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Thread: Ex-7 - Electric RX-7 conversion

  1. #91
    Low Bitrate no_hazmats's Avatar
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    Mar 2009

    4-wheel motors

    A typical 15" wheel is about 77" in circumference. That means it makes 823 revolutions in a mile.

    At 15 mph, that's 823/4 = 205 RPM,
    at 30 mph -> 411 rpm, and
    at 60 mph -> 823 rpm.

    To make an electric motor produce significant power at these low RPM's requires many many copper windings - very heavy. Or fewer windings and more Amps -> much bigger batteries. By comparison, my motor makes full power from nearly zero to over 4,000 rpm, and is capable of 11,000 rpm.

    So you'd need to have gear reduction on each wheel so each motor can be made smaller and reasonably light. That's 4 motors plus 4 gearboxes. You cannot hang that much unsprung weight on the wheel, so you would want to move the motors inboard and use axle shafts to hang the motor / gear reducer from the 'sprung' part of the car. So add 4 CV joints and axles and the bearings. And you'd need 4 controllers (albeit smaller controllers) and 4 sets of cables to the motors. And don't forget the brakes. Are you following me? Before long, you're where we are. One motor, one gearbox and a differential / axle. Yes there are losses, but overall it's relatively efficient, compared to 4 of everything.

    I've never heard of a passenger car with a motor on each wheel. I know NASA does that like on the mars rovers, but they are tiny, 1/10 of a mph vehicles designed for a planet with a bit more than 1/3 the gravity on earth - so things are 3 times lighter. So the motors are tiny.

  2. #92
    Low Bitrate no_hazmats's Avatar
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    Mar 2009
    Quote Originally Posted by 2k1Toaster View Post
    ... and also it gives you regenerative braking. Turn a motor by the shaft and you have a generator.
    Most AC cars and a few DC cars have regenerative braking. Mine does. Even with a transmission like the powerglide, you can have regeneration if you delete the torque converter. No need for direct, on-wheel motors to get that......

  3. #93
    Maximum Bitrate Crinos's Avatar
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    Mar 2009
    Kristiansand, Norway
    Out of electric vehicles, the Tesla is the only one I know of that have an engine to each wheel.

  4. #94
    Low Bitrate no_hazmats's Avatar
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    Mar 2009


    Quote Originally Posted by Crinos View Post
    Out of electric vehicles, the Tesla is the only one I know of that have an engine to each wheel.
    Nope, Tesla has a single motor in the center over the rear axle.

    2010 Tesla Roadster

    Technical Specifications

    Motor 375 volt AC induction air-cooled electric motor with variable frequency drive.
    Max Net Power:
    Base: 288 HP (215 kW) @ 5,000-6,000 rpm
    Sport: 288 HP (215kW) @ 4,400-6,000 rpm

    Max RPM 14,000

    Base: 273 lb-ft (370 Nm) @ 0-5,400 rpm
    Sport: 295 lb-ft (400 Nm) @ 0-5,100 rpm

    Single speed fixed gear with electrically-actuated parking lock mechanism and mechanical lubrication pump
    Overall Final Drive 8.28:1
    Reverse Reverse direction of motor, limited to 15 mph
    Final Drive Ratio 3.12:1

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  5. #95
    Maximum Bitrate Crinos's Avatar
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    Mar 2009
    Kristiansand, Norway
    Then I don't know of any 4WD electric vehichles

    But did one of the prototypes of the Tesla have 4 motors?

  6. #96
    Low Bitrate no_hazmats's Avatar
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    Mar 2009

    Drive #3

    Problem I was seeing was definitely hitting the limits imposed by the parameters in the controller.

    For drive #2, that was:
    Battery Limit: 210 Volts
    Ramps Starts at Limit + 20 V = 230 V

    I changed the numbers for Drive #3 to:
    Battery Limit: 192 V (80% of 240 volt nominal)
    Ramp Starts at Limit +12V = 204 V

    As a reminder, this means that as battery voltage drops down to 204 V the controller begins limiting the power. It decreases power linearly until the battery voltage = Limit, and at that point power = 0.

    I drove about 18 miles, the same two circuits as #2. This time I had the laptop in the car recording a log file and displaying a few parameters as I drove. This is REALLY cool because I can run the program from within RR and both see and log performance data.

