I am not too familiar with motors so hoping someone could shed some light on this one. When a fan is sized for an example 30HP based on CFM required and the fan is specified at 50HP with a brake HP of 30HP (Design) the motor is adjusted to operate to output the CFM of a 30HP fan but will the current draw still be 50HP? or 30HP? assuming an across the line start no VFD, two speed etc.
General Motor Question
- Thread starter Mike01
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Motors driving centrifigal fans and pumps are loaded by the volume of material the impeller is moving. Putting a damper or restriction in the pipe or duct will reduce the volume of flow and put less load on the driving motor.
LarryFine
Master Electrician Electric Contractor Richmond VA
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My instincts say somewhere between the two, but it should be closer to the latter, like an under-loaded transformer.
Here's my understanding: A motor operated at less than its rated HP will provide the Brake HP the motor mechanically drives + its loss/efficiancy at that operating point.
For your example the motor will not take 50HP from the electrical source when driving a 30HP mechanical load.
For a 50HP motor, its efficiecy at 100% load may be 90%, at 50% load (25HP) its efficiency may be 85%, so the motor takes less from the power line but is not reduced linearly since the efficeincy also decreases.
As Larry said, the 50HP motor will take less than 50HP when it is driving a 30HP mechanical load, allthough without doing the numbers I think it would be on the order of 40HP.
For your example the motor will not take 50HP from the electrical source when driving a 30HP mechanical load.
For a 50HP motor, its efficiecy at 100% load may be 90%, at 50% load (25HP) its efficiency may be 85%, so the motor takes less from the power line but is not reduced linearly since the efficeincy also decreases.
As Larry said, the 50HP motor will take less than 50HP when it is driving a 30HP mechanical load, allthough without doing the numbers I think it would be on the order of 40HP.
gar
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From a table in Bailey and Gault p211 for a 3 phase induction motor rated 35 HP.
Output HP vs efficiency
48.1 0.879
36.8 0.880
28.0 0.875
19.5 0.856
10.3 0.782
This is plotted on p213 along with current, speed, and power factor.
.
From a table in Bailey and Gault p211 for a 3 phase induction motor rated 35 HP.
Output HP vs efficiency
48.1 0.879
36.8 0.880
28.0 0.875
19.5 0.856
10.3 0.782
This is plotted on p213 along with current, speed, and power factor.
.
GeorgeB
ElectroHydraulics engineer (retired)
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You raise 2 different issues reading between the lines, starting and running.
Peak, 2 or 3 cycle inrush (flux development) for a 50HP motor vs a 30HP motor will be roughly in the 5:3 proportion. But you never mentioned a 30HP motor, so that is not of interest.
With similar motors (same family from same manufacturer), the CURRENT of the 50HP loaded to 30HP will be higher than a 30HP loaded to 30HP. The POWER will be very close to the same.
Taking a Baldor CM4115T, at 50% load it is 93.1% efficient, 33.8A; at 75% load, it is 93.6%, 45.6A.
DARNIT, they don't give performance data on the 25, 30, or 40 versions of that motor ... but a 25HP U-frame at 100% is 92.4%, 30.35A ... less efficient (smaller motors GENERALLY are), lower amps. So what we see here is that THIS 50HP uses less energy at half load than THIS 25HP at full load ... but I was unable to do an apples:apples comparison with web data. This contradicts my premise above ... dumb engineer ...
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