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Old 25-02-2015, 17:26   #16
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Re: Conductor size question.

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Originally Posted by svlamorocha View Post
The power (watts) consumed by a motor does not remain constant when you change voltage.
I am not questioning your statement just trying to understand.

I understand Watts to be "the work being done" and mathematically equals volts X amps. How does the capacity of the motor to do 1500 watts of work change with lower voltage? I can see that capacity dropping with lower voltage but not increasing.
Is it simply that when the motor approaches a locked rotor state that it basically short circuits causing a spike in current, and with that spike, volts x amps (watts) is higher?

The question still, is the voltage drop on the 6 ga. cable for that short distance enough to cause the current to spike over 150 amps?

I guess the voltmeter and current probe will supply the answer.
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Old 25-02-2015, 18:18   #17
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Re: Conductor size question.

The 6 Ga cable is a short run (inches) therefore you don't need big wire.
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Old 25-02-2015, 18:41   #18
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Re: Conductor size question.

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Originally Posted by Cheechako View Post
The 6 Ga cable is a short run (inches) therefore you don't need big wire.
My 6 ga. run is about 10 feet (round trip). 10% voltage drop for up to 20 feet calls for 1 ga at 150 amps. 6 ga. at 10 % voltage drop is good to 70 amps. This is going by the chart previously posted in this thread.

I really just need to measure this and find out what is really happening. Either it really is drawing over 150 amps or the breaker is not performing the way it should. If it is drawing that much current it is likely due to voltage drop on the smaller wire. I cant see the 3/0 part of the run dropping that much voltage.

I hope this is helpful the op, didn't mean to hi-jack his thread.
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Old 25-02-2015, 19:48   #19
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Re: Conductor size question.

I thought he said the wire internal to the motor was the 6 gage.... :>)
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Old 25-02-2015, 20:33   #20
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Re: Conductor size question.

A windlass under heavy load draws a LOT more amperage than one which is running under lesser loads.

Most electric motors have a "LRA" rating, i.e., a Locked Rotor Ampere rating. This is the amount of amperage the motor will draw while stopped, or under an excessive load. The LRA can be anywhere from about 3 to 8 or more times the "RLA" or Running Load Amps.

Example: a windlass which draws 100 amps in normal operation can draw up to 800 amps in a locked rotor condition. This is enough to easily pop a breaker or fuse rated at, say, 150% of the normal amperage draw.

The LRA condition or near LRA condition can occur with improper use of the windlass, i.e., trying to break out an anchor with the windlass (you should never do that) or trying to pull the boat up to the anchor with a strong opposing wind and/or current.

Here's a graph showing amperage draw of a Lewmar Concept 2 windlass, such as the one on my 42' sloop:

Click image for larger version

Name:	ConceptII_Amps.jpg
Views:	99
Size:	77.6 KB
ID:	97933

You can see that with a load of 200kg the windlass draws about 100 amps, but when the load increases to 800kg the windlass draws upwards of 250 amps. That is sure to pop a breaker or fuse. The breaker which Lewmar provided with this windlass is rated at 90 amps.

FWIW,

Bill
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Old 25-02-2015, 21:43   #21
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Re: Conductor size question.

In general situation with DC motors the current the motor pulls is dependent on the load. A 1200 watt motor might draw 100 Amps at 12 Volts but it will not develop 1200 watts at 10 Volts unless it is overloaded, it may only develop 1000 watts at rated current. The exact current/ load/ speed/ Voltage is dependent on the design of the motor (series wound great starting torque poor speed regulation) permanent magnet/ shunt wound (lower starting torque better speed regulation). Low voltage generally just reduces the power available due to reduced speed and torque. Lower speed just means it will take longer to recover the anchor lower torque may cause the motor to stall and trip the overcurrent device.

It is not unusual to find short lengths of smaller wire inside appliances. They get away with that because of short duty cycle/ high temp insulation. The voltage drop with these short lengths (usually inches) is not significant. Current rating of conductors/ breakers feeding motors should be generous in sizing because of the real possibility of overload. That said most windlass/ thruster motors are rated for low duty cycle use and the mfg will provide a small breaker to keep the user from overheating the motor. Voltage drop at 12 Volts high Amperage systems can be considerable and often dictate the wire sizing rather than current rating.

Anchor windlass live in a bad neighborhood. A good lube job can get the torque back into recovering the anchor rather than overcoming system friction.

