Tapping off VFD Output
Tapping off VFD Output
(OP)
Guys,
I am wondering if it is to code to use the tap rule on the secondary side of a VFD which is controlling a 400HP motor, to feed a much smaller fan/blower that will be in the same general area as the larger 400HP motor.
I'm searching for the NEC Article that might deal with this, other than the tap rule section of 240.
Thanks in advance-
I am wondering if it is to code to use the tap rule on the secondary side of a VFD which is controlling a 400HP motor, to feed a much smaller fan/blower that will be in the same general area as the larger 400HP motor.
I'm searching for the NEC Article that might deal with this, other than the tap rule section of 240.
Thanks in advance-





RE: Tapping off VFD Output
Thanks
RE: Tapping off VFD Output
RE: Tapping off VFD Output
Does the blower need to track the speed of the larger motor? If not it will never work. If the blower is supplemental cooling for the large motor, it needs to be full voltage, full speed at all times, not tracking the speed of the other motor.
Best, by far, is to treat the two motors independently.
RE: Tapping off VFD Output
I think that is the intent here, because what is happening is that we are replacing an existing VFD to drive the motor and integrate with the control system, because the existing system was not working properly for control, and they ended up using full line voltage to drive the motor (this is for HVAC system) and then controlling air flow with some dampers.
So the heat issue would not pop up in existing conditions, and that is why I had no idea there was any additional fan/blower being added to cool off the existing 400hp motor. I need to double check this is the intent of the mechanical engineers' design and that they aren't referring to the blower fan INSIDE the VFD.
Anyways, just planning on tapping the feed to the VFD before the 480V 60Hz is conditioned, and running the tap to a magnetic starter/disconnect for the ~5HP motor.
RE: Tapping off VFD Output
They cannot be speaking of the VFD's fan as that is already driven by the VFD's internal wiring.
Also, as you note, the big motor would need more auxiliary cooling the slower it went, so that fan would need to speed up as the large motor slowed down, not slow down.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Tapping off VFD Output
The typical scenario is to run the aux. fan from a separately derived source, usually at constant speed.
But I would also evaluate your need for an aux. cooling fan on an HVAC motor. most of the time, HVAC loads are centrifugal, meaning variable torque, and at speed less than 30% or so, the pump or fan stops moving an appreciable amount of water or air anyway, so there is little point in running the motor. I have yet to see anyone need an aux. motor fan on an HVAC application.