DC Motor Voltage Usage

[BlowingOldBoots]

I am trying to understand how voltage is used on a DC Motor. I have a DC Motor for my autopilot, an old Neco motor. The motor has a coil and windings powered through 2 x carbon brushes on a commutator (slip) ring. Electrical voltage on my boat is 24V. With the vessel along side the pontoon, I engage the auto helm and then command the motor to turn 10 degrees. The voltage measured across the two cables fluctuates between 7 - 9 Volts as the motor powers up and rotates the rudder. Being alongside and not moving through the water, there is very little force on the rudder.

What I am trying to understand, do DC electric motors just take the voltage that is needed, based on load, up to the maximum available i.e. 24V. I know that DC motor speed can be controlled by increasing or decreasing voltage, but that is a deliberate act, where available voltage is varied. If I think about Power (W) = Volts and Current, if the required force to turn the rudder is low, then the 24V is applied but current required is very low. Or, the minimum current required to move the motor and rudder is applied and voltage take is low. I just don't understand how the motor works.

Why do I need to know - I am trying to establish if it his the motor or the Raymarin Evolution ACU400 that is causing my auto helm to stop working after about half an hour to an hour.

Appreciate any guidance on this matter.

Thanks,

Alastair

[claymore]

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.

[BlowingOldBoots]

Thank you very much Claymore. I owe you and Dr Muz whisk(e)y and beer. I removed the motor today and cleaned the commutator and carbon brushes with thinners. There was a lot of dust build up and the commutator looked dirty; now looks clean. Raymarine suggested that common cause for the “loss of drive” alarm is poor carbon brush contact. Also, if the motor does not respond to a command to turn for more than 20s, then the alarm sounds. Tomorrow I shall go for a test sail and feed back. I will also move the digital compass as it may just be too close to my iron keel as it sits low in the boat. Quite easy and quick to move. The explanation has given me a bit of confidence that the ACU 400 brain is doing what it is supposed to do.

[claymore]

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

[claymore]

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

[BlowingOldBoots]

I am afraid that I got waylaid by a Gardenshed and my own bone idleness. This weekend is big McFuck hedge cutting weekend (or at least today is), then next weekend I have a work assignment on a thin pole, floating in the middle of nowhere. Outside chance that tomorrow will be a trial run because it is forecast not to be pouring.

Please keep calm and and don't hyperventilate or blackout for my sake!

[claymore]

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

[BlowingOldBoots]

A wee trip round Shuna by Autopilot and all is fine. Autopilot did what it was supposed to do.

When making a turn by 40 degrees, voltage increased to 14.5V, then dropped to zero when heading was achieved. It’s obvious now that the ACU400 outputs variable DC voltage as required and current will rise up to 30A (system max) as needed.

The NECO motor must be voltage controlled for speed, one of the 3 methods DC brushed, no magnet motors are controlled; flux and current being the other two, according to YouTube.

Proof Point
This exercise has triggered a though. My old drilling rigs, built in the 70s, were DC motor operated, aka traction motors, originally from train power plants. These drilling rotary table speed, was controlled by a rheostat from 0-600V. The current was what it was. If the rotary table struggled and slowed, more voltage was dialled in and current rose.

I think cleaning the carbon brushes and commutator did the trick. That was 2 hours of autopilot work, it’s never worked for that length of time before loss of drive response alarm.

Thanks Claymore and Muz.

[claymore]

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

[BlowingOldBoots]

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

I am sure a dram or too can be offered.

[claymore]

[BlowingOldBoots]

I do try to understand how stuff works. So for prosperity and if anyone searches for this issue in the future, here are some comments from Raymarine to another customer, via their forum, where they describe how the DC voltage is applied by the ACU: -

https://forum.raymarine.com/showthread.php?tid=11177

... They don't drive a straight 12V or 24V, they pulse the output with a high frequency (something like 40kHz from memory) in order to provide a softer start and stop and finer control over the movement. This is very standard in motor control systems. You will almost never measure a straight supply voltage on the output from one of these pilots with a multimeter ...