tHERMAL SCIENCE
cooling tower performance
FA N M OTO R OV E R A M P I N G
Overamping
Overamping occurs when the amperage drawn by the fan motor
exceeds the fan motor nameplate amperage. In other words, more
horsepower is required than the recommended maximum for the
motor. Overamping is often thought to be caused by incorrect fan
pitch, but it is likely the result of other issues that are not inherently
related to the fan. Before attempting to repitch the fan blades,
please refer to the Troubleshooting list on page 2.
If following the troubleshooting list does not solve the problem, the
issue may lie with the fan pitch. The blades are pitched to draw
approximately the same volume of air year-round while consuming
power that is less than the motor nameplate. However, the air
density is higher in winter, and the fan will draw more horsepower
than in the summer to move the same volume of air. In addition,
the initial trial pitch is calculated using theoretical models, meaning
it may need adjustment after taking into account deviations in the
system. If incorrectly pitched, the fan can draw more than the
nameplate horsepower of the motor, leading to overamping. When
full nameplate amps are drawn at design conditions, the cold water
temperature will be in alignment with the contractual guarantee.
Please follow the fan user manual for the correlation between air
density and motor amperage.
Finally, it is important to note that overamping can be acceptable in
certain situations. Particularly, if the ambient conditions are much
colder and/or the range is much lower than design, the increased
air density may cause overamping. All fan motors are selected
with a 1.15 safety factor for operation at the maximum ambient
temperature, so a small amount of overamping at or below the
design ambient conditions will not cause the motor to overheat.
The relationship between power draw and amperage for 3-phase
power is shown by the following equation:
hp = V x I x √3 x fp x
η
746
kW = V x I x √3 x fp x η
Where V = voltage
I = currents (amps)
fp = power factor
η = efficiency
For example, assume an 1800 rpm motor has a nameplate of
40 hp, with a voltage of 460 V, and nameplate amperage of about
49 A. Efficiency is normally about 95% while the power factor
usually has a value of about 0.85. If the fan is overamping by 5%,
it is drawing about 51.6 A, or 42 hp. This assumes the voltage is
relatively constant. It is important to note that the power factor
is not constant for any particular system. Therefore, a calibrated
wattmeter is preferred when measuring power because it
automatically measures the motor power factor.
➠
Troubleshooting
Possible Cause of Overamping
1. Air density variation due to insufficient heat load or
colder ambient temperature than design
Solution
- Ensure cooling tower is operating at full design duty
at design ambient conditions
- Calibrate wattmeter/voltmeter/ammeter
2. Inaccurate instrumentation used in measuring
power/current draw
- Measure after cooling tower has been operating
continuously for 15 minutes (high current during
motor startup)
3. Low voltage
- Check system wiring (connections, length of wires,
etc.)
4. Power loss from point of measurement to motor
- Measure power as close to motor as possible
5. Overload heater tripping
- Measure voltage (low voltage causes trips,
particularly during start-up)
- Ensure heater is in correct location
6. VFD operating incorrectly
- Run the system in bypass mode; if overamping
ceases, the VFD should be inspected
7. Incorrect wiring
- Rewire system if it does not match the wiring
diagram
8. Incorrect motor speed
- Replace motor if speed is incorrect
9. Incorrect gearbox ratio
- Ensure the gear ratio is the same as the engineering
record
- Rotate fan by hand to manually count reduction ratio
10. Incorrect number of fan blades
- Compare fan blade count to that of the engineering
record; fan replacement is necessary if different
11. Incorrect blade pitch measurement procedure
- Ensure the proper measurement procedure is
followed (reference fan user manual for specific fan
model)
12. Oil in gearbox is viscous due to temperature
- Ensure adequate warm-up time for the gearbox
(typically 15 minutes minimum)
SPX COOLING TECHNOLOGIES, INC. | OVERLAND PARK, KS 66213
P: 913 664 7400 F: 913 664 7439 spxcooling@spx.com
spxcooling.com
In the interest of technological progress, all products are subject to design and/or material change without notice ISSUED 07/2014 TR-018
COPYRIGHT © 2014 SPX Corporation