Motor Selection: Centrifugal Blower for Exhaust Hoods

As a mecanical engineer, I'd think about it looking at the fan I'd have to install. What you have here is a relation between them regarding speed. One has more torque to another, and can drive a larger fan, which, in its turn, will have a higher blade tip speed. Check out the fan you have and choose the motor for the one you need. As it is a centrifugal blower, and of course you can speed it up as desired, choose the 1440 rpm one to increase flow. Without details about the blower, I am assuming the 5HP motor can drive the blower, because it will have proportionally less torque.

The power consuption, by the informations you posted, are nominally the same. Maybe someone from electrics has a tip.

As was previously stated, it has much to do with the amount of resistance provided by the fan, which relates to the torque available from the motor. the lower speed motors have higher torque values in general.

for example: as a rule of thumb: @ 1800 rpm, a motor develops 3lb-ft-per-hp @ 1200 rpm, a motor develops 4.5 lb-ft-per hp.

Another factor is simply the efficiency of the motor which varies from one manufacturer to another.

Will the blowers be switching on/off frequently? or constantly running?

single phase? 3 phase?

from my experience 960 rpm & 1440 rpm are slightly less common that 1200 & 1800rpm motors.

To get a "credible" answer, you might try contacting the manufacturer, and speaking with someone in their technical/engineering department about your application.

www.harzellfan.co

can supply all your answers.

I'd get there catalogue for fan types and engineering data.

I've dealt with them for years.

MR. GUY

If u strictly confine to motor aspects only a 1500 or 4 pole is more efficient than a 960 which is 6 pole.you can see this in motor manufacturers catalogue.When freq is 50 HZ 4pole m/c has 1500 syn rpm and 6pole has 1000 which corresponds to 1800 and 1200 when a 60Hz supply is given.Thi sis t clarify a postwhich states 1200 and 1800 are more common.This depends upon the supply frequency

yes, generally speaking, in America with 60hz standard supplied power 1800rpm and 1200rpm are more common. I cannot speak for the rest of the world where 50hz is standard supplied power.

If it's an existing fan and drive arrangement, the 960 rpm motor uses less than the 1440, because the power drawn by the fan varies as speed³. But of course the fan performance is also much lower.

If you know how fast the fan needs to run for the required performance, first check that 5 HP is enough to drive it at that speed. 5 HP suggests a smallish fan so the speed is likely to be high, > 1440 rpm. In that case it's better to go for 1440 rpm motor as step-up ratio on the V-drive is lower. Also as others have said, efficiency tends to fall somewhat with increasing number of poles.

Cheers..........Codey