Solve Motor Coil Current at Startup & Full Speed

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Discussion Overview

The discussion revolves around calculating the current in a motor coil during startup and at rated speed, given specific resistance and voltage values. Participants explore the application of relevant electrical equations to solve the problem.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant poses a question about the coil current at startup and full speed, providing resistance and voltage values.
  • Another participant emphasizes the importance of selecting appropriate equations for the problem, suggesting that the choice of equations is critical.
  • A third participant proposes calculations for the current at startup and rated speed, using Ohm's law and the relationship between voltage, current, and electromotive force (emf).
  • A later reply notes that the proposed calculations may only hold true for an unloaded motor at design speed, indicating potential conditions that could affect the results.
  • Another participant expresses agreement with the calculations provided but notes an issue with the formatting of the equations presented.

Areas of Agreement / Disagreement

While there is some agreement on the calculated values for the coil current, the applicability of the equations and conditions under which they are valid remains uncertain. Multiple views on the correctness of the approach exist, and the discussion is not fully resolved.

Contextual Notes

Participants have not fully clarified the assumptions regarding the motor's load conditions, which may affect the accuracy of the current calculations. The formatting of equations has also been noted as a potential barrier to understanding.

Who May Find This Useful

Individuals interested in electrical engineering, particularly those studying motor operations and circuit analysis, may find this discussion relevant.

Lucas123
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Homework Statement
electric machines
Relevant Equations
I do not know
The question I'm in doubt about is

The motor coil has a resistance of 10 Ω and is powered by a voltage of 120 V. When the motor is operating at rated speed, its emf is 70 V. Thus, it is questioned:

a) What is the coil current when the motor is started?
b) What is the coil current at rated speed?

What equations can I use to solve it?
 
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You must make the attempt (rules of the website) . The difficult part is to decide which equations apply. What are likely candidates and why?
 
I thought about solving it like this:
a) V = Ri -> 120 = 10i -> i = 12 A

b) E = V - Ri -> 70 = 120 - 10i -> i = 5 A

But I don't know if it's correct
 
If I remember correctly (I do not do motors regularly) that will be true for an unloaded motor running at its design speed. Somebody will correct me as required... please.
 
I get the same answers you do.

[But your equations look funny here (odd characters), I can't sort them out.]

Cheers,
Tom
 

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