Immersion heaters in series

[design4p]

Hi,  Is there any out there who can asssist. 

I want to connect a wind turbine to two immersion heaters in series but switchable.  A 400 watt and a 600 watt.  With a 1KW turbine I want to have three possible heating situations.  A 400 watt heater or a 600 watt heater or a 400 + 600 watt in series.

I know from school that the resistances in series just add together.  But when these are connected like this what would be the overall wattage be and would one heater element be taking more current and therefore be hotter that the other (and maybe burn out).

Any suggestions would be most welcome.

Regards John

[Alan]

Hello John

You do not mention the voltage of the turbine. ?

Or the voltage of the heaters. ?

information is required to answer your query

Regards

Alan

[Alan]

If it was a 48 volt turbine

The 400 watt in series with the 600 watt 
would present a load of 240 watts.

Regards

Alan

[Alan]

You would need four switches.

If the turbine is D.C. and you are using thermostats on the heaters they will
burn out / catch fire unless they are designed for D.C.



http://steamboilers.homecall.co.uk/Untitled3.bmp

You do not require the 400 and 600 heater to provide a load of 1000 watts ?

Regards

Alan

[johnrae]

If you have a 1kW turbine with only 240 watts of load connected it will over-rev at its highest rated wind velocity and possibly self-destruct.

It begs the question "Why do you wish to switch the immersion heater load"

If you have a good reason for doing so, why not switch 400, then 600 and finally 400+600 in parallel

jack

[Alan]

If you switch One and Two and Three
The turbine will be loaded at 1000 watts.

As Jack says. The turbine will never be
matched to the wind conditions. If your
not there to flick the switch or a big gust
of wind comes along. The blades will come
off.

A far better solution is to use pulse width
modulation and allways match the load
to the voltage the turbine is producing.

Regards

Alan

[billi]

Alan  is there a easy to understand   explaination of PWM ,    maybe a website
Does the Pulse stands for short  impulses  to charge a battery or  pass extra power to a dump   and is that regulated by a relay ?

Thanks Billi

[knighty]

I'm sure Alan can explain it much better than I an

(it's feels odd saying that..... my real name is Alan)


pulse width modulation is pretty simple (the idea is anyway)

you run at full voltage, but turn it on and off very quick

so, instead if giving it half the volts, you give it the full volts but half the time - by turning the supply on and off

if you have a 12v fan and you want to run it at half speed, instead of trying to run it at 6v (it might not run at all) you turn the 12v supply on and off (very fast)

the speed of the fan depends on the ratio of on/off time, more on time = faster, more off time = slower

imagine turning something on and off again so fast that it ran almost as normal....

then you start to pause for a tiny fraction of a second before before you turn it back on - it would still run, but a bit slower

[billi]

thanks kn(m)ighty  Alan 

I  just learn a bit more about this my self  and starting to get the idea ,

What is switching this  voltage /Amps so fast


Its just because i consider to buy a Solid state relay 80 A for DC switching  , cause my MPPT chargecontroller has an additional "PWM relay" that can activate a SSR and drive a diversion /dump element  , but its hard for me to believe that a relay can survive so long when its constantly switched on and of a few times per second


Billi

[knighty]

erm... I have no idea what is switched on/off....

but it will be going on/off pretty damn fast.... so fast that you'd never know it's happening.... like 50,000 times a second...

but I guess it depends on what you're switching on/off


are you sure your charge controller puts out an on/off signal for PWM ?   it could be a variable output 0 to 5v etc ?

[billi]

....some say and call it  a PWM signal ,  and still its strange to me that i find not too much on the net  , but the controller is also sold as a dump load controller  so it should work similar to the Morningstar  dump load controller  , just  that   one still has an MPPT controller as well to handle the PV

It keeps the charging voltage stable to the programmed setpoints , while diverting excess power to a dump   

i posted that video here befor  from this guy , who  did some experiment with that aux relay feature of that controller  , i  guess its hard to follow

http://www.youtube.com/watch?v=0RklbVgzHtI

Billi

[Dyslexicbloke]

PWM power control of a purly resistive load isnt hard ... FET's are cheap and relativly easy to drive if you keep the freequency low'ish, they can also be paralelled easily and switching losses at 3 or 4 hundred Hz will be minimal.

There are cheap PWM controllers designed for low voltage, low power DC, check out Vellerman - just the first link I found for one:- (http://www.apogeekits.com/pulse_width_modulator.htm)

If you used this board as a fet driver, slight mods required but not much, you would be most of the way there.
Many solid state relays (SSR's) are packages with FET's or IGBT's packaged with a low voltage driver and could also be driven with the above assuning they were rated at an apropriate freequency.

Your bigest issue will be deriving a control siganl to tell the PWN driver what to do but even that could probably be knocked up from a couple of opamps or a simple microprocessor chip.

Al

[ericw]

Most Solid State Relays are designed for AC operation and are not suitatble for use on DC.  They use Triac or SCR's which once switched on do not turn off until the load current flowing through them falls below a small value (holding current). On an AC circuit this normally happens twice per cycle.
 
To 'chop' a DC current then FET's are needed.

[billi]

Hi 

I was looking at those DC relays http://www.power-io.com/products/hdd.htm

[Dyslexicbloke]

Yes those would be some of the many DC capable FET based SSR's.
Some of them would work well from the board I mentioned.

I would build from scratch but if one dosnt do much in the way of electronics, bolting modules together is a great half way house.

Al