Can VFD Short Circuit Protection be done Electronically

[JPing]

Forgive me if this has already been answered. I'm new to the forum and couldn't find anything while searching.

My situation is this:

I have a 150HP motor currently, that MAY be upgraded in the near to future to 300HP. As such I am designing the system to currently service the 150HP load, but by spending as little $$ as possible prepare for the 300HP motor. As such, my system looks like this:

600A
^
----o o-----| 300HP Harmonic Filter |-------| 300HP VFD |----- 150HP MTR

The cable from the 600A breaker thru the filter and to the VFD is sized for the 300HP load. However to save cost now I would like to size the cable from the VFD to the Motor for the 150HP load (as it is about 300 feet of cable).

The VFD comes with input fuses sized for 300HP (500A).

Here is the question: Is it permissible to use the current limiting features in the VFD to protect my "undersized" wiring between the VFD and the motor.

Note: I know that I can swap out the 400A fuses in the VFD with 225A or 250A fuses, but I still would like to look into the simpler "electronic" method.


Second question. Should my main breaker (the 600A) be a circuit breaker or a Motor Circuit Protector (MCP)?

You help is greatly appreciated.

 

[Cold Fusion]

I would consider this a regulatory issue, not a design issue.

Art 430, part X says that parts I-IX apply unless modified. There is nothing modifying the short circuit protection articles. I don't see an electronic solution. So, assuming the FLC from T430.250 is 180A, per T430.52, you are stuck with:
Thermal-Mag, 250% FLC >> 450A
Non-Delay fuses, 300% FLC >> 540A (next size up is 600A)

Check the fuses in the VFD. Often they are fast acting non-delay fuses - in which case, 600A is okay.

Q2: MCPs can only be used as part of a listed combination motor controller, complete with overloads. Maybe you are because you want the contactor, if so it will be fine. If not, you will be installing a thermal-mag circuit breaker.

Suggest you read art. 430, part IV.

cf

 

[JPing]

Thanks CF,

I'll do some more reading in the aforementioned code section.

The fuses are indeed "Very Fast Acting" (Bussmann JJS-400 type) fuses.

So if I got this straight, I can leave the 600A thermal mag ahead of the Harmonic Filter, and leave the 500A fuses that the VFD is equipped with and be in compliance?

When/If the system is upgraded to 300HP the only thing I need to change is a second set of parrallel conductors from the drive to the motor, and change the VFD settings. Groovy.

 

[Cold Fusion]

Be careful about believing me. You need to understand it. Even if I didn't miss anything, it's hard for me to be exactly sure of the installation from my side of your monitor.

There are other issues about the disconnect being in-sight of the controller. The CB is likely your disconnect and the VFD is the controller. Can you lay eyeballs on the disconnect when standing in front of the VFD?

Just curious, compared to the cost of this project, fuses are pretty cheap. Why wouldn't you change them out to a lower value if you could?

cf

 

[JPing]

Yes, they are next to each other. And yes, I will do my due dilligence rather than taking your word for it, but someone helping me know where to look and giving a gut check is greatly valued. Thanks again.:grin:

It's honestly not a price concern, it is more of a real world, coordination, fuses getting lost, everything happening the way it's supposed to issue (i.e. one less thing to babysit). Also, I was also just curious about the issue and so it became a bit of an academic thing.

 

[weressl]

Forgive me if this has already been answered. I'm new to the forum and couldn't find anything while searching.

My situation is this:

I have a 150HP motor currently, that MAY be upgraded in the near to future to 300HP. As such I am designing the system to currently service the 150HP load, but by spending as little $$ as possible prepare for the 300HP motor. As such, my system looks like this:

600A
^
----o o-----| 300HP Harmonic Filter |-------| 300HP VFD |----- 150HP MTR

The cable from the 600A breaker thru the filter and to the VFD is sized for the 300HP load. However to save cost now I would like to size the cable from the VFD to the Motor for the 150HP load (as it is about 300 feet of cable).

The VFD comes with input fuses sized for 300HP (500A).

Here is the question: Is it permissible to use the current limiting features in the VFD to protect my "undersized" wiring between the VFD and the motor.

Note: I know that I can swap out the 400A fuses in the VFD with 225A or 250A fuses, but I still would like to look into the simpler "electronic" method.


Second question. Should my main breaker (the 600A) be a circuit breaker or a Motor Circuit Protector (MCP)?

You help is greatly appreciated.

As the ASD's have very limited short circuit capability due to the galvanic isolation of the incoming and load sides there is only electronic protection integrated in the ASD that is available or needed. This protection is more for the ASD's self protection than for the motor. The 500A input fuses are for the ASD's protection and make sure that you coordinate the upstream protection with it. There is no need to change them.

MCP's can only be used as part of a listed combination starter usually witin the same enclosure as the rest of the motor controller components.