Wind Turbine Generator Query

[Jan Ben]

Hi,

I recently went into wind turbine as a project and I ordered a chinese wind turbine generators with charge controller.

Generator Ratings:
Rated Voltage: 240Vac
Rated Power: 1kW
Rated RPM: 140 RPM
I believe it is these ones right here - https://www.greefenergy.com/post/74

The charge controller was rated: 3kW.
it is this one - http://en.win-power.com/product/display2371.html

I haven't installed the generator to the turbine yet since I want to see and understand the system (about generators and charge controller) and how it is operating and functioning.
In a way, I'm just experimenting on it.

I think I have my setup correct as per their manual.
I checked inside the charge controller and it does have a rectifier for the generator's cable.

The problem is I am manually rotating the generator, so it generates some voltage but once it reaches the RPM and Voltage where supposedly the charge controllers allows it to draw current (if I understand MPPT and this charge controller) the generator experiences too much counter torque (I think) and it just stalls.

Also even if I connect the generator to a single rectifier then directly to the battery, it's already hard to rotate the damn thing.
I have tried a resistor as well in one and all three phases of the generator, it jams the whole thing.

I have also played with the MPPT curve parameter setting on the controller, it has 30 voltage points to set the power or current in. At any point where I give any power or current value, the moment I start rotating the generator then it reaches that point, it stalls.

I literally cannot get any current from this generator, using charge controller or directly to battery or resistor.
What might be the reason for this generator not being able to let any load/current be drawn from it?

Thank you.

[Rob Beckers]

Hi Jan,

Assuming the equipment is working as advertised (a big "if" when it comes to Chinese product), the MPPT curve will have to match the torque produced by the blades to optimally produce power vs. wind speed (ignoring losses).

Just turning the alternator by hand or drill will do squad for that, other than tell you it's producing Voltage, the MPPT curve actually has to match the turbine.

If your alternator is rated for 240V AC it won't be useful to connect directly (via a rectifier) to a battery bank. It would reach battery Voltage, and therefore see a short-circuit, at far too low an RPM to be useful.

Just to test the alternator, you could program a low-power MPPT curve into the charge controller and then spin it up. The output should match what you programmed, assuming you can produce enough torque on the alternator to keep it spinning. Ultimately you'll have to make an MPPT curve that matches the turbine's rotor though to do anything useful with your setup.

-RoB-

[Jan Ben]

Hi Rob,

Thank you for your kind reply.

Assuming I acquire a reliable generator and MPPT charge controller, adding the inputs you mentioned, if I can sum up my understanding. (feel free to correct me for my mistakes)
Wind Turbine can produce mechanical power -> Torque depends on blade and turbine design, and RPM by blade design and wind speed.
This is the input power to the generators shaft/rotor.
(Assuming no losses in transmission)
This rotates the generator shaft (Torque and RPM as input).
At no load, the generator generates voltage at rated RPM (in case input power exceeds generator rated RPM and voltages then overvoltage/overspeed protection is needed).
Now, the MPPT curve determines and limits the allowable current that can be drawn from the generator at each voltage level (voltage level relates/proportional to the RPM of the generator shaft and therefore the mechanical input power of the turbine).
This means that the current and therefore counter-torque that the turbine will experience with load is controlled such that the turbine will not stall given the wind speed.
That's why the programmed MPPT curve must match the turbine's power output characteristic.. Is this correct?

I'm really doubting the generator/alternator, I'm looking to borrow an insulation tester to check it prior to opening/experimenting what's inside the generator.
Are there better ways or other options to check and prove that the generator is no good?

I just hooked the generator to a rectifier then to a watt meter then to a battery then turning it manually to check if it can charge (give current) to the battery. However current output does not even reach 0.1A and its already very hard to turn compared to no load conditions.

I also hooked one phase of the generator output to a resistor it exhibits the same reaction.

Shouldn't I be able to draw some current at least in either case (battery, resistor) by manual spinning of generator?

