It is essential to measure the motor Kw input when to
analysis the tower performance. This measurement could
be done at the motor terminal box, but it is sometime
more reasonable to measure the motor Kw input at the
motor control center because the voltage and power factor
indicators are usually not available at the tower fan
deck. Now, the discussion will be focused how to determine
the line voltage drop from the motor control center
to the motor terminal box.
Example 20-1:
Determine the line voltage loss for 1200 feet away from
the motor control center to the motor using the example
6-1. Let's assume that the 175HP of motor is applied
for the initial conditions of example 6-1 and the motor
efficiency is 92.8% at the full load. The power supply
is 460VAC, 60 Hz, 3 phases, and 4 poles. The cable between
motor control center and the motor is "Bare Copper,
250 AWG. The measured ampere at the motor control center
was 203A.
(Solution)
First, find DC resistance in Ohm per 1000 ft of cable
length from the table of properties of conductors
- DC Resistance per 1000
feet = 0.0431 Ohms / 1000 ft
Multiplying Factor for Converting
DC Resistance to AC Resistance from the table = 1.06
Then, AC Resistance = DC Resistance x (Cable Length
/ 1000) x Multiplying Factor = 0.0431 x (1,200 / 1,000)
x 1.06 = 0.05482 Ohms
Second, determine the KW
loss per the given power supply specifications.
- KW Loss for 3 Phases =
3 x I2R / 1000 = 3 x 2032 x 0.05482 / 1000 = 6.78 KW
Third, compute the net KW
input to motor.
- Net KW Input to Motor =
Measured KW @ Motor Control Center
- KW Line Loss - Measured KW = 1.7321 x Ampere x Voltage
x Power Factor / 1000 = 1.7321 x 189 x 460 x 0.86977
/ 1000 (Power Factor = 0.86977) = 140.68 KW
(The KW could be measured
with the wattmeter or calculated after measuring ampere,
voltage, and power factor.)
Finally, determine the motor
shaft BHP. - Motor Shaft BHP = Measured KW @ Motor Control
Center x Motor Efficiency @Full Load / 0.746 = 140.68
x 0.928 / 0.746 = 174.77 HP
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