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VFD right to motor no Disco?

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zappy

zappy

Senior Member
Location
CA.
My boss said just run the conduit right to the motor. No disconnect. He had a reason, I'm not a VFD expert is that code though? Thank you for your help.
 
petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
zappy said:
My boss said just run the conduit right to the motor. No disconnect. He had a reason, I'm not a VFD expert is that code though? Thank you for your help.
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There's no code requirement to locate a disconnecting means downstream of a VFD.

There is a code requirement that every motor have a disconnecting means. That disconnecting means can be upstream of a VFD as long as it meets all of the requirements of a disconnecting means.

My suggestion is you look in the code and see if what you are doing is acceptable. He might well be right but you have not given us enough information to make that determination.
 
E

EC Dan

Senior Member
Location
Florida
Occupation
E&C Manager
Typically you don't want a disconnect between the VFD and the motor since disconnecting the circuit while the motor is running could damage the VFD. If a disconnect is used between VFD and motor, it should have early break auxiliary contacts to signal to the VFD to shut off output power right before the circuit is isolated.
 
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don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
If you must have a disconnect on at the location of a motor supplied by a VFD, you can get one with a set of early break control contacts. You use those contacts to tell the drive to go into "coast to stop" mode so that when the power conductors are opened, there is no output from the VFD.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
MOST disconnect switch aux. contacts are “early break”, meaning the aux. contact opens BEFORE the main contacts open.

I have never seen one that wasn’t, but if I make a claim that they all are, someone will show me one that isn’t…
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Challenge accepted :)
Could be mistaken but I seem to recall Allen-Bradley makes both an early break and late break auxiliary contact 800X ******
 
zappy

zappy

Senior Member
Location
CA.
EC Dan said:
Typically you don't want a disconnect between the VFD and the motor since disconnecting the circuit while the motor is running could damage the VFD. If a disconnect is used between VFD and motor, it should have early break auxiliary contacts to signal to the VFD to shut off output power right before the circuit is isolated.
Click to expand...
Yes this is what he said.
 
G

garbo

Senior Member
zappy said:
My boss said just run the conduit right to the motor. No disconnect. He had a reason, I'm not a VFD expert is that code though? Thank you for your help.
Click to expand...
I worked on & performed PM'S on hundreds of VFD'S. We tried to have drives installed close ( less then 50' ) from motors. If a safety switch is installed on the load side of a drive it shou!d have a micro switch that will open NC contacts before handle opens. I talked to several drive tech about the dangers from turning off a safety switch on load side while drive is operating. All told me that you might not blow anything up maybe 9 out of 10 times. We had four 75 HP cooling tower fans on drives. Drives were two floors below the first floor and non fused safety switches were outside next to towers. Contractor who did maintenance inside cooling towers would use the safety switch to turn a running drive on & off rather then walk a distance then down over 50 steps to use drive control panel. One day he blew up two drives. These outdoor non fused safety switches did not have a micro switch to shut down drive. Forget how much the IGBT'S cost for them but it was over a $1,000 for the IGBT'S for 125 HP drives awhile ago. So I'm all for safety but if the VFD is within say a 50' sight would never install a safety switch on drive output. I always ordered drives with a built in disconnect switch. When I had to change a bad motor I installed LOTO on drive disconnect and the circuit breaker that feed it. We had over 50 air handlers from 40 to 125 HP that drives were on the outside but had a non fused safety switch from air handler company mounted inside of walk in air handlers.
 
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