Ping: Robatoy

[RicodJour]

I think I found a new toy for you.

impressive.

R

[Robatoy]

There is no such thing as a brushless DC motor. Pulsed DC , sure...

[Kerry Montgomery]

Do you mean that there are brushless pulsed DC motors? Thanks, Kerry

[Robatoy]

Yes, I think so..isn't that the type that spins hard drives?

[Larry W]

Perhaps I misunderstand but couldn't that same logic be used to state that there is no such thing a s brush DC motor? After all the brushes and commutators are just another way of "pulsing" the DC at the correct timing.

[Robatoy]

Many brushed motors will run on either AC or DC.

[RicodJour]

Alright...does that mean you have/have used the unit, or just generally disapprove of it based on marketing claims? The video highlighted some nice features.

I am not a motor guy, as recent threads may have indicated, so I have to ask why Wiki has the first section on the differences between brushed and brushless DC motors.

this one of those semantics things, or is the article wrong? It looks like a reasonable Wiki article and there are none of the usual caveats at the top of the page seen on sketchy info articles.

R

[Robatoy]

There's an upside to a sander having some weight, unless you're holding it upside down or on a vertical surface for long periods of time. I'm certainly intrigued by the concept, mostly because they have made it quiet. The weight reduction was already available by using air driven sanders, but they're noisy..and probably a bit heavier too.... and if you include the compressor, a lot more money than the Mirka Ceros.

I have not used one, but from experience, Mirka is one of those companies that take their products very seriously so now I will have to try one. I will be in touch with a Mirka rep soon.

Price is not all that bad, considering.

Now the 'brushless' part. It probably is a marketing attempt at projecting a zero maintenance image, and that's fine. The DC part bugs me, because to me, DC is a continuous flow of energy, creating an image of linear power application, i.e. smoothness. From a technical standpoint, and I admit to picking nits, it is inaccurate although most people probably couldn't give a rat's ass about that. The range of RPM they claim, would indicate to me that the pulsing of the DC would be at a high enough frequency that the application of power would be pretty smooth.

The agility, the fit in the hand, all are factors that will determine whether or not I will like it, for there is still a cord, and a frickin' hose that I find the most annoying things of handling any sander.

And, as I have touched on earlier, lightness isn't always a plus. To have some mass between your hand and the buzzing disk functions as a barrier. (I am glad my 12ga shotgun weighs as much as it does for the same reason.)

Thanks for bringing the product to my attention.

[Kerry Montgomery]

Robatoy, Your comment about the shotgun reminded me of my nephew letting his 14 year old son fire their 12ga for the first time. The boy was asking if they could saw off the barrel, and maybe cut the stock down too, so it would be like he'd seen in the movies. After firing it for the first time, held tightly against his shoulder, and almost being knocked down, the son decided that the mass didn't need to be reduced at all! Kerry

[Robatoy]

BTW, there is a discussion on on at FOG (Non-Festool Power Tools section, and it seems positive.)

[Dan Coby]

Most electric motors have some way to drive magnets to create a rotating magnetic field. (There are also electrostatic motors which we will ignore for the purposes of this discussion.)

A 'brushed' DC motor uses brushes and a commutator to vary which windings are energized to create the rotating magnetic field. The windings are usually on the rotor and the rotor's magnetic field rotates backwards relative to the rotor. The power flow to a brushed DC motor is only 'somewhat' smooth or continuous. The actual current flow into the motor goes up and down as the commutator applies voltages to the different windings. (You cannot instantaneously change the current in a winding from zero to maximum and then back to zero.) Compare the noise from a 'universal' motor such as the ones on inexpensive table top saw versus the quietness of an induction motor such as the ones found in most better table saws. The universal motors are both electrically and acoustically noisy.

A 'brushless' DC motor uses electronics to drive the windings. Just like the brushed DC motor, the winding drive currents are changed as the motor turns. The winding drive signals may be simple on/off voltages (like you would get from a brushed DC motor's commutator) or some something more complex. I worked on pen plotters which used brushless DC motors for positioning. Each motor had two drive windings. The windings were driven with variable frequency sine and cosine signals (i.e. sine waves with a 90 degree phase difference) which produced constant torque and power out of the motor. So they were very smooth and they could be run at a wide range of speeds down to a small faction of an RPM. They were very quiet, compact, and extremely efficient.

Note: The driven windings in a brushless DC motor are usually on the stator (the non rotating part of the motor) instead of the rotor. This is done simply to eliminate the need for any electrical connection to the rotor.

Dan

[Lew Hodgett]

Many brushed motors will run on either AC or DC.

---------------------- They are known as universal motors.

Very common for hand held electrical devices found in shop and/or kitchen.

Lew

[Josepi]

I think you would find the field does **NOT** rotate but rather attempts to stay in the same place relative to the field coils.

Think about that one...LOL

[Dan Coby]

I am not going to bother to try to straighten out your annoying top posting!

However as I said in the very next sentence of my posting (which you snipped):

"The windings are usually on the rotor and the rotor's magnetic field rotates backwards relative to the rotor."

Yes. In the typical brushed DC motor, the backwards rotation of the rotor's magnetic field results in a field which is stationary relative to the stator field.

My point is that the magnetic field of either the rotor or the stator or both has to change (relative to what is causing the field) to cause the motor to move. In a brushed DC motor it is usually the rotor's field. In a brushless DC motor it is typically the stator's field. In an induction motor is is typically both.

Dan

[Josepi]

I will straighten out your bottom posting as I am not that lazy.

I understood what you meant but the iincorrect information was there. You may have learned something. The back-paddle was not necessary.

All the best.

However as I said in the very next sentence of my posting (which you snipped):

"The windings are usually on the rotor and the rotor's magnetic field rotates backwards relative to the rotor."

Yes. In the typical brushed DC motor, the backwards rotation of the rotor's magnetic field results in a field which is stationary relative to the stator field.

My point is that the magnetic field of either the rotor or the stator or both has to change (relative to what is causing the field) to cause the motor to move. In a brushed DC motor it is usually the rotor's field. In a brushless DC motor it is typically the stator's field. In an induction motor is is typically both.

Dan

On 2/2/2011 7:12 AM, Josepi wrote: I think you would find the field does **NOT** rotate but rather attempts to stay in the same place relative to the field coils.

Think about that one...LOL

[SBH]

> Pretty impressive.

Searching for something else, I came across this which appears to be a similar product.

[Morgans]

Yes there is. The center of the motor, usually called the armature is instead, stationary, and has three phase windings on it. The outside of the motor has rare earth magnets, and spins, doing the work.

The controller rotates the 3 phase windings electronically to make the case turn.

[Robatoy]

No there isn't.

There is no 'phase' to pure DC. The controller pulses the DC to create 'phase' as you put it. Put a scope on the power going to the 'DC- brushless motor and see for yourself. It has a wave-form although not a reversal in polarity such as AC per cycle.