Wow! Go on vacation and lots happens.
Quote:
Originally Posted by pnitin
Exactly ,
Same is applicable for motor driven by these motor controller, input current and output current are not identical due to switching and stored energy in motor inductance. so you can not just calculate power dissipation in switch by looking at difference in output voltage and output voltage and output current
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No! A typical FRC motor controller will not have any significant energy storage elements in them so the input current is going to be pretty close to the output current. The difference between the input current and the output current is the current consumed by controller circuitry and should be insignificant at high motor currents. The motor's inductance does not have the effect you think it has.
Quote:
Originally Posted by sastoller
It's a little different than a cell phone charger taking 110V AC input and 5V DC out...
For the conventional motor controller, you can make the assumption that Iin = Iout. Here's Why:
If you look at conventional motor controller designs, the current that drives the motor flows from the input of the motor controller, through a large output device (Power MOSFET or BJT), through the load (motor, or bank of resitors), and then back into the motor controller, through another large output device, and finally, back out the negative battery input on the motor controller.
You can think of the output devices in this case like a switch. When the output devices are "on" (transistors are in the saturation region), they have a small resistance (this is what causes the voltage drop between the input and output of the motor controller). This resistance here is in series with the load. Kirchhoff tells us that current through all components in the loop is the same. Operating on the assumption that the SD540 is in fact built like most conventional motor controllers, the input current will be the same as the output current (assume extra current consumed for control circuits etc in the motor controller is negligible). You know the resistance of the resistor bank, and you know the total power output, so you can easily calculate current through the motor controller. Now, knowing Iin and the delta V across the motor controller, you can calculate the amount of power consumed in the motor controller. This power is dissipated as heat.
As you can see, from the 50A load test on the MindSensors site, this motor controller does, in fact, get HOT (125C after 5 minutes at 50A), and still climbing... Seems to me like a lot of energy lost to heat in the SD540, not to mention a potential safety hazard...
http://www.mindsensors.com/content/7...haracteristics |
Yes! Stop talking about cell phone chargers. The circuit topology is totally different and the energy transfer mechanisms are also totally different and are not comparable with what goes on in the motor controllers used in FRC.
Thanks CTRE for posting test results that are well documented and give the test conditions. I used the values to estimate the losses in each of the motor controller using the data from the 11.05 V input case.
Iout = Vout / 0.2 Ohm
Pcont = dV * Iout
Victor SP - 12.4 W
Talon SRX - 15.1 W
Spark - 21.8 W
SD540 - 44.3 W
The high voltage drop and high watt loss in the SD540 is consistent with the data that Mindsensors has published showing excessively high temperatures on their heat sink (125 degree C, and climbing).
Does anyone have an SD540 where they have opened it up, or are willing do so, and report what sort of MOSFET's are used in it?