Do I need a magnetic contactor with a VFD?

Discussion in 'General Electronics Chat' started by RimfireJim, Jul 23, 2015.

  1. RimfireJim

    Thread Starter Member

    Apr 7, 2008
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    I'm redesigning the control system for a 2 hp 208VAC 3 ph motor that uses a VFD for braking. The only load on the motor during braking is a saw blade. The VFD (Yaskawa V-1000) has inputs for "Run" and "Stop" through N.O. and N.C. momentary switches, respectively. I have verified that if there is a loss of line power while the motor is running that it (the motor) will not automatically restart upon restoration of power - the "Run" switch must be pressed. This behavior is the same as that of a simple control using a magnetic contactor (MC) or motor starter w/o a VFD. Upstream of the VFD will be a molded case circuit breaker (MCCB) with an enclosure door interlock.

    The Yaskawa manual states on one page, "Use an MC to ensure that line power to the drive can be completely shut off when necessary. The MC should be wired so that it opens when the drive fault output is triggered." In another section it states, "The drive should be shut off in the case of a fault in external equipment such as braking resistors through use of an MC." and "NOTICE: Install an MC on the input side of the drive when the drive should not automatically restart after power loss. . . . Use the drive to stop and start the motor."

    I can completely shut off power to the VFD with the MCCB. I'm OK with the drive restarting after a power loss, because the motor won't restart automatically. The drive is used to stop and start the motor. I don't have any external equipment such as braking resistors. So, is there any reason I need an MC between the MCCB and the VFD? And, if I put one there and wire it so that it opens when the drive fault output is triggered, how would I reset the fault with the VFD powered down?

    I've attached a diagram as it stands without an MC. The SMS is a back-EMF stop motion sensor for the safety interlock.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Normally a contactor is advised per NEC/CEC and IEC, this is usually picked up when the machine is brought out of E-stop.
    Specifically, motive power should be removed in the event of a emergency.
    I have rarely if ever used the VFD fault output.
    In the case of a saw with a controlled stop, it may be detrimental to do it this way as the braking will not be in effect and the blade will free wheel down.
    Is this used in an industrial setting or personal use?
    Max.
     
  3. RimfireJim

    Thread Starter Member

    Apr 7, 2008
    22
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    This is in an industrial setting. It is a saw used to cut golf club shafts. A human operator feeds the shafts into the saw by loading them onto a carriage and manually pushing the carriage into the saw - much like a tablesaw in a cabinet shop. There is no other moving or energized equipment on the machine. It has mushroom head N.C. momentary switch for a STOP buttom; I had not planned to include an E-stop to remove energy, because the saw comes to a much quicker stop with the VFD as a brake than it would without power and there is no other energy in the system.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I believe there is a clause against removing motive power immediately on E-stop where a controlled stop is preferable to prevent Machine or personnel harm or damage in the event of removal of all power.
    Now this is done with the likes of Safety Relays that come in all kind of configurations, and I believe are mandatory in Europe, and slowly being introduced and made mandatory in N.A.
    Most industrial machines coming in from Europe are fitted with them.
    Max.
     
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  5. RimfireJim

    Thread Starter Member

    Apr 7, 2008
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    That seems like a good idea. I'll have to see if there is a way I can use the VFD to stop the motor as rapidly as possible with an E-stop setup and use our normal deceleration setting with the STOP button. I'll see if I can get some help from Yaskawa.
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    There most definitely is such a clause. In my opinion it is almost always a bad idea to remove power from a VFD. I rarely install a contactor upstream for that reason. How often is a uncontrolled freewheeling load the better than a controlled deceleration? Almost never.
     
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  7. MaxHeadRoom

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    Jul 18, 2013
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    Last edited: Jul 24, 2015
  8. RimfireJim

    Thread Starter Member

    Apr 7, 2008
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    Thanks for that link. After your and strantor's earlier replies, I've been poring over Chapter 9 of NFPA 79. It's clear to me that having an uncontrolled stop (Category 0) would be worse than having a controlled stop with or without removing power after the stop (Cat 1 or Cat 2). I think the real question becomes, "Do I need an E-stop?" Looks like I need to do that "pesky" risk assessment thing.
     
    DougNix likes this.
  9. DougNix

    New Member

    Jan 12, 2017
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    If the VFD manufacturer tells you that a contactor is required upstream of the drive, then you need to install it. They are telling you that the drive is not reliable enough on its own to be used where there are safety considerations, and a saw definitely meets that description. The key here is how the control of that contactor is handled. Integration of the contactor using a safety function that includes an NFPA 79-15 Category 1 stop is the first part in this. Category 1 allows the designer to include a time delay between the start of the emergency stop activation and the point where power is removed from the drive. The intention is that the time delay will not exceed the time needed to brake the blade to a stop as quickly as possible. Safety relays designed for this purpose have both instantaneous-off contacts and delay-off contacts. The contactor coil is connected to the delay-off output, and the drive enable line will be connected from the machine controller, through the instantaneous-off contacts and then to the drive. When the e-stop is activated, the drive enable line will be dropped immediately, and the time delay will begin. The drive programming needs to be set up so that loss of the enable line puts the drive into a maximum deceleration to a stop mode.

    If you measure the stopping time of the blade with this setup, I will hope that you will find it to be less than 250 ms or so. You can set the time delay on the safety relay for 1 second or so. If you can't achieve a stop using the drive in less than a couple of seconds due to the inertia of the blade and drive, then you need to add a mechanical brake to the system to reduce the stopping time to less than a couple of seconds. The brake coil would be energised via the instantaneous contacts.

    Finally, if you are putting surge suppressors on relay and contactor coils in the safety system, avoid diode suppressors as they will delay the drop-out of the relay or contactor. Instead, choose RC or MOV based suppressors to avoid this problem.
     
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