wich relationship is between Vp and Vrms?

Pablo_1985

Pablo_1985

100 W
Joined
Feb 19, 2009
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I know how to calculate it but, i need to know the waveform from the controllers, i know they make a trapezoidal waveform, but i don´t know the timings wich are necesary to calculate it, anyones know?

If it were senoidal, it is Vp/sqrt(2)=Vrms, and it is the first harmonic from a trapezoidal, so i could aproximate it saying that is simply down to this value...but for example a square Vp=Vrms....so if anyone knows this relation i will be really pleased, and i could do some maths with good accuracy.

Thanks!
 
It might help if you tell us why more about your interest in peak and RMS voltages.

For most practical purposes, you can think of a motor controller as supplying a constant voltage, equal to the battery voltage multiplied by the duty cycle of the high frequency PWM waveform. To model controller-motor circuit you need this voltage connected through the internal resistance of the battery and controller, as well as the back EMF waveform of the motor connected through the inductance and resistance of the motor.

You would see a rectangular (trapezoidal if you prefer) waveform if you connect a motor controller to a resistive load and zoomed in to a small time scale, but if you connect a controller to a motor and consider time on the scale the motor operates at, the waveform coming out of the controller voltage will exhibit fast rise followed by decay characteristic of an RL (resistive-inductive) circuit. At the moment of voltage rise, current will be zero and immediately start increasing rapidly, then flattening as it approaches the resistive circuit maximum.

Does this help? If not, can you provide more details about your question.
 
Ok, perfect, i thought the time wich takes to rise and down would be bigger, but if i can aproximate it like a square ones , it´s ok...furthermore, if i use a square one i am in the worst case, wich help me to see the limit... i´m not taking histeresys and foucault losses into consideration, so i think it could be a good aproximation.

It is because i´m trying to know the specs from a motor, and i´m trying to know how much power will dissipate.

Thank´s a lot
 
Here's what a typical BLDC waveform looks like:
Test motor_a.JPG

The amplitude of the waveform is determined by the supply voltage. You can simply average the ramping portion of the waveform to get an RMS value. To me, it looks to be about 50% of the peak.
 
That's a surprisingly clean signal - I'm used to working with RC motors (also BLDC) with signals like those shown in the image below. Part of the difference is the motor timing (my controller is only supplying power during a portion of each 1/6 of a cycle), but I don't fully understand the straight ramps seen on your oscillogram.
 
maybe the first one is when de controlled has a load, and de second one when it has not a load...

Thank you guys, i don´t know what could i do without you! jahaha

EDIT: you have also negative values, i thing the second one is another way of modulation, the straight ramps from the first oscilogram are because the load is very inductive, it is something typical from this kind of loads.
 
My oscillogram (the second one) was taken across the leads of an old brushless model airplane motor (Jeti Phasor 30-3). You see negative values for two reasons: First, the voltage is the relative difference between two of the motor leads. The controller switches these from positive to ground at different times during a cycle, so you will see a positive voltage at one point of the cycle and negative 180 degrees later. Second, because inductors maintain current flow after being disconnected, the voltage will jump beyond the supply (in either direction, depending on conditions) unless constrained (e.g. by diodes).
 
I don't fully understand the short pulses at the beginning of the ramp either. It looks just like the oscillogram on one of the app notes for a brushless motor I saw once, so I assume it's 'normal'.
 
Hmmm...

file.php


Just a guess, but I'm thinking this could be smoothed out if MadScientist just opened up his Etch-A-Sketch and lubed the tracking mechanism...
 
So i don´t have any idea why the oscilogram has this waveform! XD
 
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