Weber and Coulomb's MX-5

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Post by Johny » Mon, 09 Dec 2013, 18:29

Just get the tow truck driver to lift the front wheels (for an iMiev/MX5 or Vogue) and drag it around for a few kM under full regen. The only problem is the legality of sitting in the driver's seat while they do it.
Come to think of it, a flat tow is legal so just have them carry a tow rope - or call a mate with a Tonka toy (4WD).

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Post by offgridQLD » Mon, 09 Dec 2013, 18:34

Is it still legal to tow a car with a rope in QLD or any other part of the country?

I can just picture it now...Call wife and have her arrive in our 3lt TDI 4wd. Attach rope to front of Imiev. Give wife quick rundown. Watch wife ignore rundown instructions and drive off with front 1/2 of Imiev dragging behind 4wd. Image

You really do need two people who know what they are doing to make it work and even then the practice is a little dangerous.

Kurt
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Post by Johny » Mon, 09 Dec 2013, 18:49

....yeah, perhaps a flatbed tow would be cheaper.
Then again - most of the issues with flat towing are to do with keeping a little tension on the rope. That is certainly made easier with regen.
I'll have to try it sometime with the Super Snipe and Vogue - hopefully not for any need at the time. Now...how will I drive both?

Edit: Don't respond - we've hijacked this thread again!
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Post by Adverse Effects » Tue, 10 Dec 2013, 01:59

i thought stiff bar towing was legal in QLD but rope / cable was illegal

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Post by weber » Thu, 12 Dec 2013, 06:40

I can't help feeling the MX-5's instrument cluster has a certain classical beauty.

Image

So I'm very pleased that Tritium's Driver Controls unit, with it's open-source software, has allowed us to make use of all these gauges, sometimes multiple uses, by modifying that software. I finally got them all working how I want.

The speedo is cable driven. We changed the diff ratio and so, with a good deal of trouble on the part of Jeff Owen some months back, inserted a small gearbox in the cable to compensate. But when compared with GPS it reads 10% too high now. Sigh. Perhaps it did this before the conversion too. I'll have to get a different little gearbox for the cable.

As mentioned in earlier posts, the oil gauge now displays cell stress, in levels*2. So 4 on the gauge is where the system controller starts limiting the power, and 5.5 is where the alarm comes on. These correspond to
stress     undervolt overvolt undertemp overtemp
------------------------------------------------
zero        3.25 V    3.35 V     11°C     40°C
limiting    2.61 V    3.50 V      3°C     48°C
alarm       2.37 V    3.57 V      0°C     51°C
What the stress gauge shows is the worst of these properties on any cell. Undertemp stress is only recognised in charge mode.

The temperature gauge shows whichever is the worst out of motor temperature and controller temperature, where "C" is 40°C for both of them, but "H" is 160°C for the motor and 80°C for the controller. So mid readings are 100°C and 60°C respectively.

The fuel gauge will eventually be based on an amp-hour integrator, when we get the shunt monitoring circuits built. But for now it is based on average cell voltage. "E" is 3.25 V, "F" is 3.35 V and the middle is 3.30 V.

Thanks to a paper Johny directed me to I have good reason to believe that this will never overestimate the state of charge (except maybe after a long downhill run with regen) and will be correct at 40%, 80% and 100% but only if the battery has been rested (not charged or discharged) for at least half an hour. And there should still be at least 15% left in reserve, below "E".

The extremely slow response of the fuel gauge is an advantage, but it is still likely to seriously underestimate the state of charge when actualy driving, and for up to half an hour afterwards. In this temporary form, its greatest use will be for checking before you start a trip.

That leaves the tacho. This is the "everything-else" gauge. Yes it defaults to being an rpm*1000 gauge (a tacho) in drive mode. But in charge mode it shows DC charge current in amps. We have two half-packs with separate chargers and so it flips between pack-A and pack-B currents on a 3 second cycle. It shows pack-A current for 2 seconds and pack-B current for one second (so you can tell which is which). Coulomb's excellent idea. A and B are the same until you get near the end of charge.

