DarZeelon

I am brand new to this.


A brush-less motor, being a 3-phase AC motor, is theoretically a one-speed motor.

I.e. it must spin at an RPM that will bring its individual magnet poles in-line with the energized electro-magnetic poles.

How does the ESC (all I know are 'sensor-less') 'know' at what rate it must alternate the polarity of the three phases?


What if the kV is, say, 1,000 and the Voltage is 11.1, but the specific BLM cannot spin the prop at 11,100 RPM?

Is the kV changed, or does the Voltage become lower to support the RPM the motor is capable of supporting?

How can a 30A ESC and a 50A ESC spin the same motor, using the same battery pack, at the same RPM, while providing the same Hz AC for the motor to run right?

How can the same ESC be used on two different BLMs, with different kV ratings?


How many question marks can I add here?...

DaveFlynn

First they run 2 phase, in other words one of the wires is not driven and used to sense position for commutation.
You can drive a 50A motor with a 30A ESC or at 30A with the 50A ESC, but you'd have extra weight not doing anything. Generally they are variable voltage and variable frequency, the amps will be the load divided by the torque constant (Kt).
Volts = RPM. Amps = torque, normally you want to set things up so that you run out of volts before the amps get high enough to break something.
Microchip.com has some application notes that show how ESC's work, check it out.

ron_van_sommeren

Nope. Our motors are BLDC motors (i.e. BrushLess DC), in fact they operate much in the same way as brushed motors, where the brushess/commutator are replaced by an electronic device, the ESC, which does the same but this time electronically. It's the motor that tells the ESC when to switch, the controller is the 'slave'. The motor is not operated as a synchronous motor (it can though), it does not follow the (more or less) AC voltage. More explanation about how power is controlled and about frequencies
http://www.torcman.de/peterslrk/index_eng.html
-> SPEEDY-BL self made brushless controller

FAQ/sticky, 'How do brushless motors work?'
http://www.rcuniverse.com/forum/m_1558046/tm.htm

See chapter 9, electronic controllers:
http://www.consult-g2.com/course.html

At any given time, one of the phases is not energized (the phases take turns). The controller reads the voltage induced in that phase (=BEMF) to determine rotor speed and position. Sensored brushless controllers use optical or magnetic (Hall) sensors.

Current would go through the roof, in theory unlimited. Only restricted by ESC and batterie internal resistance. Your ESC would be toast, your motor would go up in smoke and your magnets would die (permanent)

kV is a motor constant. Depends on number of winds, if everything else is kept the same. But it also depends on magnet strength, stator steel laminiation quality, flux ring steel and dimensions, number of magnetpoles, number of statorpoles, stator length and diameter ...
Motor rpm is only limited by mechanical properties, if rpm gets to high, it will vibrate, tear itself apart or break magnets.
There's also Kt, torque/Ampère. Goes up with the number of winds while Kv=rpm/Volt goes down when you increase the number of winds. Keeping the rest of the motor the same of course.

Those figures indicate (roughly) maximum current for the ESC. It's the battery that has to deliver the power. Too much current will fry your controller.
o
Vriendelijke groeten Ron
• diy motor building tips & tricks
• diy brushless motor discussion group
• Drive Calculator download & discussion group
• int. E fly-in & diy outrunner meet, Nijmegen, the Netherlands

pilotpete2

Hi Dar,
The motors we use in our hobby are classed as brushless DC motors, they only look like AC motors, the fact that they have a permanent magnet rotor makes them unusable with fixed frequency (mains) AC. The ESC provides electronic commutation of DC current based on the back EMF from the phase winding of the motor not in use at that instant, so the motor not the ESC controls the timing by the rate it is turning. Speed is controlled as with any commutated or brushed motor by varying the voltage applied to it, PWM is used to vary the effective voltage by the ESC, just as with a brushed DC motor. In BLDC operation the ESC is an integral part of the motor and runs in a "closed loop" with the motor.

