How to Produce DC Electricity

[MikeM3]

Hi, to you all, I have a question that is puzzling me and has been
for some time.

Years ago if you wanted a wind generator, to supply 12v, you got
a dynamo and a fan, unfortunately it is not so simple today, this is
progress!

We do know that wind turbines, are DC Dynamos, because they can
generate, usable electricity at slower speeds.

We also know that modern cars, use AC alternators, whose output
is rectified and reduced to 12v DC.

We also know that some DC motors are brushless and not all can be
used as Dynamos, why, I don't know.

Can anybody suggest, honestly, which can be best used to supplement
a solar system, to keep the batteries as fully charged as possible.

I have read that not all brushless motors will act as generators, but in my
day, I am 83 years old and probably a dinosaur to most of you, we knew,
that if we supplied power to the DC unit, that it would run, we also knew,
that if we mechanically rotated it at any speed, it would produce electricity,
to my way of thinking, although we have progressed technically, in leaps
and bounds, in some ways we have gone backwards!

So, my question is what is the best machine for my project

Thank you for taking time to read this and hopefully answer, knowledgeably.

Mike.

 

[Simon47]

Are you familiar with a stepper motor ? If so, then a brushless DC motor is a bit similar in that electronics switch power to different windings in sequence to make it move.
That's why you can't use one as a generator.

You are also wrong by assuming wind turbines use DC generators. Some smaller ones might, but on a commercial scale they really don't want the commutator & brush gear (carbon dust & maintenance). There isn't really much difference between a dynamo and alternator for low speed operation - both scale output (IIRC) proportional to speed (for a constant rotor field current).

For a small unit, what is "best" will depend a lot on what you have to hand. Then it's a case of matching the turbine design so it's torque/speed characteristics match the generator torque/speed/output characteristics.

 

[MikeM3]

Are you familiar with a stepper motor ? If so, then a brushless DC motor is a bit similar in that electronics switch power to different windings in sequence to make it move.
That's why you can't use one as a generator.

You are also wrong by assuming wind turbines use DC generators. Some smaller ones might, but on a commercial scale they really don't want the commutator & brush gear (carbon dust & maintenance). There isn't really much difference between a dynamo and alternator for low speed operation - both scale output (IIRC) proportional to speed (for a constant rotor field current).

For a small unit, what is "best" will depend a lot on what you have to hand. Then it's a case of matching the turbine design so it's torque/speed characteristics match the generator torque/speed/output characteristics.

Thank you for your reply Simon47, I was misinformed about the wind turbine output, I have never had
anything to do with them, any generators that I worked on were diesel driven.
I was told that a DC Dynamo will produce electricity at very low speeds, but that an AC Alternator, has
to run at high speed, before it will produce any.

I appreciate the explanations, they are helpful.

Mike.

 

[Simon47]

Perhaps this is folklore from the automotive world ?
An issue with the AC alternator is that it cannot self start like the older DC dynamo.
The dynamo self starts as residual magnetism in the field creates a small output, which feeds directly into the field with no electronics in the way. Thus the field builds up and it self starts. Nothing happens until the battery voltage rises, when the regulator (electromechanical relay) pulls in and (IIRC) puts a resistance in series with the field to reduce output.
The alternator is different. There is still residual magnetism, this time in the rotor - but the resulting small output can't do anything. Firstly there's two diode voltage drops (around 0.6V to 0.7V each), but mainly there needs to be enough voltage to power the regulator so it turns on the transistor that controls the field current.
So, we have the charge warning light which connects between the ignition circuit and the field winding & regulator in the alternator. This is enough to power the regulator, turn on the control transistor, and allow some current to flow. When the alternator is spinning fast enough, this will allow it to start and it can then supply it's own field - it can use something like 4A, from memory, for full field. I've seen cases where the engine needs considerable revs to get to this stage - but once passed, the alternator will continue working at lower revs. IIRC output is proportionate to speed for a given field strength.
My guess is that this is the "dynamo works at lower speed than alternator" you had in mind.

Once you get to "bigger stuff" then things are different. For an alternator, the field is likely fed from the control circuit & supply - so not reliant on the bootstrap process described above. In wind turbines, they often use permanent magnet generators. Also, these days they are generally variable speed with power electronics to convert the variable voltage/variable frequency output to export to the grid.