cut in relay run off turbine voltage

[cardamon]

SO it has come time to address my turbine's starting difficulties.  A refresher for those who don't remember my post about the setup  a while back, The turbine has a 240 volt three phase output feeding a transformer feeding a 12 v system.  The turbine doesnt have trouble starting from a dead stop, but it seems to have trouble getting above 60 rpms or so and I am assuming the transformer is to blame for this.  My idea is to make a cut in relay system using 3 solid state relays with the 'coils' connected in a wye or delta configuration and connected directly to the turbine output and these would make/break the connection to the transformer primary.  Basically I start charging at about 200 v so I would shoot for a relay that makes at say 150 volts if delta connected or 100 volts wye.  I quick search of digi-key showed a relay with these characterisitcs is available.  I wanted to know what others thought of this as it seems simple and quick to me.  I have read what some others have done with a controller that senses the frequency and runs relays, but I am not great with electronics and dont have a ton of time to fiddle around.....

Thanks.

[GWatPE]

If the turbine arrangement has operated successfully before, and now will not, then maybe there is a problem with the transformer.  Maybe some shorted turns on the primary.  Simple check is to place a mechanical 3phase 3p1t switch where you want to place the relays.  Let the mill get to say 100rpm, and then close the switch.  If the mill slows down, then there is a problem with the loading.

Gordon.

[cardamon]

Gordon,

Just to clarify, nothing has changed.   The mill still runs as it always did, but once it has stopped, it takes a brisk wind and some gusts to get it going.  I have the feeling that I am missing out on harvesting some of the lower winds.  

Ethan

[Warrior]

The system you describe is something similar to what African Wind Power uses on their turbine (high volt version). What they use looks a little more complicated than just 3 solid state relays...maybe they also have a dump load control circuit...

Just as a suggestion, you can use only 2 relays & one of the phase wires can be hardwired directley to the transformer and nothing will happen cause it won't load up unless 2 or more phases are connected. Here's the story on the AWP:

http://www.fieldlines.com/story/2007/6/16/103132/303

http://www.otherpower.com/images/scimages/5171/awp_1.html

Oztules is a very knowledgeable guy and can probably explain how the circuit works.

Have Fun!

[Flux]

Try the machine without the transformer and see if it starts then. Iron cored PMAs have iron loss drag at start up. The transformer iron loss and magnetising current adds to this drag but if the machine won't start much better on its own the relay is not worth the effort.

For this application a voltage sensing relay will probably be adequate but you will need some hysteresis to prevent it switching on and off. It all depends on the transformer magnetising current and how much it drops the alternator volts. If you can get it to switch above prop stall speed but below cut in there may be no need for the hysteresis at all.

Normally a frequency ( speed) sensing relay would be better but again it may need some hysteresis to prevent chatter.

This is a fairly simple case and it doesn't matter if there is a lot of dead band, nothing like as difficult as star /delta switching so you should manage it fairly easily.

Flux

[scoraigwind]

Transformers do draw a small magnetising current that inhibits start-up for the turbine in most cases.  I like to use DC rectified by a small bridge to feed a mechanical relay.  I use a 3-pole relay.  I use 2 of the contacts to switch AC power, and keep one wire attached to the transformers.  The third contact is used to bypass a resistor in series with the relay coil so as to provide extra hysteresis although most relays have quite a bit of hysteresis built-in.  I use a variable resistor in series to adjust the cut-in point.

[cardamon]

Thanks for the great info guys.

Flux,

you are right, I really should do a little testing with and without the xformer to be sure confirm if it is causing significant delays.  I should be up on the tower with a handheld wind speed meter and have someone on the ground connect and disconnent the transformer on a day with steady winds around cut in.  However I have disconnect the start point from the tranformer's primary a few times in low winds and it seemed to aid startup.

Hugh,

Why do you prefer a mechanical relay? Is it because of the increased hysteresis?  And why rectify the AC for the relay?  Because of the high voltage, I may have trouble finding one that will handle 300 vdc....

[Flux]

I will let Hugh answer but I would also be more than happy to use a mechanical relay.

Ac relays are horrible and are not very happy at 50/60 Hz, at low frequencies I doubt that the shading bands would prevent them vibrating. Rectifying to dc solves the problem.

This relay is only closing on the magnetising current of the transformer if you get it in below cut in. The thing will only open at very low voltage just before the mill stops and it should break the magnetising current of the transformer with no problem, perhaps an RC network across the contacts would be desirable. The contacts don't have to withstand or break the 300V.

I see no problem with a solid state device except that I can never understand the control circuit requirements, the data sheets just don't seem to make sense and I am not sure how you decide when it fires. You need to hit it above prop stall when it is accelerating but ideally before battery cut in so that you have a rapidly rising voltage then it won't chatter ( or chop if that is the solid state equivalent).

Flux

[tecker]

Try a cap diode circuit and see how it works for this genset . The only problem will be if the cut will allow enough rpm to stay out of stall .It should be ok because the batteries won't take current until 13 volts or slightly higher .and of coarse the primary resistance is very small.

You may not even need a transformer just a dump light or water heater circuit both work  well with this circuit switch the dump in parallel with the battery bank and in series with a car battery the result is usually a float on the batteries .

[cardamon]

Flux,

Yes solid state relays are a bit confusing to me too.  The data sheet of one I looked at states that the "must operate voltage" is 150vac max and the "must release voltage" is 40vac min.  The way I read that is the voltage may have to fall to 40 volts before it releases, which is far too low.  I was concerned about the lifespan of a mechanical relay, but as you point out, if set up properly, it will not have to open or close any significant current.  Also as you point out fine tuning when it makes or breaks is easier.

Tecker,

I dont quite follow.  Do you mean that that circuit can be used to get a low voltage dc to trigger the relays from my high voltage ac?  Just wanted to clarify that.