I hope I haven't confused you even more, my ability to be concise in describing a concept is limited
Here's a link that may be of interest
http://ww1.microchip.com/downloads/e...tes/00885a.pdf
regards,
Pete

P.S. I hope that you aren't thinking of[X(] giving up on internal combustion[X(]

pilotpete2

Hi Ron,
It's high noon at the OK corral
Beat me again, Quick Draw McGraw
Cheers,
Pete.

ron_van_sommeren

I'll be there, waiting for you (And Grace Kelly behind the window, holding a rifle [sm=devious.gif]

DarZeelon

Dave,


A true 3-phase power system does not have 'square' waves, as does the voltage supplied by a brush-less ESC.

Voltage from the ESC always has one phase at +V, the second at -V and the third at 0V.


Pete and Ron, you two 'cowboys'... Don't hurt yourselves over me...[]

DaveFlynn

Dar,

It's not square waves, it's PWM. The drive electronics (3-phase H switch) and motor (delta or wye wound) are the same for a 3-phase system.

DarZeelon

Dave,


In 'normal household' 3-phase systems, the waves are sinusoidal, at 120º intervals, since they are produced by an alternator type generator.

Producing 3-phase current using a wave generator, like a brush-less ESC, allows the waves to be made more efficient; either trapezoidal, or square.

At any point, with a square wave ESC, two of the phases are phases are mirror image (+) and (-) and the third is (0).

The (0) stage is half the duration of each (+) and (-) stage.

It can best be described using this table:


As you can see, in any sequence number, one phase is (+), the second is (-) and the third is 0.

They are at 120º to each other, i.e. two sequences apart and each of the phases, in a complete sequence series, is 1/3 of the time in each voltage.

DaveFlynn

Dar,

I think you are mixing AC and DC motors. Either way the input to the motor is the same. BTW, the third wire is not 0 it's floating and follows part of a sine curve from +BEFM to -BEMF or -BEMF to +BEMF. In an industrial ESC the line in is rectified and filtered to provide 270VDC to the H-Switch which outputs 3-phase PWM (current controlled) to the brush-less DC motor.

DarZeelon

Dave,


With a square wave ESC, which as far as I know applies to all current BL ESCs, there definitely is 0V!
It lasts for 60º, is followed by 120º of full positive voltage and then another 60º of 0V, followed by 120º of full negative voltage.

If there is no sine wave, or trapezoidal wave (which a square wave isn't), 0V is an actual, time consuming part of the alternating current cycle; not a transitory point the graph only passes through...


If you scrutinize the table in my previous post #9, you would see that in none of the six sequences, do two phases both have (+), or (-).
There are always three different values and the wave has an identical shape for all three phases, just with a two sequence separation (120º).

With sine wave (or trapezoidal wave) 3-phase systems you do see situations where the phases can work against each other, where two phases are at different, low levels of (+) and the third is at a high level of (-). This may not work very well with the permanent magnets in our brush-less motors.

Like simple brushed motors, brush-less motors truly use permanent magnets, but it does not make them DC motors.
The current through their 3 winding sets still alternates, making them variable speed, variable frequency (Hz), 3-phase AC motors, by any other name.

DaveFlynn

Dar,

Did you know that you can drive these DC motors with 60Hz 3-Phase, they will turn 3600, 1800 or 1200 RPM depending on how many windings they have? They are DC because an AC ESC would weigh more than the plane. In a DC controller each phase is driven + or - or left floating. In an AC controller each phase is driven to a specific point on the sine curve + or - as required to track the sine wave.

pilotpete2

Dave,
You can try to run them on fixed frequency AC 50 or 60 Hz, but they cannot start on their own, in theory they are a simple and crude AC synchronous motor, but lack the ability to start asynchronously as a true synchronous AC motor must do. To start one on AC mains current you would have to spin the motor with an external motor to get it fast enough to jump to synchronous speed, when forced to start in this manner they will run fine until their torque is exceeded resulting in a sudden stall, requiring an external start.
Pete

DaveFlynn

Pete,

Correct it's not an induction motor so I did have to spin is by hand to get it going. I was running a motor on about 5 volts and 2 amps from a 3-phase variac. I was looking for an easy way to measure the Kt, didn't work, the number I got was too big.

DarZeelon

Dave, Pete,


Let us take the simpler model of the 'star' type brush-less motor, with 12 electromagnetic poles.
Let's also use my table from post #9.

