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Author Topic: 11.1v Lipo with 7.2v motor  (Read 5440 times)

Bkr0411

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11.1v Lipo with 7.2v motor
« on: April 26, 2011, 12:03:51 PM »

Firstly, apologies if this is a really dumb question or has been covered elsewhere...

I am building my first boat using a Graupner Speed 600 Race 7.2V motor. I visited my local model shop to get advice about a suitable ESC and battery to use. I explained what motor I was running and the guy persuaded me to go for a 11.1v lipo.

It didn't occur to me at the time, but won't a 11.1v battery fry a 7.2v motor?

Many thanks in advance,

Dave
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andrewh

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Re: 11.1v Lipo with 7.2v motor
« Reply #1 on: April 26, 2011, 12:42:06 PM »

Dave

Reasonable question-  basic answer is No.

This is becos:
11.1 Lipo is not 11.1 V (except when it is half-discharged), it is three lipo cells in series which is 12.6 V fully charged and about 9V fully discharged
Speed 600Race 7.2V  motor is not 7.2V (that is just a shorthand for the type of winding)

The limits on "Speed" motors are current, not volts and boaters (usually) keep the currents low to get 6 hours or more sailing time :}
Tell us please about the boat and propellor - which is what set the amps
hope this reassures
andrew
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Bkr0411

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Re: 11.1v Lipo with 7.2v motor
« Reply #2 on: April 26, 2011, 01:34:26 PM »

Hi Andrew,

Many thanks for your response, that's really helpful.

The boat is a MFA Piranha 2 (http://www.cornwallmodelboats.co.uk/acatalog/mfa-piranha.html) and I was planning to use the stock prop that comes with the kit (at least initially). Other than it is twin bladed I don't really know the specification, but I will check when I get home this evening to confirm the size and see if I can work out the pitch.

Once again thanks for your help,

Dave

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Bkr0411

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Re: 11.1v Lipo with 7.2v motor
« Reply #3 on: April 28, 2011, 09:08:19 AM »

It's a 35mm prop with (I think) a 40 degree pitch if that helps...
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malcolmfrary

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Re: 11.1v Lipo with 7.2v motor
« Reply #4 on: April 28, 2011, 10:10:34 AM »

Motor current is affected by both voltage and load.
A nominal 7.2 volt motor will certainly work OK on higher voltages.  It will certainly want to spin faster, and its ability to do so will be limited in the first place by the prop fitted, in the second place by just how fast it can spin before either the bearings give up or the armature parts go their separate ways or the brushes just give up. 
If the 3 cell Lipo pack produces a higher voltage than the motor's recommended maximum, then I would expect riotous performance, but not for long.  The motor would probably benefit from water cooling.  Lipos of a given capacity are generally capable of delivering a lot more current than other types, and for any given capacity, they tend to be lighter as well.
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nick_75au

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Re: 11.1v Lipo with 7.2v motor
« Reply #5 on: April 29, 2011, 06:55:34 AM »

Speed 500 race has Kv of aprox 3000, so at 11.1 volts will have a no load speed of 33000 RPM, for maximum efficiency you want the motor to draw around 10-12 amps under load at around 25000 RPM, you could allow the motor to draw up to about 18 amps with water cooling of the can and brushes.

Nick
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Bkr0411

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Re: 11.1v Lipo with 7.2v motor
« Reply #6 on: May 03, 2011, 01:11:14 PM »

Thanks everyone for your input... I was hoping to keep the build as cheap and simple as possible as it is my first one and frankly I wouldn't know where to start with installing water cooling.

Even before this issue I was beginning to think the decision to go with Lipos was a mistake, given the price of chargers/balancers/charge bags/low voltage cut-offs etc. etc.  and this has probably clinched it.

I will see what the shop say but think I may well revert to a NiMH set-up.

Once again thanks for your advice.

