I’ve rather failed to keep up the blog in the past few weeks. I have been kind of in a hole with the Generalized Electric Vehicle Control Unit software with the latest final version ever just around the corner. It’s refreshing and very enjoyable to be back in code though a little rusty. And so on a number of days I’ve simply failed to make it into the shop and neglected various other duties while focusing on getting the web interface to work etc. It’s been a little obsessive.
But its been an exciting week and gratifying. Some of our activities take quite a while to mature and several of those came together this week.
First the GEVCU progress. We’ve added some mundane but needed features – at least for the VW Thing implementation. We have completed the precharge function or completed A precharge function. I’ve actually come across an alternate strategy for this that bears a mention and so was mentioned in this week’s show.
Consider the circuit diagram below. We normally have a main contactor and a separate precharge relay. The precharge relay, often just an automotive 12v, connects the battery pack to the inverter through a precharge resistor – typically a power resistor of 50watts or more and anywhere from 50 to 750 ohms. The resistor serves to restrict the inrush of current to the input caps of the inverter controller. This is kind of necessary on ANY component that has internal input capacitors – DC to DC converters come to mind but also air condioner motor drives and more.
Once the caps are charge at a low current level through the resistor, the main contactor is closed effectively connecting the battery directly to the inverter input caps and largely bypassing the precharge resistor.
The problem is the little precharge relay. It’s just not meant for higher voltage. We recently added a Tyco precharge relay to the product line. It IS rated for high voltage, but almost doesn’t make sense at the neceassary pricing of $99. You can very nearly buy a true high voltage high current contactor for that amount of ducats.
Ryan Bohm actually came up with an alternate topology for this for his Netgain Controls WarpDrive Industrial. The system uses TWO contactors, one on the positive input and the other on the negative input of the inverter. Again a precharge resistor is provided but it is simply connected across the two positive leg contactor terminals.
The magic is of course the contactor on the negative side. No current flows through the precharge resistor and no power to the inverter until you close the bottom contactor. At that point, the circuit is closed and the current through the resistor precharges the input caps on the inverter. Once they are up to battery potential, you close the top contactor, bypassing the resistor.
This is actually a simpler configuration, and avoids the wear and inevitable failure of the little 12v relay from high voltage. HIgh voltage has in the past been 200v. We are rapidly moving to 330vdc or even 400vdc for modern drive trains.
Actually with this circuit, you can apply IGNITION switched 12v to the lower contactor and, for example, the power input to the GEVCU and to the Inverter all simultaneously. THis starts the precharge cycle. Once sufficient time has elapsed to be charged, you simply engage the upper contactor.
We added a couple of other “features” to the GEVCU. A cooling fan output for example. We have dual heat exchangers on the VW THing and they are kind of monster Deralis, with an appropriate throaty howl when they run. We don’t need them in town. We can’t drive without them on the freeway. The trick is of course the inverter and motor cooling. The DMOC645 has a pretty severe current limit that kicks in at exactly 80 centigrade. So we want to keep it below that. With our pump and heat exchangers, this is easily accomplished in town WITHOUT the heat exchanger fans. But on the freeway, the system quickly heats up to beyond 85 degrees. Fortunately, the addition of the fans to the mix is more than enough to bring the temp down. So the kicker is when to run the fans. We don’t want them howling at us while we maneuver in the driveway, and we have to have them under load.
Fortunately, the DMOC645 reports motor rotor temperature, motor stator temperature, and inverter temperature all to the GEVCU via CAN. So we can simply monitor actual temperature and when it gets to 70 or 75 degrees, use one of our GEVCU outputs to turn on the heat exchanger fans. Then, if it cools to some lower value, say 65 degrees, it will turn them off. By having TWO specified temperatures, we can avoid the hysterisis that would inevitably occur if we simply set it to 70 and let it go. It would then “hunt” rapidly around the 70C mark.
Finally, AC motors give us the gift of regenerative braking. This is actually a mixed blessing and does not achieve the energy recovery most people accrue to it. But we’ve gotten addicted to the feel of braking with our throttle. The problem is, with aggressive regen, we slow the car quite capably and even dramatically without touching the brakes. That means we never turn on the brake lights.
Brake lights are actually kind of important. When you are in a moving vehicle FOLLOWING a moving vehicle it can be eerily difficult to detect changes in speed n the car up ahead. So we’ve grown accustomed to the warning of two great big red lights that come on when the vehicle ahead slows. Without them on your vehicle, you risk having others climb into your trunk every time you slow down dramatically.
So we added a function to select one of the eight outputs for brake light. You can use this to switch a relay which bypasses the usual brake light switch. This output will switch on anytime the actual motor torque exceeds 10 Newton Meters of negative regenerative torque. So you can slow down a little without triping the brake lights. But anything significant will result in a brake light illumination.
