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newbie question on selecting a DC motor

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pascal32

New Member
Hi there! First post :)

I'm looking to select a DC motor which has a nice balance of RPM and torque. I can vary the power source, if 9V battery provides better performance than 3V of 6V thats an easy thing to incorporate. Ideally this will pulsate, though at the moment I would be happy to find a US source that provides a decent motor. Most places I have looked at provide few details making the selection tough. I'm currently using a Stevens International SVM 280 http://www.stevenshobby.com/productlisting.aspx?supplierid=5726&categoryid=9 hobby motor at 3V which is specced at 7800 RPM with "20.0" tongue (gf.cm?). Would like slightly more torque and RPM.

Do you guys and gals have suggestions on the power source (3V VS 9V), a supplier, and possibly a motor?

I can accommodate any shape, splined shaft would be preferred.

thanks!
 

spec

Well-Known Member
Most Helpful Member
Hi there! First post :)

I'm looking to select a DC motor which has a nice balance of RPM and torque. I can vary the power source, if 9V battery provides better performance than 3V of 6V thats an easy thing to incorporate. Ideally this will pulsate, though at the moment I would be happy to find a US source that provides a decent motor. Most places I have looked at provide few details making the selection tough. I'm currently using a Stevens International SVM 280 http://www.stevenshobby.com/productlisting.aspx?supplierid=5726&categoryid=9 hobby motor at 3V which is specced at 7800 RPM with "20.0" tongue (gf.cm?). Would like slightly more torque and RPM.

Do you guys and gals have suggestions on the power source (3V VS 9V), a supplier, and possibly a motor?

I can accommodate any shape, splined shaft would be preferred.

thanks!
Hi Pascal,

As you live in the USA you have an infinite range of electric motors available to you.

Your stated requirements are not very specific, so here is a link for you to look through: http://www.ebay.com/sch/i.html?_fro...ctric+motor.TRS0&_nkw=electric+motor&_sacat=0

With an electric motor, like any machine, you only get the power out that you put in, less some power which is wasted as heat. That is why motors get hot.

The voltage makes no difference (unless the voltage is very low). But what is important is the input power which is the volts available and the current available.

The other thing to bear in mind is that a motor without a load will spin at it's maximum speed and take little current, but as the load on the motor is increased the current taken by the motor will increase.

The current taken by the motor will be exactly the same if you had a voltage supply that could just supply the current that the motor demands or a voltage supply capable of supplying a 100A (automobile battery for example) or infinite current.

For example, a twentieth horsepower motor would take 750 watts/20 = 37.5 Watts on full load (it would take a bit more in practice). (One horse power= 750W)

If the motor were a 3V type it would take 37.5W/3V = 12.5A on full load and perhaps 1.25A off load

If, on the other hand, the motor were a 12V type it would take 37.5/12V = 3.125A on full load and perhaps 0.3125A off load

The choice of motor rated voltage depends on what power source you have: battery or mains power unit but, in general, a 3V motor will be less efficient than a higher voltage motor so, unless you need to use a 3V power source, I would suggest choosing a motor with a voltage of 6V, 9V, or even 12V (or any voltage from 6V thru 12V).

A low voltage motor is less efficient because more current will be taken by the motor for a given horse power, and more current means more power loss in the cables, switches, batteries (batteries have internal resistance), etc

Power loss = I * I * R
Where:
Power loss is in Watts
I is current in Amps
R is the total resistance in Ohms that the current is flowing through.

Because power loss is proportional to the square of the current, the electricity companies transmit electricity over long distances at extremely high voltages, 765,000 Volts for example, to minimize power loss.

spec
 
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cowboybob

Well-Known Member
Most Helpful Member
Welcome to ETO, pascal32!

It would be helpful to us if you can briefly describe how you intend to use the motor.

And you mention "pulsate"; by that do you mean PWM (Pulse Width Modulation)?

As an aside, very few (if any?) motors of this type and size have a splined shaft... (at least from my experience).

(Had a blue screen of near death experience :banghead::banghead::banghead:)
 

MikeMl

Well-Known Member
Most Helpful Member

Tony Stewart

Well-Known Member
Most Helpful Member
Hi there! First post :)

I'm looking to select a DC motor which has a nice balance of RPM and torque. I can vary the power source, if 9V battery provides better performance than 3V of 6V thats an easy thing to incorporate. Ideally this will pulsate, though at the moment I would be happy to find a US source that provides a decent motor. Most places I have looked at provide few details making the selection tough. I'm currently using a Stevens International SVM 280 http://www.stevenshobby.com/productlisting.aspx?supplierid=5726&categoryid=9 hobby motor at 3V which is specced at 7800 RPM with "20.0" tongue (gf.cm?). Would like slightly more torque and RPM.

Do you guys and gals have suggestions on the power source (3V VS 9V), a supplier, and possibly a motor?

I can accommodate any shape, splined shaft would be preferred.

thanks!
can U define exaxtly what you need?
___RPM per volt
___ Torque at RPM
____Amps start max @__V
 

pascal32

New Member
wow, you guys are fantastic! thanks for all the responses!

sounds like I'm most interested in current draw - based on a quick look at battery specs going to four AA's at 6V seems like the best compromise, then moving to a 6V motor

The problem is that i don't know how to spec the motor, hence the trial and error I'm going through. we are trying to create a niche blender of sorts, I know the motor we have is working, though not as well as we would like. I would love to spec the torque and speed needed, unfortunately it is rather difficult considering the various blade designs we are trying and variable mediums for mixing. based on what we have tried the current motor is slightly low on torque and we would likely benefit from more RPMs.

do you have a recommended source that shows specs on the motors? A lot of the vendors seem to be very wishy washy on the motor specs.
 

cowboybob

Well-Known Member
Most Helpful Member
... A lot of the vendors seem to be very wishy washy on the motor specs.
I think that's because the motors you're looking at (low voltage, battery operated, DC hobby types) are generally only rated for a certain RPM under "no-load", specific voltage (and sometimes current) conditions.
... we are trying to create a niche blender of sorts
(My emphasis)
That being the operative answer I was looking for, your might want to consider investigating these.

Look for "C13 Technical Data Sheet".

Also, these are "servos", meaning that for ideal usage they need to be feed a PWM signal, as opposed to straight, un-modulated DC.
 
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