This also gives feedback for the rather non-intuitive way that we change the maximum charge current when in charge mode. We turn the ignition key momentarily to the start position to move to the next setting in a cycle of 3, which are designed for charging from one 10A GPO, one 15A GPO or two 10A GPOs on different circuit breakers. 10, 15 or 20A total. We carry two 20 metre 15 A extension leads and if we can only get one GPO we use a double adapter at the chargers.

Provided there's no charging going on, turning the key to the start position momentarily will put us in drive mode. But once we're in drive mode, turning it to the start position again will cycle the tacho through displaying the following four quantities:

1. tacho in rpm*1000
2. power in kW*20
3. torque-producing component of motor current in amps*100
4. limiter number

The limiter number indicates which control loop is limiting the output of the motor controller at any instant. 0 = PWM (motor voltage), 1 = torque, 2 = rpm, 3 = battery current, 4 = battery undervolt, 5 = battery overvolt, 6 = temperature. I'll just remind you that we have an AC induction motor.

You can tell from the tacho gauge's behavior, which display mode you're in.

I'm so pleased to have these all working and calibrated at last.
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Post by coulomb » Thu, 12 Dec 2013, 13:56

weber wrote: I'm so pleased to have these all working and calibrated at last.

Yes, instrumentation is one of those things to get going "when everything else is working". And we're nearly there: air conditioning, tyres, and the high voltage controller are the main things left.

I had the instrument cluster out last EV day, trying to replace two of the indicator bulbs (for brake and charge) with LED equivalents. Alas, the LEDs were just a bit wider than the incandescents (to fit in a through-hole resistor and diode), and these ones would just not fit. I started filing one of the LEDs, but ran out of patience.

I was struck by the simplicity and robustness of the instrument cluster. That thing is probably 239,176 km old, and all the bulbs are incandescent. I'd say that they severely undervoltage the bulbs, for longer life. Indeed, they glow orange, that's how "cold" they run. That eerie green glow from the instruments at night is provided by a single bulb per instrument, with the bulb painted green. Somehow the red stripes on the tacho defy physics and glow bright red despite that paint.

We did have trouble with the tacho needle, which somehow ended up past the 8, and for several days when we powered it up , it would just strain against the zero stop, from the wrong side. Weber tried all sorts of things to get it to come good, to no avail. We were resigned to taking the instrument cluster out and taking the tacho apart, quite a job. We even speculated that this might be part of some sort of warranty voiding thing; if the tacho was found in that state, they knew you'd over-revved the internal combustion engine, so the engine warranty would be void. But before we got to taking apart the tacho, it just fixed itself with no effort on our part.

Ob. Hitchhikers Guide to the Galaxy quote: "How did you fix it?" "We just stopped fiddling with the controls."
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Post by offgridQLD » Thu, 12 Dec 2013, 14:17

There is something nice about keeping things clean and simple. Analog gauges do have a certain charm. Some of the new cars have digital dash displays that are overwhelming, Just to much going on. Green back lighting tends to be the most relaxing to look at.

Most cars tend to have a optimistic speedo readout from the factory. My Imiev is about 3% high and my 4wd is 9% high when compared to GPS.

I wasn't to sure about the stress gauge idea at first but looking at more it's growing on me. Basically bundling a few key readings into the one indicator.

I'm thinking while driving. It would be nice to know if the stress was coming from say a overheated motor or over a over temp battery. Is there anyway to define this but keep it simple? Perhaps voice alarm based on whats triggering the stress that would keep the stock dash looking clean.

Edit: Regarding the warranty avoidance indicator being built into the gauge. Every car that I have come across fitted with EFI & ignition has a rev limiter function as part of the ECU. So over revving the engine isn't possible (not that bouncing off the rev limiter is healthy practice but catastrophic warranty avoiding over revving is avoided)

Kurt
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Post by weber » Thu, 12 Dec 2013, 16:01

offgridQLD wrote:I wasn't to sure about the stress gauge idea at first but looking at more it's growing on me. Basically bundling a few key readings into the one indicator.

