Chuck Yung
EASA Technical Support Specialist
Most of us involved in the repair of electrical equipment have a good understanding of how an electric motor works–especially the stator and rotor. But the fan can appear deceptively simple. Fans are pretty interesting, once we learn a few “affinity laws”—rules that also apply to blowers and impellers. This article will review some basic facts about fans that explain how small changes to a fan can make a BIG difference in the following critical areas:
- Volume of air moved
- Static pressure
- Load
- Losses (efficiency)
These rules hold true for fan applications, impellers in pumping applications, and cooling fans on electric motors. When applied to the external fan of a TEFC (IP-54) motor, these rules offer some real opportunities for efficiency improvement.
Rules for fans
For proportional fans of different sizes, the hp required to rotate them corresponds to the ratio of diameters (d), to the fifth power. For any specific fan, the hp required to rotate it at different speeds is proportional to the speed change, cubed:
hp2 /hp1 = (d2/d1)5
hp2 /hp1 = (rpm2 /rpm1)3
To determine how effective a fan is, we have to know how much air it moves—the volume—and at what pressure. The volume of air a fan moves varies in proportion to the diameter change cubed, and in direct proportion to a change in speed:
volume2/volume1 = (d2/d1)3
volume2/volume1 = rpm2/rpm1
The second variable for airflow is static pressure. It’s much easier to move air at atmospheric pressure than at higher pressure, but the “thicker” the air, the more effective it is as a cooling medium. That’s logical; water is a superior cooling medium to air, because water is so much denser. Conversely, the lowest density would be a vacuum with no air to move. One variable that affects air density is altitude. For motors operating at altitudes above 3300 feet (1000 meters), the reduced air density requires special consideration.
If the static pressure—or air volume—is too high, a barrier of stagnant air forms a film (sort of like the ground shear that plagues airplanes) on stationary surfaces, and keeps the fast-moving air away from the part we’re trying to cool.
The ideal static pressure is high enough that the air is “thick” but not so high as to permit a barrier of dead air. Static pressure (sp) is proportional to the speed change, and to the diameter squared:
sp2/sp1 = rpm2/rpm1
sp2/sp1 = (d2/d1)2
A real-life example
Let’s look at an actual case that illustrates what can happen when affinity laws are ignored:
An end-user wanted to increase the airflow of an existing 25 hp fan application. He built a proportional larger replacement fan, and used a 75 hp motor to drive it. The original 25 hp motor was driving a 24” diameter fan at 450 rpm. The 40” diameter replacement fan was to be driven at 850 rpm. The motor burned up within a few minutes, and the fan never came up to speed. The 75 hp was obviously not adequate, but how do we explain what went wrong?
We’ll apply those affinity laws to illustrate what happened. First, consider the effects of the diameter change, involving that 5th power relationship. For the diameter ratio change from 24” to 40”:
hp2/hp1 = (d2/d1)5
(40/24)5 = 12.86, or nearly 13
That means it would take almost 13 times as much hp to drive the larger fan at the same speed.
Of course, the speed was also increased, which meant a cubed relationship. The speed change from 450 rpm to 850 rpm would require (850/ 450)3 = 6.74, or nearly 7 times as much hp.
If we assume that the fan originally required 15 hp to drive it (fans are often oversized to accelerate the relatively high inertia) at 450 rpm, then the modifications in our example would require
12.86 x 6.74 x 15 hp = 1,300 hp. No wonder that 75 hp motor burned up so fast! So, what else about fans would be useful to motor repairers?
For starters, the same affinity laws also apply to pump impellers. One special case is when we “trim” an impeller.
Trimming a fan or impeller
There are times when a replacement impeller must be trimmed (machined to a slightly smaller diameter) to reduce the hp requirement. Because the affinity laws apply to fans that are geometrically proportional, trimming the diameter of a fan—or impeller—requires a modification of those affinity laws. The adjustment is to reduce the exponent by one. For a diameter reduction by trimming, use:
hp2/hp1 = (d2/d1)4
hp2 /hp1= (rpm2/rpm1)2
The above adjustments will give reasonable results when it’s necessary to machine an impeller or fan to a smaller diameter.
VFDs and fans
The use of variable frequency drives (VFDs) means that a lot of electric motors are running slower than their original design speed. Since the effect of speed on airflow is linear (volume2/volume1 = rpm2/rpm1), the fan on a motor rotating at half speed only moves half as much cooling air across the motor. When we factor in static pressure, a fan rotating at half the rpm moves half the volume at half the static pressure, the cooling effectiveness is considerably less than at the original speed.
Other factors that affect airflow
The fan cover also plays a major role in the effectiveness of airflow. The role of the fan cover in directing air across the exterior of the motor is obvious, but the importance of openings in the fan cover is less obvious. Air flows from regions of high pressure to regions of low pressure. That’s why air moving across the motor actually turns to follow the drive end (DE) contours, and cools the DE bearing.
As Figure 1 indicates, the grid openings disrupt the air as it approaches the fan. The airflow remains turbulent for a distance approximately 3 times the width of the opening. That makes the distance from the fan to the fan cover critical. If the fan is too close to the cover, it will be considerably less effective at moving air. Given a typical 7/16” (11 mm) opening, this means the fan cover should be at least 13/8” (35 mm) from the fan. Caution!—With bulky products such as paper pulp, smaller openings are more likely to clog.
