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[tobyker]

Sorry about the title but I've just read a book about WW2 submarines.

 I'm building a Perkasa with twin Speed 600s fed from one ESC.
Obviously (even to me) the standard set up is to wire the motors in parallel. I'm initially using a 4.5 ah 6v SLA cell
(yes John, you have my order) but may go to 7.2v Nicads at some stage.

The question is, whether there is any advantage at all to incorporating in the circuit switching so I can group up or down,
 ie between parallel and series motor wiring. Presumably with series I will be able to cruise slowly round the lake all afternoon,
 but with parallel it will go faster (if I build it light enough) but not for so long. It's just that I've got 4 channels so after
 I've steered it and throttled it I've still got 2 left.

Opinions?

[Fast Electricals]

I think it would be good idea. You could select series for negotiating a steering course at low speed or parallel for a quick blast around the lake. Do you want the switch function to be manual (ie. a switch inside the hull) or controlled via R/C?

Neil

[sweeper]

Has someone got the wrong concept here?
Changing the group connection (up or down) surely refers to the connection of the cells in the batteries.
Group down (parallel) would give lower voltage but greater endurance, group up (series) gives high voltage (and thus speed) for a shorter period of time.
Series connection of the motors would remove the ability of altering the speed / directiion of a single motor to assist in ship handling.
Regards,

[Telstar]

Hi I think Sweeper has hit the nail on the head. I believe that WW2 subs. did in fact change the connection of the batteries, from parallel (lower voltage but greater endurance) to series (higher voltage but less endurance) particularly when taking evasive action, the maximum speed of a given DC motor depends on the voltage (double the voltage nearly double the revs. per minute) the current taken by the motor does not go down however so the endurance is considerably reduced. Hence reduced voltage would give enough rpm for loitering (something I am lead to believe submariners like to do) and connecting in parallel would ensure that all the cells in the battery maintained the same level of charge(if only half the cells were used, you would have one half flat and one half charged) in parallel the cells would balance out the charge. Also during WW2 the method of controlling the motor speed on DC motors (not much solid state electronics in those days) was to connect resistors in the motor circuit, to limit motor current. This was very wasteful of battery power and generated a lot of heat, the last thing I would imagine you needed in a submarine.
As for connecting motors in series, I have never come across this in full size practice, and can visualise problems. connecting in parallel would mean that unless the motors and loads (props and propshafts) were equally balanced for load, the more heavily loaded motor would try to take all the available current (power), the vessel would try to sail in circles. In full size practice(industry) every motor has its own controle/protection installed  (fuses, overloads etc. are matched to the load current of each motor and putting 2 motors on one circuit could allow one to become overloaded (prop. tangled in weed) and overheat (burn out) without exceeding the rated full load of both.
With the use of modern high speed switching solid state motor controllers the need to 'group' battery cells is much reduced since modern speed controle is less wasteful of energy, the controller output in theory being either pulsed off(no current hence no I squared R heat loss( I=0) or full on when ideally R=0 therefor no heat loss. (I know this is not true in real life because there is some if not much resistance in Mosfets when biased hard on)

Sorry Tobyker to hijack your topic however thats my twopennyworth thanks tom

[Colin Bishop]

Well put Telstar, you are quite correct about submarine practice. However in the latter part of your post was that a typo that crept in regarding connecting motors (as opposed to batteries) in series? My understanding is that this entails running a wire from the positive battery connection through both motors one after the other and then back to the battery negative connection. If you connect model motors in this way they will inevitably run at different speeds (unless perfectly matched in terms of resistance) and if one spins up first the other may not run at all.

Parallel connection means that each motor has its own connections to the battery positive and negative terminals and draws as much current as it needs. Since two motors of the same type will be quite closely matched in resistance they will turn at very similar speeds for a given speed control setting which is just what you want.

