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Service size for 2 - 16 hp motors and 1 - 10 hp motor

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    Service size for 2 - 16 hp motors and 1 - 10 hp motor

    I have 2 large centrifugal fans that are powered by 16 Hp single phase 230 volt motors. I also have a 10 Hp single phase 230 volt motor running another centrifugal fan. They will all run at the same time however, they will be started separately and allowed to reach full speed before the next one is started. See the nameplates that I have attached.

    This is in a rural setting. I have not ever worked with this large size of single phase motors. Could somebody help me out with the size of service I will need to start and run these motors. Will I have to use soft starts? Please leave your comments and thoughts.
    Attached Files

    #2
    That's one odd motor. 16HP is not an industry standard, the 1705 RPM also means it's something out of the ordinary, i.e. a high slip motor, then it's a Code D, meaning 3.3 kVA/HP Locked Rotor (starting), which is low. My guess is that it might be a de-rated 3 phase motor with a capacitor assist to get it spinning. The FLA rating puts it at about 25HP 3 phase, so after de-rating by 1/3 for single phase operation, that makes it fit at around 16HP, and the starting kVA is probably low because of the capacitor assist.

    Be that as it may, you are going to need approx. 53kVA to start that one and around 59kVA to start the 10HP (Code G = 5.9 kVA/HP x 10HP). So if the 10HP can start first (your worst case scenario), then you need 59kVA + about 63 kVA to run the 16HP, so a 125kVA service, which is a std xfmr size, 504A FLC.

    Soft starters don't work well for capacitor start single phase motors, it's generally a race to failure between either the soft starter SCRs or the motor caps.

    Being that they are centrifugal fans, I might consider using 3 phase motors and VFDs fed by single phase. You can eliminate the starting kVA altogether and the owner can probably save energy if the fans can be slowed down instead of using dampers. But if you are just being asked to install what they already bought, that's a moot point.
    Last edited by Jraef; 09-11-19, 11:33 PM.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

    I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

    Comment


      #3
      Originally posted by Jraef View Post
      That's one odd motor. 16HP is not an industry standard, the 1705 RPM also means it's something out of the ordinary, i.e. a high slip motor, then it's a Code D, meaning 3.3 kVA/HP Locked Rotor (starting), which is low. My guess is that it might be a de-rated 3 phase motor with a capacitor assist to get it spinning. The FLA rating puts it at about 25HP 3 phase, so after de-rating by 1/3 for single phase operation, that makes it fit at around 16HP, and the starting kVA is probably low because of the capacitor assist.

      Be that as it may, you are going to need approx. 53kVA to start that one and around 59kVA to start the 10HP (Code G = 5.9 kVA/HP x 10HP). So if the 10HP can start first (your worst case scenario), then you need 59kVA + about 63 kVA to run the 16HP, so a 125kVA service, which is a std xfmr size, 504A FLC.

      Soft starters don't work well for capacitor start single phase motors, it's generally a race to failure between either the soft starter SCRs or the motor caps.

      Being that they are centrifugal fans, I might consider using 3 phase motors and VFDs fed by single phase. You can eliminate the starting kVA altogether and the owner can probably save energy if the fans can be slowed down instead of using dampers. But if you are just being asked to install what they already bought, that's a moot point.
      So did you include 2 -16 hp motors in your calculation or was that 125 KVA xmfr size just for 1 - 16 hp and 1 - 10 hp?
      All motors may run at the same time.

      Sent from my SM-G930V using Tapatalk

      Comment


        #4
        Originally posted by strawburner View Post
        So did you include 2 -16 hp motors in your calculation or was that 125 KVA xmfr size just for 1 - 16 hp and 1 - 10 hp?
        All motors may run at the same time.

        Sent from my SM-G930V using Tapatalk
        No, I missed that you have 2 of the 16HP, sorry. But still, the 10HP is the worst case starting scenario, so just add another 63kVA for the 2nd 16HP. Brings you to 185kVA, so a 200kVA xfmr. You might be able to get away with a smaller transformer like a 167.5kVA if you can take a little voltage drop when starting the subsequent motors. But now you are getting into the realm of doing a TMS (Transient Motor Starting) analysis with something like ETAP or SKM Power Tools.
        __________________________________________________ ____________________________
        Many people are shocked when they discover I am not a good electrician...

        I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

        Comment


          #5
          The POCO will decide on the transformers, but on your side, expect long, hard starts. Max size the breakers or fuses. Don’t skimp on the wire size right up to the motor disconnect or starter. Pre order start capacitors now. Charge them out and have on the job site for the expected repair.

          Tom
          TBLO

          Comment


            #6
            These motors will only run intermittently for about 20 minutes at a time. They will blow air through a big propane-fired burner to disinfect truck livestock trailers. I realize the runtime is a moot point it's the startup we are worried about.
            Thanks for the responses so far!

            Sent from my SM-G930V using Tapatalk

            Comment


              #7
              I would contact the utility first and see if they are going to be able to supply you with a stiff enough service to handle a bunch of hard starting single phase motors.

              Over here, anything bigger than 10HP is 3 phase, as 3 phase services are readily available.

              Comment


                #8
                Originally posted by Jraef View Post
                That's one odd motor. 16HP is not an industry standard, the 1705 RPM also means it's something out of the ordinary, i.e. a high slip motor, then it's a Code D, meaning 3.3 kVA/HP Locked Rotor (starting), which is low. My guess is that it might be a de-rated 3 phase motor with a capacitor assist to get it spinning. The FLA rating puts it at about 25HP 3 phase, so after de-rating by 1/3 for single phase operation, that makes it fit at around 16HP, and the starting kVA is probably low because of the capacitor assist.

