Help understanding windlass power

[Scatterling]

My 700 watt Simpson Lawrence windlass has finally succumbed to corrosion and is sadly deceased. The replacement offered by Lewmar (who bought SL) is the V1. My SL unit had a 70 amp breaker which makes sense when I do the calculation: amps = watts/volts or 58 amps. My question is, why does the Lewmar, which also has a 700 watt motor, come with a 100 amp breaker? Even at 10 volts, it could still only draw 70 amps. Correct? Can I continue to use my 70 amp breaker, OR can I simply replace the 70 amp with the new 100 amp? I’d appreciate input from those who know better than I. Thanks.

[StuM]

Two thingss: "inrush current" and "locked rotor current"

An induction motor will draw more than it's rated current when starting.

How much more and for how long depends on a number of factors in the motor design, but the bottom line is that you need to fuse well above the current rating and dfferent motors have different methods of attenuating this "over-current" so they require different levels of current protection. If the manufacturer says that a particular motor needs 100A protection, that's what you should use.

[dkenny64]

is your wire large enough to handle 100amp?
if the wire size is too small. say 8ga instead 2ga you have too much voltage drop.
the windlass won't work correctly.


-dkenny64

[Jammer]

Originally Posted by Scatterling

My question is, why does the Lewmar, which also has a 700 watt motor, come with a 100 amp breaker?

Who knows what they were thinking. Maybe they think it provides "enough" protection and they don't want to stock 70 amp breakers. Manufacturers do things for all kinds of reasons.

Even at 10 volts, it could still only draw 70 amps. Correct?

No. 12vdc windlass motors are nearly always series wound, and will draw far more than their rated wattage under heavy load or while starting. Here's a page with the theory, if you want details: https://www.engineeringenotes.com/el...ineering/36546

Can I continue to use my 70 amp breaker, OR can I simply replace the 70 amp with the new 100 amp? I’d appreciate input from those who know better than I. Thanks.

If the 70 amp breaker is in good condition, you could use it and see if there is a problem with nuisance trips. If you replace it with the larger 100 amp breaker you would want to be sure the wiring is sized handle the somewhat larger load.

Originally Posted by StuM

Two thingss: "inrush current" and "locked rotor current"

An induction motor will draw more than it's rated current when starting.

DC windlass motors are not induction motors, and have different properties.

[Frankly]

My SL Anchorman is a permanent magnetic 12V DC motor. The feed is via a 70 amp breaker. It has tripped on rare occasion when probably pulling above rated load.

Give it a try with current breaker/ wire. If you start having nuisance tripping then you can upgrade the electrical side. Often motor circuits are sized at 125% of rated FL current for a lot of the reasons stated.

Motors are capable of multiple times their rated output for short periods. Anchor windlass service is one of those applications.

Frankly

[RaymondR]

Originally Posted by Frankly

My SL Anchorman is a permanent magnetic 12V DC motor. The feed is via a 70 amp breaker. It has tripped on rare occasion when probably pulling above rated load.

Give it a try with current breaker/ wire. If you start having nuisance tripping then you can upgrade the electrical side. Often motor circuits are sized at 125% of rated FL current for a lot of the reasons stated.

Motors are capable of multiple times their rated output for short periods. Anchor windlass service is one of those applications.

Frankly

Yep, way to go.

[Alan Mighty]

Originally Posted by Scatterling

My 700 watt Simpson Lawrence windlass has finally succumbed to corrosion and is sadly deceased. The replacement offered by Lewmar (who bought SL) is the V1. My SL unit had a 70 amp breaker which makes sense when I do the calculation: amps = watts/volts or 58 amps. My question is, why does the Lewmar, which also has a 700 watt motor, come with a 100 amp breaker?

Under no particular load (e.g. if you have motored up to your anchor so the chain is not under tension) you can expect your windlass to draw perhaps the 58 amps of your math calculation.

Should you use your windlass to draw your boat forward to the anchor, then the greater load will see your motor draw more current.

And should you get into the situation such that the load on your windlass via the chain stalls the motor, and you continue to keep the motor in the stalled load mode, then the motor might draw 200 or more amps (should you not have a 100 A breaker).

Most electric anchor windlass motors are not that different from the starter motor of a car. They do not have heat sinks etc. In that sense, they are not designed for long and high loads (starter motors are great at 30 second bursts).

So the 100 A breaker is there to give you some leeway in operation (of course the motor and the mechanicals will always last long if you motor to the anchor such that your windlass is only lifting the mass of the chain and the anchor, but most everyone has to cheat at one time or another). It's also there to protect the motor and the mechanicals from getting into the stalled load situation (overheating the motor, stressing the mechanicals).

That potential for high current draw is also why the usual recommendation is to have your auxiliary engine running when weighing anchor. The engine alternator will lift the voltage to 13.8 V, reduce the potential for fast current demand on your battery bank, and encourage you to motor to the anchor.

[facciatosta]

The value of the current in all dc motor is in reverse proportion to the speed of the motor and so is the torque. That's why switch is closed the current is limited only by the very low resistance of the windings and can be very high. The same , the slower the motor speed the higher the current. You do not want the breaker to trip at the worst moment. On my boat it is a 2x45 thermal fuse that suppose to close when it cools off. It is 700 W Lewmar windlass. From the safety point of view wether it is 70A or 100A makes little difference, they both happily will trip when there is short between wires.

[Scatterling]

Thank you to all who responded. I am now much better informed!