Rectifiers Question

[LukeWarm]

Hi, this is my first post.
I have a question regarding rectifiers.

Is there a way of efficiently rectifying AC at variable frequency, voltage and amperage to a smooth DC output of 12 V, 24 V or 48 V?
A bridge rectifier will produce output at a variable DC voltage and amperage dependent on the supply won't it?
I am thinking of an alternator. The output voltage and frequency will change and the DC will vary as well.
Can it be done without sending some of the resultant power to waste using load diversion using a resistor?

 

[MikeMl]

What kind of alternator?

An automotive alternator has an "alternator control unit" (ACU) which senses the car's battery voltage(input), and it modulates the average current (~1A) to the field winding (rotor) of the alternator. The alternator's output current (typically 20-30A) is proportional to the field current and rpm (its like a current source, so its output voltage is whatever it needs to be). The feedback stabilizes the alternator output such that the battery voltage is 14.5V (the correct charging voltage), and the alternator output current exactly matches that being consumed by the car's electrical loads (ECU, instruments, AC, lights, radio, etc). It matters not what the frequency is because the stator winding (three phase, multiple poles) output current is full-wave rectified to DC with very low ripple.

 

[LukeWarm]

Hi Mike ML.
I am thinking of a windmill - The power is induced using permanent magnets. It increases with the speed of rotation but so does the frequency, voltage and amperage. The power ranges from 0 to 1000 W depending on the wind. Some of the bigger domestic mills can reach a few kW.
Most circuits I have seen for rectifying the "wild" AC involve some bridge rectifiers producing a heavily rippled DC going directly to a battery. The battery apparently "clamps" the voltage to the voltage required to charge the cells. If the voltage exceeds the charging voltage, it is dissipated in a resistor wired in series with the wires to the battery. If the battery becomes fully charged, all the power is diverted to another resistor and goes to waste.
It seems less than ideal so I am keen to know if there is a better way of extracting windmill power at wild AC, converting to DC at a user-selectable or variable voltage. My intention would then be to put this stable DC into an inverter to the grid, avoiding the battery 99.9% of the time and only diverting to the battery when the grid is down.

I appreciate your help. I am very curious about this subject.

 

[RCinFLA]

Unlike solar cells (PV panels), a wind generator regulator can not just 'lighten' the load to maintain battery charge voltage regulation. Because of the permanent magnets the output current will rise as wind increases. The alternator wire diameter and number of turns are selected to best match the battery voltage. You can buy these alternators targeted at 12 vdc batteries to 500 vdc batteries. ( https://www.hydrogenappliances.com/powerpmas.html ) The higher voltage alternators have more turns with small gauge wire producing more voltage at less current. If you select the wrong alternator you can burn out the windings due to higher current at the lower battery voltage where the smaller gauge wire can not handle the current.

If an electronic regulator backs off on the load on the wind generator, the wind generator will have less torque drag and start to spin faster. It can increase its rpm's to point of mechanical destruction.

To avoid this there are two ways to keep things under control. The old western windmills turn the windmill away from direct wind direction in attempt to avoid over rpm'g.

Another way is a load diverter. A common method is to include a heater element in your water heater to be used as the energy diverter. When battery is full charged the windmill energy is switched to the heating element. This is your resistor you referred to but it still goes to a useful purpose of heating water for you.

A third way used by large commercial wind generator is variable pitch blades. They actual are directly tied to A.C. grid so must spin at a constant rpm once syncronized to grid frequency.

 

[LukeWarm]

RCinFLA, this is great information. Thanks for your post.
Our 'normal' household hot water service is powered by the mains AC. This element is probably not capable of utilising DC. Is that correct?
If you were to divert the load to a water heater, how do you ensure the correct matching of your element to the variable DC you might generate?

 

[bountyhunter]

I believe you could use an SCR approach where the gates of the SCRs are enabled for different lengths of time on each cycle, similar to PWM control. As the desired current increased the SCRs run longer. If the batteries are fully charged and the power load is light, the SCRs don't turn on and the windmill just "freewheels" where the generator output is not connected to anything. I don't know if the generators would allow that or if they would need a constant load.

The system used in cars might work if the windmills generator has an armature winding you can vary the current through which controls the stator winding current.

 

[bryan1]

If the batteries are fully charged and the power load is light, the SCRs don't turn on and the windmill just "freewheels" where the generator output is not connected to anything. I don't know if the generators would allow that or if they would need a constant load.

If a wind generator is allowed to be open circuit in a gust of wind or storm the wind genny will over speed and self destruct. What is needed is a shunt regulator to divert the power away from the batteries. Heating water using the dumpload can be done easily but the element needs to be rewound for lower ohms to suit the voltage.

The current shunt regulator and dumpload I currently use in my shed is set to dump at 28.8 volts and with a 1.5Ω resistance on the dumpload it dumps around 18-19 amps. On my shunt board I used a pair of low rds on 169 amp fets in parrallel and I only need small heatsinks on them. On a real sunny/windy day when the batteries are charged I often see the dumpload at 500C or higher. So yea one project in the pipeline is a 24 volt DC water heating element so the wasted power can be used to heat water.

Cheers Bryan

 

[bountyhunter]

If a wind generator is allowed to be open circuit in a gust of wind or storm the wind genny will over speed and self destruct. What is needed is a shunt regulator to divert the power away from the batteries. Heating water using the dumpload can be done easily but the element needs to be rewound for lower ohms to suit the voltage.

The current shunt regulator and dumpload I currently use in my shed is set to dump at 28.8 volts and with a 1.5Ω resistance on the dumpload it dumps around 18-19 amps. On my shunt board I used a pair of low rds on 169 amp fets in parrallel and I only need small heatsinks on them. On a real sunny/windy day when the batteries are charged I often see the dumpload at 500C or higher. So yea one project in the pipeline is a 24 volt DC water heating element so the wasted power can be used to heat water.

Cheers Bryan

Sounds like what you need is some kind of a slip clutch so the spinning fan could just be allowed to disengage from the drive hardware and spin free if you don't need the power. Or maybe you could put hyrdraulic clutches on the fan vanes to turn them at a steeper angle if you want the fan to slow down. Burning power up seems like a wasteful way to do it.

 

[LukeWarm]

I guess, once the battery is charged and the water has reached maximum temperature, then you can short the mill wiring to stop the mill. Or think of another load to divert to - say underfloor heater etc etc.