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Lucas capstan on Cav 32


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1 hour ago, Spirit of Wray said:

Still have the original lucas unit.

No isolation switch or breaker fitted.

What is a recommended size circuit breaker for these?

Not sure what they are rated to, but assuming 1kw, at maximum load it draws about 80A, so a 120A breaker is appropriate.  Our RC10 Maxwell has an 80A breaker fitted - it hasn't ever thrown out, but that seems a bit low to me.

Just for comparison, most anchor winches use what is effectively a repurposed automotive starter motor of he old school type (full series winding, no reduction).  These typcally draw about 300A at stall...

 

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2 hours ago, aardvarkash10 said:

Not sure what they are rated to, but assuming 1kw, at maximum load it draws about 80A, so a 120A breaker is appropriate.  Our RC10 Maxwell has an 80A breaker fitted - it hasn't ever thrown out, but that seems a bit low to me.

Just for comparison, most anchor winches use what is effectively a repurposed automotive starter motor of he old school type (full series winding, no reduction).  These typcally draw about 300A at stall...

 

And that is pretty much where the name comes from. It's a Lucas starter motor, but the winch was most likely built by someone else, like maybe James Nillson of the old Simpson Lawrence days perhaps.

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Yikes.
You mean, you need a breaker rated for the normal rated current draw of the winch and cable sizes over distance to suit the winches current draw. Otherwise, when the winch boggs down, all the current dissapates into the cable as heat and the winch will not have the power to keep it turning and will bogg down even more and the cable will get hotter and around and around we go. If the breaker is lucky, there might be enough current spare to cause it to trip. The best way to check that is to short the cable at the winch and the breaker should pop within a second or 2 at the most. If it takes too long to pop, then there is too much current dissapating in the cable. If the cable gets warm to touch, there is too much current dissapating in the cable and you need to go up a size or two in square area of cable.
 

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This is a basic electrical question. The breaker is there to protect the CABLE not the load (winch).

Cable sizing is done by a Load (amps), Volts, and distance (including return - it's a circuit, remember!). Normal allowable volt drop over the cable is 3% for essential circuits, 5% is ok for a winch. There are several good cable size calculators available online - like this https://www.fabhabs.com/dc-cable-sizing-calculator

If the system works now, and the spec of the winch motor is not available, does the cable get warm when the winch is operated under load? If not, the cable is adequate, and you can work backwards from the cable size to determine the breaker required. If it does get warm, you need a good clamp meter to read the actual draw (in amps), and can work the calc above to determine the correct cable size.

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Out of curiosity is it a straight linear increase if you run (same length) cables in parallel?

Eg, is two 35mm2 cables between contacts the same as one 70mm2 cable?

Cause AWG2/0 is a seriously FAT cable!

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there is waaaaay too much thought going into this.

A series-wound DC motor draws maximum current at stall, and current drops in a non-linear curve as armature speed increases.

If you open up a series-wound starter motor, you will see that the conductor inside is significantly smaller in cross section than the cables feeding it (unless the system was installed by a complete lunatic).  Since its a series circuit, the resistance (or to be more precise, the impedance because the current draw changes in response to changes in the magnetic fields developed in the motor and they change with respect to armature speed) of those windings sets the practical operating current of the circuit.

What does this mean in practise?  The cable is irrelevant.  As an example, Maxwell recommend 35mm2 (AWG 2) cable if the run totals 10 to 15m.  For a run of about 12.5m that means its total resistance is about 0.006 ohms. Our estimated 1kw winch has an impedance of around 0.14 ohm at its maximum power rating, so it is orders of magnitude greater influence on the circuit than the cable.

To all practical intent the cables, assuming they are sized appropriately to carry the current draw of the motor and for the length of cable run, are irrelevant when speccing the breaker current.  For a slow-acting breaker, so is the stall current of the winch. 

The relevant spec is the operating current of the winch under maximum rated load.  The breaker has to be rated sufficiently to stay closed for the entire normal operation of the winch at its rated power.  Basic math and ohms law tells us that's about 80A for a 1kw winch.  Build in whatever engineering latitude you feel inclined toward, my suggestion is near the top of the thread.  A dead short (or near-dead short) in the winch will result in a current draw that is certain to open even a 100% over-rated breaker.

IMO. YMMV. 

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IT is correct in that a breaker is normally used to protect cable, not device.
However,
The Anchor winch is possibly the only circuit on a boat, in which the breaker is there to protect the Winch itself, as well as cable. That is because there are no other items on a boat (I can think of) in which the user is likely to put into a maximum current draw situation during operation, in which the unit could be damaged by excess current draw while not in an form of fault.
If the winch ever stalls or close to stall, say Anchor jams and user thinks he can stand harder on deck switch to compensate, it draws max xurrent and produces max heat. So the breaker is in this circuit to protect winch abuse and cable is automatically protected due to being able to hand current greater than breaker trip point.
All other circuits on a boat, the breaker is used to protect the cable. Internal fuses protect individual electronics when you have multiple electronics on one circuit. Other than lighting, all other circuits tend to be individual runs to single devices.
 

On 7/04/2022 at 7:20 AM, wheels said:

You mean, you need a breaker rated for the normal rated current draw of the winch and cable sizes over distance to suit the winches current draw.

My earlier comment is pretty much as aadvarkash stated. Breaker size is selected for winch. Cable size is selected so that when winch is under load, the current does not dissipate into cable and create a voltage drop. Voltage drop equals loss of current, which equates to loss of power of the winch motor.

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On 7/04/2022 at 4:46 PM, CarpeDiem said:

Out of curiosity is it a straight linear increase if you run (same length) cables in parallel?

Eg, is two 35mm2 cables between contacts the same as one 70mm2 cable?

Cause AWG2/0 is a seriously FAT cable!

Yes.

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