D R I V E S
&
S W I T C H G E A R
O
n some electric motors, the internal and/or external fans can be fitted in
either direction. Frequently repair companies do not properly identify and
mark the fans and this results in fans being incorrectly re-fitted.
Centrifugal fans direction of rotation explained
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.
by Henk de Swardt, Marthinusen & Coutts
Fig. 1: Example of a bi-directional fan.
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 uni-
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
Item
@ % Load
Value
Unit
Power Output
100%
1200
kW
75%
900
0%
0
Item
@ % Load
Direction of rotation
Correct
Air flow rate coefficient
Current
Unit
Incorrect
3,03
3,33
"Fan losses increase: Power of"
Energy costs
3
5
35,0
46,6
56,4
kW
100%
122,4
123,4
124,3
A
75%
94,7
95,7
96,5
Fan losses
0%
35,6
35,7
35,8
Speed
100%
2985,3
2985,2
2985,1
75%
2989,1
2989,0
2988,9
Total losses
100%
64,5
76,6
86,8
75%
54,8
66,7
76,8
0%
42,2
54,4
64,2
Efficiency
100%
94.90%
94,00%
93,25%
75%
94,26%
93,10%
92,13%
Input Power
100%
1264
1277
1287
75%
955
967
977
75,4
77,6
80,8
Temperature rise
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.
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).
kW
kW
°C
Table 1: Changes when rotation is reversed.
Eqn. 1: Calculating the direct energy cost.
I
Item
Value
Unit
Power Output
1200
kW
Item
Direction of rotation
Correct
Unit
Incorrect
Fan losses increase: Power of
3
5
Fan losses
35,0
46,6
+ 24,9%
56,4
+ 37,9%
Current
122,4
123,4
+ 0,9%
124,3
+ 1,6%
A
Total losses
64,5
76,6
+ 15,7%
86,8
+ 25,7%
kW
Efficiency
94,90%
94,00%
- 0,9%
93,25%
- 1,7%
Power Factor
0,905
0,906
+ 0,1%
0,933
+ 2,9%
"Light duty
(one year)"
Energy cost
R 910 k
R 919 k
+ 0,9%
R 926 k
+ 1,7%
"Standard duty
(one year)"
Energy cost
R 910 k
R 919 k
+ 0,9%
R 926 k
+ 1.,%
kW
Table 2: Case study calculation.
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.
C o n s i d e r a 1 2 0 0 k W, 2 p o l e ,
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
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,
[email protected] D