Homebrewed Electricity > Wind
Once again in 3-part (phase) harmony
kitestrings:
There's been some discussion in the past on the "growl" or "singing" of the axials. Ours is still on this relatively low test tower, and the winds are back, so I've been watching and listening a bit more.
Ours has a distinct low-frequency growl just above cut-in. It is always there to some extent, but seems more pronounced at cut-in. We are using MPPT via the Charge Controllers, and I don't know if that has any influence. I'd estimate that it is about 25VAC RMS, 60+ DVC and maybe 13Hz. And, the sound maybe amplified through the hollow stub pole that it is mounted on and perhaps gong thru some resonant frequency at this particular speed.
My questions are:
1) Is this a function of loading - causing vibration through the stator when it starts getting hit with, I guess, pulsating loading?
2) Would it help to load it lighter - changing the pre-programmed load curve - in this range of speeds?
3) Or, is it related to the rectification of power from 3-ph to DC?
4) Does the number of poles have any effect?
I don't know that it is doing any harm, just trying to better understand what causes it, and what influences it. Related discussion:
http://www.fieldlines.com/index.php/topic,144842.msg980835.html#msg980835
~ks
Flux:
Interesting. Basically all electrical machines make noise but is often masked by fan noise in commercial alternators.
There is no doubt that machines feeding rectifiers make much more noise than alternators supplying linear loads. This I suspect has always been true but probably not recognised. Alternators hum but the frequency is low and it is often masked by cooling fan noise, If engine driven the engine noise tends to totally swamp it.
Those of us old enough to have had dealings with dynamos know that they whine quite audibly even above engine and fan noise.Your question has made me suspect that this is exactly the same case, a dynamo is just an alternator with a mechanical rectifier, albeit a many phase one.
Single phase alternators are always noisy, there is constant pulsating which sets up electro mechanical forces and there is noise from magnetostriction, the forces are so high that noise is inevitable. Much of this noise goes away with poly phase machines and in theory a 3 phase alternator is vibration free. I doubt that this is entirely true, the forces all sum to zero over time, but there must still various modes of vibration exciting noises in the core.
Axials don't have iron cores and are a bit different but there have to be exciting forces moving parts of the magnet rotors and stator in various modes.
Immediately you introduce a rectifier things become much worse, with inductive loads, conduction periods are fairly long and things may not be much worse than a linear load, but once you introduce any form of capacitive loading on the rectifier the conduction becomes very rough, occurring in short intervals when the emf is above the capacitor mean voltage. A battery behaves just as a capacitor and is every bit as bad. The worst case is just at cut in when conduction is discontinuous, current just flowing when the peaks of the 6 pulse ripple are above battery volts. Once you get above the rectifier mean ripple things get better. (Cut in is at v peak i.e. 1.414 times v rms, the dc mean is near 1.4 v rms so there is a small band at cut in where this can be most pronounced.
It is not harmful, the forces involved are small compared with the pulsating load of a single phase alternator, but can be annoying. With variable speed machines it would be very lucky if you didn't hit a resonance somewhere in the alternator or some part of the turbine or tower, just bad luck if you hit a point where the worst exciting forces near cut in hit a critical resonance.
With battery charging you can include an inductor between the rectifier and battery and change the rectifier conducting mode, it is rarely worth bothering with 3 phase, but it does really take most of the damaging forces out of a single phase alternator. It also changes the load characteristics very considerably, possibly to advantage if stalled but if running well below stall it may not be so useful.
The change in loading with a 3 phase machine will be small and a much smaller inductor is needed as you are dealing with the 6 pulse ripple of the 3 phase bridge.
With mppt I assume that the input of the inverter is a bank of capacitors so the same thing happens as with a battery. How things will behave with inductors after the rectifier is difficult to predict, in moderation it may be beneficial but id you hit resonance between the inductor and input capacitors all hell may break loose, probably need a lot of inductance to cause this.
If the inverter takes in ac with its rectifier circuits inside you have no control over the rectifier characteristics.
Basically it is normal to have some growling at some speeds, you will struggle to eliminate it, but if it is bad then shifting something off resonance may make drastic changes.
The number of poles affects the ac frequency, it will not eliminate the noise. The main noise will be at 6 times the basic alternator frequency. A very small number of poles may bring the noise lower in frequency where the ear is less sensitive, high pole counts may change the growl to a much more frustrating whine, more like a dynamo. . Load does change the noise but light loading is always needed near cut in for prop matching. There may be room for experiment with mppt by changing the cut in voltage and moving the curve but I suspect the gain may be minimal.