[Rob Beckers]

Jan, yes, your summary is correct. In essence you want the MPPT curve to put a load on the alternator (and therefore the rotor/blades) in such as way that the turbine blades are running at their optimal TSR (Tip Speed Ratio) for each RPM (and the alternator Voltage is more or less linear with RPM, so this translates to Voltage). Since power in the wind goes with the cube of the wind speed, and RPM/Voltage is linear with wind speed (or close to it for a good MPPT curve) it follows that the output power of the charge controller goes with the cube of the Voltage. Now, this flattens off once you reach higher output power levels because of inefficiencies (mainly the resistance of the windings, turning power into heat).

The optimal TSR translates to the best lift-to-drag ratio for the blades, and for the range of wind speeds we're normally interested in it's nearly constant over the entire range. It is a design parameter, by mounting the blades at a specific angle vs. hub they will need to run at a different speed. Most regular HAWT type turbines run at a TSR of around 5 - 8 (i.e. blade tips are moving at 5x to 8x the speed of the wind).

Too little load on the blades and they just speed up, ultimately causing runaway and self-destruct, too much load on the blades and they slow down to ultimately stall.

You can test an alternator by connecting it to a battery and spinning by hand. Once you reach cut-in Voltage it's seeing nearly a dead-short and you can't turn it any more. Possibly spinning with a drill and measuring open Voltage plus some curves using resistors as a load will tell you what the alternator does.

-RoB-

[Jan Ben]

Hi Rob,

Thank you for your reply.

After some testing, I finally ditched the Chinese axial flux PMG.
Learned the hard way that it is incompatible with the for the turbine and blade I am using it with.
Correct me if I am wrong but I think axial flux PMG seem to require higher torque because of their low RPM ratings.

Would like to ask if possible what's the difference and impact of choosing 24/48V radial flux PMG (high rpm, low torque generators) and 230V ones?

Would choosing 24/48V generators require higher turbine+blade efficiency?

[Rob Beckers]

No relation (that I know) between axial or radial flux alternators/generators and their suitability for use with a wind turbine. Either can work, and either one needs to be designed for the task (RPM range, Voltage and power in that range etc.). Many DIY wind turbine makers prefer axial flux because they are easier to make yourself, but that doesn't make them inherently better or worse vs. radial flux.

No relationship between alternator Voltage vs. blade efficiency. The alternator needs to match to the blades though. In particularly if you're going for direct battery charging (more leeway if you're going through some type of MPPT device that can convert Voltage between alternator and load, that make it much easier to match load and blade RPM/TSR).

-Rob-

[Jan Ben]

I see, thanks for the insight.
Majority of axial flux generators that I find available are usually designed/rated with lower RPM.
Won't having such rated power and low RPM require higher torque to operate at the rated range? Thereby they will only 'work' (convert some power) if they're used on VAWT which usually is designed to operate at low RPM high torque.
It would be interesting to find some high RPM axial flux generators though.

Does matching mean controlling the load being drawn so that the designed/target operational TSR is maintained?
I thought lower voltage generators would require higher torque from the blades since for example if the tubrine + blade can capture 300W from the wind at X m/s then for that power if you're converting it at 24/48V it equates to 12.5A/6.25A compared to converting it at 230V which is about 1.3A only.

[Rob Beckers]

Jan, indeed, matching the alternator means making the load match the torque provided by the blades at the optimal TSR. If the load is too little the blades will speed up, increasing the TSR, likewise too much load and the blades slow down, ultimately leading to stalling them.

For a turbine that couples the alternator directly to the batteries (via a rectifier) that means matching that alternator so it keeps the TSR more-or-less in the right range over the mid-range of wind speeds. For a setup where there's an MPPT charge controller (or inverter) is involved this makes the job considerably easier, since the controller can vary the load via an MPPT-table. That means it's possible a larger alternator (that can produce more power than the blades can at a given RPM) to the turbine, and match it perfectly.

It may be that many axial flux generators are made for lower RPMs. What I was saying is that there is no inherent advantage of either type. Axial or radial can both be designed to work for a given power/torque/RPM range.

Power = torque x RPM
So, yes, lower RPM means higher torque for the same power. Voltage doesn't figure into this though. Voltage and current relate as Power = Voltage x current (roughly, it's a bit more complicated for AC), so lower Voltage means higher current for the same power.

-RoB-