Sorry if I'm telling people how to suck eggs - it's good to remind myself of the difference between battery  series/parallel connections and motor series/parallel connections - two quite different things. Not many people know that, but I'll bet most of them are on this Forum 

[Telstar]

Hi Colin
No typo error if you notice the first posting Tobyker was postulating connecting MOTORS in series or parallel, for whatever reason.
My comments as you read, or missread were that in real life (industry) I have never seen motors in series, I assume that this is due to there being more problems than uses.
As for parallel
Parallel connection means that each motor has its own connections to the battery positive and negative terminals and draws as much current as it needs. Since two motors of the same type will be quite closely matched in resistance they will turn at very similar speeds for a given speed control setting which is just what you want.
if you insert a speed controller between the battery and the motors then you don't have each motor with its own connections and if you are limiting the power available to the motors the more heavily loaded motor could hijack the available power.

By the way in your posting Colin in your series comment, you are concerned about the motors being matched in terms of resistance, while in the parallel comment the two motors (of the same type) are quite closely matched.

With models (and I have made model boats since early 1970's) you (we) can take some liberties with good practice after all a model motor at 12v 20A is not much compared to a main mill motor in a steelmill at 3750HP running on 500v DC, I spent part of my early working life maintaining and repairing the latter good practice is to protect/ controle motors individually
Cheers to All (no offence meant to Colin)  Tom

[sweeper]

This is getting into the exotic world isn't it?
Something is passing me by here, if you connect two motors in series, how can one run and the other not? By simple theory the current must pass through both motors and if one was not turning this would amount to a stalled condition resulting in a plume of smoke before long. The applied voltage is across both the motors, they may not turn at the same rate but they should both turn.
As to varying the speed of large motors, rather than using resistors and thus producing heat, it was often a practice to use a tapped field winding and use the variation in magnetic flux to vary the speed of the motor.This method was particularly used with series wound machines.

Regards to all.

 

[Colin Bishop]

connecting in parallel would mean that unless the motors and loads (props and propshafts) were equally balanced for load, the more heavily loaded motor would try to take all the available current (power),

Hi Telstar, certainly no offence taken. The bit I was referring to is quoted above. Can you clarify a bit further for me please? (I'm only talking model applications here!). If you have a twin screw boat with two nominally identical motors and one screw stalls because it catches in weed then that motor will draw stall current (in an unfused circuit) but no more. As long as that doesn't exceed the capacity of the battery to deliver current, the other motor should continue to run normally - until the whole lot goes up in flames! Moving on from there, if you did have two different motors, say a 380 on one shaft and a 550 on another (no I know you wouldn't), surely each motor would operate normally drawing it's usual running current.

Sweeper, No I'm not quite sure exactly why it happens either - but it does! I'm quite interested to learn more about the theory.

[tobyker]

Crikey! Perhaps I should go back to simple steam engines. The book - "one of our submarines" by Edward Young includes the footnote "'Group up' is an order to the motor-room to move the 'grouper' switch to put the motors 'in parallel' and so apply more powerto the propellors. When the motors are 'in series' for slower and more economical speeds, the are 'grouped down'."

It may of course be that the sub captain, seaman rather than engineering branch, understood the effect of the order rather than the full technical implications.   I was going to use a spare servo but now I am more confused than ever. If I used two three-volt batteries I could swith them to parallel or series, leaving the motors in parallel. However, as I've ordered the 6-volt battery anyway, I shan't bother - and the effect I was wondering about isn't really vital. Incidentally, for this model both motors are the same hand, and being controlled by the one ESC - I'm not going for super sensitive steering control with mixers and things.

Thank you for your contributions.

