Jan 05, 2012, 05:40 PM Suspended Account Joined Feb 2011 639 Posts Hey! sorry I didn't meant to start such a controversy! I was just surprised by the "Im-Io". Because I think Io as a way to take "rotor torque" into account... so nothing different than just I on a motor with no load...
 Jan 05, 2012, 06:09 PM Senile Member Moab, Utah, USA Joined Apr 2003 7,040 Posts I agree with you, yomgui. Io is current that overcomes the motor's built in losses so it cannot contribute to the torque required to drive a load. Therefore it has to be subtracted from motor current when calculating output power. Pout = (Im - Io) * (V - (Im * Rm)). But Io has the same effect on motor RPM as load current does. Therefore as the motor's load increases the load current is just an extention of Io so Io is not subtracted in the RPM calculation. RPM = kv * (V - ( Im * Rm)). Larry
Jan 05, 2012, 06:14 PM
Senile Member
Moab, Utah, USA
Joined Apr 2003
7,040 Posts
Quote:
 Originally Posted by Martyn McKinney We seem to be having a communication problem. I have been on these threads many years and recognize both you and yomgui as posters who "get it".
I like to think I get it and from what I've seen of your previous posts you get it too. I'm just having trouble interpreting some of what you are describing. I guess we can chalk it up to semantics. In any case, thanks for taking the time to attempt to unconfuse me.

Larry
Jan 05, 2012, 07:39 PM
Registered User
Joined Dec 2002
5,609 Posts
Quote:
 Originally Posted by yomgui Hey! sorry I didn't meant to start such a controversy!
It's actually my mistake.

When I first read your post about Io, it was on a cellphone and I didn't see the name of the poster. I thought the question was simply about Io itself, not realizing that there was an error in the formula in the previous post.

After posting my first few responses from my cellphone, it was not until I got to my laptop and Larry began asking pertinent questions that I realized my error.

Regardless it was an interesting exercise for me.
Jan 06, 2012, 11:49 AM
Registered User
Antony (France)
Joined Sep 2003
3,382 Posts
Quote:
 Originally Posted by Lnagel I agree with you, yomgui. Io is current that overcomes the motor's built in losses so it cannot contribute to the torque required to drive a load. Therefore it has to be subtracted from motor current when calculating output power. Pout = (Im - Io) * (V - (Im * Rm)). But Io has the same effect on motor RPM as load current does. Therefore as the motor's load increases the load current is just an extention of Io so Io is not subtracted in the RPM calculation. RPM = kv * (V - ( Im * Rm)). Larry
Hi
I agree with Larry

In fact Io corresponds to several losses factors
a) Hysteresis magnetic losses
b) Eddy currents losses
c) ESC (MOSFETs) losses
d) Bearings friction
e) Aerodynamics drag of rotating rotor

a) and b) are major parts generally
Louis
Jan 07, 2012, 12:33 AM
We want... Information!
Hastings, New Zealand
Joined Jan 2001
5,221 Posts
Quote:
 Originally Posted by Fourdan In fact Io corresponds to several losses factors
Unless you try to run your brushless motor by connecting two wires directly to the battery, in which case you only have to worry about resistive losses.
 Jan 13, 2015, 02:18 PM Registered User United States, WI, Racine Joined Jan 2007 3,840 Posts Old thread on brushless motors revived Our brushless motors are actually three phase motors, driven by a three phase AC voltage out of our brushless ESC's. That makes our ESC's kind of like a variable frequency three phase driver. I've actually run one at work off of a variable voltage, variable frequency three phase power supply. You can even directly connect the three leads of one brushless motor to the three leads of another smaller sized brushless motor, then spin up the first brushless motor with a battery operated drill. Note, DIRECTLY interconnect the two motors, do not use jumper wires. To much resistance. The smaller brushless motor will start up and rotate. Last edited by vollrathd; Jan 13, 2015 at 07:21 PM.
 Jan 13, 2015, 06:40 PM Registered User The Northeast Kingdom, Vermont Joined Jun 2004 5,866 Posts This is a three year old post I do think you need to go back and read post 17 & 18. An ESC is not a variable frequency drive, but just a simple electronic commutator. Ron and Martyn explain it way better than I can Pete
Jan 13, 2015, 06:58 PM
Registered User
United States, WI, Racine
Joined Jan 2007
3,840 Posts
Quote:
 Originally Posted by pilotpete2 This is a three year old post I do think you need to go back and read post 17 & 18. An ESC is not a variable frequency drive, but just a simple electronic commutator. Ron and Martyn explain it way better than I can Pete
Yes, this is an old post.

I've changed the above to "kind of like" a three phase variable frequency driver. As indicated in the previous posts, the ESC monitors the position of the motor, and sends appropriate driving signals to the brushless motor via a three phase "H" type of bridge driving circuit using those Mosfets.

As to whether our brushless motors are three phase motors, they will self start and run when connected to an appropriate three phase power supply. I've done it many times with our brushless motors. To me, that is the definition of a three phase motor.

The motor RPM depends on the three phase AC frequency, and the number of poles of the motor.

FYI, the same thing applies to the common automotive alternator. They are three phase AC units, with a three phase bridge rectifier on the output. When spun up, both the Automotive alternator and our brushless motors will generate three phase voltage on their three wires, as displayed on a four channel oscilloscope.

And, connecting a 12 VAC 60 Hz three phase power to the Automotive alternators windings, while powering up the alternators rotating field windings, will spin up that alternator, at a very low RPM. Lots of poles on those things. Stuff we could do at work during lunch hour before I retired.

Back in the 1990's I designed and built several brush type ESC's for running on a 38 cell Nickel Cadmium battery to power an Astroflight geared 90 motor. Still have some of those ESC's. That system was installed on a 1/4 scale Piper Cub model. The ESC used a MicroChip microcontroller with some 10 pages of machine code in the program files to run the ESC. The software in the modern brushless ESC's is 100 times more complex than anything I did!

For those still interested in how these brushless motors work, here is a pretty good description of how they operate. The Application is from Microchip, one of the big suppliers of microcontrollers in the world. This PDF file indicates that a three phase motor must be used with their brushless motor driver.