brushless-dc-motorgeneratorwindturbine
While trying to work out which motor to use for a wind turbine project, I'm coming across torque numbers for DC motors. I'm mainly looking at a BLDC motor right now.
Clearly, for a smallish turbine that I'm aiming for, I'm looking for low startup torque.
So, is there any relation between motor torque and/or wattage with its generator torque? Or more in general, how does one decide which motor is best for a given set of blades?
Implementing a current loop should not require field oriented control. You should be able to have a PI controller driven by a current feedback (e.g. see here: http://irtfweb.ifa.hawaii.edu/~tcs3/tcs3/0405_Servo_review/onaka_docs/2000_articles_control.pdf and here: http://www.drivetechinc.com/articles/curbldc3.pdf ). I would think you need two independent loops/feedback for a three phase system. What speed are you running your loop at? What are the stability issues you're seeing? You should tune your current loop with all the other loops disabled.
Since you mention "average" current and measuring on one phase I'm wondering what exactly you're doing. Closed loop current control is traditionally controlling the "instantaneous" current and is the highest frequency/bandwidth, innermost, loop in a closed loop motion control system.
This is a bit off-the-cuff mathematically just to see if things stack-up: -
Your mass is 100 kg and your top speed is 20 kmph (5.56 m/s).
You want to accelerate at 0.4 m/\$s^2\$ - this means it'll take about 14 seconds to get to full speed.
The energy attained by 100 kg at 5.56 m/s is \$\dfrac{mV^2}{2}\$ = 1546 J and this equates to a power during acceleration of \$\dfrac{1546}{14}\$ = 110 watts or about 28 watts per motor.
Am I looking for something feasible by a dc brushless motor?
It definitely sounds feasible - I don't see why you shouldn't find a brushless motor like this but it'll be easier to find a brushed one for sure.
What Voltage/Amperage values am I to expect from a motor of these characteristics?
I think you can be looking at 80% efficiency so each motor will consume about 35 watts. 4 x 12V, 3A full-load rated motor should work but maybe I'd play conservative and go for something like 50 watts: -
I know these aren't cheap but it was virtually the first hit when I searched for "50W brushless motor". There should be plenty that are a bit cheaper.
Try this too - maxon are the supplier.
Best Answer
Generally yes. To a pretty good first approximation, the torque that a DC motor gives for a given current is equal to the torque that same machine will need to absorb to deliver the same current as a generator. The biggest difference will be the machine's friction torque, which is always a loss either way.
For an ideal DC machine, as long as there is no current, there is no torque. So in that sense "startup torque" is entirely under your control, or at least under the control of the electronics you use.
Typically what's not ideal about a DC machine is the friction torque and cogging torque. These are often not well specified in datasheets (and aren't specified at all if you're buying a motor for hobbyist or consumer use, such as an RC plane motor).
You find the speed and torque at which the blades are most efficient, and you find a machine that will absorb that torque at that speed -- or you choose to drive the motor through a set of gears (probably step-up) to spin the motor at a different speed, while absorbing the same power.
Then you figure out the speeds and whatnot for when the wind conditions aren't ideal (most especially you check what happens if the turbine goes overspeed), and you deal with those.