Fusible disconnect on load side of vfd
Fusible disconnect on load side of vfd
(OP)
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?
The vfd is equiped with overload protection on the load side already.What are the pros and cons fo such an application?





RE: Fusible disconnect on load side of vfd
RE: Fusible disconnect on load side of vfd
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.
RE: Fusible disconnect on load side of vfd
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
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Fusible disconnect on load side of vfd
RE: Fusible disconnect on load side of vfd
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.
RE: Fusible disconnect on load side of vfd
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.
RE: Fusible disconnect on load side of vfd
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
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376