Basic Brushless Motor Operation

[DrkAngel]

Simplest - most basic, explanation of how a brushless motor works.
Power is regulated by modulating the amperage applied to the motor, by the controller.
(Typically, in a regulated, 3 phase arrangement. )
This is done via a series of full voltage pulses, the width, and quantity of which, determines the power input, (amps x battery voltage).
Additional amps supplied, increases motor "torque", but does not affect the "unloaded" ** top speed.
Amperage application is limited by controller capacity - or as a function of thermal depreciation.
(Thermal resistance of wire increases, as its temperature rises.
The application of more amps, shifts efficiency lower, as heat to torque ratio, increases.)
200% amp input might increase output to 150%. (Which Increases watt output, also.)

Additional, speed, as well as rated motor output - "watts", is obtained via the increase of supplied voltage.
Ex. Upgrading from 24v to 36v will enhance output, from 500w to 750w, which raises the torque curve, as well as increases the top "unloaded" ** speed by a full 50%.

Note: The "output" or "motor rated", "watts" is a fraction of the input "watts".
1000w input might produce a, usable, 750w output, = 3/4 = 75% efficiency rate.

** "Unloaded speed", is specified, to avoid, having to, explain-factor in, wind resistance, road load, component resistance etc., which are significant, measurable, factors during actual usage.

Please Note: I'm trying to keep this as simple-basic-understandable as possible!

Subject to enhancement ...

 

[TylerDurden]

From: http://adamone.rchomepage.com/guide5.htm

"One type of electric motors for model aircraft are the so-called brushless. These motors are little more expensive but they have higher efficiency. Typically between 80 to 90%. Since they have no brushes, there is less friction and virtually no parts to wear, apart from the bearings.

Unlike the DC brushed motor, the stator of the brushless motor has coils while the rotor consists normally of permanent magnets. The stator of a conventional (inrunner) brushless motor is part of its outer case, while the rotor rotates inside it. The metal case acts as a heat-sink, radiating the heat generated by the stator coils, thereby keeping the permanent magnets at lower temperature.

They are 3-phase AC synchronous motors. Three alternated voltages are applied to the stator's coils sequentially (by phase shift) creating a rotating magnetic field which is followed by the rotor.

It's required an electronic speed controller specially designed for the brushless motors, which converts the battery's DC voltage into three pulsed voltage lines that are typically 120 degrees out of phase.

The brushless motor's max rpm is dependent on the 3-phase's frequency and on the number of poles: rpm = 2 x frequency x 60/number of poles. Increasing the number of poles will decrease the max rpm but increase the torque.

A brushless motor's direction of rotation can be reversed by just swapping two of the three phases.

Earlier speed controllers needed an additional set of smaller wires connected to the motors' internal sensors in order to determine the rotor position to generate the right phase sequence. New controllers read the so-called "back EMF" from each phase, which allows the motor to be controlled without the need of the extra wires and sensors. These new controllers are called "sensorless" and can be used to control motors
with or without internal sensors.

At less than full throttle the 3-phase pulses are chopped at a fixed frequency with a duty-cycle depending on the throttle position. At full throttle the phase pulses are no longer chopped giving the max rpm and torque. The ESC's 3-phase actual output frequency and thus the motor's rpm depend on motor's Kv (rpm / volt), the actual load and the voltage applied, as the ESC needs the EMF positioning pulses back from the motor before it sends the output pulses.

Many brushless ESC allow the user to set the Electronic Advance Timing. High advance timing (hard timing) is suitable for high pole count motors (above 6 poles, such as Jeti, Mega, Plettenberg). High advance timing gives more output power at expense of efficiency. Low advance timing (soft timing) is suitable for low pole count motors. It gives higher efficiency with some loss of output power and is recommended when long run-time is the primary goal.

A more recent type of brushless motor is the "outrunner". These motors have the rotor "outside" as part of a rotating outer case while the stator is located inside the rotor. This arrangement gives much higher torque than the conventional brushless motors, which means that the "outrunners" are able to drive larger and more efficient propellers without the need of gearboxes.

More:
http://adamone.rchomepage.com/guide5.htm

 

[TylerDurden]

From: http://en.wikipedia.org/wiki/Brushless_motor

"A BLDC motor has permanent magnets which rotate and a fixed armature, eliminating the problems of connecting current to the moving armature. An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the brush/commutator system.

BLDC motors offer several advantages over brushed DC motors, including more torque per weight, more torque per watt (increased efficiency), increased reliability, reduced noise, longer lifetime (no brush and commutator erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI). With no windings on the rotor, they are not subjected to centrifugal forces, and because the windings are supported by the housing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter.

Because the controller must direct the rotor rotation, the controller requires some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors, and therefore are often called sensorless controllers. Like an AC motor, the voltage on the undriven coils is sinusoidal, but over an entire commutation the output appears trapezoidal because of the DC output of the controller.
The controller contains 3 bi-directional outputs to drive high-current DC power, which are controlled by a logic circuit. Simple controllers employ comparators to determine when the output phase should be advanced, while more advanced controllers employ a microcontroller to manage acceleration, control speed and fine-tune efficiency.

Controllers that sense rotor position based on back-EMF have extra challenges in initiating motion because no back-EMF is produced when the rotor is stationary. This is usually accomplished by beginning rotation from an arbitrary phase, and then skipping to the correct phase if it is found to be wrong. This can cause the motor to run briefly backwards, adding even more complexity to the startup sequence. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position."

More:
http://en.wikipedia.org/wiki/Brushless_motor

 

[TylerDurden]

PWM-based motor controllers function much like "buck converters", which use switching and inductors to alter the relationship between voltage and amperage.

In motor control, the process allows speed changes by reducing average motor voltage while increasing current. The motor acts as a large inductor.

It works well: at slow speeds (low motor voltage), the controller provides more torque (higher current).

More:
http://en.wikipedia.org/wiki/Buck_converter

 

[neptronix]

^---- +1

 

[Arlo1]

good thread guys

 

[TylerDurden]

A bit more detail:

Archive version (until Nelson Kruschandl's site gets restored):

http://web.archive.org/web/20090526030425/http://www.speedace.info/solar_car_motor_and_drivetrain.htm

See "BASIC MOTOR PRINCIPLES" section.