Contactor load control - Non grid simple circuit help

[Dave B]

I am heating water direct, 0-120 vac 3 phase with my 18' axial alternator. Currently I have 3 hot water elements connected with no controller and this does work fine but I want to add a series load to assist in start up and have this bypassed to the original load after a given RPM (voltage) is reached. I want the control to be self powered no grid and would like to use a mechanical contactor that can handle the load. This is very simple to many I'm sure but I'm not sure of the best way to set up the control circuit to trigger the contactor, (ac or dc and voltage ?) My AC voltage per leg runs approx. 1/2 the RPM. I would like to switch the load some where around 60-100 rpm or 30-50 vac. Any help would be greatly appreciated.  Dave B.

[DamonHD]

At a wild guess, perhaps a 3-pole 120VAC power relay with a low-pull in 120VAC coil (pull-in at maybe 60--70% of nominal) might get close to what you want, as long as the RPMs are high enough that the relay does not get a chance to 'drop out' as the voltage crosses zero.  (You could probably get round that with a DC coil and drive it from 3-phase-rectified output so the voltage never gets down to zero when any significant power is being produced, maybe even adding a high-voltage smoothing cap.)

But I'm sure that there's someone round here that knows, rather than guessing!

Rgds

Damon

[Dave B]

Damon,

  Thanks for your reply. I had thought of rectifying one leg to DC for using a DC coil contactor. I would want to be able to adjust the trigger voltage and then also not have to worry about this trigger voltage going too high to possibly burn things up. Am I talking about a variable voltage regulator ? I know enough electronics to be dangerous and reading  a schematic and building the circuit is no problem, circuit design I'll let someone else help me with that. The simpler the better for this setup is what I'm looking for. Any other thoughts or help from others is appreciated, thank you.  Dave B.

[DamonHD]

Hi,

If you were thinking on those lines then I suspect that you should rectify all three phases (standard 6-diode stuff) to get something that never drops to zero and is much more 'DC' and thus never lets your coil drop out.

You might be able to get away with something as crude as this:

Find your coil resistance (pref pick a fairly high resistance coil, ie sensitive relay, to avoid gratuitously wasting power in the coil IMHO), and put in series with it a variable resistor of maybe 2x the resistance.  That might just about give you enough control, simply, but don't sue me!

(It might also be wise to put the normal back-EMF diode across the relay coil to protect everything else in the circuit from coil spikes.)

Rgds

Damon

[Flux]

I think you will have extreme difficulty doing this the crude way from the raw machine voltage. AC contactors will chatter at pick up and will probably object to non standard frequency. Dc coils may work better but clean pickup may not be easy to obtain and you will have to watch coil ratings. Drop out will be very low with dc coils. AC coils will never drop out clean even if you can get them in cleanly enough.

I would use dc from a regulated supply or a small battery charged from a charger off the raw power and force switch the relays or contactors with electronically triggered signals from voltage or better still frequency. I think you will have endless trouble even from a dc supply if you let the relays do their own pick up and drop out unless you choose the relays very carefully.

Chattering relays will have a very short life even switching non inductive loads.

If you must do it without electronics of any sort then it may be best to use small dc control relays with diodes across the coils and possibly capacitors and get them to switch cleanly. If you can manage that then you can use them to switch the power contactors. You may be able to use the raw supply for the power coils.

Flux

[Opera House]

DC relays generally have a 80% pickup and 20% drop out.  This should work well enough.  Use a separate DC relay to control the contactor.