Quite possible. Should be no problem. The VFD just generates the waveform, the motors just do what they always do. This would likely not be a good idea with a vector drive though.
and be mindful of cable length. The total cable length is the total between each motor from the VFD.
Other advice is to size the drive(VFD) on the current rating of each motor x 1.1 to provide a safety margin for overloads.
the 10% margin is used typically to cope with any minor overloads on the motors connected. It's a general rule and one that is there if costs permit to allow for but usually you don't have that luxury so you rely on the overload capability of the drive to cope with certain load changes in the motors.
I've never used more than a 10% de-rate, 33% seems a bit extreme. 10% allows a fudge factor for slight differences in the load that may make one motor accel and decel differently. If the motors are on dissimilar loads then maybe the 33% would make more sense, but I can't imagine off hand many applications where you would have 2 dissimilar loads that need to run at the same speed reduction.
Thanks everyone for the feedback. FYI this is for a retrofit application on a ship where they want to change the shape of a cooling radiator and run two smaller fans instead of one. Otherwise I would have just replaced the one VFD with two, but in this case it would be a whole lot easier to run the one VFD.
Two independent drives will allow you to maintain some form of operation, albeit at reduced cooling capacity, even if one fan is out of service for maintenance. As the original system used a single fan this might not be significant.
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If we learn from our mistakes I'm getting a great education!
application where you have two disimilar loads would be if you have to bring up motors on conveyors at same speed from a bar code scan tunnel to a sorter. Sorter usually has the biggest motor versus the tracking belts between the scan tunnel and the sorter. If you just put all the motors on one vfd and use the 1.33x multiplier hp the vfd would not be undersized. This way the boxes track consistantly and as long as the gear boxes on the conveyors do not have a big difference between them
I guess fans that are the same do not need this multiplier.
I guess I don't understand the whole horsepower multiplier issue. I usually size VFDs based upon total amp requirements (even on single motors since I get a lot of unusually size motors with large compressors and centrifuges) instead of horsepower. Is there any reason why adding up the total FLA (including service factor amps if they will be used) will not work? I haven't had any problems as yet.
Another comment as controlsdude surmised is that centrifugal fans and pumps are variable torque loads where conveyors, positive displacement blowers and pumps, etc. are constant torque loads which will make a difference in loading at less than nominal speeds.
Good way of killing drives unless they are specifically designed for this. I know some are supposedly able to withstand opening of an output contactor or load-break switch but not all drives will put up with this. Of course shutting down the drive for a few seconds would work too, and on a fin-fan cooler this would probably have little consequence.
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If we learn from our mistakes I'm getting a great education!
Here is my two cents on shutting off the disconnect
I believe you would get an overvoltage fault on the drive if you switched one of the disconnects off. You should as a general pratice to have an aux contact on the disconnect that pulls the enable on the vfd to prevent this type of situation either through the plc or hardwired.
I had a job where their was a backup motor and used contactors to swapped between motors with a keyswitch. Their was some interlocking between the disconnect aux and the contactors, and the enable on drive to prevent opening the output between the motor and vfd.
If your going lock out one, then you need a written procedure posted at vfd that you need to shut both off before switching over to just one.
A minor comment about two fans in parallel. Should one stop, often air from the working fan will blow a significant amount of air backwards throught the non-working fan appature. This can cause the cooling downstream to be greatly reduced.
