Bow Thruster Battery Charging

[Farey]

I'm installing a Vetus Bow Thruster in my 60' boat, with a battery next too it. The question I'm struggling with is how I charge the battery. I've got a single alternator charging the starter and dommestic batteries, with a charge splitter, and a Victron Multiplus which provides a trickle charge to the starter battery when connected to a shore line.

 

Its been suggested that I simply run a pair of heavy (10sqmm) cables from the starter battery to the bow thruster battery, to carry the charge current, with a 30A fuse in line.

But if I just do that, surely the bow thruster and the starter will try to take charge from both the starter battery and the bow thruster battery? So, what's the best way to charge the bow thruster battery? Do you set up a relay so that when either the starter or the bow thruster is operating, the charging cables get disconnected?

[John Orentas]

Farey.

 

Yes you could do as you you suggest and isolate the thruster battery though even with 10mm charge cables you may suffer some volt drop preventing it from getting fully charged.

 

I would give some consideration to not having a local battery for your bow thruster, add an extra battery to the starter circuit and run very big cables to the thruster, 35 or even doubled up to give 70mm. In this case it would be in order to use a ground return system as the batteries will be bonded to ground anyway.

Let us know if you would like a volt drop calculation.

[rog guiver]

On our last 45' boat the bowthruster had "heavy" cables from the main batteries in the stern. When we extended it to 60' the voltage drop became unacceptable and another battery (in parallel) was installed in the bow.

 

The present installation has the main alternator charging just the service batteries with the smaller alternator to charge the starting, bowthruster and sternthruster batteries.

 

The plan was to use a mastervolt triple diode unit with the sternthruster and starter batteries in the stern and the bowthruster battery in the bow.

 

The cables between the diode unit and the bowthruster battery were selected such that, with say a 10 amp charge current, the voltage drop would be minimal. The system works perfectly and the bowthruster battery reaches a full charge even though it is some 60' from the alternator.

[Amicus]

Let us know if you would like a volt drop calculation.

25678[/snapback]

 

John, any idea what the volt drop would be in a solid copper bar, say 10mm?

[John Orentas]

Amicus.

 

I doesn't quite work that way, voltage drop is related to the current flow so it is much better to start with the voltage and current flow and calculate an acceptable volt drop and then cable size.

 

To give some idea:

 

35mm sq, cable carrying 100 amps the volt drop will be 0.0493 per metre. With a return conductor bowthruster on a 60ft boat volt drop would be 2 volts. That would be just about acceptable given that the 100 amp figure would no doubt be a 'maximum load'.

 

The 35mm sq means 35 sq. mm cross sectional area.

 

Your copper bar 10 x 10mm would of course be 100mm sq. mm

[Amicus]

Thanks John, I was thinking of 10mm round bar. I feel a ponder coming on.

[ChrisPy]

I took John's advice. I have Vetus 75kg thruster (7.2KW). Cables twin 474/0.45 running the length of the cabin (2 x 15m), with grounded return (negative thruster terminal bonded to the hull). Volt drop 1.3V. Cost of cables £100. Perfect.

[rog guiver]

I think I am right that DC appliances will draw extra current to compensate for voltages below the norm.

 

Would be interesting to see what the manufacturers say about accepting a voltage drop and hence greater amperage through the windings by remote siting of the batteries.

 

Over to the experts....

[Richard Bustens]

Vetus say to use 120mm cable for 0-45' i have used that to the batteries about 9' both ways and 16mm cable to join the engine batteries to the thruster ones

[ChrisPy]

I think I am right that DC appliances will draw extra current to compensate for voltages below the norm.

 

Would be interesting to see what the manufacturers say about accepting a voltage drop and hence greater amperage through the windings by remote siting of the batteries.

 

Over to the experts....

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Not in general, I think. For most devices the current is generally proportional to the voltage. The power will drop by the power of two. That's why motors run slow and lights dim when batteries get flat. Traditionally you control the DC motor in a train set by dropping the voltage through a rheostat controller.

 

There may be systems with special wiring to compenstae, but I wasn't aware that Vetus Thrusters are like that.

