Thrusters short circuting the panel

[Kevin White]

Hello electrical genius's. I had all 4 batteries replaced by a "mechanic". The first time I put the boat back in the water, the thrusters turned on but then when I went to use them they tripped the circut breaker. The mechanic that installed was in a motorcycle accident and is incapacitated. I'm thinking and hoping that it's a simple fix and wires are crossed or the wires that support the system at the battery were put on two positive posts? Any ideas or has this happened to anyone before? Thanks, Kevin

 

[scross]

Kevin,
This won’t address your current issue but something to consider for next time the batteries need work. I mark every single wire going to the batteries with colored electrical tape. Amazon sells sets of 10 different colors of electrical tape. I try to use a different color for every wire/cable going to each specific terminal if possible. Then I take photographs of all the batteries with the colored tape in place. That gives me confidence when the whole project has been reassembled that it’s been done correctly.
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[Kevin White]

As it turns out, the brand new West Marine AGM battery was bad and was short circuiting the panel. Should I be concerned about AGMs? Has anyone ever heard of anything like this happening before? Thanks, Kevin

 

[vertigo]

As it turns out, the brand new West Marine AGM battery was bad and was short circuiting the panel. Should I be concerned about AGMs? Has anyone ever heard of anything like this happening before? Thanks, Kevin

That's not quite making sense. How was that battery not "short circuiting the panel" the rest of the time?

 

[Wee Venture]

As it turns out, the brand new West Marine AGM battery was bad and was short circuiting the panel. Should I be concerned about AGMs? Has anyone ever heard of anything like this happening before? Thanks, Kevin

That's not quite making sense. How was that battery not "short circuiting the panel" the rest of the time?

I’m guessing the new (bad) battery was not able to handle the thruster load. It may have had a bad cell or two that created an under voltage condition. With the thrusters running under their rated voltage they started to stall and therefore drew enough current to blow the circuit breaker.

John

 

[Kevin White]

Thank you. Sorry I wasn't completely clear on the switch board. Only the Thruster switches were flipping when the thrusters were engaged.
West Marine battery replaced and I''m hoping that was the problem. On another note, is there a reason that that battery, or any battery would drain with all circuit board switches, AC and DC in the off position. I was told by a West Marine manager that it sounded like it to him that there was something that was drawing power even with the switches in the off position. If that's the case, how would I even begin to know what was on, when it was supposed to be off. Any ideas. The boat is officially mine and I guess I'm on my own now.

 

[vertigo]

As it turns out, the brand new West Marine AGM battery was bad and was short circuiting the panel. Should I be concerned about AGMs? Has anyone ever heard of anything like this happening before? Thanks, Kevin

That's not quite making sense. How was that battery not "short circuiting the panel" the rest of the time?

I’m guessing the new (bad) battery was not able to handle the thruster load. It may have had a bad cell or two that created an under voltage condition. With the thrusters running under their rated voltage they started to stall and therefore drew enough current to blow the circuit breaker.

John

Not to be a contrarian, but that theory sounds entirely implausible to me. Perhaps my understanding of batteries and DC voltage is incomplete, but I don't see how diminished voltage or capacity in a battery can in any way cause an increase in the load on that battery. Perhaps someone can explain it for me. :|

I would guess that the problem is elsewhere, and hope won't fix it.

It would also aid clear understanding if the OP would clarify whether the use of thruster was actually "short-circuiting the panel", or merely tripping the thruster breaker. Several posts are inconsistent on that detail.

 

[Wee Venture]

Yeah, it’s counterintuitive. It doesn’t follow ohm’s law (amps = volts / resistance in ohms). This is the way I understand it; someone please correct me if I’m wrong or there is a better way to explain it:

DC motors have a very low actual resistance in ohms. The thing that limits the current is the reactance, or “counter emf.” As the rotor spins, magnetic lines of force are inducing a counter emf in the coils, limiting current. If you were to physically slow the spinning shaft down, there would be fewer lines of force cutting through the coils, therefore lower reactance, so current would increase.

When you decrease the voltage to the motor it cannot maintain the same power output without increasing amps. If the motor’s torque with the reduced voltage is now insufficient to get the job done (in this case turning the thruster prop to overcome the resistance of the water), the rotor will start to slow or stall. As it does, reactance decreases and all you are left with is the resistance in the windings which is negligible. So then you either blow a circuit breaker or the motor burns out.

Again, anyone please feel free to add or subtract from this.

John

 

[Red Raven]

I believe what John has described is correct. The only thing I would add is to look at it not as Amps = Voltage / Resistance (the reactance here complicates that formula a bit). Think instead in terms of power (Power = Amps x Voltage). The thrusters require a fixed amount of power to operate, the voltage is low and dropping, thus the Amps have to go up to produce the required power, thus breaker trips do to increased Amps. Just another way to say what John stated without getting into the reactance.

Does this help?

