Weber and Coulomb's MX-5

[coulomb]

After doing some modifications to the BMU software, we've come to the conclusion that we need timer-interrupt driven communications for our BMUs. Sadly, this means turfing out the Forth interpreter; it has served its purpose. Its replacement will be all assembly language.



You might not be able to see much in the above photo, but I'm connecting the comms outputs of one BMU to its own comms input, and connecting the RS485 receiver across all that.

The software is configured to cause a "real time" interrupt every quarter of a second. That interrupt routine does some futzing with a semaphore to see if it is already echoed, and if not, gets the received character, adds one, and sends that character.

So every quarter of a second, the software sends a character and receives it at the same time, adds one to it, and stores it in RAM for the next interrupt.

My first attempt was to merely echo the character as soon as it is received, but that emits 960 characters per second, and TeraTerm can't keep up. I could not tell if the mess on the screen that resulted was because TeraTerm is only echoing every umpteenth character, or because the software wasn't working. It was impressively fast, going through the whole 256 byte "character set" almost 4 times per second, with many beeps and cursor movements thrown in.

With the quarter second interrupt, things are much more civilised, and I was able to see the whole upper and lower case alphabet come out in order, then it goes quiet at around the 0x80 character (TeraTerm is written by a Japanese, and although I've set the language to English several times, it doesn't seem to save (or believe?) the change, and it goes into this mode where characters over 0x7F are treated as some sort of unicode character (I think). I also saw cursor movements, perhaps as a result of an escape sequence.

So basically, it's working fine. This was running the software at 4 MHz; we calculate that there is just barely enough time for one received and one transmit interrupt (let alone a timer overflow interrupt) in the 104 us of one communications bit period.

Oh, as I write this, I've just thought of a nasty flaw in our timing arguments. I won't bother the forum with it here (and embarrass myself at the same time).

[weber]

Here is a series of photos showing the welding of the aircon/power-steering/alternator/water-pump pulley off the original ICE to a taperlock hub that will be used to mount it to the non-drive-end shaft of the induction motor.

[weber]

The newly fitted rollbar and a test fit of the battery box that will be mounted to it.

I'll let Coulomb explain.

[coulomb]

I'll let Coulomb explain.

Wha?   

Well, these are the highest cells, formerly known as the neck breakers. But now Weber's gone and bought this fancy roll-bar thing, so we have to find another name for them.

I think a roll-bar is a great thing to have in a performance oriented vehicle with a soft roof and eight battery cages to throw around in a roll or collision.

There was going to be 2 rows of 18+14 cells in that area, but now with the space taken up by the roll-bar, it will be one row of 19. We'll make up for it by adding another row of 14 into the under-the-bonnet pack. We've already cut it up ready to add the new row. (It's easier to add a row at the moddle of a box than at the end, since the ends have verticals that the middles don't).

The roll-bar battery box (ah, the new name is obvious) will have two pieces of 50x50x3 angle iron at the front to hook onto the racing harness attachment bolts. The rear will be attached on each side to one of the solid roll-bar attachment bolts. The extra verticals to attach to the 50x50 angle will give the unusually long and "thin" (single row) box more stiffness with two extra verticals.

However, these extra verticals need a bolt and captive nut in the lid. We don't have other boxes that need a captive nut in the middle of a row. It will be an 8mm nut; some boxes have 10 mm. Even so, the nut will have to share space with the BMUs and battery terminals. The nut will be carefully insulated behind some construction of 1 mm polycarbonate yet to be completely designed. It will be a challenge getting it all to fit and be safe.

I've already suggested a nylon nut. They don't weld well , and besides the bolt will probably protrude enough to be a problem. It could also be nylon, but then the strength is lost.

[coulomb]

I should add that the "newly fitted" roll-bar isn't really fitted properly yet. Certain parts are supposed to be sycaflex-ed to hold them temporarily in a place that can only be accessed by taking out the seat belt reels. Alas, these pieces keep falling off. Newton tells us that boats are made out of this stuff, it's really good, but either we got a bad tube, or it's not meant to adhere to the shiny galvanised or zinc-flashed (whatever it actually is) parts from the roll-bar kit.

