Disconnects and VFDs

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cvirgil467

Senior Member
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NewYork
Beaten to death, I know.

One vfd controls multiple motors in an air handling unit. Are disconnects required at each motor if VFD is located within sight of the motors?

I have heard locating disconnects between VFDs and motors can damage the VFD if the disconnect opens while the motor is running. Can anyone shed some light on why?

Thanks.
C
 
If the disconnect for the VFD is at the VFD and can be locked in the open position and is within sight of the motors it can serve as the motor disconnect.
 
cvirgil467 said:
I have heard locating disconnects between VFDs and motors can damage the VFD if the disconnect opens while the motor is running. Can anyone shed some light on why?
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I can not tell you why but I can tell you that a small aux. control switch can be added to the disconnect switch and wired to the VFD control circuit to prevent VFD damage.
 
VFD manufacturers recommend no disconnects or starters between the VFD and motor. If you were to close or open the disco while the VFD is in operation it would likely trip out the VFD. They are very sensitive to fluctuations in voltage and current.
 
iwire said:
I can not tell you why but I can tell you that a small aux. control switch can be added to the disconnect switch and wired to the VFD control circuit to prevent VFD damage.
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Unless the aux contact is wired to cause a freewheel stop it will not do much to protect the VFD. The timing between the aux contact and the power contacts opening is not normally sufficient for the output of the VFD to disappear. Though, the aux contact will prevent the VFD from feeding into an open circuit.
 
jim dungar said:
Unless the aux contact is wired to cause a freewheel stop it will not do much to protect the VFD. The timing between the aux contact and the power contacts opening is not normally sufficient for the output of the VFD to disappear. Though, the aux contact will prevent the VFD from feeding into an open circuit.
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Well you better tell AB 'cause they where very firm about having us install lots of them. wired back to the MCC where all the VFDs where located.:smile:

Maybe they where dropping a contractor on the front end of the VFD, I really don't know.
 
don_resqcapt19 said:
If the disconnect for the VFD is at the VFD and can be locked in the open position and is within sight of the motors it can serve as the motor disconnect.
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I disagree. As long as the VFD disconnect can be locked in the open position it doesn't have to be within sight of the motors.

430.102 (B) Exception says where such a location of the disconnecting means for the motor is impractical or introduces additional or increased hazards to persons or property, disconnecting means shall not be required as long as the controller (in this case VFD) is lockable in the open position.

And FDN No.1 says motors associated with adjustable speed drives are increased or additional hazards.
 
iwire said:
Well you better tell AB 'cause they where very firm about having us install lots of them. wired back to the MCC where all the VFDs where located.:smile:

Maybe they where dropping a contractor on the front end of the VFD, I really don't know.
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Wow, AB making you spend extra money.:D

Since PWM drives came out, I have never seen any other manufacturer require aux contacts as much as AB does. And I don't think I have ever seen any manufacturer require the aux to be of an early break design. I believe the requirement has more to do with "starting" the VFD onto an open circuit more than it does opening a circuit that is running.
 
jbond said:
I disagree. As long as the VFD disconnect can be locked in the open position it doesn't have to be within sight of the motors.

430.102 (B) Exception says where such a location of the disconnecting means for the motor is impractical or introduces additional or increased hazards to persons or property, disconnecting means shall not be required as long as the controller (in this case VFD) is lockable in the open position.

And FDN No.1 says motors associated with adjustable speed drives are increased or additional hazards.
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I should not have said the disconnect needs to be lockable. I agree that you don't need a lockable disconnect if the disconnect is within sight of the motors. The use of the exception is very limited under the current code, but if you meet the restrictions of the exception, a remote lockable disconnect at the VFD would meet the code rules.
 
jbond said:
430.102 (B) Exception says where such a location of the disconnecting means for the motor is impractical or introduces additional or increased hazards to persons or property, disconnecting means shall not be required as long as the controller (in this case VFD) is lockable in the open position.
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Hold on there 430.102(B) exception (a) does not use the word impractical.

430.102(B) exception (a) uses the word impracticable.

The meanings are entirely different

impractical = difficult

impracticable = imposable

You must always have a disconnecting means in sight of the motor unless it is imposable or introduces more hazard.

or under exception (b) written safety, supervision, qualified yada yada yada.
 
cvirgil467 said:
Beaten to death, I know.

One vfd controls multiple motors in an air handling unit. Are disconnects required at each motor if VFD is located within sight of the motors?

I have heard locating disconnects between VFDs and motors can damage the VFD if the disconnect opens while the motor is running. Can anyone shed some light on why?

Thanks.
C
Click to expand...

Multiple motors should not be controlled from a single ASD, unless the manufacturer specifically permits it. In that case follow the manufacturers instructions.
 
iwire said:
Hold on there 430.102(B) exception (a) does not use the word impractical.

430.102(B) exception (a) uses the word impracticable.

The meanings are entirely different

impractical = difficult

impracticable = imposable

You must always have a disconnecting means in sight of the motor unless it is imposable or introduces more hazard.

or under exception (b) written safety, supervision, qualified yada yada yada.
Click to expand...

Bob,

Could you please let us know where you find the above interpretation? All reference books, dictinaries and Thesaurus's I find make no such distinction. That would be 'impossible'. Your unwelcome visit at a place would be and 'imposable event'.
 
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