I'm trying to explain why partially closing off a furnace fans return air grill, reduces, not increases, the motors current. Motor is conventional 1/2hp, 115v. I had assumed that restricting the air flow would have increased the torque and accordingly increased the amps, but obviously I was wrong.
Motor performance
- Thread starter cosmos
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kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
The basic answer is that you are reducing the mass flow across the blades. This decreases the difference between the pressure across the blades. The motor does not have to work as hard to overcome the work that is being done and therefore current is going to go down. However, the fan is no longer operating as efficiently, and therefore the reduction will not be a linear relationship.
On large fans without VFD's, the control system will close the dampers to reduce the starting torque required to overcome the acceleration of the air. The motor will then only have to overcome it's own inertia, and that of the fan and shaft.
On large fans without VFD's, the control system will close the dampers to reduce the starting torque required to overcome the acceleration of the air. The motor will then only have to overcome it's own inertia, and that of the fan and shaft.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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Think of it as the air within the fan spinning around with the blades instead of being displaced by them.
rt66electric
Senior Member
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- Oklahoma
reply to cosmos
reply to cosmos
In reply to the decrease of amps in fan. I watched a PBS show on vacum cleaners, and the reason is simple. Closeing off the intakes creates a vacum. There is less air resistance to the blades. Lower resistance/ Lower amps to turn the blades.
reply to cosmos
In reply to the decrease of amps in fan. I watched a PBS show on vacum cleaners, and the reason is simple. Closeing off the intakes creates a vacum. There is less air resistance to the blades. Lower resistance/ Lower amps to turn the blades.
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