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  • Switch for 3 phase motor

    I have a switch for a 3phase motor .5 kw. This switch puzzles me because on of the phases actually is passed through the switch directly to the motor. The switch has the other two phases connected and it allows me to stop the motor in the middle position and turn the switch either direction for CW or CCW turning of the motor. Does that seem correct that one phase is always going to the motor. There is no sound or buzz/hum coming from the motor when it is off but will something burn up in the motor if I use this switch?
    Location: The Black Forest in Germany

    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  • #2
    Yes, it will work. Is it safe? Only if the person servicing it turns off the main power breaker or fuse box that kills all three legs. I am assuming the motor is internally protected in some way because otherwise you have no overload or overcurrent protection, especially on the leg that is "hot" all the time. Except for the 3 Phase branch circuit or motor circuit breaker.
    To reverse a 3 phase motor only need to swap any 2 phase leads around, and your rotary switch does that.

    Switching only 2 of 3 legs is popular on HVAC applications. Or only one of 2 hot legs for single phase 230 volt compressors.
    Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

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    • #3
      It's permitted now to control a motor as you see it. Having one phase always connected does no harm. It's just a cheap way to make a motor control. This is common on HVAC condensing units that run on 230V-single phase and 3-phase. They commonly have a single pole or 2-pole contactor. The sole advantage is that a ground fault on the passed thru phase will trip the overcurrent protection whether the motor is running or not. I don't know when UL and NEMA decided it was OK to do this. I'm sure there are some regulations on how the practice is applied.

      RWO

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      • #4
        Switch is meant to reverse the motor, AFTER, a motor contactor turns power on/off
        Green Bay, WI

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        • #5
          It also makes things cheaper when using a solid state control like a soft start. No need to switch three phases when killing two is just as good.

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          • #6
            Originally posted by Rich Carlstedt View Post
            Switch is meant to reverse the motor, AFTER, a motor contactor turns power on/off
            No, you can plug reverse, thats one of the huge benefits of three phase.

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            • #7
              Originally posted by macona View Post
              No, you can plug reverse, thats one of the huge benefits of three phase.
              True, but that doesn't mean you leave a machine with a 240 Volt hot motor lead!
              Illegal to do so.

              rich
              Green Bay, WI

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              • #8
                Originally posted by Rich Carlstedt View Post
                True, but that doesn't mean you leave a machine with a 240 Volt hot motor lead!
                Illegal to do so.
                rich
                Quote me a section of the NEC. Its done a lot on HVAC equipment.
                Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

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                • #9
                  Sheesh.....

                  We got safety nannies everywhere! Misinformed ones....

                  You do NOT HAVE TO BREAK ALL HOT WIRES to control a motor. Not even under EC rules, and B-F is in Germany, where they are under EC rules, NOT UL.

                  Now, under UL/NEC rules, you DO have to have some sort of "disconnect" which DOES totally disconnect all power wires and allows work to be done on the machine. This can be a switch of a specified type (lockable) OR a plug and socket.... I assume the EC rules have a similar requirement.

                  Obviously if the machine is unplugged, it is not powered and is safe to work on. The equivalent of a lockable disconnect is a lockable cover for the plug that prevents it being plugged-in.

                  But a standard switch cannot be a disconnect, since it isn't lockable. So do not try to force a "switch" to work like a disconnect, there is no need.

                  Under EC rules, the symbol for the "on" and "off" cannot be the "I" and "O" unless all power wires are disconnected by the switch. But for regular switches all that is required is that they turn the item on and off...

                  What B-F has sounds like a standard "drum switch" of the type used for on/off/ reverse on single phase and 3 phase equipment.
                  Last edited by J Tiers; 01-14-2014, 08:47 PM.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #10
                    Three phase is a four wire circuit. What you have sounds like a fwd-off-rev switch for a DC motor. It will reverse a three phase motor if one hot leg is wired through, but that leaves the motor hot when off. I know of no code that allows that.

                    As to posts stating otherwise. 220 single phase in the US, is 120-0-120 and requires a 2 pole switch or contactor with a through wired neutral. Reversing the two hot legs will not reverse the motor. To do that the start windings have to be reversed.

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                    • #11
                      Three phase is EITHER 3 or 4 wire circuits...

                      The switch is a typical fwd-off-rev drum switch.

                      The confusion is motor control with machine isolation - they do not have to be the same.

                      Jerry's post #9 spells out the code/issues.
                      Last edited by lakeside53; 01-14-2014, 09:50 PM.

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                      • #12
                        Originally posted by wmgeorge View Post
                        Quote me a section of the NEC. Its done a lot on HVAC equipment.
                        430-102. (b)
                        which says that a disconnecting means must be provided and shall be located in sight of the motor.
                        Doesn't matter what HVAC does, we are talking machine shop machinery, and safe practice
                        "Disconnecting" does not mean leaving a hot connection
                        Rich
                        Last edited by Rich Carlstedt; 01-14-2014, 11:27 PM. Reason: spelling
                        Green Bay, WI

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                        • #13
                          Originally posted by Rich Carlstedt View Post
                          430-102. (b)
                          which says that a disconnecting means must be provided and shall be located in sight of the motor.
                          Doesn't matter what HVAC does, we are talking machine shop machinery, and safe practice
                          "Disconnecting" does not mean leaving a hot connection
                          Rich
                          A service disconnect and an operator switch are two different things. What happens inside a machine is separate from the service to the machine.

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                          • #14
                            Originally posted by macona View Post
                            A service disconnect and an operator switch are two different things. What happens inside a machine is separate from the service to the machine.
                            +10

                            This is a HUGE area of mis-understandings.....

                            IF IT IS NOT LOCKABLE, IT AIN'T A DISCONNECT. and so it doesn't have to follow those rules..... but you DO need a LOCKABLE disconnect somewhere..... so you do not have to wire up the machine "hot", and so you can KNOW the machine will not be started up while you have your head and hands inside.
                            1601

                            Keep eye on ball.
                            Hashim Khan

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                            • #15
                              Rich I am going to stir this one time only...

                              You are both right and wrong. As stated by J Tiers and Lakeside, A disconecting "DEVICE", either a switch, plug and socket or other approved device, is REQUIRED as a DISCONECTING MEANS. The switch BF has is a CONTROLING device and therefore does NOT have to disconect all 3 phases to "Control" the motor or machine. The NEC and the EI and CSA all have very specific rules about electrical equipment and the way it is installed. A disconect is not nessisarily a control device nor is a control device nessisarily a disconecting device. An example refering to J Tiers post... If you unplug your machine and lock a cover over the plug you have met the requirement for a disconecting means. Npw would you consider pluging and unpluging your lathe every time you needed to start or stop it, of course not, you would use the "Control" switch. Anyway Rant off, the important thing is to be AWARE of your equipment and be safe.
                              Robin

                              Happily working on my second million Gave up on the first

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