. The 1.73 factor is the square root of three and this takes the three phase thing into account. So when the math is done, the factor is not 2 as my example above might suggest, but somewhat less. It would not be in the equation for power on a single phase system.

                But the manufacturer's ratings on the name plate should be accurate and you can use them.

                If you want more Google "three phase power calculation". Lots of explanations out there.
                Paul A.
                SE Texas

                Make it fit.
                You can't win and there IS a penalty for trying!


                • #9
                  I have a BIG 10hp single phase motor that drives a BIG four cylinder compressor and its in the same circuit as my welder, 50amp. I have yet to blow the breaker. Oh, but I didnt mean to say I run both at the same time, just what size circuit its on. I do run the plasma and a 7.5hp compressor off a 50amp circuit at the same time though and havent popped it yet either. My run from the panel to the outlet is about forty feet with #6 wire. JR
                  My old yahoo group. Bridgeport Mill Group



                  • #10

                    I think that you are not trying to fill a reservoir to 145 psi. You are trying to fill a 20 CF or so bottle to 2200 psi.

                    Getting to 2200 is harder than getting to 145. The compressor is more strongly built, the pistons will be smaller, and stronger. Bigger motor to run.

                    To oldtiffie, reservoirs to hold 2200 psi would equate an oxy tank. Many years ago, I had engineroom long accumulators on my ship. NOT HP. Ship's pressure, about 150psi. Still enough, with the capacity, to start the Diesels.

                    Regardless, I'd go with single phase.

                    You are trying to do something most of us have no experience with. Some of us didn't even realize what you are trying to do.

                    Fourth post you say it is to fill scuba tanks, 5 more still think it is shop air.
                    I don't think I would want a 2200 psi bomb near me, regardless it be a spherical reservoir or cylindrical. Oxy/Argon bottle are either indate or hydro'd when I refill.

                    I would rather a hose or tubing burst than that reservoir, in MY shop.

                    Good luck with what you decide on.




                    • #11
                      Pressure vessels.

                      Thanks George. I missed the scuba tank bit.

                      I think I'd want to know what the performance criteria was before I even thought about filling Scuba tanks.

                      I am ex-Navy and I recall that even in Shore establishments, filling Scuba gear for divers or fire-fighters had huge requirements as regards tests and cleanliness/purity of the air. It was easier to get it done by Contract.

                      In the gun-turrets we had large 4,000psi (yep, no "typo") receivers to re-charge the accumulators and intensifiers as well as the breech air-blast gear. When operating we had at least one diesel and/or steam turbine operating just to power the compressors which serviced the high and low pressure air ring mains.

                      I have no idea what the requirement is now for Scuba and similar filling equipment.

                      My LPG bottles as well as my argon, oxy and acet bottles are all in test (10 years) as is my compressor receiver. The gas supplier here owns the argon and oxy/acet bottles and keeps them in test. I ditch/dispose of my LPG bottles and compressed air receivers as soon as they get to their 10-year test period and buy new ones. The LGP fuel bottle in my car has just had its first 10-year re-test since it was fitted - and I have the certificate. All this is mandatory in OZ. Some choose not to comply. That's OK by me as that is on their heads - not mine.


                      • #12
                        I could be off but i got a quote for 700.00 bucks for a VFD (Variable frequency drive) that could power a 3hp motor. 21 amps. Single phase in 3 phase out. You can do a lot with VFD's. Install an unloader valve hooked up to the VFD so the motor starts unloaded then the valve closes. All timed by the VFD.

                        Good luck.


                        • #13
                          On a side note, have you worked out the filtration system for this setup?

                          Breathing air systems are serious business and pumping unfiltered wet air into storage tanks or scuba tanks will lead to equipment failures at best and dead divers at worst.

                          Originally posted by dockterj
                          Forest - This is powering a compressor that will hopefully fill scuba tanks. It won't be running very often and won't be running on/off like a shop compressor. I thought it would be cheaper to find some surplus 3ph motors than to find a 1ph 5hp-7hp motor. The compressor really should have a 10hp motor on it but I'm pretty sure it will work running half speed with a 5.

                          I guess the next thing I need to do is visit some surplus shops and see what I can find for motors.
                          Brett Jones...


                          • #14
                            Looking at the amps of a 3 phase is confusing when compared to single phase
                            A more simple way of looking at 3 phase loads is to use the KW rating
                            of the motor. It can be foiund on the name plate, and reflects the "Power" required to operate the motor.
                            I set up a three phase 440 volt system for a friend.
                            He also had 60 amp 220 service which is 13.2 KVA (KW)
                            I fed it into a 11KVA transformer to make 440 V
                            There, it went into a 10 KVA VFD (200 bucks surplus) to feed the 7KVA motor, which is 6 HP rated ( Foriegn built)
                            The real value of the VFD is soft start, with amperage limiting capabilites, which saves the 60 amp breaker from popping
                            Green Bay, WI


                            • #15
                              Carl-- I think you should go check that plate on your 3ph motor. My guess is that its missing a "1" (actually a 15HP motor)??

                              Here is a 5HP (Dayton) that draws 11.8 at 220ish. Granted at 87% efficiency, its a higher efficiency motor than some, but a more typical 80+% efficiency motor still has current specs not that much higher.


                              When current is specified for either a 220v single phase motor or a three phase motor, it is the current per "phase". That is (in the case of 220v single phase) a motor rated at say 9 amps is drawing 9 amps from each current carrying conductor....and the circuit breakers that protect this same circuit are designed to trip near their rated current on either side of neutral. If something shorted only one side to ground, both "sides" (relative to neutral) will be tripped. In fact, with all of the circuit breakers I have seen, they are actually just tied by a physical interconnection of the trip levers.

                              Forrest is correct about circuit breakers beginning to trip well below their rated current. In fact, those which have tripped a few times become more prone to trip early. They should be considered to need replacement when they get to where they trip to easily even though they may still function. Code suggests that calculations for wire (and circuit capacity) should be based on 80% of rated load for a given size of wire. Staying at or below this value will help with overly sensitive breakers...and reduce losses and corresponding low-voltage conditions.

                              The thing you *do* have to cosider in calculating load using three phase motors off an RPC is that the total load on the single phase circuit powering it will be full rated load of the running load motor plus the current used by the idler motor on each of the "provided" phases, plus one half of the current provided on the "generated" phase. One of several good reasons to use balancing caps between each "provided" phase and the third generated phase is to reduce (often substantially) the current that is drawn.

                              In effect, that third generated phase current is actually carried on the other two. This becomes important if you want to protect your idler motor on the input side because you cannot use the nameplate current to determine correct fusing or heaters. An inductive type ammeter from Harbor Freight is something like $10 on sale, and is a worthwhile investment in testing things.

                              Paul Carpenter
                              Mapleton, IL