I'm thinking while driving. It would be nice to know if the stress was coming from say a overheated motor or over a over temp battery. Is there anyway to define this but keep it simple? Perhaps voice alarm based on whats triggering the stress that would keep the stock dash looking clean.
The repurposed oil pressure gauge is purely battery stress, or more specifically cell stress, as it is the stress level of whichever cell in the pack happens to be most stressed at any instant. You can easily tell the difference between voltage stress and temperature stress as voltage stress is instantly responsive to changes the load or charge the cells are getting, while temperature stress only changes very slowly. And you can tell the difference between undervoltage and overvoltage stress by the seat of your pants. If you're decelerating or regen-braking down a hill, it's overvoltage.

Motor temperature is displayed on the temperature gauge (aggregated with controller temperature). There's no easy way to tell the difference between motor and controller temp. We have to plug the netbook into the ethernet socket under the glove-box to see that. But in our experience so far the reading has always been due to motor temperature, which only reached 123^C (from data logging) thrashing it up the steep side of Mount Coot-tha. No motor fan. But I'm keen to learn how it goes from Landsborough up to Maleny.

Voice feedback would be a great idea when selecting which quantity is displayed on the tacho.
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Post by Renard » Thu, 12 Dec 2013, 19:29

weber wrote:
Motor temperature is displayed on the temperature gauge (aggregated with controller temperature). There's no easy way to tell the difference between motor and controller temp. We have to plug the netbook into the ethernet socket under the glove-box to see that. But in our experience so far the reading has always been due to motor temperature, which only reached 123^C (from data logging) thrashing it up the steep side of Mount Coot-tha. No motor fan. But I'm keen to learn how it goes from Landsborough up to Maleny.


This agrees with my experience, that controller temperature is so unlikely to be an issue, that it need not be reported. After all, the controller will still shut down if it gets too hot.

My own preference is against multimode instruments; they can be confusing to interpret at moments of sudden stress, even though they seem like a good idea at the design stage.

Landsborough/Maleny will be an interesting test.
Renard

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Post by offgridQLD » Thu, 12 Dec 2013, 20:10

weber wrote: But I'm keen to learn how it goes from Landsborough up to Maleny.


If you ever need some backup on call when testing in that neck of the woods perhaps a charge point or heaven forbid 4wd and car trailer let me know.

I don't find that run up the hill to bad. I can hold my hand on the motor of the Imiev shortly after climbing the that hill. It's warm , like hot cup of coffee warm (though it's liquid cooled) It takes about 30kw of power to hold 70kph up that 5km hill. 1100kg Imiev with about 225kg of cargo/passengers. I pull 2C from my battery for about 5 min doing it.

There is a live unlocked 15A outlet on the side of a stainless steel box in the middle of the lands borough train station car park. I don't have permission to use it but its good to know its there.

I have felt the motor just as hot if not a little hotter than the hill climb after 1hr of driving on the freeway at 100-110kph.

Kurt
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Post by Jeff Owen » Thu, 12 Dec 2013, 22:56

offgridQLD wrote: There is a live unlocked 15A outlet on the side of a stainless steel box in the middle of the lands borough train station car park. I don't have permission to use it but its good to know its there.
Kurt

I think you will find that it is a 10A outlet.

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Post by offgridQLD » Thu, 12 Dec 2013, 23:07

Could be 10A come to think of it I just assumed that 99% of the industrial style outlets they install for maintenance they usually go for the 15A for versatility.

I did plug a meter into it once quickly a few weeks back (bored waiting at the train station) so I know its live.

I can usually get to landsbourough on 1/2 a tank/battery Image so haven't needed it yet.

Kurt
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Post by weber » Mon, 16 Dec 2013, 04:43

Coulomb and I have had a few jam sessions lately and have composed the following circuit for monitoring the battery current using a shunt (for the purpose of integrating amp-hours for the fuel gauge) and monitoring the insulation resistance to ensure that the battery is still floating relative to the chassis. We call it an "IMU" where the "I" stands for both Insulation and current (via its conventional quantity symbol "I").

It is a modification of one of our celltop Battery Monitoring Units and so communicates via low cost plastic optic fibre which automatically provides high voltage isolation.

Image

The isolation testing will be done by turning on each reed relay in turn, with its 100k 5W 500V resistor, and checking, via the optocoupler, whether more than 5 mA flows through the 150 ohm resistor to the chassis.

