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[pwhavens]

Not wanting to study the formulae  , would someone be so kind as to explain in simple terms what happens when a higher voltage, say 7V, is applied to a lower voltage rated motor, say 6V?  For example, I have a 6V motor and a 7V battery pack.  Can I use them together, or should I find a 6V battery pack?

Thank you in advance.
P

[Shipmate60]

In very simple terms the higher the voltage the higher the speed. IE a motor will spin twice as fast as if the voltage is increased from 6 to 12 volts.
6 volt motors will usually happily accept 7.2 volts.

Bob

[dougal99]

The simple answer is that the motor will run faster. I don't think applying 7v to a 6v motor will do any damage. Most batteries when freshly charged will provide more than the rated voltage.

I wouldn't consider applying too much voltage though, as you can burn the motor out. No doubt people more knowledgeable than I will give better guidance.

HTH

Doug

[Bunkerbarge]

Staying away from formula's  I don't think 7v would be a problem for a 6v motor at all.  It would run slightly faster and could ultimately reduce the life of the motor but probably only if you were running it continuously and for model boat use we rarely run any motors for a greta length of time.

I take it you are talking propulsion here, in which case you are only going to get the full 7v anyway at full speed, which is usually only a proportion of the time.

[Martin (Admin)]

To sum up:

The motor will spin faster.
The motor will also consume more ampage, (power, current, wattage).
Battery will not last as long if it's the same Ah, (Amps per hour).
Motor will run hotter.
Motor won't last as long.

..... but most of these are relatively by small amounts. 

[Sandy Calder]

I'd add to that last post and say figures are proportionate to the change in voltage.(bar age)

Quoted voltage for a model motor is only a ballpark guide to stop you overheating the motor.It's added by distributors

The limiting factor in motor rating is temperature so you could multiply the voltage by ten so long as you reduce the load to keep the power losses within the rating. (and the bearings too)

[andrewh]

P

Everything folk have replied is true - go for it!

Good news - the motor cannot read.  It HAS no voltage.  all it has is, say, 22 turns of .057 wire on each pole. 
The manufacturers may call it a 6v motor (if it is a  "speed" motor the 6V means that you can run it AT FULL LOAD at 6V with a life that will be mesured in thousands of hours. 
Generally we boaters never nearly approach this load, so this is virtually meaningless

Similarly I'm intrigued by your 7V battery pack - if this is a buggy pack of 6 NiCd or Nimh cells it will be 7.2V nominal, about 6V under heavy load (say 20 amps) and about 8.1 ot 2 V when fully charged.

You will be fine - but please tell us more if you need more info, and post the pics anyway!

andrew

 

[Seaspray]

Find out the type of motor you have firstly.

Then you'll find all it's ratings. This will give you all the info from  max/min voltages to use max revs at those voltages, stall speed. maximum efficiency @ x amps and maybe size of prop to use. Check out some of the motor  charts at Electronize. There easy to read as they are an X Y chart.

Battery type is an other point to consider

Seaspray

[Sandy Calder]

stall speed ?

[dougal99]

stall speed ?

Only in a Flying Boat 

[Martin (Admin)]

stall speed ?

Maximum current ( and temper) when motor jams when boat gets stuck in weeds! 

[dougal99]

stall speed ?

Maximum current ( and temper) when motor jams when boat gets stuck in weeds! 

And there was I thinking that was the stall current 

Surely the stall speed of a motor is zero? 

Doug

[Sandy Calder]

I thought it was a simple typo but I checked back with Johnson motors.

It possibly explains the chart at the foot of this page. 
http://www.johnsonmotor.com/Performance-Charts.266.0.html

[Shipmate60]

Sandy,
The chart doesn't mean much without figures!!

Bob

[Seaspray]

It should have read "stall and speed and maximum"

I associate "stall speed" with "torque converters".



Seaspray

[Sandy Calder]

Sandy,
The chart doesn't mean much without figures!!

Bob

I pasted it to highlight the part of the Johnsonmotor hyperlink that was relevant....scroll down.
It is irrefutable proof that motors must rotate after a complete stall
How else would they manage to supply power? (green curve)
Don't give yourself a headache over it!

[pwhavens]

Thank you gents!  I'm using the Dumas 6V motor on the Lord Nelson Victory Tug.  The directions say use a 5 cell saddle pack and I've had some difficulty determining what that is.  Your answers indicate that I've got leeway in selecting batteries.

I've already learned about stalling in weeds during the maiden voyage of my steam powered fantail launch.  It's clear water for me now!

P

[stallspeed]

Even small motors have some form of thermal inertia don't they?

You run them on 12 volt for a while it should not do the motor harm.

[craftysod]

Cos they get hot and burn out,i have found running a 12v motor on 12v 7amp batt and the motor is smokin after 10 mins running

[malcolmfrary]

Cos they get hot and burn out,i have found running a 12v motor on 12v 7amp batt and the motor is smokin after 10 mins running

In this case, either the motor was broken in the first place, or too great a load was being offered.
I have had quite startling performance from a motor that said max 6v on the label when running it off 7 cells (8.4v), but its life was short.  In the same hull, an 8.4 volt motor has been perfectly happy for years.
In a similar hull, same type of motor, same battery, larger prop, short run times and a hot motor (and a molten fuse holder, but thats another subject).

[OMK]

In a similar hull, same type of motor, same battery, larger prop, short run times and a hot motor (and a molten fuse holder, but thats another subject).

Malcolm,
Now that is intriguing, because I too experienced the exact same on two identical hulls. Both had identical motors, identical batteries and ESCs, the same propshafts, same props', yet one of them would always melt the fuse holder after a few minutes runtime. Even after swapping out parts from each hull, the problem would always persist. Out of sheer desperation I even swapped out each propshaft, in case one was binding. But that same fault was always there.
I have never been able to identify the root cause.

Did you finally nail the problem? If so, could you kindly throw some light this way?

[malcolmfrary]

I just fitted a smaller prop and ditched the tubular fusehoder for a blade type.  The smaller prop allowed the motor to rev up and improved run times. 
The blade type fuse presumably had lower contact resistance and thus less heating effect on its housing.  Unless you are using very low current, the bayonet type are a waste of time and space for us.

[Sandy Calder]

In a similar hull, same type of motor, same battery, larger prop, short run times and a hot motor (and a molten fuse holder, but thats another subject).
 .....................
Did you finally nail the problem? If so, could you kindly throw some light this way?

It's called "over the hump syndrome"
Just shine your torch on the graph up top but I reckon you and a few other reply posters know that.
Can nobody explain how a motor outputs power when it has stalled?
Regards
Sandy Calder

[OMK]

Malcolm,
Sorry, buddy -  I feel a bit of a dork now. Refering to your quote, just after I hit the send button was when when I spotted the tell-tale clue after the third comma. "Larger prop".
Like I said, if'n I had spotted it sooner I wouldn't have troubled you to re-explain what was already clearly obvious.

Sorry about that. But thanks just the same for replying.

[malcolmfrary]

Nothing wrong with feeling a bit of dork, depending on which bit and if there is one nearby and you both consent.