Variable speed draft fan conversion

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sbleiweiss

New Member
Aug 19, 2009
60
Central MA
I have a WoodGun E140 and would like to be able to control the speed of the draft fan. The boiler has a single phase Baldor 1/3 HP motor running at 3450 RPM. I have not been able to find a cost effective drive (VFD) that will work with a single phase motor. There are some very low cost VFD units that work with 3 phase motors and 3 phase motors are not that expensive, so this seems like the way to go.

I am wondering if anyone has experience with the low cost ($100) VFD's such as this:
http://www.automationdirect.com/static/specs/gs1drives.pdf

The other concern I have is with motor sizing. The small 3 phase motors are rated for HP at 1800 rpm. A 1/4 HP motor at 1800 PRM makes more torque than my 1/3 HP motor, however the 3-phase motors I was looking at claim they can maintain their rated HP at 2x the base speed (or 1/4 hp at 3600 RPM). I would have expected the HP output to go up with RPM. In any case, I am wondering what will happen with the VFD if the loading of the motor exceeds the output rating of the motor or VFD? Will the motor be shut down, or will it be slowed down to a sustainable power level?

Has anyone else done a similar conversion?
Thanks,
Scott B.
 
scottb said:
I have a WoodGun E140 and would like to be able to control the speed of the draft fan. The boiler has a single phase Baldor 1/3 HP motor running at 3450 RPM. I have not been able to find a cost effective drive (VFD) that will work with a single phase motor. There are some very low cost VFD units that work with 3 phase motors and 3 phase motors are not that expensive, so this seems like the way to go.

I am wondering if anyone has experience with the low cost ($100) VFD's such as this:
http://www.automationdirect.com/static/specs/gs1drives.pdf
Nice looking unit.
The other concern I have is with motor sizing. The small 3 phase motors are rated for HP at 1800 rpm. A 1/4 HP motor at 1800 PRM makes more torque than my 1/3 HP motor, however the 3-phase motors I was looking at claim they can maintain their rated HP at 2x the base speed (or 1/4 hp at 3600 RPM). I would have expected the HP output to go up with RPM. In any case, I am wondering what will happen with the VFD if the loading of the motor exceeds the output rating of the motor or VFD? Will the motor be shut down, or will it be slowed down to a sustainable power level?

Has anyone else done a similar conversion?
Thanks,
Scott B.

You could have twice the power at twice the RPM with torque held constant, but heat losses are more or less a linear function of power up to a point. After that motors react like old Volvos, you can give 'em more gas but all they'll do is just get hotter. At any rate to keep heat dissipation more or less constant you have to keep power more or less constant.

So if you really need 1/3 hp at 3450 rpm then you're out of luck with a 1/4 hp motor, but I would measure the actual current draw first to get an idea of actual power usage.

The VFD you pointed to should shut down if you draw too much current. Presumably it would be up to you to limit the speed to be sustainable.

--ewd
 
The boiler has an air damper, but I end up having to run the boiler with it all the way closed or just a tiny bit open. It has a hole in it, so fully closed is not zero air. I wanted to be able to slow things down a little more while still allowing some secondary air into the boiler. It's an experiment...

The VFD takes in single phase power and gives 3 phase output for the motor. This is a pretty common feature for the smaller VFD's.
 
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