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Old July 16th, 2019, 12:27 PM   #1
JeffKiper
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Disconnect on VFD output

Does anyone know if there is disconnect requirement for early break switches on the output side of a VFD for the controls?

We have talked about it once years ago and I can't find that thread. I want to say it wasn't a NEC requirement but a manufacture requirement.
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Old July 16th, 2019, 12:52 PM   #2
Ken Moore
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Jeff,
I don't think it is a requirement, just good practice, if the disconnect is thrown when the drive is "working", the early break switch can send a "safe stop or safe torque off" signal to the drive a few milliseconds before the motor is disconnected from the drive.

On a couple of SIL-3 applications, I have seen Safe/Stop, Safe Torque off, used with an additional contactor downstream of the drive along with an encoder. On a Safety stop, the Safe Stop signal is sent to the drive, then if the shaft has not stopped in a certain time (milliseconds) the downstream contactor is opened. The goal is never open an downstream contactor under load if you can avoid it.

Last edited by Ken Moore; July 16th, 2019 at 12:56 PM.
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Old July 16th, 2019, 03:10 PM   #3
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I would agree with Ken Moore that disabling the drive before the disconnect opens is preferable. However, todays IPMs do a pretty good job of protecting themselves. It's not like the bad old days when disconnecting a drive under load was almost a guaranteed blown drive.

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Old July 16th, 2019, 03:42 PM   #4
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Quote:
Originally Posted by Ken Moore View Post
Jeff,
I don't think it is a requirement, just good practice, if the disconnect is thrown when the drive is "working", the early break switch can send a "safe stop or safe torque off" signal to the drive a few milliseconds before the motor is disconnected from the drive.

On a couple of SIL-3 applications, I have seen Safe/Stop, Safe Torque off, used with an additional contactor downstream of the drive along with an encoder. On a Safety stop, the Safe Stop signal is sent to the drive, then if the shaft has not stopped in a certain time (milliseconds) the downstream contactor is opened. The goal is never open an downstream contactor under load if you can avoid it.
Quote:
Originally Posted by kamenges View Post
I would agree with Ken Moore that disabling the drive before the disconnect opens is preferable. However, todays IPMs do a pretty good job of protecting themselves. It's not like the bad old days when disconnecting a drive under load was almost a guaranteed blown drive.

Keith
I agree on all counts; not a requirement, but a very good practice even though modern drives that use the latest generation of IPM have more protection than in days of yore. The arc that happens when you open a set of contacts under load can be damaging to transistors. The modern IPMs include suppression and shunt paths for that energy, but that can fail eventually.

All NEMA KS-1 designed knife switch type disconnects use handle operated aux contacts that are, by design, always early break compared to the main contacts. But if you use the low cost rotary type IEC disconnects where the aux contacts are driven off of the same cam as the mains, that is not always the case and you must make sure to order the early-break aux. contacts.
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Old July 16th, 2019, 04:19 PM   #5
JeffKiper
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You guys rock. I'm looking at an application with a few drives that will have field disconnects. Most of our small machines don't require field disconnects.
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Old July 16th, 2019, 05:18 PM   #6
James Mcquade
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disconnects are required for all loads when they are more than 50ft away from the main power disconnect or are not within sight of the main disconnect when they are in that 50 ft range - NEC 70.

all our motor disconnect have aux contacts for feedback to the plc and scada systems.

james
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Old July 16th, 2019, 05:18 PM   #7
Nethop
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We use Square D EIK032 switches in the local disconnect. DPDT. Wire one set to the enable cct. on the VFD and one set to the PLC for status.
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Old July 17th, 2019, 05:32 AM   #8
Ken Moore
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The disconnects are required, but....there is an exception in the code, if the facility has a formal lockout tagout policy/procedure.
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Old July 17th, 2019, 06:07 AM   #9
EICS
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What is a IPM ? Im an Aussie and have not heard that term?
Inverter power module im guessing. We call them IGBT's
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Old July 17th, 2019, 09:04 AM   #10
mike3475
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IPM: Intelligent Power Module.
basically IPM is same as IGBT motor drive module.
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Old July 17th, 2019, 09:49 AM   #11
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Quote:
Originally posted by mike3475:

basically IPM is same as IGBT motor drive module.
...with the addition of protective devices and functions, reporting capability and interface isolation, all built into a single package.

Keith
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