Besides, we find it useful to be able to take the roll-bar in and out while fitting this battery box. Eventually, we'll get the stuff to stick, or it goes in for the last time and doesn't come out again for a long time.

Maybe there are many products with the sycaflex name, and we got the wrong one for this application. I see with a quick search to check the spelling that there are heat activated versions, and probably many more.

[Jeff Owen]

Weld the extra uprights to the lid and bolt them to external tabs on the bottom angles.
You could also use short countersunk bolts from the inside of the lid, bottom angles or both. Weld the bolts in place with clearance on the cells. If you weld one end of the uprights, you may need to drill the hole in the other ends slightly oversize to get them over the captive bolts. If you bolt both ends, the holes can be a closer fit.

Sikaflex will hold the roll-bar mounts in place. Try degreasing with a solvent such as acetone. You may need to use a primer on the metal.

From some old Technical Data Sheets, Sikaflex-291 should be suitable. Sika Primer 210T looks like the one if you need a primer, but I don't think this is required.    

[rhills]

I use 291 on my boat and it's very sticky stuff. I agree with Jeff, you probably need to degrease the metal a bit. At least some of the sheen you mention is likely due to machine oil that very little of anything would stick to.

[weber]

To help the MX-5 cope with the extra weight, one of the things we're doing is upgrading the brakes. We're replacing the NA1.6's four-wheel 230 mm disks with the 250 mm disks and matching calipers from an NA1.8.

When we went to put the new disks on the rear, we found the existing splash covers were not quite big enough to fit the 250 mm discs inside them. Not surprising really. But what was surprising was that to get them off we'd need to remove the suspension uprights and hubs and put them in a press and press the wheel bearings out. This seemed a little extreme for a splash cover. This series of photos shows how we expanded the existing covers in stages, to make them clear the disks.



The first stage was to force the cover to admit the disk by levering it out around the disk with a screwdriver as the wheel nuts were tightened to pull the disk in. Then clamps and scraps of plywood were used to pull the dust cover hard against the disk and use the cover's own curvature to help expand it. Unfortunately this also permanently bent the cover too far past the disk, so a piece of plywood with a circular cutout was used to bend the cover back into its normal position by tightening the wheel nuts.

Then some 1 mm galvanised steel bracing strap was wrapped around the disk and inserted inside the cover and the wheel nuts were tightened to pull the disk in and expand the cover further. Then the nuts were loosened off and a second piece of 1 mm strap was hammered in between the disk and the first strap and the nuts tightened again to expand the cover even further.

Because the cover can only expand its circle by widening the opening where the caliper fits, the result is that the part farthest from that opening must bend inward to move away from the disk, and the parts near the opening must bend outward, overlapping the disk more than they did originally. You can see this in the last photo.

The disk still scrapes lightly at a few points, so I will try another day, to expand the cover further by inserting a third piece of 1 mm strap.

[Electrocycle]

the usual method is to cut the lip off the edge

[weber]

the usual method is to cut the lip off the edge

What with? I thought about that, but couldn't figure a good way to do it except oxy or plasma, which I don't have. And I figured it wouldn't be so good at keeping the water out. The 3rd layer of strap did the trick. No scraping now.

[Electrocycle]

usually careful use of an angle grinder and / or tin snips.

[weber]

Newton made a visit to Tritium and Ultramotive on the weekend. They are right next door to each other now. He took these lovely photos of some long lost or long awaited components of the MX-5.

The first photo shows our ABB 3GAA131008-HSE motor (blue) coupled to Tritium's dynamometer, which uses one of Ultramotive's motors (black).



The second photo shows the beautiful new flywheel with integral taperlock hub that Ultramotive have made to our design.

[woody]

Is that wimpy cable driving your motor? Or is that just monitoring...

[EV2Go]

Was wondering that myself woody. Not knowing how an electric dyno works I wasn't sure if the black motor was turning the blue motor and they got a reading that way somehow...

[weber]

Hee hee. No. That's temperature monitoring. Power isn't connected yet, as there are still problems with mounting the shaft encoder. The encoder may be hard to make out in the photo as its body is the same diameter as the motor shaft.

[Tritium_James]

The dyno works by running the two motors against each other. One is driving, and one is regenning. There is a motor controller for each motor, and they have their DC links connected together, along with a battery pack and a power supply to top up any losses.