Any suggestions or questions will be gratefully received.
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Post by 7circle » Mon, 16 Dec 2013, 11:16

Great work Photonic brothers.

My 2cents...
I'm concerned a wiring or relay bad connection could stop faults being detected.

So it could have a self test mode of continuity from HAZ+ to HAZ- through the two relays.

Have you considered the HAZ- using another sense 150R and opto to Chassis?

With both relays closed then powering both optos to turn on, confirming both test points have no bad connections.
Then when one relay is closed you have a higher reliability that the system can detect a fault.

As there are no spare pins, the 2 optos could make 4 level analogue input as simple D to A.

I would go a bit flashy and add LEDs to opto outputs and relay coils.

Perhaps even manual switches across the relay FETs.
(...Maybe magnets on the push buttons to allow operation inside sealed junction box.)

But as you have it in the IMU V1 circuit, should leakV be pulled high to Vdd 5V then into U1 with Vcc at 2.5V?
And why 470R pull down, that's excessive?

If 12V supply is lost how long do you want shunt integrator to keep active.
Can the isolation DC/DC charge up a bigger cap than the 1uF, like a diode that feeds a cap to the 5V/2V5 dc/dc.
A 10 second buffer could be good, just for the uCRO and shunt amplifier power.
(ICE cars make me think of the nuisance glithes on 12V bat, your EV may be sweet, but others may not have stable 12V. )

Is it worth protecting the 12V input with fuse and reverse polarity?

Is this Insolation detector for 350V or 700V battery pack?

Considering the HAZx sense 100k resistor will need 500V for 5mA.
Perhaps 10k's would be better needing 50V to activate a fault alarm.
The 150R with 5mA is 750mV. Is that enough?
What's the curve for the infra-red opto LED?

Are you using this opto for its analogue transition?

Data sheet time. Is the part number VDL628A?
Not coming up on google. ...
vishay sfh628.pdf
Looks it. ..
LED Vf 0.95 V for 0.1mA.
LED Vf 1.125 V for about 5mA
But it can maintain much more 10mA.
So 150R looks low to me for 5mA trigger level.
And get the 470R current up to trigger the digital input.

Guess it depends on what is your Min and Max HAZ V you want.

Oh... are faults likely to be intermittent?
Do you want to capture if it early?
What a can of worms I always open.

Just like a photon seems to consider all paths to it's target, you guys have already considered all options and shown the resolved design.   heehee.

Typing on phone so finish off my 2cents (more like 7cents) at that.
7c

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Post by CometBoy » Mon, 16 Dec 2013, 14:10

Some nice ideas in that design and I’m glad to see you are looking at leakage currents - should be on all conversions.

When I look at this in 2008, I was going to make up a simple circuit design similar to the idea that Ian Hooper discussed with acmotor and others back in 2008. I then read about the EV leakage standards in the US (less 0.5 MIU’s) and never decided on the leakage current I should trigger at – I see you use a 150 ohm resistor so I guess you are going for something around 10 to 15 ma? The other problem was what to do with the fault condition once I detected it? In the end I decided it could only be a warning indicator and not a level 1 fault condition. So my result was just leaving 2 test points in a convenient location that I could measure any leakage once a year. I actually could start to see some leakage after a couple of years – easily fixed by cleaning out the area around the brushes (obviously only an issue for DC motor conversions). Never did decide on a final value and only looked for a change.

These are some of the things I found when doing a google on it:

-     7.2 ELECTRICAL ISOLATION
Propulsion power shall be isolated from the vehicle chassis such that leakage current does not exceed 0.5 MIU. Charging circuits shall be isolated from the vehicle chassis such that ground current from the grounded chassis does not exceed 5 mA at any time the vehicle is connected to an off-board power supply. Supplier should provide details on grounding and isolation methods.

-     5.7 WATER DURABILITY
Vehicles should be able to drive through two (2) inches of standing water at a speed of
20 mph without damage, without becoming inoperable, and without battery to chassis leakage current exceeding 0.5 MIU per UL Standard 2202. Vehicles should be capable of setting in eight (8) inches of standing water for 15 minutes without damage, without becoming inoperable, and without battery to chassis leakage current exceeding 0.5 MIU per UL Standard 2202.