Dave
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essex2visuvesi

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Re: 11.1v Lipo with 7.2v motor
« Reply #7 on: May 04, 2011, 06:25:54 AM »

Why didn't you go with 7.4V 2 cell lipos?
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Bkr0411

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Re: 11.1v Lipo with 7.2v motor
« Reply #8 on: May 04, 2011, 10:51:15 AM »

I just bought what the guy in the shop advised me to... It was only when I got back home and thought about it that this issue occurred to me. In retrospect a 2 cell Lipo probably would be a good solution.
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derekwarner

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Re: 11.1v Lipo with 7.2v motor
« Reply #9 on: May 04, 2011, 12:05:06 PM »

 >>:-( ...Bkr0411  ..print this thread out...& return to your local hobby shop & politely confirm to the owner that you belong to a model vessel WEB site of 6 thousand + members   >:-o >>:-(

If the hobby shop owner has provided you with advice that a 11.1 V litho pack  would be suitable for a 7.2 V motor........ask him to read the members responses

Then ...politely ask him if an exchange to a 7.4 V litho pack could be arranged.........Derek O0 ...
 
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essex2visuvesi

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Re: 11.1v Lipo with 7.2v motor
« Reply #10 on: May 04, 2011, 01:19:35 PM »

>>:-( ...Bkr0411  ..print this thread out...& return to your local hobby shop & politely confirm to the owner that you belong to a model vessel WEB site of 6 thousand + members   >:-o >>:-(

If the hobby shop owner has provided you with advice that a 11.1 V litho pack  would be suitable for a 7.2 V motor........ask him to read the members responses

Then ...politely ask him if an exchange to a 7.4 V litho pack could be arranged.........Derek O0 ...
 


+1 to that and better put than I could
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Bkr0411

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Re: 11.1v Lipo with 7.2v motor
« Reply #11 on: May 04, 2011, 01:40:39 PM »

Thnanks Derek and Essex2visuvesi... I'm planning to go on Saturday. I'll let you know how I get on.
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snowwolflair

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Re: 11.1v Lipo with 7.2v motor
« Reply #12 on: May 04, 2011, 02:26:22 PM »

some programmable ESC for LiPo's can generally have their output voltages set to limits.  In which case they can be set up for 7.2volt motors.
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red181

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Re: 11.1v Lipo with 7.2v motor
« Reply #13 on: May 04, 2011, 10:21:05 PM »


Spec taken from a retailers website:

 Graupner Speed 600 Race 7.2v  Our Price 15.64
 
 
 
 
Click to enlarge Product information A high-speed motor with integral cooling fan for racing use in high-speed model aircraft or model hydroplanes (boats). Suitable for up to 7 NC cells of 1.7 Ah capacity. Suitable propeller: SPEED PROP 15 x 13.5 cm. Specification Nominal voltage 7,2 V Operating voltage range 6 ... 8,4 V No-load rpm 22635 min?1 No-load current drain 2,4 A Current drain at max. efficiency 17,7 A Current drain when stalled 130,3 A Max. efficiency without gearbox 70 % Length of case, excl. shaft 57 mm Diameter 37,5 mm Free shaft length 6,5 mm Shaft diameter 3,17 mm Weight 235 g
 
Assuming this is the motor, the nominal voltage range is 6v to 8.4v, so with your 3 cell lipo at 11.1v, the voltage is too high and you risk overheating the motor.

If however, the motor was for example a 8.4v graupner bb turbo, the voltage range then increases to 14v. I tonight ran that motor on a 11.1v lipo, and the performance was way better (speed and runtime) than a 7.4v nimh pack, as the discharge rate of the lipo pack (ie, how fast it can give up the juice!) is way better than a nimh pack.

Unfortunately it would appear your retailer has provided duff info >>:-(
 
 
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mikeljames

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Re: 11.1v Lipo with 7.2v motor
« Reply #14 on: May 11, 2011, 02:37:05 PM »

I think it will be better.
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