Induction Motor Problem, How to find Rc

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Discussion Overview

The discussion revolves around calculating various parameters of a three-phase induction motor, specifically focusing on how to determine the core resistance (Rc) in the context of line current, power factor, shaft torque, output horsepower, and efficiency. The conversation includes aspects of theoretical calculations and practical approximations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant questions how to find the core resistance (Rc) necessary for calculating the line current and equivalent impedance.
  • Another participant notes that the core losses are constant at 262 W and asks how to incorporate this into the calculations.
  • There is a suggestion to express the fixed losses as an equivalent line current and add this to the calculations after determining other parameters.
  • Some participants express uncertainty about the validity of approximating the core resistance as negligible after accounting for core losses separately.
  • One participant seeks clarification on which equivalent circuit is being used in the calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to handle the core resistance in the calculations, with multiple approaches and uncertainties expressed regarding the treatment of core losses and their impact on the equivalent impedance.

Contextual Notes

There are limitations regarding the assumptions made about core resistance and its potential neglect in the equivalent circuit, as well as the dependence on the chosen equivalent circuit for calculations.

TheRedDevil18
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Homework Statement


A three phase 220 V , 60 Hz, six poles 10 hp induction motor has the following circuit. Parameters on a per basis referred to the stator:
Rs = 0.334 Ω,
R’R = 0.147 Ω,
Xs = 0.498 Ω
X’R = 0.224 Ω ,
Xm = 12.6 Ω
The rotational losses including the core losses amount to 262 W and may be assumed constant. For a slip of 2.8 % determine (a) The line current and the power factor (b) The shaft torque and output horsepower (c) The efficiency

Homework Equations

The Attempt at a Solution


To get the line current, I need the equivalent impedance. But how do I get Rc (core resistance) ?, Is their a way to get it or is it just neglected because it is very large ?
 
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TheRedDevil18 said:
the core losses amount to 262 W and may be assumed constant.
You're wondering how to involve this detail in your calculations?
 
NascentOxygen said:
You're wondering how to involve this detail in your calculations?

For question a), it says determine the line current, so IL = Vphase/Z, but they haven't given Rc. They also said that the core losses and rotational losses amount to 262W
 
Can you express these fixed losses as their equivalent in line current and then add that in after you have done the other calculations?

N.B., I'm not sure this is the accepted approach, but it seems a valid approximation.
 
NascentOxygen said:
Can you express these fixed losses as their equivalent in line current and then add that in after you have done the other calculations?

N.B., I'm not sure this is the accepted approach, but it seems a valid approximation.

Not sure how to do that, I have this equation

IL = Vphase/Zeq

Where Zeq would be the equivalent impedance of the circuit, but then I would have to neglect Rc
 
TheRedDevil18 said:
Not sure how to do that, I have this equation

IL = Vphase/Zeq

Where Zeq would be the equivalent impedance of the circuit, but then I would have to neglect Rc
Once you have taken account of Rc losses separately, you can then neglect Rc in the equivalent circuit. Which equivalent circuit are you using?
 

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