[Telstar]

Hi Colin
In your remark with two nominally identical motors connected to a theoretically unlimited supply (bear with me a moment) if one of the motors for whatever reason took h heavier load ( as you say weed on the prop) then it would take more current from the supply which being unlimited would continue to supply the other motor (as you say, I agree with you) however if you start to limit the current, by inserting a speed controller the supply is not unlimited, the slower motor would generate(wow something different here) a lower back emf(voltage) and tend to lower the voltage across the good motor thus causing it to slow down. when stalled the motor current is limited by the resistance of the circuit only(motor wiring controller etc. however once the motor starts to turn, it acts as a generator, and produces a back emf (voltage) which opposesthe supply (battery) voltage this is the main way the supply current is limited since the current through the motor is supply volts-back emf volts divided by the motor resistance.   a model motor with permenant magnet field on no load on 6v can generate a back emf in the order of 5.9volts, with a motor resistance of about 0.15ohm the no load current would be 6-5.9/0.15 =.67A  (near enough for a guess)
As load is applied the motor slows down, the back emf reduces and the motor takes more current until at zero speed back emf is zero, stall current is max ie. 6/0.15=40A 
As to Sweepers dilemma, if connected in series it is possable for one motor to start to run first and generate enough back emf to limit the current too much for the second motor to get under way, while the max rpm of a DC motor is governed by the applied voltage, its ability to turn (torque) is proportional to the current it draws, and any limiting device in the supply (be it intentional or just bad connection) will effect its ability to turn
I would do a drawing ("a picture paints a thousand words" Telly Savales I think?) but not being computer literate I don@t know how to include it here

Hope this helps disperse the fog ,or is that a different thread
cheers Tom

[Colin Bishop]

Thanks for taking the trouble to give that explanation Tom. I think I just about grasp it on first reading but I will read it again a couple of times to make sure I fully understand. I think that, in common with many others on this Forum, I have a good working knowledge of how to wire up a model boat (I've had plenty of practice!) but when it comes to the theory behind it all I tend to flounder once it gets beyond the basic level. Electronics has always been something of a mystery to me and I take my hat off to those with proper electrical training and the ability to design circuits etc. My talents, such as they are, lie elsewhere! Anyway, your forebearance is much appreciated and I expect some others have learned a bit from your posts as well!

Cheers,

Colin

[Telstar]

Colin If you want to play with a small motor, rig it on the bench with some kind of flywheel, then with your voltmeter across the motor connect a battery to spin it up. The voltmeter will read Battery volts. Then disconnect the battery (while leaving the voltmeter connected to the motor. You will see the voltmeter read the back emf, this will reduce as the motor slowes down. A simple demo. The flywheel is only to allow time to see this before the motor stops. Cheers Tom

[Welsh_Druid]

Forgive me if I take this slightly off topic but it has raised a query in my mind.  My latest boat is a scale motor schooner so of course it has motor propulsion as well as sails. When the motor is not running but the boat is sailing fast  the prop is being probably driven by the speed of the boat. Will this be generating a current ? Will it feed back to the battery ? If so - will it charge the battery ?

Don B.

[Colin Bishop]

You will see the voltmeter read the back emf,

Because when you disconnect the power the motor is acting as a dynamo until it stops, same principle as regenerative braking isn't it? I should have made the connection(sic!). Thanks, Tom  

Re Don's query, I suppose it would depend on a) whether there is a speed controller in the circuit which prevents current flowing back to the battery and b) the revolving prop would have to generate a voltage in excess of the actual battery voltage which is very unlikely unless Don is sailing in a hurricane! Have I earned a smartie?

[Telstar]

Hi Welsh_Druid
In theory yes. But you would have to spin the prop at least at the same rpm as it was doing when being driven from the battery, (to get up to battery volts) and also you would need to arrange some kind of switching arrangement to change from drive to charge. In theory if you were driving your schooner and a gust of wind speed ed you up (your boat I mean) then the prop/motor would try to charge the battery, but would only in all likelyhood just reduce the power taken by the motor.
PS whats the schooner, I'm looking to make a multi sail as the next build, and I am looking mainly for plans since I prefer to scratch build
cheers Tom

[Welsh_Druid]

PS whats the schooner, I'm looking to make a multi sail as the next build, and I am looking mainly for plans since I prefer to scratch build
cheers Tom

Thanks Tom.  See " Scale Sail"  posting 6th November "Motor Schooner"

Don

[Martin (Admin)]

Regarding Group up / down, Wikipedia says:

http://en.wikipedia.org/wiki/Submarine
"The motor could have more than one armature on the shaft — these would be electrically coupled in series for slow speed and parallel for high speed (known as "group down" and "group up" respectively)."

