Help for Motor to Generator control circuit

[Liam]

Hello there!

I'm making a circuit that will be attached to a small wind turbine garden ornament. We don't want the turbine to stop spinning.

The circuit needs to be able to charge a battery when the wind is good, but i switch over to this battery for power when the wind is bad.

I will connect a generator a diode (that will then lead the current into the battery) which leads to a relay. This relay effectively will sense the wind strength as it will switch to a contact only with the battery, and no diode, when the current from the generator is low. This will then allow the battery to discharge back into the generator, which now becomes a motor.

My question is: What will happen to the current flowing out of the battery, through the relay when it's not connected to the diode, as the motor spins faster due to wind and battery power? Will it decrease or increase or stay the same?

This leads me to also ask.. how will i get the relay to switch back when the wind picks up again?

 

[Grossel]

My suggestion is to mount a tachometer on the shaft. That way the speed can be recorded by a controller or it can be converted to a voltage. Then you'll have a sort of controller that will control a Buck converter. I think the buck is capable to also get the power to float the other way too (from motor/generator to battery) but I'm not completely sure about that.

Or, a simpler way is that the Buck converter juust keep the voltage over the motor/generator constant. In that case, the rotation speed of the wind turbine will be higher when it's picking up wind.

However, the most simple solution is just to connect the wind turbine to be connected to a battery directly. But then you should have a motor that have a rated voltage slightly higher that the battery. That way, you increase battery time AND lifetime of the motor. And it's also neccesary that rated voltage of the motor is higher to make it possible to generate enough voltage to charge the battery.

 

[Liam]

My suggestion is to mount a tachometer on the shaft. That way the speed can be recorded by a controller or it can be converted to a voltage. Then you'll have a sort of controller that will control a Buck converter. I think the buck is capable to also get the power to float the other way too (from motor/generator to battery) but I'm not completely sure about that.

Thanks Grossel. The tachometer is a great idea, but how would you use the buck converter exactly?

Would i just be able to use a zener diode that'd break down every time the voltage from the turbine got to a certain level? Maybe i need some sort of transistor that only goes on at a certain voltage....

 

[crutschow]

Why not just connect the turbine directly to the battery? It will charge the battery when it's output voltage is greater than the battery voltage, and will run as a motor when its speed drops below the battery voltage.

 

[Liam]

Why not just connect the turbine directly to the battery? It will charge the battery when it's output voltage is greater than the battery voltage, and will run as a motor when its speed drops below the battery voltage.

So Carl, the battery wont try to discharge when the turbine is spinning at a good rate?

 

[crutschow]

So Carl, the battery wont try to discharge when the turbine is spinning at a good rate?

When the turbine is spinning fast enough from the wind so it's output voltage is higher than the battery's, then it will charge the battery.

 

[MikeMl]

Why not use a Schottky diode between the genny and the battery? If the wind is blowing hard enough, it will charge. If not, the diode prevents the battery from discharging into the genny.

 

[crutschow]

Why not use a Schottky diode between the genny and the battery? If the wind is blowing hard enough, it will charge. If not, the diode prevents the battery from discharging into the genny.

But he wants the turbine to keep spinning when the wind slows. See his first sentence.

 

[Liam]

Why not use a Schottky diode between the genny and the battery? If the wind is blowing hard enough, it will charge. If not, the diode prevents the battery from discharging into the genny.

That's what i wanted!! but i didnt know such a diode existed.

Cool. I'll research this diode thing. hopefully it's what i was after. Thanks!

 

[crutschow]

So I obviously misunderstood your first sentence. I thought the turbine was ornamental and you wanted it keep running even when the wind slowed.

As MikeMl posted, a Schottky diode should do what you want. Using a diode with a high current capacity will give a lower voltage drop and minimum power loss when charging.

 

[chemelec]

This Circuit will help you.
**broken link removed**

 

[Liam]

Oh wow. I love this forum everyone is so helpful!!

So the schottky diode wont let the battery discharge to run the genny as a motor? Because (yes you were right crutschow) i want the turbine to be spinning all the time - A self sustaining ornament - so the schottky wouldnt be different in that case. How does a schottky differ from a normal diode?

I talked to my electrotechnology teacher and he's under the impression that attaching the motor directly to the battery will have complications... although he didnt say what sort of complications. What do you think he was talking about?

Chemelec, that circuit you provided a link to is crazy!!!! Sorry i've never designed a circuit before so that looks quite daunting to make sense of. But i'll try and make some sense of it. Thanks for your help.

Is there some sort of transistor that'd only allow current to flow through once it reached a certain level? Cause that'd be perfect.

 

[crutschow]

A Schottky is a type of diode with a metal-semiconductor junction that has a lower forward drop than a standard P-semiconductor to N-semiconductor junction diode (about 0.4V versus 0.7V) so is has lower loss when conducting. The purpose of the diode is to only allow current to flow from the generator to the battery, not from the battery back to the generator.

Perhaps the complications your teacher was referring to was overcharging the battery. If that could be a problem then your would need to add some sort of regulator circuit or something similar to what chemelec suggested.

 

[Liam]

I suppose i shall have to do some experimenting wont i. Do you believe that the circuit that chemelec suggested is necessary for my application?