This way we can circulate large powers around the dyno (~160kVA), only draw a few kW from the mains, and not need massive load banks and battery capacity.

[coulomb]

Not knowing how an electric dyno works I wasn't sure if the black motor was turning the blue motor and they got a reading that way somehow...

They may get some motor constants from reading the back EMF from the motor spinning, I don't know.

I assume that they will have two Tritium Wavesculptor200s with their DC buses paralleled to the same battery pack. So the carbon motor is configured as a generator by requesting negative torque, and regenerates most of the power from the motor back to the pack. That way, they only have to have their battery pack supply the losses, rather than all the motor's power and losses, and you don't have 100 kW of heat to get rid of somewhere.

Maybe one day they'll dispense with the battery pack and use 3 Wavesculptors, one configured to work bidirectionally to/from the 3-phase mains. So then the three controllers are connected to 3 3-phase machines: the motor under test, the dynamometer motor/generator, and a rather large generator in Gladstone connected by a complex set of transformers, transmission lines, and switchgear Then the final power-down regenerates the momentum of the two local machines back to the mains.

All that lovely equipment.

Edit: added comment re final regeneration to mains.

[EV2Go]

Ok so is a perpetual motion machine (for those who don't understand the concept of friction).

The batteries power the motor, the motor turns the generator, the generator returns power back to the batteries, and the batteries supply the difference... brilliant.

Only two questions how do you determine the load results:
Do you somehow increase the field strength on the generator making it require more input power to turn?
Do you increase whatever it uses to creates the load to the point of stalling the drive motor (guessing not or it might burn out)?
I am visual person so not very good at electrical things. Just trying to work out how you get a max power reading.

[woody]

Friction + Resistance (Electrical Friction?

From a high level point of view, you ask the controller to regeneratively brake as hard as it can, and ask it how many kW of braking it is doing.

From an electrical point of view, you feed AC into the regenerating motor at a frequency close to, but lower than one which matches the speed of the rotor. This means the current induced in the rotor causes a magnetic force against the rotation of the rotor, and induces a reactive current in the stator which delivers power back to the controller.

[coulomb]

Ok so is a perpetual motion machine (for those who don't understand the concept of friction).

Heh, I thought of that while writing, but decided not to mention it.

Only two questions how do you determine the load results:
Do you somehow increase the field strength on the generator making it require more input power to turn?

I guess so. You just request more (in absolute terms) negative torque, and the controller (on the generator/dyno) sorts out what to do with slip, phase angle, voltage, and from those last two you set field strength. If it was a DC generator, then yes, you would just increase field strength, which increases the volts per RPM, which increases the current charging the battery pack.

Do you increase whatever it uses to creates the load to the point of stalling the drive motor (guessing not or it might burn out)?

My understanding is that exceeding breakdown torque on an induction motor does not harm it, except that you are way beyond the continuous power dissipating ability of the motor, so you can't do it for very long. It's important to keep an eye on the revs, since the motor's torque will go from flat out to nothing very quickly, so the generator could suddenly drive the motors in reverse to maintain the requested negative torque.

And yes, I think that TJ will probably want to push the motor to breakdown torque, so he knows where it is, and can program the controller to only go to say 85% or 90% of that value. I suppose the specifications of the motor tell us where that point should be theoretically, but it's nice to actually measure it. Also, I'd love to know what the efficiency is like near breakdown; it may be that we don't want to let it get to more than 80% or even 70% of breakdown, if the efficiency gets really bad.

Just trying to work out how you get a max power reading.

Ideally, you would measure torque mechanically, but that's a hassle at this power level. However, the carbon motors loafing along at a mere 100 kW    are very efficient, around 97%, so taking 103% of the electrical power regenerated by the carbon motor will be within a percent or so of the mechanical power of our motor. [Edit: as pointed out below, measuring torque is not so hard, and is actually how it is done.]
Electrical power is easy to calculate, voltage times current times power factor times the square root of 3. (OK, it sounds harder, but it's really easy.) The controller makes this calculation routinely, I believe, or at least it's no trouble at all to calculate it.

