Calculating a simple generator's output power (wattage)

In summary, the conversation discusses building a simple electrical generator using wave power and the theory behind it. The individual has two basic questions regarding calculating the generator's maximum current and wattage and how its output power is related to the input mechanical power. They mention using Faraday's law to calculate the induced voltage and being comfortable with Ohm's law but are confused about calculating the current and maximum wattage. They also mention posting their questions in a physics forum and receiving limited responses. The conversation ends with a discussion about the relationship between input and output power and a mechanical analogy. Ultimately, it is confirmed that the EMF field created by the generator will cause a magnetic force to act upwards on the magnet and slow its descent, adhering to the
  • #1
Steve S
5
0
Hi All,

I am thinking about building a simple electrical generator (to use wave power), and I am trying to make sure I clearly understand the theory and expected results before starting the project. I have two basic questions:

1) How do I calculate the generators maximum current and wattage, if i know the induced emf?

2) How is this generators output power related / limited by the input mechanical power?

At the moment the concept is a simple renewable energy source which is the prime mover, acting to drive a magnet up and down a cylindrical coil, with N turns. So I believe this should be a simple classical problem.

I am clear that the voltage induced is calculated by Faraday's law - and I am comfortable with how this would be develop. Based on my initial setup of 200 turns, 6000gauss magnet, a cylinder of radius 4cm and a magnet travel speed of 0.25m/s i get a voltage of ε = 4.824V

I am conformtable with Ohms law, however, where i am confused is how I calculate the actual current and generators maximum wattage. Suppose for arguments sake I have a 1 Ohm load resistor, and neglect the impedance of the generator coil - I think that the induced current would be :

4.824V / 1 Ohm = 4.824 Amps
as P = V^2 / R
P = 23.27Watts

But if I reduced the loads resistance to say 0.5 Ohm, i get:

4.824V^2 / 0.5 Ohm = 46.54Watts

Similarily if I put a load resistance of 0.01 Ohm, I get a figure of 232.7Watts

So I am unclear as to how the generator can seemingly produce more power, by reducing the load resistance. Surely the actual power produced is limited by the amount of input energy coming in from the magnet?

Is there a way to calculate the maximum theoretical output and if so, can someone provide some guidance on how to link the input mechanical energy to the output power?

For reference I have also posted this in the Physics forum, and so far haven't had much success.
 
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  • #2
If you "try" to draw more output power than the mechanical input, then its rotation will slow down, producing a lower voltage so that output power will always be less than input. A generator becomes much harder to turn when you have it connected to low resistance load. There is no magic here.

You need to have wires thick enough to carry the maximum current without overheating.
 
  • #3
Thanks for the response. I'm aware this would happen with a conventionally driven alternator, where the prime move is a gas turbine or diesel engine - but in the case I'm looking at the mechanical input power is essentially fixed i.e. the magnets physical action is caused by gravity, or a flow of water etc..
 
  • #4
Steve S said:
Thanks for the response. I'm aware this would happen with a conventionally driven alternator, where the prime move is a gas turbine or diesel engine - but in the case I'm looking at the mechanical input power is essentially fixed i.e. the magnets physical action is caused by gravity, or a flow of water etc..

Here is a mechanical analogy. You are pushing a 10 lb sled with a certain amount of force and a certain speed. If I put another 50 lbs on the sled a few things can happen.
1.you don't change the input force. Therefore your speed decreases.
2. you increase your input force such that the speed stays the same.

So in the case of the generator, if you are applying a constant force to the generator, it will have to slow down when the load increases.
 
  • #5
I understand your mechanical analogy - but the generation system I am describing is one that creates an impulse of power - it is not a traditional rotating magnet in a field (or vice versa).

In my scenario a magnet is dropping, due to gravity, through a coil - so there would be no effect that would cause the magnets speed to slow? Or does the EMF field created cause a magnetic physical force acting upwards on the magnet to slow its descent?
 
  • #6
Steve S said:
Or does the EMF field created cause a magnetic physical force acting upwards on the magnet to slow its descent?

That is correct. If it were not, you would have broken the laws of physics as we know it.
 

1. How do you calculate the output power of a simple generator?

To calculate the output power of a simple generator, you can use the formula P = VI, where P is the power in watts, V is the voltage in volts, and I is the current in amps. This formula assumes the generator is supplying a purely resistive load.

2. What factors affect the output power of a generator?

The output power of a generator is affected by factors such as the size and strength of the magnetic field, the speed of rotation, and the number of turns in the wire coils. The type and quality of the materials used in the generator also play a role. Additionally, the load connected to the generator can impact its output power.

3. How does the load connected to a generator affect its output power?

The load connected to a generator can affect its output power in two ways. First, the type of load (resistive, inductive, or capacitive) will determine the amount of current the load draws from the generator, which in turn affects the output power. Second, the load's impedance (resistance) will determine how much of the voltage supplied by the generator is actually used, thus affecting the output power.

4. Can a generator's output power be increased?

Yes, a generator's output power can be increased by increasing the strength of the magnetic field, increasing the speed of rotation, or using more turns in the wire coils. However, these changes may also require adjustments to other components of the generator to maintain its efficiency and prevent damage.

5. What are some common units of measurement for generator output power?

The most common unit of measurement for generator output power is watts (W). Other units that may be used include kilowatts (kW), horsepower (hp), and joules per second (J/s). In some cases, the unit may also be specified as volt-amperes (VA), which is equivalent to watts for purely resistive loads.

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