Curt

 

[vertigo]

I believe what John has described is correct. The only thing I would add is to look at it not as Amps = Voltage / Resistance (the reactance here complicates that formula a bit). Think instead in terms of power (Power = Amps x Voltage). The thrusters require a fixed amount of power to operate, the voltage is low and dropping, thus the Amps have to go up to produce the required power, thus breaker trips do to increased Amps. Just another way to say what John stated without getting into the reactance.

Does this help?

Curt

Yes, that explanation makes sense to a point, but I can't but suspect that it's not quite right, because it presumes that the power must be provided to make the thruster work. My limited experience is that as a battery becomes weaker the motor simply works less and less, due to lack of available power, until it does not spin at all.

If the power were constant, then I'd agree that with diminishing voltage the amperage draw would have to increase to keep the power steady. But I can't think of why power must be steady, and my crude observations are that it is not.

I suppose I'll have to run some tests with my multimeter connected while operating thrusters to see if the amp draw goes up as the voltage drops. That'll be a fun experiment!

 

[Red Raven]

Perhaps this will help.

https://www.ecmweb.com/design/article/20901278/the-highs-and-lows-of-motor-voltage

Curt

 

[vertigo]

Perhaps this will help.

https://www.ecmweb.com/design/article/20901278/the-highs-and-lows-of-motor-voltage

Curt

Thanks for that link, Curt. I enjoyed reading the article, but even though it states the same relationship that you described, it left unanswered the question of why the net power must be constant, and amperage increasing to compensate for decreasing voltage.

I did, however notice this part:
"Low voltage can lead to overheating, shortened life, reduced starting ability, and reduced pull-up and pullout torque. The starting torque, pull-up torque, and pullout torque of induction motors all change, based on the applied voltage squared. Thus, a 10% reduction from nameplate voltage (100% to 90%, 230V to 207V) would reduce the starting torque, pull-up torque, and pullout torque by a factor of .92.9. The resulting values would be 81% of the full voltage values. At 80% voltage, the result would be .82.8, or a value of 64% of the full voltage value. What does this translate to in real life? Well, you can now see why it's difficult to start "hard-to-start" loads if the voltage happens to be low. Similarly, the motor's pullout torque would be much lower than it would be under normal voltage conditions.

On lightly loaded motors with easy-to-start loads, reducing the voltage will not have any appreciable effect, except that it might help reduce the light load losses and improve the efficiency under this condition. "

I'm not suggesting that Watt's Law of Power is wrong, but only that it may be erroneous to assume that the power will remain constant as the battery's capacity is diminished, but that instead the power will decline. That seems to be what we observe on the water, with the thruster's power output becoming weaker as the battery voltage drops.

It seems to me that the answer to the OP's anomaly must be derived from something other than Watt's law.

As always, I'm eager to learn what I'm missing.

 

[Red Raven]

I agree it may depend on the the definition of “lightly loaded” and whether the thrusters normally would meet that definition. I do know that if you get something in the thruster (and your shear pin is in good shape) it will blow the ANL fuse (and thus proving the current increases with load) I have had that happen. This does not, however, mean the current will go up with voltage without a similar “high” load. On the other hand I have drawn the battery down significantly with the thruster operation and have seen reduced power so I agree the jury is still out on this one being a battery issue. I was only trying to add another way to look at John’s analysis. Perhaps there was something else going on in this case.

Curt

 

[Wee Venture]

Thanks Curt for the article. That was interesting.

I don’t know either if the problem the OP was having is due to the battery or something else. But the battery still seems to me like a reasonable hypothesis. Remember we’re not talking about an old battery that may be sulphated. An old battery that has lost capacity has little available current so it might not cause the same problem. Likewise a seriously discharged battery.

I’m no battery expert but I could imagine a brand new battery with one or two bad cells out of six might be low voltage but have plenty of available current. But I admit I’m reaching the limits of my useful knowledge.

I don’t know either whether the thrusters qualify for “lightly loaded” but it seems to me that if you have a small motor that is trying to overcome inertia to push a boat sideways, with a prop in a tunnel that doesn’t allow for appreciable slippage, that could easily go beyond a light load.

As far as whether the power remains constant or diminishes with decreasing voltage, it has to do with reactance and the load on the motor. If the load is sufficient to slow the rotor when the voltage decreases then the reactance will decrease and the current will increase to get it back up to speed.

If the load on the motor is sufficient to cause the rotor to stop (e.g., a glob of seaweed gets caught in the thruster prop) then the reactance goes to zero and the stall current is limited only by the resistance of the coils (negligible) or by the fuse. If you measure the resistance of the motor with an ohmmeter and apply ohm’s law it will seem like at 12 volts it should blow the fuse every time.

At least that is my understanding with all of the appropriate disclaimers.

John

 

[Kevin White]

Thank you every one. This problem has been resolved. It was a faulty battery. The thrusters are working great.