Fusible disconnect on load side of vfd 5

[elrowsland]

Are fusible disconnects allowed on the load side of a vfd?
The vfd is equiped with overload protection on the load side already.What are the pros and cons fo such an application?

 

[DickDV]

If you have a single motor powered by the VFD, then the fusing makes no sense. On the other hand, if you have multiple motors feed from a single drive, then the drive can no longer properly protect each individual motor. The solution to that is to install an overload block and, often a disconnect and/or a fused disconnect to each motor.

 

[ozmosis]

A fused disconnect on the output of your VFD is not a good idea. A disconnect on the output, of any description, needs careful consideration. It is generally advised that a disconnect is interlocked with an immediate stop (coast stop/pulse disable) of the VFD so that when opening the circuit, the VFD is not outputting a voltage/frequency to an open circuit and if the disconnect closes again, the VFD is not actually running otherwise you are literally going DOL to your motor from the output of your VFD and this is not good.
There are companies who say "opening/closing a contact on the output of a VFD is ok" but this is not a recommended practice.
As DickDV says, multiple motors will need some sort of protection in the form of a thermal overload or similar, to feed back to the VFD in the event one or more motor reaches an overload state.
One to one motor/VFD is not necessary for additional protection as the VFD should provide this.

 

[jraef]

I don't like fuses on the output side of a VFD for any reason. My feeling is that they add additional risk of VFD damage by opening under load (which would be the only reason of course) and simultaneously unbalancing the current. I see no reason to place fuses there anyway, the VFD will have plenty of circuit protection for the conductors and motor windings.

Even with multiple motors on the output, I prefer circuit breakers or better yet, IEC style Motor Protective Switches (which also provide the OL function) that open all 3 phases at the same time. They can also provide a signal to the VFD to shut down if necessary, and can be interlocked with an Under Voltage Trip so that you cannot re-energize them if the VFD is running.

JRaef.com
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[LionelHutz]

jraef; You'd better recommend a brand of motor protective switch then. I've seen a few different types that would not work on the output of a VFD. The instantaneous trip coil was overheating. I was at a site and they had one brand that was tripping so they tried another and it was worse. I believe they were using around 4 or 5 amp switches.

 

[DickDV]

Using an example of 10 5hp motors all running in parallel on a 60hp drive, you simply have to have some kind of branch circuit current-limiting device on each motor branch or you have significant fire hazard. The drive cannot be expected to recognize short circuit failures or overcurrent conditions on a single branch and release in a timely manner.

Overloads for the individual motors are, of course, necessary but do little to protect against fast acting failures. Most quality modern drives will tolerate fuses opening in their output without damage. There may be better technology out there but fuses have worked for me for years without drive problems.

The most extreme multi-motor system I have been involved with is a 125hp drive powering 48 1.5hp motors on a steel mill roller table. Four quadrant operation is normal. Due to the frequent mechanical failure of the individual rollers, we had to have the ability to pull any locked up roller off line with a disconnect on the fly. That was eleven years ago and the system is still running with disconnect opening under load an almost daily occurence.

 

[rbulsara]

elrowsland:

Just to answer the original question strictly from Code (NEC) point of view, yes a fusible disconnect will be permissible on load side of a VFD. In fact NEC requires a local disconnect for a motor, if the disconnect in the controller (VFD in this case) is not 'within sight' of the motor. This disconnect does not have to be fusible, but it is not prohibited.

All technical comments made by other posts are still valid.

 

[jraef]

LionelHutz,
I have experienced this as well, in fact with most bimetal OL relays on the output side of a VFD, but in each case a load reactor cured the problem. A load reactor on a multiple motor application is a must IMHO, it also slows down the rise time of faults to a point where most VFD circuits can tolerate it within the reaction time of the IOC trip of the drive.

To further DickDVs point however, in that multi-motor application it is absolutely necessary to have SCPDs on each motor circuit, I just prefer the MPS devices because combine the OL and SCPD in one package and they open all 3 phases.

JRaef.com
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