    I didn't actually put the gauges in because I'm a dummy. I took them to work to mount them into a home made panel, and I left them there. But the PC gave me accurate voltage and an estimate of battery Amps.

    Starting voltage (Stationary) => 269 V. This is a few volts low, ideally would be about 273 or so. (20 x 13.8 = 276). But it's about what I'd expect.

    After one 8.5 mile loop, the voltage (standing still again) was about 235 or so.

    About 6 miles into the 2nd loop I began tickling the ramp towards the limit, I could feel it, bUt I could also see it on the display. I went an additional mile for good measure and came home.

    Voltage was around 200 at the end.

    So, while I'm not exactly happy with the mileage, at least I know that the controller was and is doing what it's supposed to do.

    I'll put up more details later....

  7. #97
    Fusion Brain Creator 2k1Toaster's Avatar
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    Mar 2006
    Colorado, but Canadian!
    18miles is more than I actually expected from those batteries, not bad though.

    I am still saving up for a lithium pack to EV-erize my car. Go lithium or go home! j/k
    Fusion Brain Version 6 Released!
    1.9in x 2.9in -- 47mm x 73mm
    30 Digital Outputs -- Directly drive a relay
    15 Analogue Inputs -- Read sensors like temperature, light, distance, acceleration, and more
    Buy now in the Store

  8. #98
    Variable Bitrate
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    Mar 2005
    Quote Originally Posted by no_hazmats View Post
    DoD?? In any event I don't intend to kill the batteries prematurely.
    Depth of discharge. The optimas blue tops are supposed to be fairly resilient in that regard so you can use more of the energy actually in the batteries in comparison to other Pb batteries.

    My calculations, using various EV tools out there considering vehicle weight, battery size, Drive type etc.; came out to anywhere between 80 and 100 miles. My conversations with people who have converted and are driving similar cars (Porsche 914's) with identical drive system and similar Flooded LA's suggest that the online calcs and estimates are overstated. Scaling one guys real-world experience to my battery capacity (not accounting for weight or any other differences) suggested 78 miles. He gets 60 miles with a smaller (lower voltage lower WH FLA) pack. I think he was told by the supplier to expect over 100.

    I think that "Force to over come rolling resistance" might be too simplistic. Yes you can get a theoretical upper limit, but it doesn't take into account start / stop, regenerative energy, losses, terrain, speed, or many other factors. So I don't really know what value there is in knowing that theoretical upper limit. I just want a car I can drive to/from work and to green events in may area that will promote EV's and renewable energy in general.
    It is overly simplistic on purpose. Then you can calculate the upper limit and know if the calculators are worth a darn. Your 18 mile range suggests maybe not

    Good luck as you move forward. BTW transmissions are a significant efficiency loss. Even fancy pants new ones. If you Google the prius PSD and oak ridge and you can probably find a PDF listing the efficiency losses.

  9. #99
    Low Bitrate no_hazmats's Avatar
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    Mar 2009

    RX-7 Electric Conversion Update

    I know I've been silent on this thread for a few weeks, and thought I'd just post a quick update and some pics I took today. Actually, just the pics. Enjoy:

    Here's a close up of the RX-7:
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    Really - it IS an RX-7 under there! See the mirror?
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    Need proof? Okay:
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    My House after the first storm about 23":
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    All our cars at the end of the driveway to minimize plowing work. A VW Passat, two VW Jettas, and in the background the ElectraVan on the right and the RX-7 on the left:
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    My biggest greenhouse. Former dimensions: 150' x 30' x 20' tall. (100m x 10m x 7m high)
    New Dimensions: 150' x 30' x 0' tall. (100m x 10m x 0m high)
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    Total snow fall in 5 days: 39" (1m) We're just not set up to deal with that much snow in such a short period of time. This is an all-time record in Maryland for the most snow in one winter (and we have 6 more weeks!)

    I'll be working on the car sometime in the er, future.

  10. #100
    is back. FKA Robert Wray
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    Fiberoptic's Avatar
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    Jul 1978
    Baltimore, MD
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    That sucks. Familiar sight for maryland this week though.

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