A
bout the author
Drives, motors and switchgear
Based on the affinity laws, flow changes with speed while pressure
is proportional to the square of speed. The power required is propor-
tional to the cube of the speed. The latter is most important because
if the motor speed drops, the power drops by the cube.
Figure 4: Affinity laws – flow/ speed.
Figure 5: Affinity laws – pressure/ speed.
Figure 6: Affinity laws – power/ speed.
In this example, the motor is operated at 80% of the rated speed. This
value can be inserted into the affinity laws formula to calculate the
power at this new speed:
HP
2
N
2
3
HP
1
N
1
N
2
3
80
3
N
1
100
0,80
3
= 0,512
0,512 = 51,2 percent
Therefore, the power required to operate the fan at 80% speed is half
the rated power.
Figure 7: Flow and pressure relationship.
Figure 8: Flow and power relationship.
Conclusion
Choosing a soft starter or a VFD often depends on our application.
Soft starters are smaller and less expensive when compared with
VFDs in larger horsepower applications. Larger VFDs take up more
space and are usually more expensive than soft starters.
That being said, while a VFD is often more expensive up front, it
can provide energy savings of up to 50%, thereby producing more
cost savings over the life of the equipment.
Speed control is another advantage of a VFD, because it offers
consistent acceleration time throughout the entire operating cycle
of the motor, not just during start-up. VFDs can also provide more
robust functionality than soft starters offer, including digital diagnostic
information.
It is important to note that a VFD can initially cost two to three
times more than a soft starter. Therefore, if constant acceleration and
torque control is not necessary, and your application requires current
limiting only during start-up, a soft starter may be a better solution
from a cost standpoint.
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Kyle Danielson is the Variable Speed Drive & Soft Starter product manager
at Eaton in Wadeville, Germiston, South Africa.
Enquiries: Tel. 011 824 7400 or email
.
Electricity+Control
July ‘13
6
