Why CB's in series with fuses on VFD's?
Why CB's in series with fuses on VFD's?
(OP)
Why do many motor circuits have circuit breakers in series with fuses? I'm referring to the main 480VAC fuses & CB ahead of the starter or drive.
I understand the different characteristics of fuses and CB's but it seems like having them in the same circuit would cause more problems than it solves.
thanks for your replies
I understand the different characteristics of fuses and CB's but it seems like having them in the same circuit would cause more problems than it solves.
thanks for your replies





RE: Why CB's in series with fuses on VFD's?
RE: Why CB's in series with fuses on VFD's?
RE: Why CB's in series with fuses on VFD's?
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Why CB's in series with fuses on VFD's?
RE: Why CB's in series with fuses on VFD's?
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Why CB's in series with fuses on VFD's?
Most of the fuses I see on the ac input in the drive are just standard current-limiting fuses, as best I can recall.
If the drive has a UL label and was tested with fuses, it may have to be protected with fuses regardless of how good the upstream protection is.
RE: Why CB's in series with fuses on VFD's?
For full or partial voltage starting motor circuits (not VFD) you would need a fairly high current semiconductor fuse to not blow it while drawing the higher motor starting currents. Typically, the value will be 3-4 times the FLA of the motor. This high a value isn't allowed by code as the circuit protection means. SCR's would be protected by a semiconductor fuse that would end up sized like this if you were bothering with this type of protection. There's usually no need to use semiconductor fuses these days.
Of course, it is hard to find fusible disconnects for some fuse types so it may be easier and cheaper to just use a breaker or molded case switch and a fuse block to create a fuse protected package so maybe it's as simple as this.
RE: Why CB's in series with fuses on VFD's?