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Old May 22, 2009, 07:41 PM
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How do speed controllers work?

Folks,
I have a dumb question. How exactly do speed controllers change the RPMs on motors? Do they alter the voltage, the amperage or both? The reason I ask this is because I am interested in a particular motor and I want to reduce the voltage going to it. The motor is rated at 24 volts, but I would like to run a 12 volt system and hopefully reduce the RPMs to an acceptable level. I was concerned I could not find a speed controller that would handle forward, reverse and a 24 volt system.
Rob
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Old May 22, 2009, 08:07 PM
Veni, Vidi, Feci
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First on the motor: If it's designed run quickly on 24v, it may run at a suitable speed for you on 12v-- all depends! In any case, it will run about half the speed and produce half the power on 12v.

ESCs "chop" the supplied voltage... if supplying 12v, then you will get pulses of 12v out. At slow speed, the pulses are short with a wide space between... faster speed has longer ON pulses & shorter OFFs... at full speed, it's just straight 12v.
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Old May 22, 2009, 08:35 PM
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Rob,if you have not actually tried this motor out you had better tell us what it is and give some info on what you are driving with it.
If you just read a motor does 12,000 rpm ,no-load, then it is likely to have a piss poor torque.Put a large prop on one of those and you actually get a lower speed than one that says 6,000 rpm
Oh,there are escs that run on 24 volts
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Old May 22, 2009, 10:27 PM
Grumpa Tom
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ESC : Electronic Speed Control

Fancy name for a switch. That is all it is. It switches the power to the motor on and off. It does not vary the voltage at all. It turns it on, and off, and on again at the frequency listed in the specifications. For example, if it is a 1000 Khz (kilohertz) frequency it turns the voltage on and off 1000 times per second.
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Old May 23, 2009, 10:01 AM
KC8WPF
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Euclid, Ohio, United States
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So far, every answer has ASSUMED that the word electronic was in the originl question, because esc's are more efficient than mechanical speed controls.
Quote:
Originally Posted by Kampfleiger
I have a dumb question. How exactly do speed controllers change the RPMs on motors?
Mechanical speed controls, like the Dumas 2022 or 2023 use a variable resistor to limit the voltage going to the motor, the resistor converts the excess power to heat.
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Old May 23, 2009, 10:32 AM
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Guys,
Here is what I was looking at: http://www.allelectronics.com/make-a...PROCKET/1.html There are several reason I was thinking about a set of twin motors like these.
First, I want a direct drive solution. This motor at half the RPMs will give me exactly what I need 1000 - 1500 RPMs (its a big model). If I could run it at 12 volts, cutting its max RPM in half, I think it would be just about right.
Second, its cheap.
Third, I bet its pretty robust given its original purpose.
Rob
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Old May 23, 2009, 05:22 PM
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Guys - thanks so much for the feedback.

Pat and Kmot - I would never have guessed that the mechanism used to regulate power to the moter was handled in that fashion. Very interesting.

Micro - In my posting just above this, is a link to the motor in question. Basically, here is what I need. My model is a 1/48th scale replica of the USS Nevada at the time of Pearl Harbor. Being 1/48th scale (550 lbs), it will require something like 200 watts of power. I want some extra for maneuvering and 2x actual scale speed. So, I am figuring 2 screws in the range of 100 - 200 watts with an RPM range of say 1000 to 2000. I also want it to be direct drive to eliminate cost, noise and power loss associated with gearing.

Bob - its going to be electric, after all, this is the 21st century .

Thanks so much for everyone's help
Rob
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Old May 23, 2009, 05:23 PM
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By halving the supply voltage the current is also halved. This means a QUARTER the power. Actual rpm depends on the load imposed by the prop.
Mechanical speed controls work by restricting the current, the apparent change in voltage is only what you measure. They also need to be a good match to the motor.
An electric "chopping" device, on the other hand, only needs to be capable of handling the max power - generally, what will handle a lot will also handle a little, Except for some "performance" types that lack low speed control, cutting in suddenly at about 1/4 power.
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Old May 23, 2009, 05:45 PM
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MFR,
I understand what you are saying relative to the current (watts = volts * amps). The approach would be to maintain the same amperage and only reduce the voltage. That means wiring the batteries is parallel (I think, or is it in serial? I forgot my basic electronics ). MFR, I like the way you think, "what will handle a lot will also handle a little". Thats what I was thinking.

If there is a source for a 24 volt ESC that will handle the current and go in reverse, at a rational price, then all of this is a mute point.

Rob
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Old May 24, 2009, 01:21 AM
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Quote:
Originally Posted by Kampfleiger
MFR,
I understand what you are saying relative to the current (watts = volts * amps). The approach would be to maintain the same amperage and only reduce the voltage. That means wiring the batteries is parallel (I think, or is it in serial? I forgot my basic electronics ). MFR, I like the way you think, "what will handle a lot will also handle a little". Thats what I was thinking.

If there is a source for a 24 volt ESC that will handle the current and go in reverse, at a rational price, then all of this is a mute point.

Rob
Rob,I was going to confine my opinion to the motor but I see you know what kind of motor you need.There are a few with that spec and you will get some at a car scrapyard.

You and mfr02 narrowed the ESC choice down a bit.
A versatile 24 volt ESC with a 25 amp continuous rating that "can handle a lot" but with low end control and does not have a minimum 30% start? One that can also limit the power by a variable span? One that would cut off on radio loss and handles high stall motors like a 700bb turbo.

The 25 amp version of this one meets that.
Feature summary and advanced setup download
Is $37 all in "rational" ?
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Last edited by microgyros; May 24, 2009 at 04:40 AM. Reason: corrected the bad linkto rcgroups post
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Old May 24, 2009, 06:40 AM
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Provided you dont exceed what the battery is capable of in the first place, adding another battery in parallel doesn't increase the current - this is dependant on the load. Same motor, same load. The extra battery gives an increased run time.
If you go the 24 volt route, it would probably be best to use a separate RX pack - unless there is a really fancy BEC, there will be an awful lot of power to get rid of.
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Old May 24, 2009, 07:51 AM
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Micro,
That ESC is perfect! And yes, the cost is definitely rational. I especially like the auto shutdown feature upon signal loss. Thats very important for a 550 lb model. The setup instructions are excellent - perfect for a novice like myself. So, I think I now have the complete power solution for BB36. Thanks so much for your input.
Rob
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Old May 24, 2009, 08:19 PM
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If you want your ESC to NOT work, send it to me, Im good at burning them up, even though I use them within design parameters

Thats an interesting ESC, what kind of support does it have? I mean MFG support.
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Old May 25, 2009, 01:48 AM
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Last edited by 1Ironhorse; May 25, 2009 at 02:38 PM. Reason: Sorry for trying to educate everone.
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Old May 25, 2009, 03:06 AM
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Quote:
Originally Posted by 1Ironhorse
The graphic shows a PWM signal at 50% duty cycle, or 50% power.
You spoiled a good post there!

Mike
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