Tapping off VFD Output

[jimgineer]

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-

 

[jimgineer]

And by the way, the load that would be supplied from the tap is another motor, is right next to the 400HP motor, and is much smaller. Both are 480V. In terms of controls, I'm thinking this will be fine because the small blower fan is only going to need to be running when the big 400HP motor is.

Thanks

 

[dpc]

Sounds like a really bad idea, in general. Each motor must have separate overload protection, as a minimum.

 

[davidbeach]

You're looking to buy a lot of trouble for a little bit of cost savings. You will be far better off with separate VFDs. The VFD provides all of the required overload protection, so with two motors you need independent overload protection for each, but if you then trip one of the motors, particularly the larger motor, neither the VFD nor the other motor will like the results.

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.

 

[jimgineer]

This is something I need to coordinate with the Mechanical engineers on the job but there is some confusion as to whether the fan/blower that is required is the one that is integral to the VFD, or a separate one that cools off the larger motor going in. I was reading another thread on VFD's and it looked like if the motor is being underdriven or driven lightly by the VFD then it could potentially heat up the motor, requiring a separate cooling system to cool off the fan.

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.

 

[itsmoked]

That is a better solution.

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

 

[jraef]

That's right. Running an aux. fan to cool the motor off of the same VFD output is not going to do any good. The fan will slow down with the motor, so right when you need the extra colling, it will stop providing it.

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.