Magnetic field and generator power output

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Discussion Overview

The discussion centers on the relationship between the magnetic field strength in a generator and its power output, exploring whether decreasing the magnetic field can lead to an increase in power output through changes in frequency and current. The scope includes theoretical reasoning and mathematical relationships related to generator operation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that decreasing the magnetic field could increase the frequency of the generator, leading to higher power output, based on their mathematical reasoning.
  • Another participant counters that to maintain a constant electromotive force (emf) with a lower magnetic field, the frequency must increase, but this does not result in greater power output.
  • A follow-up question is posed regarding the scenario of maintaining constant torque instead of constant emf, asking if lowering the magnetic field would necessitate an increase in frequency that could enhance total power output.
  • It is noted that maintaining constant torque with a lower magnetic field would require an increase in current, which must come from the current source.
  • Clarification is sought on whether the current in the equations used is consistent across different contexts, specifically relating to the mechanical force driving the generator.
  • A participant proposes that if the current increases with a decrease in magnetic field, the frequency must increase by a greater amount than the decrease in the magnetic field.

Areas of Agreement / Disagreement

Participants express differing views on the implications of decreasing the magnetic field on power output, with some asserting that power output does not increase while others explore conditions under which it might. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference specific equations and relationships but do not reach a consensus on the implications of their findings, leaving open questions about the dependencies between magnetic field strength, frequency, current, and power output.

dansmith170
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TL;DR
Could decreasing the magnetic field increase a generator's power output?
Hi,

I am confused about whether decreasing the magnetic field used for a generator could increase the generator's power output.

I used four equations:

1. Torque = Force x radius
2. Torque = NIAB (N = number of turns, I = current, A = area of armature, B = magnetic field).
3. emf = NAB(2*pi*f) (emf = electromotive force, f = frequency of rotation of armature.
4. I = emf / R (R = resistance)

Using these equations, I found that frequency is inversely proportional to magnetic field ("f" is proportional to "1/B^2"). If that is the case, then decreasing magnetic field should actually increase frequency, correct?

A generator should produce more power when the frequency is greater. Well then, a generator with lower magnetic field, provided that it has greater frequency, should produce more power.

I get the feeling that I made a mistake somewhere in my math or reasoning, would someone please help me out on this?

Thanks.

Citations to sources for relevant equations:
https://courses.lumenlearning.com/suny-physics/chapter/23-5-electric-generators/
http://spiff.rit.edu/classes/phys213/lectures/amp/amp_long.html
 
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To keep a constant emf, if the B field is lower, the frequency has to be increased. The frequency compensates for a low B field in order to maintain an emf. The power is not greater.
 
osilmag said:
To keep a constant emf, if the B field is lower, the frequency has to be increased. The frequency compensates for a low B field in order to maintain an emf. The power is not greater.
osilmag, thanks for your response. Suppose I do not want to keep a constant emf, but I do want to keep a constant torque. In that case, would lowering the B field mean increasing frequency (f) in a way that increases total power output of the generator?
 
If you wanted a constant torque when your B field lowered, your current would have to increase. That has to come from your current source. Is the current in equation 2 the same as in equation 4?
 
Last edited:
osilmag said:
If you wanted a constant torque when your B field lowered, your current would have to increase. That has to come from your current source. Is the current in equation 2 the same as in equation 4?
Yeah, same current (I) in both equations.

The current source is, I think, from the mechanical force that is spinning the generator's armature (maybe it is a wind-powered generator). In that case, the generator's armature spin, measured in frequency (f) should determine the current (I) magnitude.

In other words, given that current (I) is proportional to (B) times (f) (see equations 3 and 4), if I decrease (B) but (I) increases, then (f) must increase by a greater amount than (B) has decreased by.

Agree?
 
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