MRO Magazine

Tech Tips: Why premium efficiency motors may not be the lowest cost solution for fans

In this issue's episode of the Fan Guys, there were numerous instances of power being transmitted. We saw the lads pushing the canoe through the water, the lightning bolt transmitting energy from sky ...


April 1, 2003
By Jim Wywrot

In this issue’s episode of the Fan Guys, there were numerous instances of power being transmitted. We saw the lads pushing the canoe through the water, the lightning bolt transmitting energy from sky to ground, as well as the hyper-drive of the space ship.

When people think about the transmission of power, they usually think of doing it as efficiently as possible to minimize efficiency losses. So today we will discuss efficiency, or more specifically, the economies of efficiency.

As no one has unlimited reserves when it comes to capital or maintenance spending, we continually make choices. So we begin with a question. Given a choice, does it make more sense to always purchase premium efficiency motors with a less-efficient fan, or would it be better to spend any extra money on a more efficient fan and use a standard or EPACT efficiency motor?

Let’s use the example of a dust collector application fan moving 25,000 cfm at 14″ static pressure. Looking at a selection of radial bladed wheels, we would see a typical selection range as follows (see Table 1).

We can see that a Size 33 or 29 can be operated with a 100-hp motor, but the Size 26 requires a 125-hp motor. Using these criteria, we might eliminate the Size 26 fan, leaving us two options that we can analyze easily.

Looking at purchase costs we would see the following (see Table 2).

From this comparison we may think the premium efficiency motor, with a reasonably efficient fan (the Size 29), is a good compromise overall.

But in order to answer the question of which version makes more sense –a) use the highest efficiency fan with a EPACT efficiency motor, or b) use a lower-efficiency fan with a premium efficiency motor — we need to look at energy costs.

Compare one year’s energy costs based on 10 cents/kWh as an average of peak and demand charges (see Table 3).

From this table, we can see that given a choice, it is more economical to pick the highest efficiency fan with a standard efficiency motor, than to use a less efficient fan with a premium efficiency motor. In this particular case, the difference is a savings of about $4,096 in energy costs for each year of operation. This occurs because, while the premium efficiency motor gives a 0.7% increase efficiency, the more efficient fan option provides a 4.5% efficiency improvement.

Bear in mind that each application will be unique and must be looked at on a case-by-case basis. In addition, there are some other advantages of premium efficiency motors aside from energy savings.

The intent here is to show that a blanket policy stating premium efficiency motors shall be used in all cases — with no corresponding policy of purchasing the most efficient equipment — may not be the best use of available funds.

Table 1

Size1 Wheel Speed Efficiency HP Installed
Dia. (in.) (rpm) (per cent) Motor
33 58 902 63.40% 86.9 100
29 51 1082 58.90% 93.5 100
26 45 1298 52.80% 104.0 125

1 The fan size number used refers to the inlet diameter.

Table 2

Size Fan EPACT Motor Purchase Cost
33 $13,000 $4,000 $ 17,000.00
29 $11,000 $4,000 $ 15,000.00

Size Fan Prem. Eff. Purchase Cost
33 $13,000 $5,000 $ 18,000.00
29 $11,000 $5,000 $ 16,000.00

Table 3

Size Motor Fan Eff. % Motor Eff. % Energy Cost Per Year
33 EPACT 63.40% 94.3% $ 60,221.42
29 Prem. Eff. 58.90% 95.0% $ 64,317.76

The Fan Guys is a creation of Jim Wywrot, P.Eng., Mark Bugdale and illustrator Richard Comely (creator of Captain Canuck). For more information, visit www.fanguys.com.