Electric Porsche 944

The ultimate EV question, AC or DC

by on May.24, 2015, under Electric Car

In the world of DIY EV’s there are many choices to be made, type of donor vehicle, automatic or standard, clutch or  clutchless, battery size, high voltage or low voltage, but one of the biggest ones is AC or DC motor & controller/inverter.  I have the benefit of having used both in my 944, same car, same battery, same transmission set up.

Water cooling3

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I’ll list the main points first and go over them in more detail individually.

– Cost

– Performance

– Reliability

– Driveability

– Maintenance

– Heat

– Efficiency

-Range

Cost:
Almost all EV Conversions have a budget.  Paying retail the DC system is almost always cheaper.  On paper this gets even more one sided if you consider a used forklift motor and performing some modifications to make it “more” suitable for EV use.  On the other hand with more OEM EV’s on the market and the availability of surplus and salvage pieces the prices can drop significantly below “retail”.  On the other hand if one of the HPEVS systems fits your performance needs they are priced quite fair.  My DC system was purchased new and used some of the higher end pieces available.  The Warp 11HV was not a cheap motor, the Soliton1 was also a fairly pricey piece.  Due to the demise of Azure Dynamics my AC system was less than half the cost of my DC system.

Performance:
If you simply look at 0-60 times and HP/Torque numbers the DC system was in a different league, with almost 300ft-lbs of torque and 270HP the car was fast and powerful, 0-60 in 4.8 seconds with traction problems, with the right suspension set up I’m sure the car would have gotten into the mid to low 4’s.  The AC system while still fun, simply doesn’t come close, 220ft-lbs but less than 150hp.  It’s more than enough for a daily driver in a vehicle of this size, but it’s no longer faster off the line than almost anything I could pull up against.

Reliability:
On the surface it’s no question, the AC system has no brushes or wear items with the exception of bearings.  Many of the AC motors are sealed including the Siemens motor I purchased.  This virtually removes the possibility of contamination, either dust/dirt/rocks/snow/rain/fluids from damaging the motor.  The electronics are actually more complicated than the DC system, so long term reliability of the electronics will depend on the design and ruggedness.

Driveability:
A very minor pet peeve of the DC system was how well the car coasted, normally that would be a good thing, however if the traffic in front of me let off the gas the engine would slow them down slightly, when I let off the throttle the car coasted really really well, so I had to use the brakes a lot to avoid getting too close to the vehicles in front of me.  Same with going down a hill, the car just coasted too well and I was always catching the vehicles in front of me.  I was always on the brakes with the DC motor, being on the brakes means wasted energy and momentum.  The AC system is about as opposite as a vehicle can get.  I almost never have to touch the brakes now, regen is set up on the throttle pedal and I can drive the car 90% of the time without removing my foot from the throttle.  I capture quite a bit of energy that would otherwise be wasted as heat and traffic jams are really not that bad anymore.  I can speed up and slow down using the throttle pedal and cruise at any speed, there is no minimum speed like a manual and I’m not riding the brakes like an automatic.  I couldn’t be happier with the driveability of the car.

Maintenance:
For the first time in my EV I’m working on new things and tidying up loose ends that I never got to before.  The car just works, I’m not worried about brushes or heat, I look at the pack temperature if it’s chilly to make sure I’m ok to use regen (charging batteries below zero is bad, that includes regen charging) and just drive.

Heat:
With a liquid cooled motor and controller and the fact I went back to the stock radiator and two loops there is no worry about heat.  The inverter typically reaches 5 degrees above ambient temperature at the end of a drive, the motor fluctuates based on load but comes down quickly with the cooling system.   Again I no longer have to worry about heat and how long my motor is going to last.  The stock radiator is overkill but keeps things nice and cool even without the use of a fan.

