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  • E-Stop switch?

    Hey guys! I have a "twist- lock type E-stop switch that I was going to use before my VFD. There'll be 240V running into the switch. There is only one pole in this switch. I just looked in the catalog link that Willy provided and the e-stop switches they show all seem to have only one pole as well. So do you only disconnect the power to one hot wire for these in a 240 volt deal? I'm thinking no here...
    Thanks!
    Russ
    I have tools I don't even know I own...

  • #2
    The e-stop should be controlling a relay that cuts main power when the e-stop switch is activated (pushed in).
    Ignorance is curable through education.

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    • #3
      Originally posted by Rusty Marlin
      The e-stop should be controlling a relay that cuts main power when the e-stop switch is activated (pushed in).
      Sheesh! Sounds like another box. Dang! I just made up a cool plate and mount for the VFD and E-stop boxes. Now I'll have to change some stuff...unless the relay will fit in the E-stop box as well.
      Thanks!
      I'll check with the local electrical supply rapists and see if they'll take my first born as a downpayment on a relay
      Russ
      I have tools I don't even know I own...

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      • #4
        What Rusty said. The relay, also called a motor starter, should interrupt the 1 or 3-phase power to the VFD. Ideally the circuit should include a start button wired in series with the e-stop then to the coil of the relay. An extra contact on the relay is wired in parallel with only the start button to hold or "seal" the circuit when the start button is released. Push the e-stop to open the circuit to the relay coil, allowing the relay to open the seal contact.

        The twist-lock feature helps to prevent inadvertent starting. The start button should have a flush or guarded or recessed head so you can't bump it and start the machine.

        About the relay:
        The simplest circuit uses the line voltage in the control circuit, with the coil voltage being the same as that supplied to the VFD. Just connect one leg to the start switch and the other leg to one of the coil terminals, with the above described circuit in between.
        However, this circuit has certain drawbacks if all the circuitry is not contained in one enclosure. If external wiring is not protected a cut wire may present you with the line voltage and a lethal shock. Also, this circuit may violate the electrical code in certain localities.

        A better option would be to add a small transformer or DC power supply, powered by the line voltage to power the control circuit and select the motor starter relay coil voltage to suit.

        This has the added safety advantage that if power fails while the machine is running, the start button has to be pushed to restart rather than starting in its own if you forget to turn it off.

        Also, things like guard switches can be added in series with the e-stop for other safety measures.
        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
        ~Practitioner of the Electromechanical Arts~

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        • #5
          What Rusty Marlin said,most manual switches aren't designed to handle the current that you will be controlling.Should not be too expensive ,but like you say another PITA.
          Home, down in the valley behind the Red Angus
          Bad Decisions Make Good Stories​

          Location: British Columbia

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          • #6
            Torker, your VFD will have a low voltage input that can be used with the E-stop. Check the manual on which input it is and there should be a schematic on how to wire and program it. Usually the way it works is the VFD requires voltage into this input before it will start running and if it looses voltage when the switch is tripped, it will shut the drive down. It is very dangerous to use the e-stop on the leads to the motor, it is meant to be used only with control voltage and will most likely fail fairly quickly if wired inline.

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            • #7
              Originally posted by mbensema
              Torker, your VFD will have a low voltage input that can be used with the E-stop. Check the manual on which input it is and there should be a schematic on how to wire and program it. Usually the way it works is the VFD requires voltage into this input before it will start running and if it looses voltage when the switch is tripped, it will shut the drive down. It is very dangerous to use the e-stop on the leads to the motor, it is meant to be used only with control voltage and will most likely fail fairly quickly if wired inline.
              Be aware that in the (unlikely) event that the VFD has an electronic control malfunction the e-stop may not be effective.
              Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
              ~Practitioner of the Electromechanical Arts~

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              • #8
                I dont think you want to have an emergency stop switch on the 220 single phase line feeding the VFD .

                A disconnect to isolate the VFD yes,asuming it is used when the vfd is not running the motor ie motor stopped , vfd on stand by

                an emergency stop should be wired in to the vfds aux strip

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                • #9
                  The Estop on my VFD ramps down the VFD output at the same rate as simply shutting off the VFD. It is not instantaneous, nor can it be without hazard to the VFD. The VFD power supply must be drained down before the VFD shuts off power to the motor. The VFD must deal both with its own power supply and the power dumped into it by the motor on shut down. Doing this requires time, usually at least 0.5sec to several seconds depending on how the VFD is programmed, the dump resistor(s) if any extrinsic to the VFD and the load on the VFD. Think of it like an Estop on a table saw.
                  Steve

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                  • #10
                    Legally an e-stop must interrupt the main power input.
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                    • #11
                      Originally posted by Evan
                      Legally an e-stop must interrupt the main power input.

                      On CNC machines the e-stop will usually only power down the axis drives.

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                      • #12
                        Originally posted by Evan
                        Legally an e-stop must interrupt the main power input.
                        Correct, in principle. However, I have seen a trend in automated equipment toward interrupting only the motion producing circuits. This from both US and Canadian machine tool builders. The various limit switches, photo sensors pushbuttons, etc. all retain control power while in the event of an e-stop, the motors, valves, heaters or anything else that could cause motion or injury are powered down. This is different from what I learned years ago in designing and building automation, and makes it doubly important to shut down and lockout the main disconnect or excersize extreme caution when doing electrical troubleshooting.
                        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                        ~Practitioner of the Electromechanical Arts~

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                        • #13
                          The contacts in an e-stop push button are not designed to handle the mains power to a VFD. It should control a properly rated contactor supplying the drive.
                          As Wes said, common practice is to remove the mains supply for the DC bus or other motion producing power sources such as pnuematics, while leaving the control power on the controller and sensors. As an additional level of safety, new servos are incorporating reduntant internal safety contacts that disconnect the IGBT gate inputs. This allows higher SIL level compliance that is being pushed by various governmental agencies.
                          Greg

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                          • #14
                            Originally posted by thistle
                            I dont think you want to have an emergency stop switch on the 220 single phase line feeding the VFD.

                            an emergency stop should be wired in to the vfds aux strip
                            VFD's have a specific low-voltage E-Stop input. Read the Fine Manual -- they show how to wire the E-Stop.

                            Also, make sure you use shielded signal wire, with the shield grounded to the VFD's ground.
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                            • #15
                              It's ok to leave low voltage logic powered up. Low voltage is anything below 40 to 50 volts depending on local codes. All of the machines I have worked on, and they are numerous, have a main power relay that disconnects all input power to the drives. Solid state relays are not acceptable, they must be mechanical contactors. For smaller single phase equipment it is acceptable for the interlock or e-stop to switch the main power. For anything over 12.5 amps a relay must be used.

                              The e-stop or interlock must be fast acting, without time delays.

                              The electrical codes are extremely fussy about this sort of thing. It doesn't make a real difference in a home shop (other than your own safety) but Russ has at least one employee and should be adhering to the code.
                              Last edited by Evan; 01-15-2007, 02:41 PM.
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