Weber and Coulomb's MX-5

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Weber and Coulomb's MX-5

Post by Squiggles » Fri, 30 Oct 2009, 12:49

Is this box going to compromise the crumple zone?

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Post by coulomb » Fri, 30 Oct 2009, 14:11

Possibly. But virtually any battery bank in the back will not crumple as well as an empty boot. Just carrying something solid in the boot will also compromise crumple safety.

However, we are retaining that rather large chassis rail at full strength, which would have resisted crumpling anyway, and still will, though not as much as a solid block of batteries in a steel cage.

I guess the safest arrangement is like the Tesla Roadster: all the batteries and drivetrain concentrated in the middle. But this is impractical in most conversions.
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Post by Squiggles » Fri, 30 Oct 2009, 14:14

I was just wondering what the engineer is likely to say about that.

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Post by weber » Fri, 30 Oct 2009, 17:51

We wouldn't be doing it if it wasn't already approved by our engineer.

[Edit: Grammar]
Last edited by weber on Fri, 30 Oct 2009, 06:53, edited 1 time in total.
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Post by Squiggles » Fri, 30 Oct 2009, 21:34

Did a bit of reading, apparently it is the front crumple zones that are the big concern.

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Post by coulomb » Sat, 31 Oct 2009, 07:12

The MX-5 moves!

Image

Alas, not under electric power. Circumstances favoured a move from Coulomb's to Weber's house. The seat was bolted in, gearbox dropped, various loose items firmed up (amazing how many loose bolts you find), and the car was towed to the other side of the city.

It was a bit of a sight, with no bonnet, boot, or bumper bar, and things dangling off it everywhere.
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Post by Squiggles » Sat, 31 Oct 2009, 14:08

coulomb wrote: The MX-5 moves!

Circumstances favoured a move from Coulomb's to Weber's house.
Definition of Circumstances.
a. Weber's garage is more spacious.
b. Coulomb's wife wants her car back in the garage.
c.
d.

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Post by coulomb » Sat, 31 Oct 2009, 14:45

Heh. It has all to do with the amount of time we can spend on the project. At present, Weber has way more time to spend on it than Coulomb. However, in true alternating current fashion, these availabilities (potentials? potential contributions?) could reverse in the next few months.

Ok, that was a feeble simile. Image
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Post by coulomb » Sun, 01 Nov 2009, 23:22

Squiggles wrote: Definition of Circumstances.
a. Weber's garage is more spacious.
b. ...

Bingo!   Image

The luxury limousine laboratory almost backs onto Brisbane Forest Park. Room with a view!

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Post by coulomb » Tue, 22 Dec 2009, 02:25

Sometimes less is more. Weber had the idea to try three resistors instead of the four that will fit, and that we had been using until now:

Image

Where the maximum temperature was 93°C with four resistors, the maximum is now 78°C (ambient temperature 27°C in both cases). Note that we're still using a higher value resistor in the middle; the heat seems to be reasonably well shared across the three resistors that way.

It seems that resistors need a few millimetres of free air to radiate their heat from, and this seems to be a larger effect than increasing the power load by a considerable 33%. The 10 ohm resistors will now be dissipating 1.3 W @ 3.60 V.

For the curious, the 3R3 and 3R9 resistors were connected in 2S2P format, hence the more than 33% change in resistance. We could have used a 1R2 in series with a 4R7 and 5R6 in parallel, but the bypass current would have been below the psychologically important 1.0 A. Besides, three resistors in parallel is neater and easier to lay out. Similarly, three 1R2s in series would have resulted in exactly 1.0 A @ 3.6 V, but the middle resistor would have become hotter than the others, and replacing it with 1R5 results in less than 1.0 amps again.

In other progress, we finished one of two brackets required to attach the rear battery rack to the wish bone bolts:

Image     Image

The gussets are for strength and to keep our engineer happy.

The long slot is to match the long slots in the suspension joints, which adjust the height of the lower rear suspension supports, thus adjusting the camber. By copying the slot in the suspension joints, the bracket should not move when the suspension is adjusted.

The angle of the bracket is to follow a lip on the metalwork that the bracket will rest against. The radius on the bottom right of this bracket is to follow the bend in that metalwork (unfortunately not quite visible in the photo). The bracket fits quite snugly against the metalwork. Two pieces of 50 mm x 50 mm angle will weld to the rear battery box to bolt to these brackets.
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Post by acmotor » Tue, 22 Dec 2009, 04:21

[quote="coulomb"] ............Note that we're still using a higher value resistor in the middle; the heat seems to be reasonably well shared across the three resistors that way......
QUOTE]

What happens when you put the next cell alongside ?
What is the clearance to its resistors ?
Will this mean a re-proprtion of values ?
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Post by Electrocycle » Tue, 22 Dec 2009, 05:19

I'll think you'll find the wishbone mounting bolts will be an area of fairly major vibration relative to the rest of the chassis - so you might need some sort of rubber / urethane mount there - depending on how much flex your battery box has / can handle.
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Post by coulomb » Tue, 22 Dec 2009, 05:24

acmotor wrote: What happens when you put the next cell alongside ?
What is the clearance to its resistors ?

