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  1. #171
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by dom View Post
    Bear in mind that excessively low voltages can damage electric motors.
    Surely voltage reduction is the standard way of controlling electric motors?

    Richard

  2. #172
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    Default Re: Anchor thread with a lesson

    Simple electrical science.

    The amps drawn will go up as the voltage drops.

    Too much current-amps-in the winding may cause significant overheating.

    1000W at 250V = 4 Amps

    1000W at 200V =5 Amps.

    Vics volt drop will make a similar difference to the Amps drawn. About 20% increase.

  3. #173
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by RichardS View Post
    Surely voltage reduction is the standard way of controlling electric motors?

    Richard
    Provided the voltage is kept within the defined parameters usually stamped somewhere on each motor, then yes.

    But go too low or too high and problems can occur. On the low side, as the voltage falls the motor operating at a fixed power output will draw more amps roughly in proportion to the falling voltage. When amps exceed the motor's rating, a dangerous heat build-up can occur within the motor, something a suitably specced breaker is designed to interrupt in a timely manner. Vic has a non-spec breaker, in which light he should be careful to keep the voltage to Lofran's spec, whatever that is. Failure to do this over an extended period of time could damage the motor.

  4. #174
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by rotrax View Post
    Simple electrical science.

    The amps drawn will go up as the voltage drops.

    Too much current-amps-in the winding may cause significant overheating.

    1000W at 250V = 4 Amps

    1000W at 200V =5 Amps.

    Vics volt drop will make a similar difference to the Amps drawn. About 20% increase.
    I do not believe that a standard DC motor with brushes will behave like that. If you lower the voltage, the revs of the motor will decrease and the torque will reduce but the current will not increase in that way because the motor will no longer draw 1000W.

    Richard

  5. #175
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by RichardS View Post
    The loads are additive, load of chain + load of wind and waves on bow + load of boat in water. If wind/waves is zero then a large force is excluded so no problem in that case and we would not be so prescriptive about not towing the boat.

    Richard
    After stern-to berthing it is imperative to pull the boat using the windlass when recovering the anchor. Those who motor forwards are those who catch others' chains and anchors. Releasing the lines from the wall starts the boat moving, upon which the windlass has an easy time pulling the boat onwards and recovering chain. With wind from astern I will sometimes engage astern in order not to over run the anchor.
    Answers to some technical queries at new website http://coxeng.co.uk

  6. #176
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by vyv_cox View Post
    After stern-to berthing it is imperative to pull the boat using the windlass when recovering the anchor. Those who motor forwards are those who catch others' chains and anchors. Releasing the lines from the wall starts the boat moving, upon which the windlass has an easy time pulling the boat onwards and recovering chain. With wind from astern I will sometimes engage astern in order not to over run the anchor.
    That is indeed an occasion when we might well use the windlass to pull the boat forwards unless there was a strong wind on the nose .... but I'm a bit confused about the "catching others chains and anchors" as presumably that would also only happen if one over-ran the anchor so that would be a bad idea in any circumstances, save one.

    Richard

  7. #177
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by RichardS View Post
    I do not believe that a standard DC motor with brushes will behave like that. If you lower the voltage, the revs of the motor will decrease and the torque will reduce but the current will not increase in that way because the motor will no longer draw 1000W.

    Richard
    Its science. You cant alter that. If voltage reduces, amps drawn go up, heat in the winding increases. The 1000W will try to be drawn-Amps equals W divided by Volts. Volts drop, Amps go up to try and keep the science going.

    I spent a major part of my life working with Lucas brush kit, both dynamo's and starters.

    Many starters overheated the windings through excessive cranking with poor batteries. The low voltage caused more amps to flow in the circuit so overheating it.

    The prats turning the key, however, were part of the problem...........................
    Last edited by rotrax; 19-07-19 at 15:07.

  8. #178
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by rotrax View Post
    Its science. You cant alter that. If voltage reduces, amps drawn go up, heat in the winding increases. The 1000W will try to be drawn-Amps equals W divided by Volts. Volts drop, Amps go up to try and keep the science going.

    I spent a major part of my life working with Lucas brush kit, both dynamo's and starters.

    Many starters overheated the windings through excessive cranking with poor batteries. The low voltage caused more amps to flow in the circuit so overheating it.

    The prats turning the key, however, were part of the problem...........................
    But not all electrical loads obey Ohms law in the way you suggest.

    Anyway, I'm sure that we've both done our Google searches for "running DC motors at lower voltage" and this was my top hit:

    Operating a motor at a voltage below nominal generally has no detrimental effect on performance. In fact, running a motor at lower than nominal voltage (and, therefore, slower than nominal speed) can result in less brush and commutator wear (for brushed motors), lower current consumption, and longer motor life. On the other hand, running a motor at a voltage higher than nominal increases current draw and can cause the motor coils to overheat, decreasing motor life.

    I'm happy to leave it there as it's not really relevant to Vic's issue.

    Richard

  9. #179
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by RichardS View Post
    That is indeed an occasion when we might well use the windlass to pull the boat forwards unless there was a strong wind on the nose .... but I'm a bit confused about the "catching others chains and anchors" as presumably that would also only happen if one over-ran the anchor so that would be a bad idea in any circumstances, save one.

    Richard
    We have seen it happen so many times. Helm too enthusiastic steers off course a bit, finishes up dragging his anchor across several others while getting back to his own.
    Answers to some technical queries at new website http://coxeng.co.uk

  10. #180
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    Default Re: Anchor thread with a lesson

    Quote Originally Posted by vyv_cox View Post
    We have seen it happen so many times. Helm too enthusiastic steers off course a bit, finishes up dragging his anchor across several others while getting back to his own.
    That's interesting. I wouldn't have thought it that likely that one could easily pull the anchor sideways-ish until one was getting pretty close to it but your experience with stern-to/bows-to mooring is much greater than mine so it's a valuable reminder.

    Richard

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