Explaining the Increase in Fan Amperage with Installation of a V-­‐Belt Tensioner White Paper By Stephen Heston, Product Development Engineer A publication of Fenner Drives
© Fenner Drives, 2013. All rights reserved.
A common question posed by HVAC technicians is, “If installing a tensioner on a belt-­‐driven fan should improve efficiency, why does the amperage draw of the motor increase when a tensioner is installed?” Short Explanation: The reason the fan amperage increases is that the tensioner reduces belt slip, which brings the fan wheel up to a higher rpm. At this higher rpm, the fan is moving more air, so the power draw of the motor increases. It actually increases with the speed cubed! Detailed Explanation: In order to understand exactly what is happening, we need to closely examine the principles of fan operation. We need to look at the reduced fan speed caused by a slipping belt, the resulting change in brake horsepower of the fan, the change in efficiency of the belt, and finally, the effect that these changes have on the motor torque. Consider the centrifugal fan system shown in the diagram below. !! = !" !"!#$%&#'" !"#$% !! = !∆! FAN AIR POWER (AHP) !! = !!" !! MOTOR SHAFT POWER !! = !!" !! FAN SHAFT POWER (BHP) This fan is designed to operate at 2500cfm with a static pressure rise of 2in wg. Assuming standard temperature and pressures of 70oF and 1atm, this gives an airflow power of WA1=0.94hp. This corresponds to Point 1 on the manufacturer’s performance curves shown in Chart 1, giving a mechanical efficiency of ɳF=55% and a brake horsepower of: !!! =
!!!
ɳ!
= 1.70 hp 2 Chart 1: Fan Performance Curve 3 3 2.5 Sta]c P (A) 1 Sta]c P (B) 2.5 2 2 System Curve Efficiency 2 1.5 * 1.5 1 1 * 0.5 0.5 0 BHP Brake Horsepower Sta]c Pressure (in. wg), Efficiency 3.5 0 0 1 CFM (x 1000) 2 3 4 5 This is the initial operating point of the fan (Point 1 on the chart). However, over time, tension decay in the v-­‐belt will cause increased slip in the drive. A white paper providing a detailed analysis of this phenomenon can be found on the Fenner Drives website at www.fennerdrives.com/white-­‐papers/. From the Fan Laws, we know that power is proportional to the cube of the quotient of the speed, ie: !!! = !!!
!! !
!!
This is the power required at the fan shaft. Assume the fan is driven by an A-­‐50 v-­‐belt, tensioned at 70 pounds using a 1.25:1 pulley ratio. Over a period of time, tension decay causes belt slip to increase from 1% to 3%, causing the fan to drop from its initial operating speed of ω1=1386rpm to ω2=1344rpm. The brake horsepower required at the new operating point (2) will then be: !!! = 1.70 ℎ!
!"## !"# !
!"#$ !"#
= 1.55 hp Assume an initial efficiency for this belt drive of (ɳD= 96%). The motor operates at (1750) rpm and an efficiency of (ɳm= 85%). The electrical power required by the motor (disregarding power factor) is equal to the fan brake horsepower divided by the product of the drive and motor efficiencies: !!!
!!! = ɳ
! ɳ!
= !.!"!
.!" .!"
= 2.10hp 3 With the change in rpm of the fan, if the belt drive were operating at the same efficiency as (1), the motor power would be: !
!!!
=
!.!! !!
!.!" !.!"
= 1.90 hp However, factoring in a reduction in the belt drive efficiency of 2% due to the increased slip, the actual electrical power required is: !!! =
!.!! !!
!.!" !.!"
= 1.94 hp So, in this example the slipping belt caused an increase in power draw of 0.04hp due to the loss of efficiency, however it caused a larger offsetting reduction in power draw of 0.2hp due to the reduction in airflow. In other words, the difference between WE2 and WE2’ is less than the difference between WE1 and WE2. The important point to note is that the increased belt slip does result in decreased drive efficiency due to frictional losses. However, these frictional losses are not visible because the change in fan speed causes a larger offsetting reduction in motor amperage. So to an untrained eye, it appears that a poorly tensioned v-­‐belt uses less power, but this is only because it is actually doing less work. In this case, it is moving less air. In cases where reduced airflow is acceptable, the pulley ratio of the drive should be changed to reduce the fan speed, and a tensioner should be used to maintain optimum efficiency. Allowing the belt to slip is not a reasonable way to control airflow. It results in wasted energy and decreased belt life. 4 Contact Headquarters 311 West Stiegel Street Manheim, PA 17545 US www.fennerdrives.com Technical Support ae@fennerdrives.com Product Information customercare@fennerdrives.com Telephone US: +1.800.243.3374 US: +1.717.665.2421 UK: +44 (0) 1924 482 470 The information in this document is subject to change without notice. No part of this document may
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Fenner Drives assumes no liability for any damages or injury incurred, directly or indirectly, from any errors, omissions or discrepancies between the technical product documentation and the information contained in this document, or by misuse of the product. ©2013 Fenner Drives FDWP002-­‐001 5