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