Hi everyone. I ´ve bought this dual dc motor driver in Cytron https://www.cytron.io/p-10amp-5v-30v-dc-motor-driver-2-channels to control 2 dc motors with PWM and Arduino. I´d like to know what output voltage of the motor driver should I see reaching the motors. I´ve some doubts about some dc motor parameters too.
For example, I want to control a 12V dc motor which has a current of 3A with a load of 500N, if I introduce a duty cycle of 50%, what output voltage of motor driver should I measure? I´m very confused with this topic because I´ve seen a lot of different ways of calculating this.
First of all, I´d like to check the dc motor parameters:
Power = Voltage * current = 12V3A = 36W
Resistance = Voltage/Current = 12V/3A=4ohm
Is this resistance constant? How can I measure this motor resistance with a multimeter? Do I need the motor to be working in a specific way to measure this resistance correctly with the multimeter?
Once I have the motor parameters, I´m going to explain my doubts. For example, with a duty cycle of 50%, I´d have the following calculations with the average voltage:
Vave = 0.512V=6V*
Iave = Vave/R=6V/4ohm=1.5A
P=6V1.5A=9W
Why do I have a power of 9W? That is the 25% of the total power, shouldn´t I have the 50% of the power, I mean, 18W?
If I use the RMS voltage, I´ve the following calculations:
Vrms=square root(duty cycle/100)*V=square root(0.5)12V=8.48V
Irms=Vrms/R=8.48V/4ohm=2.12A
P=8.48V2.12A=18W
If I use the RMS values, I´m obtaining the correct power I think, but my question is, why is there that difference? What values do I have to use? Will my dc motor have a power of 18W(50%) or 9W(25%) with a duty cycle of 50%? Will my dc motor receive a voltage of 8.48V and a current of 2.12A or a voltage of 6v and a current of 1.5A (or 3A to obtain 18W)?
I´m not sure of the calculations, so the first question is if they are correct.
And for finishing, will I need a trueRMS multimeter to measure the correct values?
Thank you very much,
Alberto.
You need to have a true RMS DVM to measure a PWM signal and express it in volt.
AFAIR RMS only applies to sine waves. The effective voltage of any curve is the voltage integral over time.
The effective output voltage of a rectangle is defined by the supply voltage times duty cycle. H-bridges double the output voltage swing. Unknown is the current drawn by the load, that's why (better) driver modules have a current sense output (shunt resistor) usable for output current control.
For what reason do You intend to use any voltage measurements?
Railroader:
For what reason do You intend to use any voltage measurements?
I want to know what voltage is reaching my motor when I modify the duty cycle and know at what power it can do the job
Don’t forget you are dealing with a motor , not a resistor - DC motors have inductance and generate a back emf related to its speed .
So it will distort a PWM waveform as it generates between pulses , and react differently to different loads and speeds and therefore appear to have different inductance/ resistance under these different conditions and your measured voltage might not be what you expect
Have a look at electrical models if motors .
With a freewheel diode over the motor the motor voltage will be a nice rectangle, varying from 0 to 100% with the duty cycle. The motor current has a very different shape, depending on load, speed, and PWM frequency and duty cycle.
aserrano96:
I want to know what voltage is reaching my motor when I modify the duty cycle and know at what power it can do the job
I suggest You relate to the PWM factor. 255, usual max, gives full power and 0 gives nothing.
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