Probably more than you want to know,
Frank
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Old 25-02-2015, 22:00   #22
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Re: Conductor size question.

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Originally Posted by Terra Nova View Post
Use 4/0.
This.
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Old 26-02-2015, 04:31   #23
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Re: Conductor size question.

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Originally Posted by Wind River View Post
I am not questioning your statement just trying to understand.

I understand Watts to be "the work being done" and mathematically equals volts X amps. How does the capacity of the motor to do 1500 watts of work change with lower voltage? I can see that capacity dropping with lower voltage but not increasing.
Is it simply that when the motor approaches a locked rotor state that it basically short circuits causing a spike in current, and with that spike, volts x amps (watts) is higher?

The question still, is the voltage drop on the 6 ga. cable for that short distance enough to cause the current to spike over 150 amps?

I guess the voltmeter and current probe will supply the answer.
Others replies explain relationship between available power and voltage at given speed ( = more voltage gives you more power and available torque). This is not particularly inportant for cable sizing.


Another poster talks about "locked rotor condition" that can multiply the current by a factor of say 5 to 8.

Now lets' s put together both concepts. If batteries are weak then windlass has less power available to pull chain, hence the rotor locks and then, the current is much higher than if batteries were good.

In addition ALL that locked rotor current time the voltage at the motor goes into heat that wants to burn the windings.


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Old 26-02-2015, 05:17   #24
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Re: Conductor size question.

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Originally Posted by Wind River View Post
My 6 ga. run is about 10 feet (round trip). 10% voltage drop for up to 20 feet calls for 1 ga at 150 amps. 6 ga. at 10 % voltage drop is good to 70 amps. This is going by the chart previously posted in this thread.

(snip) I cant see the 3/0 part of the run dropping that much voltage.
Those tables are only perfectly accurate when the cable from battery to load is all of the same gauge. In your case you have 60ft of AWG 3/0 and 10 ft of AWG 6, and the smaller gauge portions is long enough to cause significant drop.You need to look at the drop in each portion and be mindful that total drop is the sum of the two. Then the table is not that convenient and you can just use a formula.

Assuming 150 amps draw and that the lengths are round-trip then you have 5% drop in the AWG 6 portion and another 5% in the AWG 3/0 portion., for a total of 10%.
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Old 26-02-2015, 08:30   #25
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Re: Conductor size question.

Thanks everyone for the explanations. It makes perfect sense.

What seemed strange to me is how fast this breaker trips. It doesn't even seem to give me an indication I am overloading it.
When retrieving my anchor I am motoring up to the the anchor and bringing in the loose chain. I can tell when the slack chain is taken up when the windlass just starts to slow (ok that is an indication). At that point I stop the windlass and continue to motor forward over the anchor or just let wave action break it out. Most of the time this is uneventful with no breaker trips. When bringing the anchor up to the bow roller, there is a direction the swivel can be in that will not come up through the opening in the bowsprit and I have to adjust the direction of the anchor before continuing. If I wait a split second to too late to realize that the swivel is not in the correct orientation, it will trip the breaker. Just a split second! This is why I questioned the old breaker.

Maybe everything is doing it's job and operator error is to blame. Seems a bit of a nuisance though. Not a big deal when I have someone below flaking the chain and they can just switch it back on, but if I were alone it becomes more of a problem.

Does anyone know of a reasonably priced clamp on DC ammeter that can log the high current so I can see the point at which the breaker tripped?
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Old 26-02-2015, 08:53   #26
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Re: Conductor size question.

I had a similar problem with nuisance trips with my 90A breaker and the Concept II windlass (see graph above), despite totally correct anchoring practice.

I never tried to break the anchor free using the windlass. Rather, I'd get up directly over the anchor, usually by using the engine. Then, I'd use the engine in slow forward or reverse to break the anchor free of the bottom.

I also avoided overloading the windlass by various strategies, like powering up toward the anchor, then taking in some chain, then powering up a bit more and take in more chain, etc. Usually, I'm alone in this practice, but have UP/DOWN switches both on the foredeck and in the cockpit.

However, on occasion I'd misjudge the load on the windlass a bit and the breaker would trip. Then, I'd have to go below and all the way forward to the V-berth where the breaker was located next to the windlass battery bank, reset the breaker, and go up on deck and all the way forward again.

