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Old 01-12-2021, 11:08   #1
nwn
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spike current

Does anybody know what the spike current is for a bow thruster?
I have a Lewmar 185TT 4.0 kW thruster. I have measured the current to 168 Amps when it is running. However, it uses much more when it starts because the resistance is smaller.
The electronic relays connected with my BMS should be able to handle that current. In my case it did not and I got roasted Mosfets out of it. The contactors (relays) were rated for 300Amps and that was clearly not enough. So now, I have to get new contactors and need some advice on how much the spike current could be.
To my knowledge, the spike current can be calculated from the resistance in the motor measured when there is no voltage on it (=not turning). My problem is that the boat is in Greece and I am in Denmark.
So informed guess work would be appreciated.
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Old 01-12-2021, 14:56   #2
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Re: spike current

Some informed guesswork

https://www.lewmar.com/node/11676?v=25384
12v uses 400A fuse

https://www.lewmar.com/node/11676?v=25385
24v uses 250A fuse
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Old 01-12-2021, 15:21   #3
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Re: spike current

I can't help you with an exact number of the current spike, yet I expect it to be out of range of the common MOSFET switches, especially given you already blew a 300 A one.

A mechanical contactor like the Bluesea ML-RBS might be a better fit? The continuous ratings are about the same as MOSFETs but they tolerate surge loads far better. Another approach might be to connect the thruster motor directly to the batteries and let the BMS only drive an enable signal for the controller (if possible, of course). Or, given how rarely the bow thruster is driven and the negligible energy it consumes, omit BMS protection altogether (I assume you are using lithium batteries).
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Old 01-12-2021, 15:32   #4
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Re: spike current

Most tunnel thruster motors are series DC motor (the armature and field are connected in series). Series DC motors typically have large starting currents (think starting a trolley car/ train). Propeller is a low inertia load but still a series motor. If BMS means lithium batteries (ie low internal resistance) then you might be in a difficult situation.

Might want to look at a separate fused tap just feeding the thruster directly from the battery bypassing the BMS protective relays/ contactors.


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Old 01-12-2021, 15:59   #5
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Re: spike current

No need to guess. Take your Ohmmeter and measure across the motor leads when disconnected or at at least OFF.
Start current will be just less than I = system V/Ohms

Is there no one watching the boat that could measure this?
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Old 02-12-2021, 00:45   #6
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Re: spike current

I have never seen a bow thruster only draw 170a. most are in the 500a range. is this 12v or 24v?

lithuim and bowthrusters gernally do not agree with each other. stick an agm battery in the bow.
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Old 02-12-2021, 05:12   #7
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Re: spike current

Thank you for your suggestions. I agree, the best solution is to measure the resistance in the motor it self. I will do that myself if COVID 19 restrictions allow me to go to Greece in January. Meanwhile I have written to Lewmar. They should be able to tell me, if they have a good service unit.
If the spike current is too high for the Mosfets, the simplest solution is probably to feed the bow thruster directly circumventing the contactor.
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Old 02-12-2021, 05:13   #8
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Re: spike current

Let me add. It is 24V and I am using 8 Winston Life04 batteries
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Old 02-12-2021, 06:30   #9
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Re: spike current

Winston has a very wide range of cell sizes. From website: 40 to 2800 Ah. The size, no doubt affects rated current. My Relion battery is 100 Ah and specs a max load of 100A. As one example.
And, BMS capability is another question.
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Old 02-12-2021, 06:41   #10
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Re: spike current

To Karst
I have 100Ah Winstons with the following characteristics
Optimal discharge current < 50 A 0.5 C
Maximal discharge current < 300 A 3 C, continuous for max 15 minutes from full charge
Max peak discharge current < 1000 A 10 C, maximal 5 seconds in 1 minute
so that should not be a problem, unless the spike current is above 1000Amps.
The problem lies in the capacity of the contactors, the fuses and what you may have of diodes on the way to the thruster, as far, as I see it.
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Old 02-12-2021, 06:47   #11
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Re: spike current

Diodes? anyway, how did the contactors fail? my experience with high current DC inductive loads is that the contacts weld shut, due to arcing on the break cycle. AC loads are much easier to break due to the zero crossings which tend to quench the arc.
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Old 02-12-2021, 06:57   #12
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Re: spike current

I had used a contactor from Chargery based on Mosfets. It was rated max 300Amps. I clearly need something rated higher, but the question how high. The alternative is to draw current directly from the battery for the thruster.
I have a similar installation all with Chargery BMS and conductors for my 12V installation and that works really good. I will describe the system in a later post.
I only mentioned diodes because it is my my plan is to connect another battery in a parallel position but separated by ideal diodes on the charge and consumption side to double my battery capacity.
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Old 02-12-2021, 07:15   #13
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Re: spike current

OK, I'm slowly getting on board. My use of contactor is mechanical solenoid type. MOS would be a SSR or solid state relay in my industrial world.
YES, totally different failure modes.
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