What size windlass battery and where?

What I asked you is whether, in your opinion, a 24V windlass connected to a 12V supply would either draw a high current or a low current. The correct answer is that it will draw a low current and barely work. You don't need to introduce 18V or falling voltages or any other complications. It's just a variant of a series-wound dc motor. All motors start off 'stalled'. Series wound motors have a very high starting torque at the expense of a high starting current. For a given supply voltage, the current is load dependant. But, if the supply volts fall, so the current and power fall.

By the way, since these windlasses can run unloaded, the motors cannot be straightforward series wound or they would over speed and destroy themselves; it might be a compound winding but the characteristic will be essentially series to get the high torque required at low speeds. I presume, but I couldn't be 100% sure, there is more than one way to skin a cat - even a Ship's Cat /forums/images/graemlins/smile.gif They could be compound or even shunt wound with suitable gearing. I am going to take my Tigress apart shortly and might find out.

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The outcome of that belief (which is what I accept and is what happens in my experience because the breaker opens when the motor is slowed or stalled through overload) is that the motor draws more current when the voltage is reduced.

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The reason the breaker opens when the motor is slowed or stalled is because the load has increased, thus increasing the current. The supply voltage is always assumed to be, for all practical purposes, fixed, at 12V. If the supply volts fall, then so will the current.

It isn't going to be easy for you to prove it to yourself unless you have a high-power variable supply but I assure you that if you connect up a variable supply to a windlass you will find the current falls as the volts fall. I had hoped that since you seem to have access to lots of kit you might have easily been able to connect a 24V windlass to a 12V supply and see for yourself. It won't take more than half and hour. Don't worry, it won't damage anything, I promise you /forums/images/graemlins/smile.gif
 
Windlass motors are usually either series wound or increasingly frequently now permanent magnet. My own is series wound.

John
 
On a note of caution - I used to run both the windlass & bow thruster from the domestic battery - After destroying 3 auto pilot computers Raymarine suggested that the bow thruster or windlass was creating voltage spikes which caused the auto pilot to fail (it is aslo powered from the domestic battery)
I have now powered both the windlass & the bowthruster from the engine battery both fed with large csa welding cables - i couldn't afford tinned marine cable that long and taht size.
 
Even without the windlass, etc on the service bank I think it is still also a good idea to fit a cheap automotive MOV surge/spike suppressor across the pos and negative of the service DC supply (at the DC board) - they are cheap and probably add to the protection.

Our windlass is also off the cranking battery but on the service side we still have some biggish inductive loads (the freezer DC drive draws at up to 40 A when running so is probably quite a thump at start and stop, plus a big alternator). We have never had a problem with any electronics, not even resets/restarts, etc, so maybe the surge/spike suppressor helps, or maybe have just been lucky ('cos it seems I don't know what I am talking about).

John
 
Ships_cat
Where did you get the spike supressor?
I fitted an X-Act voltage regulater to the auto pilot power supply - which so far seems to have solved the problem - However I will not know for sure until the auto pilot is destroyed again - then I will know it did'nt work
 
Hi Niord

I have tried to find a link that shows the exact type of thing I am referring to and a supplier in UK but no real luck (I am not Europe). The following may help lead you to identifying the right thing and a supplier -

This link - http://www.autosurge.com/Product.phtml describes a similar one but I don't know how that one works (but I assume it uses MOV's or similar newer devices) and also seems expensive. But it describes the service I mean. The one in the link is 18 v and I don't recall the voltage of our own one - so if you equalise your batteries at near to or higher than that it should be installed where it won't see that voltage (for us, we have a main DC isolation switch with batteries and charger output to them on one side and the spike protector and DC board is on the other service side so can be isolated from seeing the chargers along with all the other DC equipment not liking to see an equalisation voltage ie when we open the DC Isolator the spike protector and all DC distribution system is isolated from the batteries and chargers)

It should be like the one in the above link with just two wires - one goes to positive and other to negative buses at the DC distribution board so in parallel with everything (in the link's installation instructions, referring to vehicles and small boats, says fit between positive and negative terminals of battery, however on larger boat with a DC switch/distribution board it should go there).

There are other much more complicated ones which are both expensive and are high current devices that fit IN line not across them and I am not suggesting those. There are others also that will dump the output from an alternator or protect against lightning and those are not the ones I refer to either.

Our own is just in a small box about 60mm square and 30mm deep, two lightish gauge wires (one to -ve, other to +ve) and uses MOV's and on it has, from memory, "MOV Voltage Spike Protector". The boatbuilder sourced it when the boat was built but I would be quite confident that would be easily sourced as an after market device from any auto electrician's supply wholesaler or from any auto electrician that does after market work such as for RV's, etc or a good marine electrician. I suspect that mine was much cheaper than the one in the link above.

John
 
Fitting a spike suppressor is an excellent idea - I really should fit one and maybe will, now, as I have my bowthruster and windlass running off the service battery - BUT through around 10m of cable, via dedicated fuses and isolators which will help considerably.

MOVs - Metal Oxide Varistors - are devices that clamp the voltage across them to a specified level. Electrically, they look like two zener diodes back-to-back so note that an MOV will not, by itself, clamp large negative-going spikes. To provide adequate protection to dc circuits you need an ordinary diode across the MOV. This combination then clamps the supply to -0.6V (the forward voltage of a silicon diode) and +Vm where Vm is the clamping voltage of the MOV. In the range that interests us for boat electrics, you can buy 12V MOVs which clamp at 18V. Now, the problem is whichMOV?

The spikes we are talking about come from the commutators of motors or the switches that turn them on and off. These motors have 'inductance' and a key feature of an inductor is that current flowing in it cannotchange instantly.

So, if you have a switch in circuit with an inductor passing current, when you open that switch the voltage will rise to whatever is needed - kilovolts if needed - to ionise the air and cause a spark which keeps the current flowing. That is why contacts spark when you open them (capacitive circuits spark when you close the contacts for a similar but inverse reason).

Now to prevent these high voltage spikes from doing any harm we need to dissipate the energy in them; whatever we use to clamp the spikes will have to be able to dissipate energy. Have a look here MOV data The two key bits of information you are looking for are Vnom - choose 18V - and peak current Itm. On this table you'd have choice of Itm 250A and Itm 1000A A bowthruster could be carrying 500A or more so you might have up to 1000A as a breaking current, so that would be the best choice.

Where to put it? Well, you can put them right by the sensitive equipment (not a bad idea) or right across the heavy equipment (not a bad idea) so why not put them across both? Don't forget the diode across them!! Your diode also needs to be able to take 1000A peak. Not mean, but peak.

I have had a quick look at Maplins and can't see any MOVs there but maybe someone can check as I have to go out. If anyone finds a source and wants help in selecting the right one, please post the details here (in public, not a PM as it is best to share these things) and I will have a look for them.
 
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