Newbie E-Stop and Safety Relay Questions


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    Member Project_Hopeless's Avatar
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    Question E-Stop and Safety Relay Questions

    My DIY build will use a simple AC router. So on the AC side I'll have an e-stop button controlling a relay connected to the router.

    So on the stepper drive side do you interrupt the ac to the PSU or the DC supply side, or both?

    Is a single pole enough for the router AC side or should you use a 2-pole?

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    Community Moderator ger21's Avatar
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    Default Re: E-Stop and Safety Relay Questions

    If it's a 120V router, then you just need to cut power to the hot side.

    Never cut DC power, you'll fry the drive.

    Thgere's really no point in killing power to the stepper drives, as steppers will stop as soon as the step pulses stop, which is much sooner than the capacitors in the power supply drain.

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    Default Re: E-Stop and Safety Relay Questions

    I would cut AC power to anything that moves. Not, just the router but the stepper drive also. I am also assuming that you are using the E-Stop switch to send a signal to the motion control board too.

    You can never assume that things will stop moving because you sent them a signal. Cut their power as well.

    As far as the disconnect relay, it should be one intended for the disconnect purpose. Also, please correct me if I am wrong someone, a 20 Amp 3 phase contactor is not rated for 20 Amps on each leg, it is 20 Amps total.

    Page 3.
    https://literature.rockwellautomatio...d556_-en-p.pdf

    Last edited by maxspongebob; 04-07-2019 at 10:55 PM.


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    Default Re: E-Stop and Safety Relay Questions

    If you read further contact ratings are per pole! However don’t assume that all poles are the same they can have different ratings though you don’t see that much on safety relays. It is always a good idea to reference documentation whenever possible, If not documentation the devices labeling.

    As a side note it is a mistake to believe safety relays make a machine more reliable. At work they are the most commonly replace item in the control cabinets. They MIGHT make for a safer machine but even here there are always issues that require careful consideration.

    Quote Originally Posted by maxspongebob View Post
    I would cut AC power to anything that moves. Not, just the router but the stepper drive also. I am also assuming that you are using the E-Stop switch to send a signal to the motion control board too.
    Yes cut AC power but also drive enables.
    You can never assume that things will stop moving because you sent them a signal. Cut their power as well.

    As far as the disconnect relay, it should be one intended for the disconnect purpose. Also, please correct me if I am wrong someone, a 20 Amp 3 phase contactor is not rated for 20 Amps on each leg, it is 20 Amps total.
    It is always possible that I’ve missed a relay where this is true but all that I’ve worked with are rated on a per contact basis. What you have to be careful about here is that a relay (especially 3 phase power relays) will have three sets of contacts rated for power and one or more rated for “pilot” duty. The safety relays Ive worked with generally have contacts all rated the same per pole. Heavy current switching is handled by external relays. That being said I would not be surprised at all to find safety relays with high power contacts.
    I’m not sure what you are referencing on page 3. Nothing about contact capacity is mentioned there.



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    Default Re: E-Stop and Safety Relay Questions

    A contractor is rated per amps per contact, so a 20 amp 3 phase contractor is rated the same. Never ever break the output legs of a VFD going to a 3 phase motor or you will fry the VFD/ Inverter. Best safety switch on a machine is the guy/gal running it.

    Retired Master Electrician, HVAC/R Commercial. FLA Saturn 2 4x4 CNC Router Mach4


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    Default Re: E-Stop and Safety Relay Questions

    Quote Originally Posted by Project_Hopeless View Post
    My DIY build will use a simple AC router. So on the AC side I'll have an e-stop button controlling a relay connected to the router.

    So on the stepper drive side do you interrupt the ac to the PSU or the DC supply side, or both?

    Is a single pole enough for the router AC side or should you use a 2-pole?
    If you are using 120v then you only need a single pole if you at using 240v then you need a 2 pole

    Mactec54


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    Community Moderator Al_The_Man's Avatar
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    Default Re: E-Stop and Safety Relay Questions

    Quote Originally Posted by wmgeorge View Post
    Never ever break the output legs of a VFD going to a 3 phase motor or you will fry the VFD/ Inverter. Best safety switch on a machine is the guy/gal running it.
    It will if output power is present, I have actually done it with two contactors on the output for two motors that operated alternately, but used the At-Zero-Speed output to interlock the two contactors to ensure they did not switch when output was present.
    Al.

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