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Freewheeling

FREEWHEELING

I was talking to a KISS owner the other day.  He had recently purchased a boat with a KISS wind generator installed.  He said that there were three other KISSes in his marina, and that all the owners had told him that it was necessary to turn the generator out of the wind and tie it down when the winds get over 25 knots and the generator goes into freewheel.  Apparently, this rumor has become common knowledge, and like much common knowledge, is untrue.

Why and How

All wind generators have to have some way of preventing overheating.  Most modern marine wind generators use rare earth magnets, which are sensitive to overheating.  Even older generators with earlier magnets can loose their magnetism if they get hot enough.  Manufacturers use a lot of different techniques to prevent overheating.  On the KISS, we use two thermal sensors inside the housing that are bi-metalic discs that open the circuits, stop energy production, and allow the generator to cool.  When both are open, the output drops to zero, and the generator turns much faster than normal, causing additional noise and vibration.  When one opens, the output is unbalanced, and the generator vibrates more than normal and the output drops by 2/3.

The on-off switch on the KISS connects the coil windings in the stator (shorts them out), creating a very large electrical (and thereby mechanical) load.  When the circuits are opened by the thermal sensors, the coils cannot be shorted, and so the on-off switch will not turn the generator off when it is freewheeling.

What to do

There are more than one way to deal with this;

1)  Do Nothing.  When the KISS cools, it will start making power again.  It will be making more noise and vibration while freewheeling.  It will not be damaged.  The thermal sensors we use are rated for something like 500,000 on-off cycles.  We were stuck in 30 – 50 knots winds off shore with no engine or useable sails (it’s a long story).  The KISS kept the SSB/ham, all the deck lights, and VHF radio going all night, cycling on and off it averaged 11 amps.

2)  Turn the switch off and wait.  The KISS will cool, and when the thermal sensors close, it will shut off.

3)  When the winds get up to a steady or average 25 knots, pull the generator part way out of the wind with a tether attached to the tailfin.  In order to avoid approaching rapidly spinning blades, the tether should be set up before hand.  We sat in the Tobago Keys for a few days with nothing but a submerged reef and 3000 miles between us and Africa.  Winds were well above 25 knots.  As the boat sailed around on the anchor, the wind generator (pulled part way out of the wind) would speed up or slow down, but it never went into freewheel.  On a dark and stormy night, alone in the cockpit, without the additional tether rigged, I would probably go for options one or two.

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