 

That reminds me of a British electrical 'technician' I lived and worked with in Nigeria. We suffered frequent brown-outs when the mains voltage dropped and the lights dimmed. He would run around switching off the lights. He said that if a lamp was rated at 100W and the voltage dropped then the current would increase to compensate !! He did not understand Ohm's Law.

[John Orentas]

Chris.

 

Interesting point that about the train set controller. Rheostats are not very good at controlling d.c. motors, some years ago I had a chap design some speed controllers for the direction motors on helicopter searchlights.

 

The way they work is by the used of 'chopped' d.c. You leave the voltage as it is, 28 in this case and electronically very rapidly switch it on and off, varying the on /off time proportion gives the speed reduction, the idea being that as the motor always sees a full voltage and maintains it's torque even at low speeds, they worked very well.

 

I understand from a TV programme that model train controllers now work in the same way and give a very realistic slow acceleration from a standstill.

Edited July 20, 2005 by John Orentas

[ChrisPy]

Chris.

 

Interesting point that about the train set controller.  Rheostats are not very good at controlling d.c. motors, some years ago I had a chap design some speed controllers for the direction motors on helicopter searchlights.

 

The way they work is by the used of 'chopped' d.c. You leave the voltage as it is, 28 in this case and electronically very rapidly switch it on and off, varying the on /off time proportion gives the speed reduction, the idea being that as the motor always sees a full voltage and maintains it's torque even at low speeds, they worked very well.

 

I understand from a TV programme that model train controllers now work in the same way and give a very realistic slow acceleration from a standstill.

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you're right, John. I forgot about that. I was back in my youth for a minute.

 

Radio control cars and boats are similar. You can get a variable resistor (printed copper or coil) for a tenner that fits on a servo, or you can spend thirty on an electronic pulse controller.

 

hey we were explaining why the bow thruster current will drop off with voltage drop, not the other way round.

[rog guiver]

Further to decrease in voltage leads to increase in amperage....

 

With regard to a bowthruster motor have a look at http://www.ecmweb.com/mag/electric_highs_lows_motor/

 

The main points are covered in the chapter covering the effects of low voltage.

 

When you subject a motor to voltages below the nameplate rating, some of the motor's characteristics will change slightly and others will change dramatically. To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. The amount of power the motor draws has a rough correlation to the voltage 2current (amps). Thus, when voltage gets low, the current must increase to provide the same amount of power. An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.

[ChrisPy]

. To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line.

25778[/snapback]

 

and that is not the case, is it?

 

you're getting back to the 100W light bulb scenario. the 100W rating of the light bulb is not fixed. the resistance of the element is fixed. at normal voltage the bulb consumes 100W, so by Ohm's Law you can calvculate that the resistance is about 420milliohms. At 115V the bulb will consume 25W. at 460V the bulb will consume 400W before it goes POP.

 

like the light bulb, the 7.2kW rating of my 75kgf thruster is not fixed. It is what the motor consumes under normal conditions at 12V supply, at which point it will provide a thrust of 75kgf.

 

With 10V it will produce no more than 7.2 x 10/12 x 10/12 kW = 5kW, and that is about 52kgf of thrust. If you upped the volts to 15V you would consume more than 11kW and would get more thrust, until the motor burns out. The motor and speed characteristics will actually exaggerate those numbers.

[John Orentas]

Rog.

 

Your link though interesting in itself refers to alternating current motors, where you are concerned with phase slippage and all kinds of things that do not apply to d.c. motors. Have you ever been aware of your car starter motor being overloaded when the battery is a bit flat ?

[rog guiver]

Damn, I stand corrected!

[maffi mushkila]

Chris.

 

Interesting point that about the train set controller.  Rheostats are not very good at controlling d.c. motors, some years ago I had a chap design some speed controllers for the direction motors on helicopter searchlights.

