brushless-dc-motorcontrolmotormotor controllerpwm
I'm using ESC(Electronic speed controller) to control outrunner brushless dc motor.
As you know ESC detects ppm signal between 1ms and 2 ms approximately. So my ESC detects signal between 4% and 9% duty cycle at 50 Hz as an input.
However, it is unknown what kind of relationship exist between this input signal and output signal that drive BLDC motor.
Is ESC linear? How ESC converts input signal into required voltage for motor?
A BLDC motor is a 3-phase AC motor, which is why there are three wires.
An ESC is a set of 3 half-bridges that drive the motor phases to create a rotating magnetic field. Below is a schematic from here that should give you some idea.
If you were to feed PWM to two wires of a BLDC motor it would do very little except get hot and stay in one place.
Ideally, the output would like this (the frequency is varied to change the motor RPM, 60Hz is shown).. image borrowed from here.
A video showing what the non-ideal waveforms look like is here:
A BLDC motor is similar to other motors in that the torque produced is proportional to the current, and the speed is proportional to the applied voltage (assuming commutation is done correctly.) Of course there are second order effects, but that's the basic idea.
So if your motor was ideal and lossless, running at high speed with no load would be zero power output and would require no current. With a real motor windage and other losses increase with speed even at no load, so there is more current required to overcome those losses.
The first-order increase in speed comes from an increase in effective applied voltage, which comes from appropriate commutation and PWM duty cycle.
Best Answer
Most ESCs convert the servo pulse into into a linear PWM range.
However it's not quite as simple as that, because ESC's are designed to work with a variety of radio control transmitters which may have slightly different pulse ranges. Also, many transmitters have adjustable center and end-point settings. Several methods may be used to 'calibrate' the ESC to a particular transmitter:-
Auto-calibrate: The ESC assumes that the initial pulse width received represents 'low throttle' (motor off) and records it as the low end-point. As the throttle is advanced above half way, the highest point reached so far is taken as 'full throttle'. This procedure has to be followed each time the ESC is turned on.
Manual Calibration: The ESC is put into calibration mode by starting at full throttle (>1.5ms), then going to low throttle (<1.5ms) after a few seconds. These settings are recorded in an eeprom, so the calibration only has to be done once. Default settings that should work with most transmitters (eg. 1.2 to 1.8ms) are often pre-programmed into the ESC at the factory.
Using a programming card or PC link cable: End-points can be set to specific values and/or switched between auto-calibrate and fixed end-points. Some ESCs also allow setting different throttle curves (eg. exponential) or a 'governor mode' where the servo pulse width represents an rpm that the motor speed is to be locked onto.
Fixed end-points: The ESC may have two or more fixed end-point ranges, which be can be selected using throttle programming or with a programming card.
The ESC's instruction manual should tell you which method is used and how to do the calibration.