Flux
clockmanFRA:
Hiya KS,
As you know I have 3off 3.7m, (12footers) diameter Hugh Piggott design Turbines.
My No1 has a deep Hum (sort of deep moan as the Mrs say's) at cutin and the noise is constant no matter what the speed, air gap is about 0.6mm each side of my coils.
No2, has an air gap at about 0.9mm and has a gentler quieter Hum.
No3, has an air gap at about 0.75mm and has a Hum noise between the No1 and No2 turbines.
No1, has logged me more power than No2 and No 3 more than No 2.
Note all windings are the same, all casting moulds are the same, all blade designs are the same, the only difference is the air gap.!
kitestrings:
"Good, good, gooood, good vibrations....nah, nah, nah, nah, naaaah, nah, nah, nah... " Oops, sorry, I got distracted there for a moment.
Well, I've got something new to research, because I confess I have no idea what "magnetostriction" is, though the word appears to a melding of magnetic forces and friction.
Much of the rest makes sense to me, in particular:
--- Quote ---once you introduce any form of capacitive loading on the rectifier the conduction becomes very rough, occurring in short intervals when the emf is above the capacitor mean voltage. A battery behaves just as a capacitor and is every bit as bad. The worst case is just at cut in when conduction is discontinuous, current just flowing when the peaks of the 6 pulse ripple are above battery volts.
--- End quote ---
If I put my ear right against the Sched 40 tube, it really is a rough, drumming of sorts. There are blade noises that can clearly be linked to a one per rev cycle. If I had to guess I'd say this was more like a one per magnet pair pulsation. It does seem less intense as the rpms increase.
--- Quote ---With mppt I assume that the input of the inverter is a bank of capacitors
--- End quote ---
I'm not sure here if you are more broadly including grid-tie applications, Flux, or mean the converter? I think you know that we're not grid-tied. Our system has two, independent stud-mounted diode rectifiers. Each set of six feeds one of two MS Classic controllers. The controllers allow the input voltage to free range up to about ~160-170 VDC, and then buck the voltage down to the nominal 48V bank.
I doubt that I'll take any pre-emptive measures...adding inductors and such, as this is likely closer to this turbine than we'll normally be by 200-300' (not to mention insulated walls and ambient noise of the wind itself). Still, our power curve is pretty linear from cut-in to furling, and I'm thinking it may help to not be quite so steep out of the gate. A bit lighter loading early might help both noise at cut-in and blade matching. I may also try dampening the amplification through the stub tower/tube.
clockman, this is interesting, you've got the benefit or comparison to near identical machines. Our gap is also fairly close, and the coils sectors are identical, so I'm not thinking there is any unnatural imbalance, or anything necessarily wrong. This discussion has been helpful to understand better 'what' it is.
If you think about it we have many of the contributors - rectification, capacitance (batteries), current flow in six-pulse peaks at cut-in (as Flux explained), no white-noise cover of drives, fans, motors - and then, we have a hollow tube connecting this variable frequency, energy pounding device to the ground. It may be then no surprise at all that it springs to life with a "grunt".
Thanks for the feedback.
~ks
joestue:
All electrical motors except for the homopolar motor produce a torque ripple.
Hub motors, outrunners, and other small high torque motors are often designed to produce a trapezoidal waveform, and these motors should work well rectified with a single three phase diode block.-but they also have high iron losses and high cogging torque.
it should be possible to build an axial flux, trapezoidal waveform designed for low torque ripple when feeding the usual 6 diode block into a battery without an inductor in between.. but you would probably have to build it with trapezoidal magnets and trapezoidal coils, and that would be difficult and expensive.
A sine wave machine directly rectified into a battery will produce something like 100% torque ripple initially at "cut in"
some of you here may see a marginal efficiency improvement placing an inductor after the bridge rectifier for the worst of cases.
as the power draw increases the torque ripple will drop from 100% to 16%, combined with the frequency increase this is why the problem goes away.
For the exceptionally bad cases.. find yourself a three phase transformer and wind it for a 1:1 wye-delta ratio (which means the actual turns ratio is 1:1.73)
and connect that transformer to the turbine, and the battery or whatever, with its own separate diode block.
such a transformer need only be rated for 57% of the total power demand, and you can play the usual games such as series ac motor run capacitors in series with the primary to prevent startup issues. such a wye to delta transform shifts the phases 30 degrees and you will have a 12 pulse rectifier which delivers sufficiently flat DC with almost perfect sinewave input currents.. torque ripple should be reduced to a maximum of 5% assuming sine wave input.
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