We also did some work on the wireless website configuration screens to make them more informative and to fix a lot of things that were just broken. One of the things we added was a kWh meter. The DMOC645 also reports battery voltage and current and we can use this to calculate energy use in kiloWatt hours or kWh.
Actually we calculate it in kiloWatt milliseconds and store them in EEPROM. We DISPLAY them in kiloWatt hours by dividing this number by 3,600,000. And so each time you get in the car and start it up, it retrieves the stored kWms and then starts adding to it each timer increment based on the power coming out of the pack.
The problem with these kind of meters is resetting them. We actually do NOT have a way to monitor charging as yet so we cannot measure the power coming back in from a charger. I have cars where you have to manually “reset” the meter and the problem is that I forget to reset it. This results in a very innacurate meter.
For GEVCU, we allow you to enter a fully charged pack voltage. For LiFePo4 systems, this is often 3.34v per cell. That’s the charge on the cell after it settles a few hours after charging. So we ould normally set our fully charged voltage to something beneath that by a small amount. Like 3.30v per cell. In this way, the kWh meter is reset on any tick where the pack voltage exceeds the entered voltage.
And so whether you have just finished charging and are at 3.55 per cell, or if it is some hours later and 3.34v per cell, the meter continously resets.
What we know, is that if you get in the car and back down the driveway, by the time you reach the road you will bleed off the surface charge on your cells and the voltage will fall below 3.31. At that point, the kWh meter begins its count.
It is also interlocked by negative torque, so if you spike the voltage using regenerative braking, it will NOT reset.
We actually have about 30 GEVCUS already delivered with earlier software without these features. Unfortunately, as we are based on Arduino, there is no easy way to magically upgrade the software in them. You have to install Arduino and recompile the source code to update it. Fortunately, that’s not as hard as it sounds but I have done an instructional video showing how it’s done.
The other interesting thing this week was receipt of our first Scott Drive. This is a 150kw maximum AC inverter from New Zealand that has been characterized for the Siemens 1PV5133 motor. And unlike some of the testing, this has actually been run on a test bench WITH a load generator.
While this drive has been bench tested with the Siemens, it has never yet been used with a Siemens in anger in a vehicle. We are currently discussing becoming a dealer for the Scott Drive and assuming that all happens, we’ll be commencing one soon. The Scott Drive DOES feature a CANbus connection but it is unclear to me at this point whether we can actually control the drive with it or if it is just used to interact with other devices – such as a BMS.
We have actually had a Rinehart Motion Systems inverter for a couple of months. GEVCU development has rather precluded our tearing down our one test bench to do any of that.
And finally, we now have 10 UQM Powerphase 100 systems from teh CODA bankruptcy. We have to develop an object module for the GEVCU to drive it.
And so this past week I made a command level decision to go into test benches in kind of a big way. We are building one for the UQM and another to test compatible inverters for the Siemens. Indeed, we could well wind up with a test bench for each inverter we carry. Questions are going to come up and often the only way to answer them is to go to the bench and try it.
The UQM bench kind of fell into our lap. It turns out it uses the same eGearDrive shaft and coupler as the Siemens so it was trivial to couple the UQM motor to one of our Siemens motors. SImilarly Hauber’s dual Siemens allows us to run a Siemens motor for test, with a Siemens motor generator for load.
I did imply that we would do this by adding a 3-phase rectifier to the Siemens motors to produce DC. And that was the intent. But we also needed some capacitors to excite the stator windings, which is no big deal. But Paulo ALmeida has been persuasive that we really need a couple of our dwindling supply of DMOC645s as it would allow us to both control the load, AND to display it on a GEVCU wireless web site display. He’s right again of course.
This will also allow me to experiment a bit with something I’ve been wanting to try. Some version of the AC propulsion strategy of connecting 240vac to the motor windings and using the inverter to control charging a battey pack with regenerative braking from the grid. I’ve always wanted to play with that and these test benches ought to sratch the itch.
Recall we received two oddball A123 battery packs from the Better Place battery raid. We’ll use these on these two benches to get a smooth 360v source for them.
So overall, the test benches are going to be a bit costly to implement – kind of like an entire car build on a bench. But we think it is a future direction we need to go.
Meanwhile, Damien Maguire has an inverter working as well. So we are quickly moving from having a bunch of motors and no inverters, to having inverters to pick from.
I must say the size of the Rinehart Motion Systems inverter is just tiny compared to the others. This little inverter with a Siemens motor and some of the new CAM72Ah cells from CALB would make a very small light package for a little yellow sports car I’m thinking.
Speaking of packages, we are running a one week sale on the High Performance Electric Vehicle Systems AC35x2 system. This lists for $9395 and we normally offer it at $8900 just to come in under $9000. For this week, we are going to offer it at $7400 with FREE SHIPPING to lower 48 US commercial addresses or nearest UPS terminal.
Till Friday noon.
Stay with us.