Actually, the efficiency of the carbon motors has been mapped well, so at each power point, you can look up the efficiency and come up with an accurate figure.

I assume that the efficiency map was figured out from two identical carbon motors back to back; one motoring and the other, as here, the dyno. The efficiency of the two machines at the same RPM and torque will be very close to identical, so you can take the difference of the two electrical power levels (into the motor and from the generator) and halve that figure.

Edit: 95% -> 97% for the Carbon motor. Possibly 98%.

[EV2Go]

From a high level point of view, you ask the controller to regeneratively brake as hard as it can, and ask it how many kW of braking it is doing.

Of course that SHOULD have been obvious.

From an electrical point of view, you feed AC into the regenerating motor at a frequency close to, but lower than one which matches the speed of the rotor. This means the current induced in the rotor causes a magnetic force against the rotation of the rotor, and induces a reactive current in the stator which delivers power back to the controller.

So you just set the shaft speed for a given RPM point,load it up electrically until it wants to alter the speed and take a reading.

Anyway I didn't mean to sidetrack the thread, so back to the very interesting build (pity I don't understand half of what is said though).

[coulomb]

... you ask the controller to regeneratively brake as hard as it can, and ask it how many kW of braking it is doing.

As hard as it can? I think that carbon motor would reverse spin our "little" motor till the rotor shattered! [Edit: not so, as Weber points out a few posts later. If there was a gearbox inbetween, then it could at least stop the motor dead.] Perhaps you mean as hard as it can with a lower limit of say 80% of the motor's initial RPM.

My picture of the process is that you start with zero regen torque, spin the motor up to the test speed, then set a low regen torque, and slowly increase the regen torque in steps, taking power measurements after each step. The motor controller would be set for a constant RPM, the dyno controller for constant torque.

[Edit: as TJ points out below, you can also do it the other way, regen machine at constant speed, and motor machine at constant current (= almost constant torque). It seems they do it that way.]

You might then repeat this test sequence at a different motor RPM, for say 3 RPMs and maybe 5-10 power levels.

Maybe you could set it up to do this automatically and generate a family of graphs with power and efficiency verses torque, but if I were doing that, I'd have my hand hovering above the big red kill switch    Automatic control of machines at this power level... it just sends shivers up my back. Once it was fully debugged, I suppose, but how long would it take to get there? Still, that's what circuit breakers are for, I suppose.

[Tritium_James]

The blue motor is mounted to a plate which then mounts to the dyno through a slewing bearing, and the plate is prevented from rotating by a loadcell. So we know torque. We also know motor speed exactly (both motors have position sensors) so power is simply torque x speed.

As far as the mechanics of the test go, the motor running in regen tries to maintain constant speed, and the motor running in drive runs at constant current (proportional to torque). We then step through various currents on the drive motor, and speeds on the load motor, logging currents, temps, actual speed, etc in both motors as we go.

[weber]

Ideally, you would measure torque mechanically, but that's a hassle at this power level. ...

If you look at the photo above and follow the skinny black cable at the bottom you'll see it connects to a silvery box that connects to a radial arm from the silvery flange that the blue motor flange is bolted to. The silvery box is most likely a load-cell, measuring force. And Torque(Nm) = Force(N) * Radius(m). And the RPM can be measured from the shaft-encoder on the non-drive end of the motor shaft. And Power = Torque(Nm) * Angular_velocity(rad/s), where Angular_velocity(rad/s) = RPM(rev/min) * 2pi(rad/rev) / 60(s/min).

[EV2Go]

The blue motor is mounted to a plate which then mounts to the dyno through a slewing bearing, and the plate is prevented from rotating by a loadcell. So we know torque. We also know motor speed exactly (both motors have position sensors) so power is simply torque x speed.

As far as the mechanics of the test go, the motor running in regen tries to maintain constant speed, and the motor running in drive runs at constant current (proportional to torque). We then step through various currents on the drive motor, and speeds on the load motor, logging currents, temps, actual speed, etc in both motors as we go.

Ok so the torque is messured mechanically... From the description of the mechanical portion of the test, it sounds like the motor has the minimum mechanical stress applied. If I have understood this correctly it sounds like you use just enough resistive force to achieve the necessary RPM at various loads?