Finally, several years ago I replaced the 90A breaker with a 200A ANL fuse. No blown fuses...all works as it should.

Bill
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Old 26-02-2015, 09:16   #27
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Re: Conductor size question.

Quote:
Originally Posted by Wind River View Post
Thanks everyone for the explanations. It makes perfect sense.

What seemed strange to me is how fast this breaker trips. It doesn't even seem to give me an indication I am overloading it.
When retrieving my anchor I am motoring up to the the anchor and bringing in the loose chain. I can tell when the slack chain is taken up when the windlass just starts to slow (ok that is an indication). At that point I stop the windlass and continue to motor forward over the anchor or just let wave action break it out. Most of the time this is uneventful with no breaker trips. When bringing the anchor up to the bow roller, there is a direction the swivel can be in that will not come up through the opening in the bowsprit and I have to adjust the direction of the anchor before continuing. If I wait a split second to too late to realize that the swivel is not in the correct orientation, it will trip the breaker. Just a split second! This is why I questioned the old breaker.

Maybe everything is doing it's job and operator error is to blame. Seems a bit of a nuisance though. Not a big deal when I have someone below flaking the chain and they can just switch it back on, but if I were alone it becomes more of a problem.

Does anyone know of a good clamp on DC ammeter that can log the high current so I can see the point at which the breaker tripped?
If you want a precise and long-lasting clamp get one with the Fluke brand. Make sure it measures DC amps (not just AC amps) and has a "Min Max" function. I like the Fluke 325 and 374 that sell for between $200 and $250.

If you are cheap like me you can make do with an overstock Radio Shack 22-172 that you can get for about US$40 from Ebay or Amazon. It does not have "maximum" feature but it gives me what I need, including DC amps for starter/windlass/alternator, RMS volts, temperature, frequency, capacitance, etc.
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Old 26-02-2015, 09:28   #28
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Re: Conductor size question.

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Originally Posted by btrayfors View Post

Here's a graph showing amperage draw of a Lewmar Concept 2 windlass, such as the one on my 42' sloop:

Attachment 97933

You can see that with a load of 200kg the windlass draws about 100 amps, but when the load increases to 800kg the windlass draws upwards of 250 amps. That is sure to pop a breaker or fuse. The breaker which Lewmar provided with this windlass is rated at 90 amps.
To Lewmarīs partial credit, in the manual they say that the specified rating of the breaker depends on the model (ie 90A for Concept 1 and 150 for ocncept 2). I say partial credit because in the diagrams that apply to both models they include only the part number for the 90A breaker.
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Old 26-02-2015, 09:32   #29
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Re: Conductor size question.

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Originally Posted by btrayfors View Post

Finally, several years ago I replaced the 90A breaker with a 200A ANL fuse. No blown fuses...all works as it should.

Bill
Donīt you worry about the fuse not protecting against burning the windings when the windlass locks? A 200A ANL fuse will take 250A forever without blowing... I would expect that Lewmarīs recommendation of a 150A breaker should be related to what they know about the windings. I would appreciate your wisdom.
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Old 27-02-2015, 12:01   #30
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Re: Conductor size question.

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Originally Posted by svlamorocha View Post
Donīt you worry about the fuse not protecting against burning the windings when the windlass locks? A 200A ANL fuse will take 250A forever without blowing... I would expect that Lewmarīs recommendation of a 150A breaker should be related to what they know about the windings. I would appreciate your wisdom.
Yes, good question.

Lewmar recommends a 150A breaker for the Concept 2 and 3 vertical windlasses. However, I found that nuisance trips were a problem, even though I was operating the windlass correctly. Thus I went to a 200A ANL fuse, which solved 2 problems for me:

1. no more nuisance trips;
2. the possibility that the 150A (not 90A as I earlier mentioned) was underperforming, i.e., tripping at less than its rated current.

I'm extremely careful not to operate the windlass with too high a load, or to keep the button pressed if the windlass slows way down or stops. I'm paranoid about this. I also don't speed thru anchor retrieval, but take my time and try to tax the windlass as little as possible.

It's installed in a clean and dry place, and so far has been perfect. It has its own battery bank (two T-105s) located under the V-berths forward, and has 1/0 cables running about 8 feet (one-way) to the windlass motor.

With this arrangement, I can come to anchor under sail or retrieve the anchor without using the engine if desired....something I like to do.

Bill
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