 

What we are missing here is that a Rheostat is a current controller not a voltage controller. Model electric trains normally have permanent magnet fields. The field strength never varies. The Rheostat only adjusts the current through the armature. Increasing or decreasing the armature current increases or decreases the speed of the motor

 

As far as the light actuators are concerned these motors are normally 'series wound motors. The characteristic of such a motor is high speed low torque. This is then converted by the use of a reduction gearbox into low speed high torque delivered to the mechanical load.

 

In order to change the speed of a motor you adjust only the current through the field or the armature, not both. Since in a series motor the field and armature are connected 'in series', reducing the current through field also reduces the current through the armature this has little or no effect. A series motor will only draw the current required by the load it is driving. If the mechanical load increases so will the current and visa versa for a decreasing mechanical load.

 

Parallel motors are a different box of spanners altogether. And Rheostats are perfect for controlling those.

Edited July 22, 2005 by maffi mushkila

[John Orentas]

Maffi.

 

In both examples the motors are the permanent magnet type, the fundamental problem is how to reduce the speed without the inevitable power / torque loss that occurs when you simply attempt to reduce the voltage through a rheostat

[maffi mushkila]

................................we were explaining why the bow thruster current will drop off with voltage drop, not the other way round.

 

Might I venture to suggest that the problem here is the limitations of the generator.

 

In a generator system if you draw more current from the generator the voltage falls. This is sensed by the voltage regulator and more current is supplied to the field increasing the O/P of the generator.

[maffi mushkila]

Maffi.

 

In both examples the motors are the permanent magnet type, the fundamental problem is how to reduce the speed without the inevitable power / torque loss that occurs when you simply attempt to reduce the voltage through a rheostat

 

I reiterate a Rheostat is a current controller not a voltage controller.

 

Voltage is dropped across a resistance and current is passed through it.

 

In your case John I suspect that the increasing current increases the back EMF ergo you will loose power/torque. Don't quote me on that I'm rusty on BEMF.

Edited July 22, 2005 by maffi mushkila

[John Orentas]

Maffi.

 

"I reiterate a Rheostat is a current controller not a voltage controller.

Voltage is dropped across a resistance and current is passed through it".

 

And what is a rheostat but a variable resistor in this case. Your mention of back EMF reminded me of a further refinement, when the motor is switched 'off' the two input conductors are connected together effectively stopping the motor dead with no run-on at all.

[maffi mushkila]

Maffi.

 

"I reiterate a Rheostat is a current controller not a voltage controller.

Voltage is dropped across a resistance and current is passed through it".

 

And what is a rheostat but a variable resistor in this case.  Your mention of back EMF reminded me of a further refinement, when the motor is switched 'off' the two input conductors are connected together effectively stopping the motor dead with no run-on at all.

 

Like most thimgs John it's all about terminology and understanding. If I wrote all my posts here in Arabic few if any would understand so what would be the point.

 

When talking about electricity we should all use the language that has devloped so we all understand.

 

Rheostats are placed in series with a load to control current.

 

Dubrys are placed in parallel with a load to control voltage

 

You are quite right in that real electric trains use BEMF braking

[maffi mushkila]

Rheostats are placed in series with a load to control current.

 

Dubrys are placed in parallel with a load to control voltage

 

What I was expecting John was for you to ask what was a 'dubry' so I could reply "I rest my case" but you are obviously to smart to be drawn in to that one.

[DHutch]

I'm installing a Vetus Bow Thruster in my 60' boat, with a battery next too it. The question I'm struggling with is how I charge the battery.

We have a vetus unit in our bow, with a 'starter' battery next to it (and two domestic batterys)

 

We have the output of our alternator wired to a DPDT switch, which then feeds to eather the bow batery, or to the boat side of the battery selector switch.

- This allows us to charge the bow battery OR which ever of the domestic baterys we're running off (one, or other, or both)

 

That said, we have a very unusal set up, with no sarter baterys, and only one (specially wound) alternator.

- If i had a normal diesal setup, it would have two alternators, one for the starter batery and one for the domestics, and then run the bow thruster off the engines starter battery using suitable cabling.

 

 

Daniel

[DHutch]

Also, a good supply off heavy cable in arc-welding earth cable.

- Its really quite thick, and relativly inexpesive if you look around.