                Be that as it may, you are going to need approx. 53kVA to start that one and around 59kVA to start the 10HP (Code G = 5.9 kVA/HP x 10HP). So if the 10HP can start first (your worst case scenario), then you need 59kVA + about 63 kVA to run the 16HP, so a 125kVA service, which is a std xfmr size, 504A FLC.

                Soft starters don't work well for capacitor start single phase motors, it's generally a race to failure between either the soft starter SCRs or the motor caps.

                Being that they are centrifugal fans, I might consider using 3 phase motors and VFDs fed by single phase. You can eliminate the starting kVA altogether and the owner can probably save energy if the fans can be slowed down instead of using dampers. But if you are just being asked to install what they already bought, that's a moot point.
                That motor is not unusual at all to me, they are used on crop storage drying/ventilation quite often, especially in places where there isn't three phase available.

                POCO here likely supplies service with the three motors (and no other significant load that will run at same time) mentioned with 50 kVA transformer.

                Soft start would makde matters even worse with this motor as it usually has the centrifugal switch drive a small DP contactor with 230 volt coil to switch the start capacitors instead of directly switching them. Low volts from soft start may never allow enough voltage to pull that contactor in to close the capacitors into the circuit during starting.
                I live for today, I'm just a day behind.

                Comment


                  #9
                  Originally posted by strawburner View Post
                  These motors will only run intermittently for about 20 minutes at a time. They will blow air through a big propane-fired burner to disinfect truck livestock trailers. I realize the runtime is a moot point it's the startup we are worried about.
                  Thanks for the responses so far!

                  Sent from my SM-G930V using Tapatalk
                  How long will they rest between cycles? Too frequent starting on regular basis may not be good for this motor, don't know for certain but something to consider.
                  I live for today, I'm just a day behind.

                  Comment


                    #10
                    Originally posted by kwired View Post

                    That motor is not unusual at all to me, they are used on crop storage drying/ventilation quite often, especially in places where there isn't three phase available.

                    POCO here likely supplies service with the three motors (and no other significant load that will run at same time) mentioned with 50 kVA transformer.

                    Soft start would makde matters even worse with this motor as it usually has the centrifugal switch drive a small DP contactor with 230 volt coil to switch the start capacitors instead of directly switching them. Low volts from soft start may never allow enough voltage to pull that contactor in to close the capacitors into the circuit during starting.
                    Yes these are basically grain dryer fans made by GSI.
                    In all due respect to the previous posts about the higher KVA Transformers I was wondering if something like this in a rural setting with three phase not being readily available if it would work
                    It's a little unfortunate that the customer purchases equipment without showing the electrician the electrical requirements first. LOL!

                    Sent from my SM-G930V using Tapatalk

                    Comment


                      #11
                      Originally posted by kwired View Post

                      How long will they rest between cycles? Too frequent starting on regular basis may not be good for this motor, don't know for certain but something to consider.
                      I think the motors will rest at least an hour between Cycles and maybe more.

                      Sent from my SM-G930V using Tapatalk

                      Comment


                        #12
                        Originally posted by kwired View Post
                        ... POCO here likely supplies service with the three motors (and no other significant load that will run at same time) mentioned with 50 kVA transformer. ...
                        Huh. Is that because, with nothing else on the service drop, the VD from starting them is acceptable?
                        __________________________________________________ ____________________________
                        Many people are shocked when they discover I am not a good electrician...

                        I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

                        Comment


                          #13
                          Originally posted by Jraef View Post
                          Huh. Is that because, with nothing else on the service drop, the VD from starting them is acceptable?
                          VD is horrible on most single phase supplied grain bin sites I have ever been to during starting of larger fans. You not only have the high current issue, but they are driving large enough blower wheels that it takes a little time to accelerate also. Start capacitor replacement seems to be a higher maintenance issue on such applications compared to many other applications.

                          And yes often there is no other loads on the service that are effected much. Rural voltages typically run near 250 unloaded, that helps a little compared to being on the bottom end of what is acceptable.

                          I have a site about a mile from where I live and IIRC there are four of these fan motors plus the original bin had a single 10 HP motor on it. I know we had to rework the service when we added the last bin and I placed a 400 amp service disconnect on that site, with spare conduits for future service disconnects because if they add just one more bin like the others 400 amps isn't enough. Pretty sure the padmount that got installed then is a 75. Before the last bin was added it was served by pole top transformer and a 200 amp meter main on the POCO pole. At that point there was pretty much OP has, two 16 hp motors and a 10 hp. There was other unloading augers and such, but those typically don't run when fans are running (on same bin), and I doubt it was more than a 50 kVA transformer then, could even have been a 37.5.
                          I live for today, I'm just a day behind.

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                            #14
                            Yeah, I almost posted a 200 amp service until Jraef posted a 200kva trany--- 200000/240 = 833 amps for 3 motors that draw 62, 62 and 40 amps... Seems crazy
                            They say I shot a man named Gray and took his wife to Italy
                            She inherited a million bucks and when she died it came to me
                            I can't help it if I'm lucky

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                              #15
                              Originally posted by Dennis Alwon View Post
                              Yeah, I almost posted a 200 amp service until Jraef posted a 200kva trany--- 200000/240 = 833 amps for 3 motors that draw 62, 62 and 40 amps... Seems crazy
                              200 amp main breaker will hold during starting - as long as you only start one at a time. His large transformer was to lessen voltage drop at the source during starting, probably to levels I never see for such applications.
                              I live for today, I'm just a day behind.

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