Fuse and Cable

VFD implies it is a 3 phase AC motor then right? If that's the case, then the VFD (if it was made any time in the last 10 years) is protecting the motor for both short circuit and overload, you don't need anything on the output side of the VFD. So all you need to do it protect the line side of the VFD.

Since you are obviously in the US, you are required to size the circuit conductors (and therefore the OCPD) feeding the VFD at NO LESS than 125% of the max amp draw of the VFD, regardless of the motor connected to it (430.22 I think). The VFD data sheet would tell you the max amp draw, and for a VFD they are always referring to the DC bus anyway, so the value would apply here. You don't need for them to be time delay fuses, and in fact most VFD manufacturers will tell you to use high speed "semiconductor" fuses. If you do that though, size them to the maximum allowed by code, usually 250% of the VFD rating.

But also double check your voltage issues. Many people forget that the DC voltage needed for a VFD is a lot higher than the AC voltage, because you lose the RMS effect in charging the DC bus. So if you are feeding a 480V rated VFD, the DC voltage you need to give it will be 678VDC, which then moves you out of "low voltage" as far as wiring and such. If it is a 240V drive, you need to feed it with 340VDC minimum, so from a wire insulation standpoint 600V rated wire is fine, but from a fuse standpoint, fuses come either as 170VDC class or 500V class and above, you will need to use 500V class fuses.

Battery voltage is 48Vdc going to vfd.

VFD rating is 375 - 500A ( 2min RMS) 48VDC.

My motor full load current is 178A ( FLC-design current) , 160A ( name plate FLA).

I m trying to provide a dual time delay fuse at the battery side.

So should the fuse rating depends on motor name plate reading or design FLC?

If so should the fuse rating be lesser than cable ampacity( 125% of FLC)?

And am i right with the fuse protection given for short circuit and overload in the first post? If not please correct me.........

The overload setting is there to protect the motor.

The fuses are there to protect the wiring.

British Standard 7671 and evermore shall be so.

i have two VFDs of: 1) 375A/20.8KVA (2 min RMS) and

2) 500A/27KVA ( 2min RMS).

If my induction motor FLA is 180A, then from the above two VFDs which one will be most suitable? Sorry the efficiencies are not mentioned, i need to know for deciding

1)the input fuses rating and

2)input /output cable ampacities.

Please suggest yours fuse and cable ratings. Thank you.....

I should have asked this question first before going further detailed that,

How do we size fuses and cables for VFD?

Is it wrt to Motor FLA or VFD current rating? I m confused because in NEC 430.122 it has mentioned about both.

I m asking these questions because i m new to mthis job in practical and i dont want to make mistakes..........

I would start with the drive manufacturer's specifications.

For the drives that we commonly use, AB Powerflex drives, the user guide published by AB has a table of the maximum recommended fuses, type and size, and also a set of recommendations for cable types and sizes.

I believe that, for the reasons that have been given by other posters, it is prudent to size the feeder protection and cables to the drive to the rating of the drive, even if it is intended to only part-load the drive with a smaller motor. Any less, and all it takes is for someone to replace the small motor with one that fully loads the drive without up-ratinging the feeder cabling, and you have a situation that is ripe for a meltdown.

I have a question which is running through my mind, and is mostly concerened with the input fuses for inverter. Is an inverter just like a transformer? ie the inverter consumes current from the battery depending on inverter rated value and delivers the output depending on motor load value by suitable switching configuration for constant V/hz value?

or

Does the inverter consumes current from the battery depending on motor load? If so which shouldn't we size the fuse according to motor FLA ?