Efficiency:
This is where things start to get interesting.  In theory there isn’t much difference between an AC system and a DC system, perhaps a few percent which could easily be overcome by driving style.  In reality there are a large number of factors that together have significantly increased the efficiency of the car.  First there is power, sure it’s not an apples to apples comparison, the DC system had almost double the power, but with double the power it makes it easier to “waste” energy, accelerating a bit faster than you should, passing or driving a bit faster than ideal.  Many people would choose the DC system over an AC system to get higher peak power, so I’d say this is a fair comparison to leave in.  Next is regen, now this depends on where you live, how you drive and how regen is set up in your vehicle.  For me with regen on the throttle I use it all the time before mechanical brakes.  When I first set up the GEVCU I didn’t have the brake lights connected so I have a very small amount of regen configured, it basically mimicked engine braking and let me slow down when traffic slowed down without riding the brakes.  To come to a complete stop I had to use mechanical brakes almost all the time.  Energy use was already down significantly compared to the DC system.  A conservative drive in the 944 with DC was 300wh/mile, right where the rule of thumb says I should be for a roughly 3000lb car.  I was able to get it down to 275wh/mile with ultra conservative driving, but this took a lot of work and just wasn’t much fun.  On the other hand a few heavy accelerations put energy use up around 350wh/mile and spirited driving resulted in almost 400wh/mile.  From day one and very limited regen the AC system was around 250-275wh/mile with average driving, no attempt to  drive conservatively.  With more aggressive regen enabled that average energy use dropped below 225wh/mile, and conservative driving resulted in 200wh/mile.  This is all mixed driving with some 60km/h, some 80km/h and 100km/h zones.  The comparable numbers are, between  300-350wh/mile for the DC system is around 225wh/mile with the AC system in average driving, and 275wh/mile for the DC system vs 200wh/mile for the AC system with ultra conservative driving.

Range:
The range of an EV is just an extrapolation of it’s efficiency but I wanted to point out how significant the difference is with the same car and battery.  The pack is 16kwh, but ideally shouldn’t be discharged past 80%  very often, this gives us 12.8kwh of energy to use on an everyday basis (I don’t use that much but we’re comparing numbers here).  We’ll use 325wh/mile as an average of the normal driving with the DC system, this results in 63km or just shy of 40 miles of range, and conservative driving bring that up to almost 75km or 46miles.  Due to the efficiency increase the AC system is 91km or almost 57 miles with average driving and 102km or 64 miles of range with conservative driving.  Now of course your results will be different, but this is the difference between a 1986 Porsche 944 with a Warp11HV and Soliton1 vs Siemens 1PV5135 and DMOC645.

 

Conclusion:
I’ll start by saying I wish the car had a bit more peak HP, it’s not bad, but I think that a 150-200kw AC system would be amazing in this car, sure it would decrease the range without very careful driving but it would be worth it for day to day use.  Second I’m amazed at the difference in energy use switching from DC to AC* (*in this specific situation/car/components).  I love this car, no question about it, if I were to do it over again I would have kept the clutch in the AC system, first gear is amazing, but like an ICE you need to shift fairly quickly and without a clutch that’s a 2 second event which kind of kills your speedy acceleration.  I have yet to test it but I’m pretty sure my 0-60mph time would be faster in 2nd gear only instead of being quicker off the line but a slow 1-2 shift.  I would probably be happier with the power level of the car if I had kept the clutch.  This issue didn’t exist with the DC system, it had unbelievable power from 0-5500rpm with no obvious decrease in power within that range.  1st gear was unusable and traction was even an issue in 2nd gear at times.  In short I would never build another daily driver with a DC motor, but I would use a  DC motor for a toy, or something that might make it to the track or autocross event if an AC system at the desired power level didn’t exist or was simply too expensive.  (I still have the 11HV, and one day I might repair it and use it in something else with a Zilla 2k, unless of course Tesla drive systems become available and usable with 300kw+)

 

 


4 Comments for this entry

  • Randy

    Very informative post. I am running a Warp9 with a Synkromotive controller with one of the Better Place Modules and the last drive I checked, was around 270 watts/mile for a 2650 pound Fiero. The maximum range so far on one module has been 24 miles, so I am figuring with the other two modules installed, should be around 70 miles or so. If that could be increased with an AC motor. If the Fiero got similar results to yours, it should have over a 90 mile range with an AC system.

    • admin

      The numbers surprised me, but that’s what’s I’m getting consistently. The DC numbers weren’t that bad, but the AC numbers are very good for a vehicle this size. I’m running 65% regen meaning a possible 195nm of regen torque, this is important in recapturing as much energy as possible, I may even increase this with testing.

  • CZ-EV

    Robin,

    I am going to try to get a few miles on my DC motor set-up but will be working toward switching out to an AC motor. I have a Miata with 175k miles that would make a great platform for an AC build.

    Also saw you posted the 944 on the EVTV Registry, nice quality build! Hope to see it at EVCCON this year.

    Randy

  • Fred Behning

    Uh-oh, now you’ve got me thinking again (a dangerous thing). I decided to dispense with the clutch on my 924S (http://porscheev.blogspot.com) largely based on your comments. Can you give me some idea how yours drives in everyday use without the clutch now that you’ve had a few miles with the Siemens? Do you shift or just start and stay in second or third gear? How about freeway driving? Sorry for all the questions, but I still have time to switch back.

    Also hope you can make it to EVCCON this year. I’m pushing to have mine ready, so we can compare notes.

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