DC Motor Voltage Usage

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I referred this knotty problem to my good friend and crewmember Dr Muz who gave it due consideration and then came back with the following:
I hope it helps!

The answer is not an easy one to explain and the bottom line is that I have no idea why it would work for half an hour and then stop. They will have to check the obvious things about whether the motor is overheating; if there is any friction in the rudder system that is overloading the autopilot, etc.

Autopilots are a 3-Term control system reacting to 'Proportional', 'Integrative' and 'Derivative inputs as the boat changes course, or goes off course due to Yaw, Pitch and Roll movements affecting the course to be steered. The system relies on electrical feedback signals to drive the motor in proportion to the amount the vessel is off course, sensing the amount of offset and as rudder is applied proportionally reducing the amount of rudder applied as the vessel approaches its 'on-course' heading. The reason I am telling you this is that the motor drive supply is altered to create a high current flow when the vessel is significantly off course and reducing the current as the vessel returns to its course. The motor also has to be reversed dependent upon whether the deviation from course is to Port or Starboard.


The typical motor drive arrangement is that the motor is connected in a 'Wheatstone Bridge' configuration , where the drive motor in the Autohelm system is connected like Vg. When the vessel is on course and no-helm is required the current flow is zero. Any off-course shift is detected and a correcting current in the right polarity is applied to spin the motor and drive the rudder with a correcting push to turn the vessel towards its course. If the vessel Yaws beyond its course direction this is sensed and a reverse spin is given to the motor to apply the rudder in the opposite direction.

Best advice is to look for obvious problems, such as how hard is it to turn the rudder i.e. is it harder to turn that normally expected and is the motor having to work harder than expected so that after half an hour it has over-heated.

Apart from getting the rudder to move one way or the other in response to a course offset when alongside the pontoon there is no-way you can test the system because there is no change of compass heading so no feedback signal to make a course correcting adjustment. So you can only test it is a 'live' situation i.e. not on a pontoon.

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You are welcome - the reward will be in my telling Muzzy that his diagnosis led to successful treatment and outcome

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The good Doctor and I are waiting with baited breath!!

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This isn't easy for either the Muzz or myself - do be quick

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If you don't need an invoice we can accept other currency

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