Fried DC motors - under voltage

[KnotYet]

A DC motor has lower resistance when it is stalled or barely turning, due to the commutator
and brushes allowing current to flow through more than one set of windings at rest or low RPMs.
In theory, this could trip a breaker even at somewhat lower than nominal battery voltage.
It might not happen in every example but I can imagine it could in certain circumstances.

 

[DDW]

Here is what I recorded by measuring voltages at the batteries and amperage with a clamp meter:

Old Northstar MS-210, Top battery: Volts: 9.0 Amps: 350
Old Northstar MS-210, Bottom battery: Volts: 9.9 Amps: 350

New Northstar 190FT Red, Top battery: Volts: 11.4 Amps: 425
New Northstar 190FT Red, Bottom battery: Volts: 11.14 Amps: 428

Jim

These numbers look the same to me. 350 - 350 and 425 - 428. All well within the accuracy range of a clamp ammeter (unless a very expensive lab unit).

I think a DC motor driving a propeller isn't all that simple a system. What operating point did they choose? Maximum power? Max RPM? Something in between? And when you lower the voltage, you change all those things.

 

[JDCAVE]

These numbers look the same to me. 350 - 350 and 425 - 428. All well within the accuracy range of a clamp ammeter (unless a very expensive lab unit).

I think a DC motor driving a propeller isn't all that simple a system. What operating point did they choose? Maximum power? Max RPM? Something in between? And when you lower the voltage, you change all those things.

Well, the response of the thruster was very different. Much more response with the new batteries. I guess you kinda had to be there to experience it.

 

[O C Diver]

Ted - I thought I had proposed a possible explanation - repeating: the startup at lower voltage takes longer. The breaker has a delay in the trip and this "timeout" could be getting exceeded at lower voltages.

Just a theory, but something is happening and (as JDCave says) lower voltage pushes less current.

Nick

Ok, but the breaker popping was instantaneous. The windlass maybe flinched before the breaker popped.

Ted

 

[O C Diver]

Well, the response of the thruster was very different. Much more response with the new batteries. I guess you kinda had to be there to experience it.

When my thruster batteries were at the end of life, I had the thermal overload trip well before expected. I attributed it to the windings generating more heat or possibly less cooling from the fan at a slower speed. Wondering if you saw a reduction of time with the thermal overload switch?

Ted

 

[Nick F]

Ok, but the breaker popping was instantaneous. The windlass maybe flinched before the breaker popped.

Ted

Thanks Ted - interesting. It's a possibility that at the lower voltage the windlass was not able to start turning at all and this caused the trip, but I may be clutching at straws here.
Nick

 

[JDCAVE]

My thrusters never failed or timed out from overheat. However the “Balmar Digital Duocharge” fuse popped once or twice. I replaced with a 30 amp fuse. But the batteries were beginning to lose “jump” so I replaced the Northstar MS-210 with the Northstar 190FT Red (which were used batteries from a cell tower). These have been fine since. But I baby them. Only little taps on the thrusters unless I really need them when docking. Before, I used to run them a lot.

That said, it’s a bit of a McGivor set up by one of the POs I think. It’s a 24 volt thruster with a Cole Hersey relay of some sort that combines the 12 volt batteries in series when the thrusters are activated. I think the Duocharge DC to DC charging is insufficient for the most part, although it does provide some charging when running. I can see that is the case from the display. I definitely monitor the charging carefully, because the new equivalent batteries are #)&*@%@ expensive!

Getting back to the experiment. Primarily, I now have some idea as to the baseline amps and volts at the batteries to compare as the batteries age—a load test if you will.

Jim

 

[JDCAVE]

This is the thread on what I originally reported:

House & Start Bank Design Issue

Without hijacking the thread too much, I had two 6v batteries in each main engine start bank and I was told by a very experienced ABYC marine electrician to replace them with one 12VDC 8D and not to use 2 x 6VDc. One set had failed anyway so no great loss. Has anyone heard this from other...

www.trawlerforum.com

Jim

 

[DDW]

Well, the response of the thruster was very different. Much more response with the new batteries. I guess you kinda had to be there to experience it.

Possibly, higher rpm at the higher voltage? That is what you would expect of DC motors.

 

[O C Diver]

It's my impression that DC motors at design or slightly higher voltage work more efficiently. Also load is a function of amperage draw. Just start an engine on a 0 degree day versus one that's been running for hours and then shutdown. Both windlass and bow thrusters essentially start under some percentage of a load which may generate a momentary surge.

Ted