Emergency stop control of Variable Drive

[sparky48]

I am trying to find the code Number that requires a control contactor to drop off the 480 VAC power to all drives when a e-stop is pushed. Most of the equipment is wired this way, but now being told this is causing a problem. I need the code section that covers this before someone gets hurt.

 

[friebel]

Re: Emergency stop control of Variable Drive

sparky48, in my opinion you will not find what you are looking for in the N.E.C. The code is not a design manual. You will need to be more specific in what kind of problem is an Emergency stop button causing.This certainly sounds like a design issue to me, not a code issue.

 

[tony_psuee]

Re: Emergency stop control of Variable Drive

Sparky48,

You are not going to find that requirement in the NEC, you may need to look in NFPA 79, which is the code for industrial equipment, something used to make something else.

Tony

 

[bthielen]

Re: Emergency stop control of Variable Drive

In our machinery design we try to meet the requirements of IEC 60204-1 for category 2 safety circuits. This requires us to incorporate into our design a safety contactor on all motors including VFD driven, Servo, etc. Where one must be careful is in the location of the safety contactor relative to the VFD. Some VFDs recommend opening the line side of the VFD and some recommend opening the load side of the VFD with the safety contactor. Most prefer the latter. Some specify minimum time intervals between open circuits. You can run into problems with drives overheating, not resetting, and damage if done incorrectly. If you're having problems, check the OEM of the VFD and make sure your system is designed properly. One last note, most VFDs, if not all, highly recommend opening the enable circuit with the safety contactor to avoid damage to internal components.

Hope this helps,

Bob

 

[sparky48]

Re: Emergency stop control of Variable Drive

Thanks for the help on the VSD's. We use Allen Bradley drives. All equipment that comes to us prewired has the E-stop control a MCR to dropout all voltage to the drive, and I believe that is correct. The problem is when we wire a panel with drives we depend on the 120vac control circuit to stop motor. That does drop out enable but a problem with the drive could allow equipment to run with e-stop pushed. They are trying to save a few dollars on their bids, but I like the safer equipment method. They are telling me to prove that a MCR is needed. The MCR on the safe panels also kills all ouputs from the PLC equipment, we do not do that either.

 

[bthielen]

Re: Emergency stop control of Variable Drive

That does drop out enable but a problem with the drive could allow equipment to run with e-stop pushed....They are telling me to prove that a MCR is needed.

I think you answered the question yourself. Without the MCR, a fluke failure in the drive, such as a shorted output transistor, could cause power to remain available to the motor. The MCR adds a certain level of safety in the same way the safety circuit removes power from the PLC output modules as you described. Solid-state electronics, in my experience, are more prone to fail closed than hard, physical contacts, which can weld themselves, of course. I believe this is the reason NFPA and IEC recommend the added security of the hard-wired circuitry. Incidentally, most of the Allen-Bradley drives we use prefer the MCR on the load side rather than on the line side (there are some exceptions, especially with older models). If the line side is opened and closed too often, the drives can be prone to overheat due to the constant repeated inrush when the DC bus is recharged on power-up.

Bob