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There seems to be some confusion in the electric motor repair industry regarding the rotation direction of a motor’s cooling fans. We have seen cases where the fans removed from the motor were incorrectly installed. This paper explains the basic principles behind correct fitting of fans.
For general applications the rules listed here should hold true.
Centrifugal fans can in general turn in both directions. The performance of the fan may however not be identical for both directions. Centrifugal fans can only blow air radially outwards.
Bi-directional fans
A “bi-directional fan” has straight blades, perpendicular to the shaft, which may or may not extend to the boss
(inside section of the fan that holds the fan on the shaft).
This type of fan can rotate in both directions, without any change in fan power consumption and air volume.
These fans are frequently used on a “slow” speed motor, i.e. a motor with a rotational speed of less than
1300 rpm.
Uni-directional fans
A “uni-directional fan” may have straight or curved blades. The blades are however at an angle to the shaft, which may or may not extend to the boss
(inside section of the fan that holds the fan on the shaft).
Fig. 2 shows an example of a unidirectional fan with curved blades.
Fig. 3 shows an example of a uniby Henk de Swardt, Marthinusen & Coutts
Fig. 1: Example of a bi-directional fan.
Fig. 3: Example of a uni-directional fan, with straight blades.
directional fan with straight blades.
In these examples, the correct direction of rotation is clockwise as viewing the fans from the inlet side (as shown currently in the Figs).
Using these fans in the opposite direction from that shown here will result in an increase in the fan’s power consumption, fan noise and also the air volume it moves.
July 2008 - Vector - Page 45
Fig. 2: Example of a uni-directional fan with curved blades.
Fig. 4: The “backwardly curved blades” principle is used in determining the direction of rotation for angled or curved blades.
The principle used for determining the direction of rotation is “backwardly curved blades”. Fig. 4 shows how this principle is used.
Consequences of incorrect direction of rotation
Although the increase in air volume is good, unfortunately in most cases the increase in fan power usage outweighs

I
Item
Power Output
Item
Fan losses increase: Power of
Fan losses
Current
Total losses
Efficiency
"Light duty
(one year)"
Power Factor
Energy cost
"Standard duty
(one year)"
Energy cost
Item
Power Output
@ % Load
100%
75%
0%
Value
1200
900
0
Item @ % Load
Air flow rate coefficient
"Fan losses increase: Power of"
Fan losses
Current
Speed
100%
75%
0%
100%
Total losses
Efficiency
Input Power
75%
100%
75%
0%
100%
75%
100%
75%
Temperature rise
Correct
Direction of rotation
Incorrect
3,03 3,33
35,0
122,4
94,7
35,6
3
46,6
123,4
95,7
5
56,4
124,3
96,5
2985,3
2989,1
64,5
54,8
35,7
2985,2
2989,0
76,6
35,8
2985,1
2988,9
86,8
42,2
94.90%
94,26%
1264
66,7
54,4
94,00%
93,10%
76,8
64,2
93,25%
92,13%
955
75,4
1277
967
77,6
1287
977
80,8
Table 1: Changes when rotation is reversed.
Value
1200
Correct
35,0
122,4
64,5
94,90%
0,905
R 910 k
R 910 k
46,6
123,4
76,6
94,00%
0,906
R 919 k
R 919 k
Direction of rotation
Incorrect
3
+ 24,9%
+ 0,9%
+ 15,7%
- 0,9%
+ 0,1%
+ 0,9%
56,4
124,3
86,8
93,25%
0,933
R 926 k
+ 0,9% R 926 k
Table 2: Case study calculation.
Unit kW
Unit kW
A kW kW
°C
5
+ 37,9%
+ 1,6%
+ 25,7%
- 1,7%
+ 2,9%
+ 1,7%
+ 1.,% the benefit of an increase in air flow.
In practical terms, it is difficult to estimate exactly what the increase in flow and power will be for a specific fan without extensive finite element modelling.
Let’s assume some values for a specific application and study the effects on the motor’s performance.
Consider a 1200 kW, 2 pole,
6600 V motor that has been designed to turn in an anti-clockwise direction.
This motor is coupled to a load that requires a clockwise direction of rotation and is electrically connected accordingly. Let’s also assume the three (two internal and one external) fans will produce as much as 10% more air flow (even with the increased pressure drop through the heat exchanger) because of the incorrect direction of rotation. In general the increase in power would be between the cube and the power of five of the increase in flow, thus the power consumption of the fans would increase by between 33 and 61%.
Let us compare the performance characteristics for the correct and incorrect direction of rotation of the fans for both the best case of 33% increase in fan losses as well as for the worst case of 61% increase in fan losses.
Even though the air flow increased significantly through the motor, which results in better cooling of the motor, the much higher increase in the fan losses far outweighs this improved cooling and results in a motor that consumes more power and runs at a higher temperature.
The motor’s efficiency is greatly reduced and the full load current is increased.
An interesting development is that customers may experience overheating problems as a result of higher operating loads, as demands on production increases. We have seen cases where motors have run for years at less than full load power and at a lower temperature.
The customer did not know that the fan losses actually increased the running temperature of the motor, since the overall temperature was well below the limits. However, when production demands increased, and the load at which the motor ran increased, it was found that the motor actually could not drive the driven load at the full load power without overheating. On
July 2008 - Vector - Page 46
Unit kW
Unit kW
A kW investigation it was found that the direction of the fans was incorrect. We corrected this mistake and the motor could deliver the correct full load power while still complying with the specified temperature rise.
Energy costs
To put the cost of incorrect fan fitment into perspective, the direct energy costs will be calculated using equation 1.
The rate for the energy will be used as 23c/kWh. The calculations will be annualised, assuming running times of firstly for a light duty, 12 hour per day,
5 days per week, and secondly for a normal duty, 24 hours per day, 7 days per week. (See table 2).
Eqn. 1: Calculating the direct energy cost.
where: h = hours running per day d = days running per year
If we apply this calculation to the case study, we can see the additional costs as a result of the lower efficiency.
Although the percentage change looks relatively small, the result on a population of a few hundred or a few thousand motors of various sizes can have huge implications on the energy bill of the company. And these additional costs could have been easily avoided.
Conclusion
In order for the electric motor to operate at its peak designed performance, the fan needs to be correctly installed and designed to suit the specific application.
It must be noted that the influence of the direction of rotation on the air flow and fan losses become less important in slow speed motors.
In many cases the direction of rotation in relation to the construction of the external fan can easily by checked by means of visual inspection on site.
Whenever a motor is overhauled or repaired it is recommended that the correctness of the orientation of the internal and external fans should be checked.
In instances where there is doubt, it is better to contact experts in the motor repair and manufacture industry to ensure the motor and plant’s long term reliability and dependability.
Contact Henk de Swardt,
Marthinusen & Coutts,
Tel 011 616 2320, henkd@mandc.co.za D