CB for VDF

[elec_eng]

I am little confused how to size the cb for the VFD with bypass..VFD 60 hp, 480V, 3ph..

Should the circuit breaker sized based on the table 430.52 because of the bypass circuit

or

can it be 125% of the input current to the VFD? ( somehow I thought the cb can be sized this way...was this only for the conductor?)


When the VFD is in bypass mode, how do you provide the overload protection?


How about the disconnect for the motor and controller?

 

[don_resqcapt19]

I am little confused how to size the cb for the VFD with bypass..VFD 60 hp, 480V, 3ph..
Should the circuit breaker sized based on the table 430.52 because of the bypass circuit
or
can it be 125% of the input current to the VFD? ( somehow I thought the cb can be sized this way...was this only for the conductor?)

I think you need to meet the rules for both. I expect that you can find a OCPD that is equal to or less than the maximum value in 430.52 and is also equal to or less than 125% of the VFD's rated input current.

When the VFD is in bypass mode, how do you provide the overload protection?

A standard starter with an overload relay is often used in the bypass circuit.

How about the disconnect for the motor and controller?

The rules in 430.102(A) and (B) must be complied with.

 

[ibew501ed]

What is a vfd with a bypass?
is this a choice either adjustable OR across the line in case the VFD fails?
or is it start it up get it running and THEN throw it in full run

 

[TxEngr]

IBEW - the bypass allows you to run the motor 'across the line' in the event of a vfd failure or if the vfd is simply not needed. Typically you have a set of electrically and/or mechanically interlocked contactors that the motor T-leads attatch to and a switch to allow you to pick which device is in control. These can be purchased from the manufacturer this way or, more often, built at a fab shop or on site. The key is to prevent the load side of the drive from seeing the full voltage when in bypass - you can blow up a drive that way.

elec_eng - I agree with don that you should be able to size a single breaker to meet all needs. If not, you could use two circuit breakers, each sized per code feeding their respective devices. Since the load side is normally connected to a pair of interlocked contactors, you can have the two different breakers. An advantage to this is isolation of portions of individual circuits if they are not being used so if the output contactors are in a different structure, you might be able to replace the drive while running in bypass mode if a proper lockout can be made.

TxEngr

 

[elec_eng]

That makes sense...Thanks Don and TxEngr.