-     For propulsion power systems with voltages greater than or equal to 48VDC, the system shall be isolated from the vehicle chassis such that leakage current does not exceed 0.5 MIU.

Acmotor is most likely familiar with MIU’s and UL Standards but I never understood why they seemed to be adopted as the standard?

All academic anyway as we have no standards on electrical work when getting our DIY projects registered here anyway. But that’s another issue.

Bruce

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Post by weber » Mon, 16 Dec 2013, 22:56

Thanks 7Circle! Everything you said was spot on. If I haven't addressed something in the following schematic please remind us again. And of course if you think of anything new ....

Thanks Comet Boy for sending me back to the regulations and the data on human effects of DC current. NCOP14's 20 mA seems to be around the maximum let-go current for DC. I note that neither NCOP14, nor our circuit, deal with motor insulation failure and the AC voltages that might then appear on battery terminals. Ours is intended for testing the unloaded battery only. And yes, it will definitely only be used to raise a warning alarm.

Zoom to see more detail. e.g. In Firefox, Ctrl and plus-sign or two-finger spread on trackpad or touchscreen.

Image

[Edit: Since rev 2 schematic: Changed "Leak" to "Touch" and added fuse and reverse protection on +12V.]
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Post by weber » Mon, 16 Dec 2013, 23:53

7circle wrote:I'm concerned a wiring or relay bad connection could stop faults being detected.

So it could have a self test mode of continuity from HAZ+ to HAZ- through the two relays.

Have you considered the HAZ- using another sense 150R and opto to Chassis?

With both relays closed then powering both optos to turn on, confirming both test points have no bad connections.
Great idea! Done, assuming we can fit the extra opto.
As there are no spare pins, the 2 optos could make 4 level analogue input as simple D to A.
Instead I freed up an analog input by losing the dedicated LED drive output.
I would go a bit flashy and add LEDs to opto outputs
Not really appropriate since we are treating these as analog signals.
and relay coils.
Done.
Perhaps even manual switches across the relay FETs.
(...Maybe magnets on the push buttons to allow operation inside sealed junction box.)
The reed relays themselves can be operated directly by magnet if their clip-on shields are removed.
But as you have it in the IMU V1 circuit, should leakV be pulled high to Vdd 5V then into U1 with Vcc at 2.5V?
A serious boo boo. Fixed. Thanks.
And why 470R pull down, that's excessive?
That should have been marked "Select on test". But now that I've calculated everything properly, and with your self-test mode, I think we can fix all resistors and have the micro calibrate out the Opto CTRs, since we have chosen optos with CTRs selected to have only a 2:1 range (63% to 125%).
If 12V supply is lost how long do you want shunt integrator to keep active.
Can the isolation DC/DC charge up a bigger cap than the 1uF, like a diode that feeds a cap to the 5V/2V5 dc/dc.
A 10 second buffer could be good, just for the uCRO and shunt amplifier power.
(ICE cars make me think of the nuisance glithes on 12V bat, your EV may be sweet, but others may not have stable 12V. )
Good idea. I put back the 1 farad supercap on the 2.5 V supply that we have on all our BMUs (CMUs) anyway. Gives about 10 minutes!
Is it worth protecting the 12V input with fuse and reverse polarity?
Coulomb suggested that too. Will add if we have the space. We're trying to fit this whole circuit in almost the only space available, the space immediately above the shunt.
Is this Insolation detector for 350V or 700V battery pack?
360 V, as I should have shown on the schematic earlier. We will have two of these devices. One for each half-pack.
Considering the HAZx sense 100k resistor will need 500V for 5mA.
Perhaps 10k's would be better needing 50V to activate a fault alarm.
The 150R with 5mA is 750mV. Is that enough?
What's the curve for the infra-red opto LED?
We've got rid of the 150R now, so all the test current goes thru the opto LED (design max 20 mA). So its IV curve becomes irrelevant and only its CTR matters.
Are you using this opto for its analogue transition?
We are using it in an analog manner, but we chose this opto for its selected CTR range (63% to 125%). They don't need to be AC input now. But what the heck.
Data sheet time. Is the part number VDL628A?
VOL628A and now more specifically VOL628A-2.
But it can maintain much more 10mA.
So 150R looks low to me for 5mA trigger level.
And get the 470R current up to trigger the digital input.

