Caluculate Voltage Drop Due to Conductor Resitence on a Circut with an A/C Motor

[eyust]

I have written this problem myself to present to my professor. He teaches HVAC and Electrical for the department of Construction Management. Some of the problems and solutions we do in class are inconsistent with the material I was taught in physics. He really appreciates my criticism and I actually found a few errors in the material he has been teaching for the past 15 years. I am, however, a bit confused when it comes to electric motors.

Homework Statement

In this problem you are given a simple circuit with a voltage of 120v. It has three resistors in series. R1, R2, and R3. R1 and R2 is your line resistance. R3 is http://www.grainger.com/Grainger/DAYTON-Direct-Drive-Blower-Motor-3LU91?Pid=search" .

Given:

Vt across entire circuit is = 120v
R1 = R2 = .25 ohms
Single phase A/C circut
PF= 1

Motor Specs:
1 HP
Voltage: 115v
Full load Amps: 14.7
Hz: 60
Single Phase motor
Efficiency: High

Calculate total Voltage drop due to R1 and R2 when the motor is at a constant full load

2. Homework Equations

R=V/I

Rs= R1+ R2 + R3 +...

I=V/T

The Attempt at a Solution

Use R=V/I to find the resistance applied to the circuit when the motor is at full load.

115/14.7= 7.823 ohms

Find total R

.25 + .25 + 7.823 = 8.323 ohms = Rt

***Will this always be the resistance applied by the motor at full load if only voltage across the motor changes? Based on the spec of the motor and properties of A/C motors is this a correct assumption?***

Use Rt and Vt to calculate It

It = Vt/Rt

120/8.323= 14.418 amps = total current

Use Total Current, R1 and R2 to calculate voltage drop due to line resistance.

R1 + R2 = Rline

Rline = .25 + .25 = .5ohms

Voltage drop = It * Rline

Voltage drop = 14.418 * .5 = 7.209V

 

[anathema]

First of all, it is great that you are questioning and critically analyzing the material being taught in class. This shows that you have a strong understanding of the concepts and are able to identify errors or inconsistencies.

In terms of your question about the resistance applied by the motor at full load, it is important to note that the resistance of the motor will vary depending on the operating conditions. In this case, the motor is at full load and therefore the resistance will be different compared to when it is not at full load. The resistance will also vary based on other factors such as temperature and age of the motor.

Your calculation for the total voltage drop due to R1 and R2 when the motor is at full load is correct. However, it is important to note that this is not the only voltage drop in the circuit. There will also be a voltage drop across the motor itself, which will depend on its own internal resistance and the amount of current flowing through it.

Overall, your approach and calculations are correct. Keep questioning and analyzing the material in order to gain a deeper understanding of the concepts.