Electronic speed control by voltage monitor

[Usman]

I have built a 5KW/48VDC battery-charger wind turbine with an electronic furling (upwards) mechanism + a mechanical brake (in case of emergencies or servicing, when onsite personnel is onsite).

I am investigating a way how to activate the furling mechanism at a pre-specified rotational speed rather than the wind speed, and have come up with two ideas:

1. -Either by monitoring the rpm of the PMA shaft by a tachometer sensor, but this requires additional wiring up the 90 feet tower.
   
2. - Or by monitoring the voltage output of the PMA using a voltage monitor so that when the output voltage jumps over a pre-specified value for a pre-specified amount of time, the electronic furling mechanism activates.
   
   

My question is, if I were to use the voltage monitor instead of an rpm or tachometer sensor, would the output voltage of an PM alternator is always directly proportional to the rotational speed, no matter the charging load on it? Or if the voltage alters in free wheeling (when no load is placed onto the rotor or battery-bank is not connected Vs. when connected to a battery-bank). I know that the load placed over the rotor by the batteries alters with the charge status, temperature, rotor rpm etc.

I am afraid these parameters may have an effect in the output signal from the PMA i.e. it may output 50 VAC at 500 rpm under certain conditions where as 55 VAC under slightly different conditions at the same 500 rpm?

Please advise.

Thanks.

[bob g]

are your rectifiers down at ground level? or on the generator?

if on the ground i would sense the rpm (frequency) before the rectifiers or maybe even off one of the diodes to determine the rotor speed.

i don't think you are going to get a solid rpm based on output voltage, because the battery loading will effect the voltage, and it probably will not be linear in any event. however unloaded the rpm/voltage curve is linear or should be nearly so.

bob g

[Usman]

Thanks,

Thats what I was cautious of!

But the problem is to source an rpm or frequency relay or monitor. A voltage moniotor is widely available but rpm monitor or tachometer realy isn't.

Yes, my diodes are placed on the ground and I am intendeing to slice in the voltage monitor just before the diodes.

If I could achieve results with 10% error, I would still opt for it and would really both to serach a freq. monitor, unless I could build a certain circuit or if someone on board could guide me through that? Any alternative suggestion?

Thanks.

[Flux]

Bob is right, volts are meaningless when clamped to a battery. Open circuit volts are proportional to speed but loaded volts are not.

Frequency is the correct choice but I suspect that even if you can find a commercial unit you will have to filter and mess about. Easy enough if you know what you are doing but otherwise a bit tricky.

Current is probably going to give you a good enough idea if you are not looking for absolute accuracy. It will depend to some extent on state of charge and stator temperature but it should be a rough guide. If you find a voltage sensor that will work at mV levels you can use it with a shunt. Otherwise you may need to use a hall sensor to measure the current.

Flux

[nothing to lose]

Probably a dumb idea, But...

What about like a mini-genny. A few magnets on a disk and a couple coils. Build it so it produces the power needed to activate your furling at the RPM's you want it to activate.

I'm thinking with no-load on it at lower rpm's it should not slow your genny or have any other bad effect. But when Rpm's hit your magic number like 500rpm this second small genny could produce 6V or 12V to activate the furling mechanism. So any condition 500rpm is 500rpm and the mini-genny will be making the same power at 500rpm every time. So even if you had a broke wire going to the batteries or take them off line, when the genny freewheels up to 500rpm even in low winds the mini-genny stills produces same volts to activate the furling mechanism.

[thefinis]

Wondering how you will keep from cycling. If you use a set electric output point to furl at, how will it know to unfurl? Using wind speed it keeps blowing and holds the machine in furl or partial furl till the wind dies down. With power output the output drops as the turbine goes into furl. It could be done by only going into partial furl till the power comes back within range but seems complex especially if anything fails.

Finis

[sdscott]

NTL; Usman has indicated that his alternator is already built, so it is a little late to add a "mini-genny" to the design, that is not hindered by load, that could be dedicated to monitoring and controlling the turbine in some way.  However, I have added just one extra coil into my design for control of a star/delta switch.  The coil is made up of relatively small copper (24awg) that I wound into one of the primary coils of the 9/12 alternator, i.e. two in-hand-14awg plus one 24awg in-hand at the same time.  This smaller wire fits nicely into the voids between the 14awg windings, keeping the overall coil-size the same as the rest in the alternator. (It does not need to be large wire, as it is required to transmit almost no power.)

Only one coil is wound this way and is brought down the tower on a separate conductor-pair (18awg?) to the power shack and into the control circuitry.  There is no need to add additional magnet costs as this coil is subject to all of the  magnetic-flux of your turbine.

The VA that this coil produces is linear (with no major filtering needed) to turbine speed and loaded only by the controller, that can be as simple as my star/delta switch, to control whatever you choose-like a furling system.  In the following pic, the 3-phase windings have ends labeled Z-A, Y-B, X-C and the winding farthest down is the "sense winding".  I used an 18awg extension-cord for the transmission down the tower to the controller and back up to control the large relays(K1,2,3) for Star/Delta switching at ~300rpm.

[sdscott]

You could use a small relay as the engage driver.  This would offer inherent hysteresis as the disengage voltage is typically much less than the engage voltage for relays.

[Ungrounded Lightning Rod]

The purpose of furling is to protect the mill.  You're protecting it from three things:

 - Burnout from overcurrent.  (Resistive heating of the coils is the main concern, and it's proportional to current squared times resistance.)

 - Tear-apart of the turbine from overspeed if the load can't keep its speed down in high wind.

 - Tear-apart of the turbine and/or supporting structure from excessive wind loading in high wind.

Furling to reduce current will also reduce torque, letting the load keep the speed in check.  And it also reduces the wind load, protecting the tower and turbine from excessive forces.

So it seems to me that, rather than trying to infer the speed (which is complicated), you should be furling to regulate (put an upper limit on) the current (whose measurement is straightforward).  That directly attacks the problem and doesn't involve complexity to tune out distortions.

However, it doesn't deal with the case where the load has come disconnected, allowing the mill to run away.  (Of course the offset-turbine/pivot-tail mechanical furling systems don't handle this well, either:  The torque load on the shaft from the current produces most of the wind load on the turbine that resists the tail until it rises.)

With ground-controlled furling you can also deal with this by ALSO furling to limit the voltage.  Then the mill will normally furl to limit the current but will also furl to limit speed if the load becomes disconnected.

Furling to limit both current and voltage measured at the ground gives you another advantage:  You can set the voltage limit to prevent overcharging - eliminating the need for a dump load.

If you want to get fancy you can control the mill's furling on a function of mill current (for protection), charging current, and rectifier (battery) voltage, to optimize the battery charging and perform equalization cycles when there is enough more wind than load.  B-)

[Flux]

This makes a lot of sense.

Flux

[domwild]

Usman,

Have a look at gizmo's circuit, which measures frequency via a Picaxe PIC. Also jimovonz has contributed a circuit which is similar to gizmo's, but without optos. Just a thought.

The AC pulse train from a PMA can be reduce in voltage via a resistor-divider circuit and diodes and the pulses can be counted.

Regards,