Guys,
if you use a Jag to drive a Victor you will get into a pickle. The two PWMs are free running and will produce all sorts of beating products. It will be very confusing to figure out whats going on. May be fun though!
What we have is really pretty simple. For the Jag we have decent documentation for the Vic we don't so we are guessing a bit for the Vics.
For the Jag, during the HIGH PWM cycle, we have:
Battery,
Current sensing Resistor,
Two Mosfets in parallel,
the Load (Motor),
Two more Mosfets in parallel.
During the LOW PWM cycle, we have:
the Motor that is shorted via two sets (in series) of two parallel Mosfets.
The Mosfets are driven hard-ON and hard-OFF at 15KHz by the driver chip.
All the other components are “in the noise” - that is they matter very little and can be ignored for our purposes here. It is fair to assume that these other components are engineered properly, and are not limiting the behavior we observe.
Based on the previous discussion in this thread it seems that the practical limitation we have is thermal overheat due to high amperage. The Jags have software that cuts out way before meltdown (perhaps too soon, sacrificing maximum power), while the Victors can burn in the field but as a result deliver more power than the Jags. This gives Victors an apparent power advantage, and it seems many teams prefer them for that. The sad thing is that the Jags actually could deliver more power than the Vics if allowed.
I've been playing with a simulator – it is really cool and answers many of the questions we posed here. I would advise all to play with this, see:
http://www.linear.com/designtools/software/
It is very noble for LT to provide this for free – its a great educational and practical tool.
LT doesn't have the exact MOSFETS we have, but you can get close, or design your own.
I entered a simplified model with just one Mosfet. I don't have a Motor model, so I assumed a Stalled-Motor equivalent of an Inductor and series Resistor. A good approximation for a CIM is L=200uH, R-80mOhms (please advise if you have better data on this). See:
http://i.imgur.com/1cNXB.png
You can see the Current through the Inductor is ~60A. In reality we were getting 55A see real picture of a PWM driven Jag and stalled CIM Search for the "Voltage vs. PercentVbus” thread for more motor pics:
http://imgur.com/0IjKf
To give more details on the simulation parameters if you want to play with it (it takes just minutes to do this):
V2 is 12VDC. R3 is the internal Battery resistance (ours measured about 20mOhms)
L1 is 200uH, and R2 is 80mOhms these numbers seem to match a stalled CIM based on some measurements we ran in the aforementioned thread. It would be great if someone has the SPICE models for FIRST motors!
I just used a Diode D1 for the flyback shunt. You can see this would dissipate 30W if the Jags didn't use the Mosfets for this! As the current through D1 is 60A during the LOW PWM.
V1 is the PWM:12V, Period=75us (for 15 Khz), I gave it a rise and fall time of 2.5us each. It runs for 200 cycles or 15ms.
R1 is an arbitrary value. You can see that the Mosfet's parasitic capacitance is spiked through this R and that is where the Capacitive loss is converted to heat – that stays in the Mosfet Driver chip – I will let you all run this to see the trace of the current through R1 – it is cool!
The Mosfet I picked is IRFH5004, this has an RDS of 2.2mOhms and I doubt this simulator takes heating into account so RDS probably stays at 2.2mOhms. In our reality we are heating these puppies driving RDS to about 150% of typical value.
The main things still missing are the forced-air cooling equations for the Jag&Vic (ie. What thermal C/W number should we use) and all the documentation that we don't have for the Vics.
And for the advanced student, here is the Fast Fourier Transform (FFT) of this PWM signal on V1 – it is mind boggling what LT Spice can do !!!
http://i.imgur.com/7Fjh2.png
You can see the main component at 15KHz, and the square wave's harmonics at 3X intervals.
This is much simpler and hugely more powerful than the Fortran punched card program I used for very primitive Fourier Analysis in my college days :-)