There are three groups of windings; one group on each 'arm' of the star.
Each group includes four stator electromagnetic poles, wound in series, between the phase input plug and the 'umbilical'; the center of the 'star'.

In each of the six sequence, two phases are energized; one is (+) and the other is (-).

In Sequence 1 the current flows from the phase 2 plug, through the windings of its four electromagnetic poles, through the umbilical, through the phase 3 electromagnetic poles and to its plug. Phase 1 does not participate in this sequence.

In sequence 2 the current now flows from the phase 1 plug, through the windings of its electromagnetic poles, through the umbilical; and continues to flow through the phase 3 electromagnetic poles and to its plug. Now phase 2 does not participate.

In sequence 3 the current continues to flow from the phase 1 plug, through the windings of its electromagnetic poles, through the umbilical, but now it flows through the phase 2 electromagnetic poles and to its plug. Now phase 3 does not participate.

Sequences 4, 5 and 6 are similar to 1, 2 and 3, but the current flow is reversed; i.e. in sequence 4, for example, it is from phase 3 to phase 2.


This means the model that shows (like in the Hacker web site) that only two of the 12 electromagnetic poles are energized at any one time, does not correctly portray reality; since eight (8) electromagnetic poles are always simultaneously energized.

DaveFlynn

Dar,

What you're saying is correct, but overly simplified. Things to look at: How long after voltage is applied to the coils does current begin to flow (convert to uH)? How many volts (RMS) are on the coils? By percentage how much over RPM / Kv is this RMS voltage? What is the switching frequency of the ESC (PWM period and resolution)?

All I'm trying to say is that the eclectronics are simple, but the software that makes them work is more complex than it looks.

DarZeelon

Yes Dave,


I am over-simplifying things.

I still do not know how kV is calculated, but I believe it is of lesser importance to the understanding of how to properly set-up a brush-less combo.

There are questions that arose as I looked into things...

Such as: In larger Hacker motors, there are 14 and even 20 electro-magnetic poles...
Neither of these numbers, unlike 12, does not evenly divide by 6...

The smaller outrunner brush-less motors, like the AXI 2820, or the KA22-20L and KA36-10XL all have 12 electro-magnetic poles on the stator and 14 permanent magnets on the inside of the can.

It is very easy to envision the order sequence, in which the electro-magnetic poles are energized, to provide the 'even-firing' motive force to the can...

Would the un-even order forced by using 14, or 20 electro-magnetic poles, not make the forces turning these motors' cans into the brush-less equivalents of a Harley Davidson V2, or the odd-firing Buick V6 of the late '70s?

If not, how not?

As to kV, if a motor cannot turn a prop at a high enough RPM, so V x kV = RPM, the voltage is reduced, so it does work out.
KV is the only constant in the system.

DaveFlynn

Dar,

The Hacker out-runners like A20, A30 ans A50 have 12 coils and 14 magnets. These are good motors, I have them. I don't know about the A100/A150/A200. There should always be an even number of coils per phase, 4 seems the most common in hobby motors. In fact all of the small motors I have have 12 coils. Some are delta and some are wye wound.

To measure Kv spin the motor at a known RPM as a generator with another motor and measure the voltage that comes out. RPM/Volts is the Kv.

ron_van_sommeren

@Dar
Have a look at this table, blue fields are good combinations, red are not:
http://www.powercroco.de/Kombinationstabelle.html

@Dave
Number of coils per phase can be anything. Depends on the number of statorpoles. A 3 statorpole motor has 1 coil per phase, a 9 statorpole motor (usually cd-rom) has 3 per phase.




Several direct-drive 3phase helicopter motor designs:
http://www.rcgroups.com/forums/showthread.php?t=587208
http://www.powercroco.de/Helidirektantriebe.html

jminica

yall need to be less tech about it. the brushless motors we use are dc and not ac if you measure the voltage it is dc current going to the motor on each lead and the esc is a fast switch changing each lead as is needed ( simple explination ). if we were using ac current along with changing the current the esc would also have to contend wit the freq of the ac current ie. 60hz off and on 60 times per second along with trying to turn off and on the pols in the motor as needed.