VFD disconnect question

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Red Wiggler

Senior Member
Is a VFD considered a controller? If so then wouldn't the code require a disconnect switch ahead of the VFD (line side)?

So if you installed a disconnect switch ahead of the VFD and someone inadvertantly opened the switch, wouldn't the motor back feed into the VFD?

Would this situation be avoided if the 3 pole breaker ahead of the VFD was lockable for maintenance purposes?

The VFD has a lockable disconnect incoorporated into the unit. This would allow for motor work, or replacement. But what about work on the VFD? Does it need an external disconnect switch?
 

tom baker

First Chief Moderator
Staff member
Take a look at Part X of Article 430, it was added in the 05 NEC:
X. Adjustable-Speed Drive Systems
430.120 General.
The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X.

Every motor, even a 1/8 hp clock motor requires a disconnect.

And take a close look at 430.102 to answer your question.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Is a VFD considered a controller? If so then wouldn't the code require a disconnect switch ahead of the VFD (line side)?

So if you installed a disconnect switch ahead of the VFD and someone inadvertantly opened the switch, wouldn't the motor back feed into the VFD?

Would this situation be avoided if the 3 pole breaker ahead of the VFD was lockable for maintenance purposes?

The VFD has a lockable disconnect incoorporated into the unit. This would allow for motor work, or replacement. But what about work on the VFD? Does it need an external disconnect switch?
Does the disconnect kill the power to the line side of the VFD? If not then you need to provide a disconnect on the line side of and within sight from the VFD. The line side breaker can be the required disconnect as long as it is lockable and within sight from the VFD. There is no "remote lockable disconnect" exception for the controller disconnect.
You could use a disconnect with an early break aux contact in the control circuit to tell the drive to shut down when the disconnect is opened.
 

Besoeker

Senior Member
Location
UK
We have been putting aux contacts on the safety switches used on either the line or load side of VFDs. Early break late make contacts wired into the drive control shuts the drive down in a more orderly fashion.
Yes, we routinely do that. Also, if there is an output contactor*, we wil have an early break contact on that.
*used where a hard wired emergency stop is required.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Yes, we routinely do that. Also, if there is an output contactor*, we wil have an early break contact on that.
*used where a hard wired emergency stop is required.
Now that some manufacturers have safety relays available built in to the drive, do you feel the need to still have a hard wired output contactor?
 

John Valdes

Senior Member
Location
SC.
Now that some manufacturers have safety relays available built in to the drive, do you feel the need to still have a hard wired output contactor?
Actually contactors on the load side of the VFD are BAD idea unless it is used for Emergency stopping or bypass. Cycling any contactor on the load side of the VFD during operation will surely trip the control.

If the built in disco incorporated in the control does not allow for safe work on the control, then a line side disco is required.
 

Besoeker

Senior Member
Location
UK
Now that some manufacturers have safety relays available built in to the drive, do you feel the need to still have a hard wired output contactor?
That's a fair point.
But yes, on the whole, I do.
However, to a large extent, it isn't my decision.
When we bid a project we generally get a fairly detailed specification. Our bid will reflect that although we may list, with justification, items where we are non-compliant or exceed requirements. Taking out line contactors wouldn't go down well with many customers regardless of the need to comply with regulations and if you want the business.....
If we win the bid, we then have to supply what we contracted to supply.
 

Ranch

Senior Member
Location
Global
Actually contactors on the load side of the VFD are BAD idea unless it is used for Emergency stopping or bypass.

EN 954-1 category 4 for the highest risk situations where a failure is very likely to lead to serious injury or death. A single fault must not cause a hazard, and all (non-excluded) faults must be detected. A second protection device (contactor) is required in case of an accumulation of faults in Secure Disable.

For these systems, tell your VFD to coast to stop, it won't mind the output contactor one bit
 
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