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Motor Theory - More Voltage?

R/C Submarine modelers

Postby Ramius-II » Mon Jul 07, 2003 5:23 pm

Okay, so I've forgotten some of the basics and here my confusion:
I have two DC motors of the same physical size. One operates on 6 volts the other on 12 volts. When I look at the current requirements, the 6 volt says it wants 3.5 amps and the 12 volt motor says it only wants .8 amps! ??? Originally I thought if you double the voltage you half the current. This seems to be 1/4th the current or maybe the square root of the power. Can anyone explain?

Thanks, Ed
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Postby raalst » Tue Jul 08, 2003 5:01 pm

you are right about half the voltage, double the amps.
because volt x amp = watts (power).

But this goes for the same motor. and the trick
is to force the current through the motor.

Motors can differ, e.g. in wire thickness, number of
magnetic "spokes" on the axle etc.
Those variations cause the motors to have varying
resistance. worse, the magnetics cause the motor
to have varying resistance during revolution.
a stalled motor will be equivalent to the wire it is made of
(i,e, a short) while the motor can have a far higher
resistance while rotating, because of the creation and
destruction of magnetic fields.

Resistance is what limits the amount of current
through the motor for a given voltage.
up the voltage, and more current will flow,
giving more power output.

hope this helps !
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Ronald van Aalst

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Postby Ramius-II » Wed Jul 09, 2003 11:35 am

Thanks Ron!
It just had me a bit lost. I was thinking in terms of Power= I2 X R.

Ed :)
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Postby raalst » Wed Jul 09, 2003 5:04 pm

Do trust yourself ! you are quite right

power = V x I
V = I x R
=> power = (I x R) x I

it is just that the R depends on the motortype
and also that large currents tend to lower V
(meaning that relative to the resistance of the
load/motor the resistance of the battery itself
becomes a factor.)
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Postby bo@ti » Mon Jul 14, 2003 2:14 am

raalst wrote:you are right about half the voltage, double the amps.
because volt x amp = watts (power).

But this goes for the same motor. and the trick
is to force the current through the motor.

Motors can differ, e.g. in wire thickness, number of
magnetic "spokes" on the axle etc.
Those variations cause the motors to have varying
resistance. worse, the magnetics cause the motor
to have varying resistance during revolution.
a stalled motor will be equivalent to the wire it is made of
(i,e, a short) while the motor can have a far higher
resistance while rotating, because of the creation and
destruction of magnetic fields.

Resistance is what limits the amount of current
through the motor for a given voltage.
up the voltage, and more current will flow,
giving more power output.

hope this helps !

Hi,

Say we have a motor thats supposed to run on 8.4v. The standard battery pack is rated 8.4v - 600mah.

If we change this battery to 8.4v - 3000mah, will the motor's RPM increase and if why?

Thanks
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Postby Trafalgar » Mon Jul 14, 2003 2:58 am

Hi,

Say we have a motor thats supposed to run on 8.4v. The standard battery pack is rated 8.4v - 600mah.

If we change this battery to 8.4v - 3000mah, will the motor's RPM increase and if why?

Thanks


Hi Bo@ti, No, the RPM should stay the same, unless the new battery had a lower internal resistace (V=IR for the battery as well), and allowed more current through at a given voltage. It the 3000mah battery is the same cell size as the 600mah, then its highly likely that it has a lower internal resisatance.

Anymore thoughts on resettable fuses? :;):
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Postby bo@ti » Mon Jul 14, 2003 6:24 am

Thanx, for confirming what I thought.

No news on the fuses nope :p Maybee just bypass'em? I don't know :D
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Postby Skip Asay » Mon Jul 14, 2003 9:13 am

"Say we have a motor thats supposed to run on 8.4v. The standard battery pack is rated 8.4v - 600mah.

If we change this battery to 8.4v - 3000mah, will the motor's RPM increase and if why?
"
With the example above, the only thing that will change is the amount of time between charges. The motor will continue to draw the same as it does on the 600mAh battery.

The current rating of a battery can be compared to a gas tank in your car. If you can go 350 miles on a 20 gallon gas tank (600mAh), you'll go 700 miles on a 40 gallon tank(1200mAh).

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Postby bo@ti » Mon Jul 14, 2003 1:48 pm

Can anybody, explain why the amp doesn`t have anything to do with the rpm?

Im on an Norwegian board, and 2 idiots keep saying that they have tested it and they know, by listening, or watching the motor, that amp DOES raise the rpm.

I even filmed the darn thing. But no response :p

Thanx :D
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Postby Ramius-II » Mon Jul 14, 2003 2:54 pm

Hi Bo:
The amps (current) does have an effect as the current is part of the overall power of the motor. If you connect your motor to a power supply with current limited output then yes, you can "limit" the power available to the motor. Turns out that flux or gauss is a fuction more of voltage than current. Therefore, the magnetic field of the coils in the motor are greater with a higher voltage that current. Basically, you can not force more current through a motor without raising the voltage. The two guys are probably raising the voltage and watching the amp meter go up at the same time.

Where most people get into trouble is by using a small wire size to their motors thus the wire acts like a resistor. The wire starts to get warm which tells you the power for the motor is being converted to heat before the motor can use it. Direct Current (D.C.) goes through the entire cross section of the wire and there are charts that say how much current a given wire size can handle. For main drive motors, I would not use less than 18 ga. wire and would lean more towards the 12-14 range.

Ed :)
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Postby bo@ti » Mon Jul 14, 2003 3:06 pm

Hi, Ed and Thanks. :)
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Postby Ramius-II » Mon Jul 14, 2003 5:27 pm

Welcome Bo: :)
By the way, I noticed that Engler now has a 12 volt version of the piston tanks! Maybe as a result of the design work I did? The Relay circuit can control either 6 or 12 volt motors and the "electronics" are fed from the receiver which is on 6 volts thus only the motors need to be changed! :D If the current draw is consistant with the other motors, that is 1/4 the amps, we now have a good extended run time.

Ed
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Postby bo@ti » Tue Jul 15, 2003 11:29 am

Hi, yes I noticed :D

But I didden't look on the price tag :p
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Postby ThierryC » Tue Jul 15, 2003 12:29 pm

Hey RamiusII and bo@ti, the running time of the typhoon is longer than 4 hours, do you really need more than that ? If you answer yes then you are a much more dedicated bubble head than I am :D
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Postby Trafalgar » Tue Jul 15, 2003 3:18 pm

bo@ti wrote:Can anybody, explain why the amp doesn`t have anything to do with the rpm?

Im on an Norwegian board, and 2 idiots keep saying that they have tested it and they know, by listening, or watching the motor, that amp DOES raise the rpm.

I even filmed the darn thing. But no response :p

Thanx :D

If you connect a 8.4v Nicad to a motor it will draw more current than if you hook it up to 9v dry cells (or even 8.4v dry cell if it existed) . Similarly some Nicad's will provide more current than other Nicads.
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