As the fan covers shown at the top of Figure 1 illustrate, there are numerous variations in fan cover design. Opening sizes and shapes vary; so does the turbulent zone between each cover and the fan. The permissible distance from fan to cover depends on the width of the openings. If inspection reveals that a fan cover is too close to the fan, the airflow can be improved.
The smaller the obstruction in an airstream, the less turbulence occurs. And the more aerodynamic the shape of the obstruction (in relation to the direction of airflow), the smoother the airflow will be. (See a, b and c in the inset in Figure 1.) A round grid profile works better than the typical squared cross-section. When considering any obstruction in the airflow, the more its cross-section resembles a wing the better it is for airflow.
Because the fan is rotating, the air exiting the fan forms a vortex. The sooner the airflow straightens out (laminar flow), the more effectively it can remove heat from the motor. Those ribs on the exterior of a TEFC (IP-54) act to straighten the airflow, in addition to their main function (which is to increase the effective surface area of the stator to increase cooling effectiveness). The leading edge of the ribs are normally rounded (like the leading edge of an airplane wing) and tapered to smooth the transition from turbulent to laminar flow. A steep taper, or a set of steps as shown on Figure 2, seems to hasten the transition to axial airflow. Some turbulence is desirable, to prevent that layer of dead air from forming. A glass-smooth exterior finish would not be as effective at transferring heat.
Opportunity to increase efficiency
In appearance, the external fan of a TEFC motor seems simple. As motor people, we care about the fan’s ability to cool the motor. As long as the fan does that, we’re satisfied.
But fan design offers an opportunity to increase the efficiency of many pre-EPAct electric motors. Fan upgrade offers a simple method for increasing the efficiency of TEFC (IP-54) motors, without even rewinding them.
The radial fan is used in nearly all TEFC motors because manufacturers—not being privy to the intended use of each motor sold—have to produce motors that can rotate either clockwise or counter-clockwise. The tradeoff for this versatility is fan efficiency. The radial fan is capable of a maximum of 60% efficiency. Contrast that to the Airfoil fan, which can reach 92% efficiency. That translates to a nearly eight-fold reduction in windage losses. For every 100 watts, it would break down this way:
100w x 0.6 = 60w useful (40w lost) 60w/0.92 = 65.2w input (5.2w lost) 40 / 5.2 = 7.7
For the service center, this offers a significant opportunity to improve the efficiency of an enduser’s TEFC motor by replacing the stock radial fan with a more efficient unidirectional airfoil design. hat means every TEFC motor driving a pump, fan, blower or other unidirectional application is a candidate for efficiency improvement.
The key
Anything that reduces losses improves efficiency. Friction and windage losses account for 5 to 10% of the total losses in a typical TEFC motor (Table 1). To put that into perspective, windage— which accounts for most of the F&W losses— adds about 9,500 watts in losses to a typical pre-EPAct 200 hp motor.
The bottom line is that the motor could be cooled with a substantial reduction in windage losses. That represents a measurable improvement in efficiency. Table 1 lists the actual breakdown of losses for several typical motors, and potential efficiency improvements with optimum fan replacement.
When any of the component losses of a motor are reduced, the efficiency increases.
The key point of this article is the sixth column in Table 1, which shows the potential improvement in efficiency attainable by changing the fan to a more efficient design.
That efficiency gain makes the more efficient fan an attractive investment, with savings that continue for the life of the motor. This simple measure is applicable to every fan-cooled unidirectional motor. Given the number of motors in service today, an across-the-board efficiency increase of 1/2 - 1% without replacing the motors would save an enormous amount of energy.
An important factor here is the motor design. Windage losses in 2-pole machines are higher than those of 4-pole and slower machines. Energy efficient motors are more efficient by design – building motors with lower I2R and core losses enabled motor manufacturers to reduce the fan size, further improving efficiency by reducing windage losses. Pre-EPAct motors are better candidates for this modification – because the windage losses are higher, although improvement to energy efficient motors is still viable. For EPAct motors, efficiency improvement of about 1/2% is possible.
Noise
A final important consideration with fan design is aerodynamic noise. For fans delivering the same volume of air, an airfoil fan or a sirocco fan produces less noise than a radial fan.
Empirical data is tough to come by, but it suggests that a sirocco fan is approximately 5 dbA lower than a radial fan moving the same volume of air. That is quite a difference, given that the sound pressure scale is logarithmic. Since every 3 dbA increase indicates the sound level has doubled, five decibels represents a substantial reduction in noise level.
Summing up
Fan affinity laws offer factual information about fan performance. These laws are consistent: Fan manufacturers routinely test one fan design and extrapolate performance data for fans of the same design, at a range of diameters and speeds. Being aware of these laws will help the motor repairer better understand airflow and motor cooling, as well as the potential for efficiency improvement.
Knowledge of the relationship between the fan cover and fan can maximize the effectiveness of the airflow that is vital to cool the motor. When circumstances warrant, a service center can improve the original design by changing to a more effective fan, and/or by modifying the fan cover to do its job better.
For pre-EPAct motors, where windage losses are higher than for comparable energy efficient motors, up to 1 point of efficiency may be gained.
For instance, consider the opportunity for a plant with thirty 50 hp electric motors operating at 91.7% efficiency, compared to EPAct motors of 93.0% efficiency. Various groups advocate replacement of those motors, at considerable capital expense. A logical alternative—replacing the external fan—could yield nearly the same gain for a much smaller investment. And most of that cost can be expensed.
For more information on fans and airflow, you might want to purchase a copy of the Fan Handbook. The book is written by Frank P. Bleier and published by McGraw-Hill.
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