Surprisingly, it will be at least 6 mm, possibly 8 mm. Also, the next cell along quite possibly won't be in bypass; I'm hoping that only 10% of cells typically go into bypass.
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Post by coulomb » Tue, 22 Dec 2009, 05:30

Electrocycle wrote: I'll think you'll find the wishbone mounting bolts will be an area of fairly major vibration relative to the rest of the chassis - so you might need some sort of rubber / urethane mount there -

Interesting point, thanks for raising it. Unfortunately, we'll have no room for a mount between the bolt and the bracket, we'll be pushing to show the one full turn of thread that the engineer wants to see. (We could replace the bolt with a longer one, and grind appropriate flats etc, but we'd prefer not to do that). However, we could put one between the bracket and the vertical on the battery cage. An engine mount or similar would seem ideal.

That will change where the vertical goes, so this is the time to think about it. Thanks again.
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Post by Electrocycle » Tue, 22 Dec 2009, 14:35

yeah I'd keep your bracket, and add a mount between that and the battery frame.

You could also look at having a diagonal brace between the ends of the subframe as a part of the battery box, which should improve the rigidity of the suspension mounts.
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Post by coulomb » Tue, 22 Dec 2009, 15:30

coulomb wrote:
acmotor wrote: What is the clearance to its resistors ?

Surprisingly, it will be at least 6 mm, possibly 8 mm.

In fact, it is 6 mm. The cells are packed tight, so that's 46 mm between boards. Each resistor is about 10mm wide; 46 - 40 = 6.

Weber has pointed out that the 6mm gap has no PCB under it, so that should allow for more air flow.
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Post by weber » Tue, 22 Dec 2009, 19:57

Electrocycle wrote: I'll think you'll find the wishbone mounting bolts will be an area of fairly major vibration relative to the rest of the chassis - so you might need some sort of rubber / urethane mount there - depending on how much flex your battery box has / can handle.

That sounds like good advice, however I'm hoping there is enough flex in the battery cage. Can you tell from the photos below.

The 3 red clamps are holding the new 50x50x3 mm mounting bracket in place. It would ultimately be welded to the battery cage and bolted to the gussetted bracked that is bolted with the wishsbone bolt.

It looks to me that because it is roughly midway between the end of the cage and the 20x3 flat that is bracing the middle of the cage lid and base, then there is the abilty to flex the 30x30x3 angles it will be welded to at top and bottom.

Image Image Image Image

This cage is looking pretty ugly with its combination of red oxide primer on some parts, surface rust on others and the white stuff is dried rust converter. We want to get it finished soon and get it all rust-converted, primed and painted.
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Post by Squiggles » Tue, 22 Dec 2009, 20:43

weber wrote:the white stuff is dried rust converter

Phosphoric acid?

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Post by coulomb » Tue, 22 Dec 2009, 21:15

Yes.
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Post by juk » Tue, 22 Dec 2009, 23:25

If you're serious about rust prevention you should use the tannic acid based rust converters. The phosphoric acid ones slow down the process but dont stop it.

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Post by Squiggles » Wed, 23 Dec 2009, 00:36

juk wrote: If you're serious about rust prevention you should use the tannic acid based rust converters. The phosphoric acid ones slow down the process but dont stop it.


acid BASED rust converters....never used them just some good old phosphoric followed by elbow grease and good quality primer. Lasts for years.

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Post by Electrocycle » Wed, 23 Dec 2009, 04:34

weber wrote:That sounds like good advice, however I'm hoping there is enough flex in the battery cage. Can you tell from the photos below.
I guess it depends on how much movement there is in the subframe.
You'd definitely want to keep an eye on the mounts and the battery frame for a while and make sure it's not being overly stressed, but it should all be pretty strong.
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Post by Johny » Wed, 23 Dec 2009, 15:40

I have pretty much given up on rust converters - it always seems to reappear after a few years. My current best treatment is a substance called Penetrol (not Penetrene). Wire brush and apply and it gets right down to the base metal under the oxide and prevents oxygen ever getting there again. It sets like epoxy after a few days. It unfortunately doesn't like some spray paints over it but in areas that I use it I hand brush enamel anyway.
I used it on my old garage steel lintels 3 years ago (almost immovable surface rust due to the rough steel) and they still look like I did them yesterday. Super Snipe battery compartment 3 years ago was almost rusted out - same story.
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Post by coulomb » Thu, 24 Dec 2009, 04:51

Our motor was due today, so very close to Christmas. Well, strike me pink! It's here!

Image     Image     Image

We ordered feet and flange mounting... oh dear. Hopefully they can just send a new front housing and we can just change it over. Well, it has the right numbers:

Image

Notice that while it's a 22 kW motor at 50 Hz and 220 V, it's officially a 25 kW motor at 250 V. Note that it's not 60/50 * 22 = 26.4 kW, presumably due to the extra iron and windage losses. Amazing how it works out so evenly to four significant digits Image

Image

Edit: cowling -> housing
Edit2: decimal places -> significant digits
Last edited by coulomb on Wed, 23 Dec 2009, 19:37, edited 1 time in total.
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Post by coulomb » Thu, 24 Dec 2009, 05:03

Just for comparison, a certain acmotor's 22 kW AC motor:

Image

But this one is 22 kW at 415 V and ~1500 RPM (4 pole) in a 180 frame. I suspect the relatively longer fan cowling and eyelet locations are just because it's an older model.
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