Guess it depends on what is your Min and Max HAZ V you want.
I've designed it now so we can measure test current with a full scale value of 20 mA and the the alarm threshold can be set in software.
Oh... are faults likely to be intermittent?
Do you want to capture if it early?
What a can of worms I always open.
We thought to only test when going to drive mode. Could also test when leaving it. Too much noise when the motor's going. Not quite sure what you're getting at here. "Capture"? "Early"?
Just like a photon seems to consider all paths to it's target, you guys have already considered all options and shown the resolved design.   heehee.
That's very kind of you, but clearly we had not. Image
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Post by weber » Tue, 17 Dec 2013, 04:06

CometBoy wrote: Some nice ideas in that design and I’m glad to see you are looking at leakage currents - should be on all conversions.
Thanks. Yes. But after re-reading this thread started by Renard in 2011
viewtopic.php?title=ncop-14-29-chassis-leakage&t=2619
and doing some research on terminology used elsewhere, I have decided it only confuses people to refer to this as "leakage" current.

NCOP14 specifies a maximum "leakage current" of 20 mA. But with a floating pack, a single insulation failure (which is what we are trying to detect) can exist without there being any current. So we are not trying to detect a leakage current. A leakage resistance maybe, but ...

What we really want to know is what current will flow through _a_human_being_ that touches both chassis and either of HazV- or HazV+. I finally found the accepted name for this, in standards other than NCOP14 -- "touch current".

Our reed relays essentially simulate a human-being touching each terminal in turn while also touching chassis. Except that a human may have a lower resistance than our probe resistors, because we must not make our probe resistors so low as to cause a hazard themselves, so we have to _calculate_ what the current through a human would be, based on our measurements. You see numbers as low as 500 ohms listed for humans in some cases. So we might as well assume a human is a short circuit, to simplify the calculations.

So I believe that the authors of NCOP14 intended the meaning of "touch current" when they wrote "leakage current". What else could they mean? Which is why I have renamed our various "Leak.." nets to "Touch.." on the schematic.
I see you use a 150 ohm resistor so I guess you are going for something around 10 to 15 ma?
Yes. In the first revision we intended to place the threshold at a touch current of 10 mA, which meant a measured current of 5 mA. But now I am using the NCOP14 threshold of 20 mA touch current which will correspond to a measured current around 10 mA.
So my result was just leaving 2 test points in a convenient location that I could measure any leakage once a year.
Fair enough. That's more than most people do.
I actually could start to see some leakage after a couple of years – easily fixed by cleaning out the area around the brushes (obviously only an issue for DC motor conversions).
Interesting.
Acmotor is most likely familiar with MIU’s and UL Standards but I never understood why they seemed to be adopted as the standard?
I hope he can enlighten us because all I've been able to find online so far is that the "MIU" is the most ridiculously-named unit imaginable. It stands for "Measurement Indication Unit". Aren't all units "measurement indication units"? In other words, its the "unit" unit, which tells you absolutely nothing about what kind of quantity it represents! Although I suppose you could contrast the "MIU" with the "FU", where the "F" stands for Fudged, Fantasised, Fabricated or that other "F" word. Image

One document claimed that the MIU corresponds to the milliamp at some low frequency, but may correspond to multiple milliamps at higher frequencies. Nothing about what happens with DC, but I expect that would be multiple milliamps per MIU too.

Another document http://www.wolfautomation.com/assets/15 ... manual.pdf indicated that there are various standard equivalent circuit networks to represent humans, each one corresponding to a different effect such as "reaction", "let-go", "burn hazard", but nothing about what these circuits look like. These networks are used in measuring instruments that indicate their units in Measurement Indication Units. I can't help feeling I'm in a Franz Kafka story when I re-read that last sentence. Image

[Edit: Spelling, grammar and punctuation. Added link to Touch Current Tester manual.]
Last edited by weber on Tue, 17 Dec 2013, 02:54, edited 1 time in total.
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Post by Renard » Tue, 17 Dec 2013, 14:18

weber wrote:   So we are not trying to detect a leakage current. A leakage resistance maybe, but ...

