AC breakers before rectifier

[Dave B]

 I previously had wound my stator for 3 phase max. 120 VAC and was preheating water with 3 120 VAC heating elements 1 each phase. Approx. 3000 Watt max. output. I have a 20A AC breaker inline with each phase after the stop switch.

 I have since wired a new stator for 24 VDC battery charging and see a max. 40 VAC per phase. My question is : Even though the max. output remains basically the same (but is now DC current) at approx. 3000 watts should the breakers before the rectifier be changed to a higher rating ? I guess I am confused about the AC load with no rectifier or a DC load after the rectifier. What affect does the lower AC voltage but higher DC current have on the breakers before the rectifier ?

  I hope this makes sense. Thank you for any help explaining this for me.  Dave B

[nick1234]

your heating elements are a resistant load

measure the resistance of the element and use Ohms law to calculate the current and maximum  wattage these elements will give at the new voltage   add a safety factor and fuse accordingly

nick1234

[Flux]

On a furling turbine, don't use breakers before the rectifier.If they trip they will run you out of control off load.

At best any breaker before the battery will only protect the cables in event of a dead short fed back from the battery and the breaker on the dc side if fitted should never be able to trip in any wind.

None of this protects the stator in any way and you risk mechanical damage if you loose load.

Similarly I couldn't see any point on them for the heating control. You need to control fire risk with the potential large current from batteries but from the wind side you have no protection, it is a current limited source.

Flux

[Dave B]

 

  Thanks Flux, OK I get that and it makes sense but here's a question. If I had 12 guage wire for each phase (star) from my alternator "in" distribution panel to each resistive load (delta) and let's just say (3) 2000 watt 120 VAC elements and my maximum total AC output was 3000 watts @ 120 VAC then this 12 guage wiring to the loads should handle the amperage fine correct ?

  Now the twist : Now I wire a new stator for lower AC voltage so as to rectify and charge a 24 VDC battery bank. Now the 12 guage wire from each phase is connected to the rectifier and I am generating DC volts after the rectifier say a maximum of 40 VDC. Now I want to connect my resistive heating load again so I wire 120 VAC heating elements in series or parallel or both to come up with the same maximum power (this time at 40 VDC) of 3000 watts.

  The big question is : does the 12 guage input wire to the rectifier need to be resized as well as the DC out wire to the load ? Along the same lines, will the same 120 VAC breaker rated at 30 Amp (3600 watts) be rated at 60 Amps for 60 Volts ? Sounds simple that breakers would work that way, I'm sure maybe frequency would play a part.   Dave B.  

[Flux]

If you take the same load at 40v that you originally had at 120v you will have 3 times the current. Things are complicated somewhat in that you are comparing a balanced 3 phase resistive load at 120v to a dc resistive load at 40v but if there is no battery involved it doesn't make a lot of difference to the cable argument.

Without resizing the cables the cable loss is going to go up about 9 times.

If you are shunting the dc resistive load across a battery then things are worse as the instantaneous current into the rectifier is carried by a pair of the 3 cables at any instant and the losses are worse.

If as I suspect you are hoping to charge the battery and when it is full, divert the dc to a heater not connected to the battery then you have several issues to contend with.

When connected to the battery you may benefit from the undersized line cable as it may match your blades better and give you more power. You will now be working in a very different mode from your original heater scheme. It will be stall dominated and control will be easy.

If you disconnect the battery and connect a heater it will work in a different mode, it now really needs a cube law heater controller to give good results, any single resistive load will be a compromise, running stalled in low wind and run away in high winds ( similar to what you had before). The cable losses now are wasting your available power and are not desirable so you would benefit from thicker cable.

I am not sure how much you want the battery power and how much you want the heat but the best simple trick to get a good compromise is to beef up the cables to the point where they cause very little loss. The machine should stall hard into the battery at this point if it is a good size alternator and you have chosen the cut in correctly.

If you now add a series dc heater in the line from the rectifier to battery you can bring it out of stall and choose a value that gives best charging current. When battery charging in high winds you will get some heat from this series heater.

When the battery is full you could use a diversion controller to dump to another dc heater as others do as a dump load. This will float the battery and you will now have heat from both heaters.

The 24v dump load heater is a low and awkward value bit you should cope ok. The series heater in the line will be a very low resistance and will be awkward to obtain and particularly so if you want to dump the heat as an immersion heater but with ingenuity it can be done ( banks of diesel glow plugs may work).

Just one other point. If you have wound this to charge effectively at 24v then if you disconnect the battery and connect a heater to the dc output I would expect much more than 40v unless you keep adding resistors in parallel to keep it stalled.

If you cut in at 8mph at 24v then if correctly loaded you would be looking at nearer 100v in a decent wind so watch rectifier voltage ratings.

Flux