[Telstar]

Hi
WOW we all can learn something on this forum. Thats one thing I didn't think of. But I think changing the motor design is a bit of a cheat.
Seriously Martin good one we all have to learn

[tobyker]

Thanks Martin, that is interesting. Perhaps I'll find something else for the spare channels to do.

[FullLeatherJacket]

I've just had an idea. If you only want your model to go slowly round the lake then just push the LH stick forward a little bit. If you want it to go faster, push it further forward...............................simple works best for me. Suit yourself.

[sweeper]

Now this is all getting a bit naughty.
If the article quoted by our forum administrator is read further it reveals that "this practice (ie the double armature) was changed in the early 1930's on US and RN boats".
Perhaps the idea submitted by Full Leather Jacket is the most common sense path to follow. :) The good old kiss principle of engineering.

Regards to all.
 


 

[Martin (Admin)]

OK, just to throw another 'spanner in the works'...

If you group up/down the batteries ( series / parallel ) you will derive higher voltage vs higher duration.
If you group up/down the motors ( in a model ie.  ) you'll get faster or slower speed - so which is best?

And just to add further spanner, higher voltage means less current draw from the motors running at the same revolutions!!!  ::)

Yes, I am well out of my depth here... get it!!!..... 'Out of my depth......' Oh please yourselves.... 

Martin.

[anmo]

Perhaps the idea submitted by Full Leather Jacket is the most common sense path to follow....

I think it usually is, or at any rate, that's what Mr Jacket often tells me.

[roycv]

Hi all, on the point of higher voltage less current theme.  A case in point are the 'monoperm' type motors.  If you compare a 6 volt one with a 12 volt one of the same type you can see from the data on the box that the 12 volt version is just a few % more efficient.
This comes from the current lost in the brush to commutator set up.
Lets say for 6 volt motor you have 4 amps running through the motor. and in theory for 12 volt motor there will be 2 amps for the same load.
The brushes are the same and can be ignored as a constant for both motors.  ( the armatures have a different thickness of wire wound around them)

Voltage is derived from current, I, multiplied by Resistance R  ....  V = I x R.   so the voltage lost through a resistance is the current I x the resistance R.  So the voltage to the motor is divided between the 2 brushes and the armature doing the work.

At 6 volts (brush resistance) R x 4 (amps) = 4R.   At 12 volts R x 2(amps) = 2R.
If you look at the power lost it becomes more evident.

Power,  W = I x I x R    so by substituting we have 4 x 4 = 16 x R   or for the 12 volt motor 2 x 2 x R = 4xR.  If the total resistance of the 2 brushes in each motor was 1 / 10th. of an Ohm (resistance R is measured in ohms), then the power lost across the 2 brushes of the 6 volt motor is 1.6 Watts and for the 12 volt motor it is 0.4 Watts.  Just 1/4 of the power compared to the 6 volt motor.
This comes off as heat which is why it is a good idea to cool the brush areas of a motor.
Regards to all Roy

[FullLeatherJacket]

Anmo

As I usually say, suit yourself. Like you, I'm old enough to remember when we didn't have proportional controls - it was either "full on" or "nothing". Has anyone else ever heard of a bloke called Doug Spreng? He invented digital proportional feedback servos and things have never been the same since (thank goodness). Seems a shame to ignore the benefits of a technology that's been with us since the early 60's.  I still wonder about computer radios though............

Sorry for drifting a bit off topic but what the heck? No-one hurt, I hope.

I wonder what Kayem thinks.

FLJ