What we really want to know is what current will flow through _a_human_being_ that touches both chassis and either of HazV- or HazV+. I finally found the accepted name for this, in standards other than NCOP14 -- "touch current".


Thanks Weber, for your work here -- and elsewhere -- of nomenclatural policing. It's very helpful for us all to have clear terminology so we understand each other.

On another matter, could you explain for me why you use reed relays rather than just (high voltage) FETS alone?
Renard

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Post by CometBoy » Tue, 17 Dec 2013, 20:45

I think this might help on the wording and correct definitions?

http://www.pattesters.co.uk/blog/2011/0 ... confusion/

I’ve been involved with PAT (or Portable Appliance Testing) training courses in different parts of the world and this was always my take...

But our Australian Standards (AS/NZS 3760) don’t use that terminology and broadly only use Class 1 and Class 2 when appliance testing.

I think maybe incorrectly ‘Leakage’ has become a more general term covering several situations.

Bruce


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Post by weber » Tue, 17 Dec 2013, 23:47

Renard wrote:On another matter, could you explain for me why you use reed relays rather than just (high voltage) FETS alone?

Good question. At first I thought, "I just know you don't use FETs for this. Didn't even consider it. Received wisdom I suppose." But maybe FETs are better these days.

The reed relays are good for 2 kV isolation and have a creepage distance of 6 mm. So I went looking and found that yes indeed some Silicon Carbide MOSFETs are good for 1.7 kV and cost about the same as the reed relays, although the creepage is only 3 mm (TO247 case). But as soon as I started to figure out how to drive the one that's switching HazV+ I realised why I was still going to use reed relays.

With a reed relay the coil is completely floating with respect to the contacts, whereas the gate of a MOSFET must be driven relative to its source, which is shared with the high voltage side. And so you have all the usual complications of high-side drive in a half-bridge, but with the added requirement to be able to turn them both on at the same time and both off at the same time indefinitely, that you don't have with a standard PWM half-bridge. It could be done, but it would take a lot more parts.
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Post by Renard » Wed, 18 Dec 2013, 01:20

Thanks, Weber. I hadn't really thought through the gate drive issues.

After I install my new fan, the insulation monitor will be my next little project -- sometime!
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Post by weber » Wed, 18 Dec 2013, 01:41

CometBoy wrote:I think maybe incorrectly ‘Leakage’ has become a more general term covering several situations.

I think you're spot-on there. Thanks for that confirming link.
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Post by weber » Wed, 18 Dec 2013, 02:03

Renard wrote:After I install my new fan, the insulation monitor will be my next little project -- sometime!

If you don't need (or want) to combine it with monitoring of battery current and/or battery voltage as we are, then there is no reason why the control and monitoring circuitry needs to be referenced to HazV- as we have. It could be referenced to Chassis and avoid the need for the optocouplers and isolated DC-DC. The reed relays would provide all the isolation needed.

It's just that it's very difficult to transfer precise measurements of small analog voltages across an isolation barrier without having active powered circuitry on the same side of the barrier as the source of that analog voltage (the current shunt, in our case). So we end up having to transfer the unprocessed touch currents across the isolation barrier instead. But these don't need any great precision or linearity since they will merely be thresholded, while the current shunt measurement will be integrated to drive our fuel gauge. And since we put the current shunt at the negative end of the battery, we can conveniently measure the battery voltage as well as current from the same reference point (HazV-).
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Post by 4Springs » Wed, 18 Dec 2013, 03:08

FYI, I have been measuring touch current (although I haven't been calling it that) every 1000 km via the method described in UN ECE 100. All readings have been from 0.02mA to 0.05mA, except for once when I did the test during extremely wet weather (when I had been driving in the rain), when I recorded a current of 0.2mA.

The NCOP mentions that this test should be performed before working on the electrical system of the car. If I were going to buy or build a unit, I'd like to install it where I can see/hear a warning from under the bonnet. When working on HAZV I'm constantly looking up at my isolation switch, and I imagine I'd like to have the same reassurance from a device like this.

Watching carefully to see if I could attempt to build a touch current monitor...

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