What determines the amount of current that can be drawn from a generator?

Question

I tried searching this site and the internet for answers to my questions but found only partial answers.

I am experimenting with generator design. I am using only one pole of the magnet so as to get pulsating DC output. My question is; What determines the maximum amount of current that a DC generator can deliver to a load? Does the resistance of the generator coils determines this?

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Guys, I am very grateful for your response to my question. My background is in Digital Electronics; My knowledge of electromagnetism is not thorough and so I would like to get answers to a few more questions.

1) Autistic, you mentioned that " ....inductive term vector adds to the DCR" What is DCR?

2) I am using one pole only of the magnets to get a DC output. However, I am reading the output voltage as AC. Why? When I set the multimeter to DC it displays milivolts; This makes sense, since no smoothing capacitor was added.

3) This is unrelated to the current discussion. When a voltage is applied to a coil (wound on an iron core), how long does it takes to becomes fully magnetized? When the current flow stops, how fast does it take for demagnetization to occur.

Answer 1

The resistance of the coils can determine the maximum output current. But there are other factors that generally put a limit on the maximum current. jms has stated some of these. Also the electrical machine has effective inductance which is sometimes called synchronous reactance. This inductive term vector adds to the DCR. Often the inductive reactance exceeds the DCR. Armature reaction also reduces current by weakening the field. Finally the maximum current is at short circuit i.e. a zero ohm load. Any practical implementation would be constrained by what is deemed to be an acceptable volt drop.

Answer 2

For an ideal permanent magnet DC generator or alternator, the maximum torque deliverable by the source of mechanical power would be the only limit to the available current, and the output voltage would be directly proportional to the angular velocity (RPM) of the rotor.

In practice, the winding resistance causes a power loss by voltage drop proportional to the square of the current, which produces heat. If the heat cannot be removed effectively the insulation that electrically separates the turns of the windings from each other will usually be the first thing that fails.

Another limit is the fact that if the aforementioned voltage drop is equal in magnitude to the output voltage, no more current can flow. This however would only occur at low rotational speeds (low voltage), or exceptionally inefficient generators.