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Old Apr 17, 2012, 09:58 AM
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United States, NY, New York
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Help!
brush 6 volts motor using lipo

hi all,

can anyone tell me how or is it possible to use lipo with 6 volts brush motor.

our club is making a fun fly/ or RC event next month and we are trying to build some RC stuff for kids to play with. We have lots of these 6 volts brush motors, mostly 450, ( size is about half our thumbs). As we all using lipos, 2 cells will be 7.4, even we put some charge and limit the run time, it will be over 8 volts and kill the motor.

I know in cars, they using 540 motor, the bigger one, we can use 2 cell without issue. what about these little 450 motors with a 6 volts limit?

thanks in advance

Mike
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Old Apr 17, 2012, 10:24 AM
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If they are that small they can't be 450..maybe 150 or 250?.... Speed 300 weighs 50g+, Speed 400 is 70g+. Voltage is not what kills motors... amps do that!

Problem with 6v motor on 2s will be premature brush wear... but if you can keep the prop/amps low enough and they'll be okay for a short time at least..
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Old Apr 17, 2012, 11:25 AM
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for a short time probably won't work for our event as we need the RC cars to survive for few hours. will it be possible to connect two brush motors in parallel to lower the amp draw? as we will be using much smaller props or we using some resisters ( or some LED lights in series) to lower the voltage from lipo 8 volts to around 6 volts ?
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Old Apr 17, 2012, 08:42 PM
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Quote:
Originally Posted by Dr Kiwi View Post
Voltage is not what kills motors... amps do that!

Problem with 6v motor on 2s will be premature brush wear... but if you can keep the prop/amps low enough and they'll be okay for a short time at least..
By "short time", the Doc likely means at least a few days...not a couple of hours. In reality, how long total will the cars run?

mw
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Old Apr 17, 2012, 10:07 PM
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Quote:
Originally Posted by bigtruck169 View Post
will it be possible to connect two brush motors in parallel to lower the amp draw? as we will be using much smaller props or we using some resisters ( or some LED lights in series) to lower the voltage from lipo 8 volts to around 6 volts ?
Putting motors in parallel won't help, as they all get the same voltage. LEDs are only good for about 0.1A, so they will burn out unless your motors are drawing very low current.

A resistor is probably your best option. You need to choose one that drops about 2V at whatever current the motor draws (eg. 2 Ohms at 1A) and an appropriate power rating (eg. 5W). Use Ohm's Law to calculate the correct value, or just try different resistors until you get 6V at the motor. Note that this only works when the motor has a load on it, but a free running 6V motor is not likely to be damaged by 8.4V (as Dr Kiwi said, it's the Amps that kill).

One advantage of the resistor over other methods such as diodes, is that if the motor is blocked then current draw and output torque will be limited. As well as reducing the likelihood of burning out the motor, it may be safer for the kids.
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Old Apr 17, 2012, 10:49 PM
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Quote:
Originally Posted by bigtruck169 View Post
for a short time probably won't work for our event as we need the RC cars to survive for few hours. will it be possible to connect two brush motors in parallel to lower the amp draw? as we will be using much smaller props or we using some resisters ( or some LED lights in series) to lower the voltage from lipo 8 volts to around 6 volts ?
Run 'em 'til they drop... you may get 10 short runs, you may get 100 short runs...but I can't see several hours continuous... if that is what you need.
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Old Apr 18, 2012, 09:16 AM
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thanks for the suggestions. I will get some diodes ( radio shacks? ) I never due with brushed motor + Lipo combo when i get in hobby 2 years ago. Too bad i didn't take my high school physics seriously back in the days and hopefully i will catch up this time.

for brushed motor, if i want to run 2 motors, i can use only one esc but both motors need to be parallel or series?


we are going to build the flying cart

http://www.rcgroups.com/forums/showthread.php?t=1094114

but ours won't be flying, just running in baseball field. so CG is not important at all. The best part is to let the kids involved in making something and hopefully it will work or duct tape it again.

If anyone has any unused old radio systems ( 27mhz, 72mhz ), brushed esc or old motors, and would like to pass to next generation for one sunday afternoon fun, i be happy to take them off ur hand

mike
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Old Apr 18, 2012, 08:47 PM
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for brushed motor, if i want to run 2 motors, i can use only one esc but both motors need to be parallel or series?

I borrowed (and slightly edited) this from another forum: (As you can see, "series" connection would resolve the problem of over-voltage... since you are concerned about using 2s (7.4v nominal) on 6v motors... but power would be seriously limited!)

***series, is running one motor after the other....The voltage is split (theoretically) equally, so you wont have full power, like you would with one motor. Say with one motor it runs off about 7 volts. With two in a series, the voltage is split, so each motor only runs off about 3.5 volts....see???

***Parallel, is running two motors side by side hooked up to the same battery. 7 volts goes through the wires on the battery, which go through the motors. If 7 volts goes through the battery wires, and the leads of both motors are connected to the battery, 7 volts now runs through the battery wire, and both the motors....you follow??? So each motor is running off of 7 volts. The consequence of this is, since two motors are running off 7 volts, the battery is using amps twice as fast.
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Old Apr 19, 2012, 10:48 AM
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base on the information from dr kiwi,
my setup will be 2 cell lipo ( full charge to 8.4) + a small resister diode to drop 2 volts to 6.4volts + 2 brush motor in parallel with brush esc.

Now another quesion, the 2 volts resister diode should soldering on the red ( +ve) wire or black ( -ve ) wire of the esc? i remember back in high school days, they did mention there is a difference to which wire for the resister for the voltage drop.

Will or should the diode get hot at all in this application?
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Old Apr 19, 2012, 07:35 PM
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It doesn't matter which lead you put the diodes in. Conventionally they would go in the positive lead, but this is not important unless the battery is also used to power some other device with a common ground.

Yes, the diodes will get hot. How hot depends on how much current is drawn and the size of the diodes. They should be rated at least 50% higher than the maximum current draw, eg. if drawing 2A (1A per motor) you should get 3A diodes (eg. 1N5402). You need 3 silicon diodes in series to drop 2V.

A problem you may find with putting diodes in the battery lead is that the ESC's LVC (Low Voltage Cutoff) operates too soon. If the ESC can be set for nicad cutoff then it should be OK, but you should check to make sure that it won't let the battery run too low.

If the LVC can't handle diodes in the battery lead then you can put them in the motor leads, but you will need to wire a Schottky diode across the motor terminals (with reverse polarity!) to handle back-emf at part throttle. A resistor would be cheaper and easier to wire in.
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Old Apr 23, 2012, 01:08 PM
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thanks all for great lecture.
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