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Old Dec 11, 2006, 02:51 PM
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Are brushless motors dc or ac?

Are brushless motors dc or ac devices?

As their are only North and South poles, Why 3 wire supply?

I know to the initiated these are stupid questions, answers will hopefully help my basic understanding and lessen my ignorance of what is happening.

Erfolg
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Old Dec 11, 2006, 02:53 PM
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DC and AC see http://en.wikipedia.org/wiki/Brushle...electric_motor for more explaination.
boomer
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Old Dec 11, 2006, 03:12 PM
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Hi erfolg,
Good question, one hint may be the fact that they are permanent magnet motors (PM) that kinda rules out AC operation, also the term "brushless" refers to a motor that while it has no physical brushes, it still runs as a commutated motor (another hint AC motors don't have Kv ratings, they are frequency dependant), but in this case with a brushless DC controller providing the commutation instead of mechanical brushes and commutator.
AC motors, such as the most familiar induction type motors are not "brushless" motors at all, they were developed about 80 years before "brushless" motors appeared in the 1970's
Here's a good link to a page that gives a clear, but understandable explanation of all the different classes of motors.
You will find that in every industry that manufacters and uses "brushless" motors, aerospace, computer, you name it, they are universally classed as Brushless DC motors.
Pete
http://www.electricmotors.machinedes...m/BDEList.aspx
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Old Dec 11, 2006, 03:12 PM
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ni'ihau
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You may think it is a stupid question but it is anything but...
This has been an often heated question here.. The answer is largely a matter of semantics versus rock solid engineering. The three wire motors we use here are very similar to a two wire (and two phase) motor commonly used to cool PC things.(which are also called brushless DC motors)
Read on and good luck!
jimbo
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Old Dec 11, 2006, 05:39 PM
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Quote:
Originally Posted by pilotpete2
Hi erfolg,
Good question, one hint may be the fact that they are permanent magnet motors (PM) that kinda rules out AC operation..
Does permanent magnets really rule out ac? The reason I question this, is that a standard dc motor runs on ac provided by its mechanical commutator. I would be interested to know how a brushless motor using ac would be different than the current design.
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Old Dec 11, 2006, 06:56 PM
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What they are is permanent magnet 3-phase synchronous motors and the esc is a DC to 3-phase inverter. BTW, you can use a brushless motor as an alternator to power a second brushless motor, I have tried it and it works.
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Old Dec 11, 2006, 07:17 PM
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as you probable know, generators may run as motors, and motors can become voltage sources. So if you turn the brushless motor, with a drill motor or ?, what you generator is a three phase ac voltage, which is what the second brushless motors run on. Now all we have to do is build a circuit so one motor produces voltage for the second motor, and use that second motor to rotate the first brushless motor, then both motors will run forever, or not.

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Old Dec 11, 2006, 10:04 PM
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Quote:
Originally Posted by himeros
as you probable know, generators may run as motors, and motors can become voltage sources. So if you turn the brushless motor, with a drill motor or ?, what you generator is a three phase ac voltage, which is what the second brushless motors run on. Now all we have to do is build a circuit so one motor produces voltage for the second motor, and use that second motor to rotate the first brushless motor, then both motors will run forever, or not.

himeros
They would run forever if they were 100 % efficient and you had no energy lost (like heat). Which, if you can create a motor 100% efficient you will be the next Nobel Prize.
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Old Dec 11, 2006, 11:27 PM
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ni'ihau
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Originally Posted by ecologito
They would run forever if they were 100 % efficient and you had no energy lost (like heat). Which, if you can create a motor 100% efficient you will be the next Nobel Prize.

..There is a catch 22 in place with regards to the nobel prize thing here.
Any detectable means of determining if the machine is still running would sap some of its energy and ultimately stop it. So therefore if you COULD make the machine..... you couldn't show it to any one.????
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Old Dec 12, 2006, 02:03 AM
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All electric motors are AC motors.

They just use different ways to generate it.
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Old Dec 12, 2006, 10:07 AM
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ni'ihau
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All electric current is DC. In any given moment (a snapshot) current will only be going in one direction.

(Hi Vintage!..) Could a one direction actuator.... like one in a "turns right or goes straight" little toy RC car.. be discribed as a motor?
jimbo
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Old Dec 12, 2006, 11:03 AM
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Quote:
Originally Posted by B.L.E.
What they are is permanent magnet 3-phase synchronous motors and the esc is a DC to 3-phase inverter. BTW, you can use a brushless motor as an alternator to power a second brushless motor, I have tried it and it works.
Yes, but not a practical or funtional one.
AC synchronous motors also have an asynchronous element, this is needed to start the motor on mains current of 50-60Hz, if you were to try to run the typical brushless motor on 60 hz it would just vibrate, if you can spin it up near enough to synchronous speed so it can lock in to synch, it will run fine, but I would call it a stretch to refer to it as an AC motor.
Now as to the ESC being a Dc to 3 phase inverter?? See what kind of AC it produces with no motor attached and no feed back from the motor for timing the commutation, a dc to ac inverter runs independent to the device it powers, personally I feel the only way to view a brushless commutated motor is that the controller is part of the motor, as it runs in a closed loop with the motor.

Vintage1,
I totally agree, in theory , but in practice it's a dc motor since that's what we feed it.

Kwok_yu,
A brushed motor with PM field is strictly a DC device, will not run from an AC scource, now on the other hand all those brushed AC motors (Got Dremel) that we use everyday around the house,( Vintage, remember that when you are Hoovering ) are not AC motors, as series wound (no PM's) brushed motors they are usually classed as "universal" motors as they will run on AC or DC, of course the variable speed control used on many corded tools limits them to AC operation, but the motor itself is capable of running on DC or AC.

This really all does come down to semantics. B.L.E.s scenario of powering one brushless motor from one running as an AC generator is a good one, it will work because the motor will be able to "follow" the generator, as the generator cannot accelerate instantaneously, or faster than the motor can in that scenario.
My own feelings are that to be considered an AC motor it must be capable of functioning on AC mains current (fixed frequency) and also run as an asynchronous (induction) or syncronous motor (again, asyncronous start), as opposed to being a commutated motor, also that the term "brushless" should only be used in reference to a commutated motor where electronic commutation has replaced mechanical commutation, and should not be applied to an AC induction motor, as they do not run in commutated mode, but through induction.
Well enough out of me, it would be more fun to "hash" this kinda stuff out in a round table setting, where we could all respond directly to each other %^&*#@%
Regards to All,
Pete
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Old Dec 12, 2006, 03:08 PM
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Quote:
Originally Posted by ecologito
They would run forever if they were 100 % efficient and you had no energy lost (like heat). Which, if you can create a motor 100% efficient you will be the next Nobel Prize.

Hey I am working on this, I just have to get the temp down to -300 C to get rid of the losses, and make them a super conductor.

Himeros
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Old Dec 12, 2006, 04:26 PM
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Originally Posted by himeros
Hey I am working on this, I just have to get the temp down to -300 C to get rid of the losses, and make them a super conductor.

Himeros
Sounds like we have a candidate
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Old Dec 12, 2006, 06:07 PM
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This is beginning to sound a lot like the "motor vs engine" debate in some of the other forums.
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