Single Rotor Generator loading

[brokengun]

I am currently designing a basic controller for my single generator machine. It will be setup for a very small pre-heating tank, so there will be no battery. I am just wondering how to load up my generator as it turns up without causing it to stall. For example if I measure the current to be 12 amps at 12 volts should I switch on 144 watts or will this cause the generator to stall? I was planning on under loading just to be safe, but I want to maximize my use of my power output.

What do you think is a safe loading tolerance? Have you guys run into problems with this?

[Flux]

The first thing you need to do is devise a means of starting the machine off load as this doesn't happen automatically as with a battery. When the prop is up to speed then you need to add load to keep the prop near the peak of its power curve.

A fixed value resistor will not give a good match. If you choose a value to suit low winds then the prop will run away in high winds. If you choose a value to suit high winds then it will stall out when the wind drops.

You can switch in resistors in a few stages as the speed increases if you want a simple scheme. For a more elegant solution you could use something like a buck converter with the pwm adjusted to load a fixed resistor proportional to the cube law.

There is a lot of latitude as the prop will not change efficiency much if it runs above or below its ideal tsr. You could just simulate a battery with a constant voltage clamp but there is no reason to drag the performance down to that level when you have the ability to probably get 3 times the output of a normal battery charging machine.

If you devise a way to start off load then a 2 step load may be good enough and speed is the obvious choice for the switching signal. If you must insist on it starting against load then the first load step will probably be too small to do much loading and you will need 3 steps.

If the frequency and voltage is high then there is the possibility of using series capacitors with a fixed value resistor  but the capacitor values are usually very inconvenient, if your fear of electronics is great enough then it may have some attraction.

Flux

[brokengun]

My controller will probably be micro-controller based and I was thinking of using some heavy duty solid state relays to switch on loads. I was going to use a hall effect current sensor to monitor the current coming out of the mill. The mill will be used basically to switch on a bunch of resistors, it will go into a preheating tank for a water system. I will be using some high output "heat tape" around an insulated tank to heat up water as much as possible before going into the main tank.

Starting without load is fine, I can do that no problem with the hall effect sensor. I was thinking I would have 3 stages or I could easily have many more to stay on the power curve.

What can I expect for performance for this mill compared with the same one as a battery charger? I didn't realize this would help the output so much. My design is based on the 7 foot otherpower single rotor one. Should I be prepared to handle a lot more current than theirs?

[Flux]

If you can deal with a micro-controller it will be fine so will be the solid state relays.

I doubt that it will be worth using more than 3 stages. For heating you really should start with a very powerful and efficient alternator, the single rotor approach is not the best starting point and in that case there may not be a large gain over the battery charger rating. I think the single rotor design is probably inefficient enough to keep out of stall in battery charging mode so you may not be able to let the speed rise a great deal over battery charging mode.

Ideally you need an alternator with the lowest possible internal resistance and in battery charging mode it would stall hard and perform badly. With heating you would load it such that the rotational speed followed wind speed so you would see about a 4 fold increase in speed from cut in to furling. The terminal voltage would be at least 3 times cut in at furling ( allowing for inevitable losses) so you could expect about 3 times the power out for the same temperature rise in the stator. In addition you would have a better match to the prop so 4 times the battery charging output should be within the realms of possibility. Taking a conventional machine designed with a lower efficiency for battery charging the gain may be somewhat less and in your case it may only be perhaps double.

A 7ft machine is very small for heating, I wouldn't have considered anything under 10ft and more likely 12ft but it depends on how useful you find a few watts of heat.

You won't see much useful heat in winds under 15mph so even a 2 step control may be adequate.

I am no digital expert so I don't know how you use current as a reference for control. I assume that you need to generate a signal proportional to something near current squared or a bit more but not quite cubed. Once you have the curve it should be ok but I would have chosen speed as it is the nearest thing you have to wind speed ( the quantity you really need but too difficult to use as a signal).

Flux

[brokengun]

I know the single rotor is not optimal and if I had the resources I would redo it and make a duel rotor with ten foot blades. This is for a project where we can't afford to use batteries. It's sort of a a school project thing, but basically no one is living on this windmill it's just a little experiment basically. We figured water heating would be a cost effective way to avoid grid tying or using lead acids and also a lot safer.

I am just worried about putting too big a load onto it and stalling the mill. The generator is currently far away from me and I'll be building the electronics here and bringing them up to test and install. So, I was just wondering if I would be able to use all the current I am making without stalling my mill.

[Flux]

It should be fine for your purpose even if you don't get a lot of useful heat.

"So, I was just wondering if I would be able to use all the current I am making without stalling my mill."

That is something of a chicken and egg statement. You must surely be able to use all the current you are making. Perhaps the question is more at what voltage you will, be able to produce that current. If it was intended for 12v battery charging then it would work into 12v with only mild stall. If you load it to below 12v then it most likely will stall. You may be able to get up to nearer 24v with optimum loading in high winds. In lower winds the ideal load will be close to 12v as that is where it was intended to run efficiently as a battery charger.

Flux