Why CB's in series with fuses on VFD's?

[dogleg43]

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

 

[rbulsara]

In the same enclosures or separate?

 

[dpc]

Often the VFD is fed from an MCC or panel via a circuit breaker. This is required to protect the feeder. In the VFD, a current-limiting fuse is often used to provide limiting of short-circuit current (along with protection, of course). The current-limiting capabilities of the fuse allows the VFD to carry a higher short circuit rating than would otherwise be possible with a standard circuit breaker.

 

[jraef]

Also, high speed semiconductor rated fuses are often used to (hopefully) prevent damage to the power electronic devices. They act much faster than circuit breakers on a short circuit, but are very expensive. So to avoid nuisance fuse blowing on thermal overloads, the fuses are often over sized based on the maximum capabilities of the semiconductor components, then you still need the CB as the thermal protection for the conductors.

JRaef.com
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[davidbeach]

In other words, the fuses are to protect the power electronics and the circuit breaker is to protect the fuses. ;-)

 

[jraef]

Well sort of I guess. It's more like, the fuses protect the expensive parts, the CB picks up the slop.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

I think the fuses also save the manufacturer from a lot of hassles when required to meet specified short circuit ratings.

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.

 

[LionelHutz]

I believe there is a problem with UL recognizing semiconductor fuses for motor circuit protection unless, for example, the VFD was tested with them and UL listed to use them.

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.

 

[mc5w]

There is also the problem that if a power semiconductor decides to die, a semiconductor fuse will not stop that but it will stop the failed semiconductor from damaging the other power semiconductors and to stop the failed semiconductor from catching on fire or exploding.