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I am going up to the Evolocity Event at Ruapuna Race Circuit,

There is a sprint and drag races - the circuit looks a bit fast for me but I will do my best

http://evolocity.co.nz/emotorsport/

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6,370 Posts

I am going up to the Evolocity Event at Ruapuna Race Circuit,

There is a sprint and drag races - the circuit looks a bit fast for me but I will do my best

http://evolocity.co.nz/emotorsport/

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65 Posts

and there is a warning sign in places(warning smooth road ahead)

other wise the weather here on sunday is the odd shower and about 18c

so sorry cant do any thing better for the weather

my home ph is 039814888

i will have a name tag at the meet

Don't forget to take pics!

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65 Posts

coming down the straight cant see the opposition

whew got it to right size

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6,370 Posts

We did the event on Sunday and drove home today

I will have to look and see if Jane managed to take any usable pictures

Anyway the controller was great

But I have made a mistake!

I went down to 130v (40S) - BIG MISTAKE!

On the track it will only reach 100kph (59mph)

So I stonked out of the corners and then my tiger turned into a pussy cat and I poodled down the straights

Great fun but not fast

I did the sprints around the track and used up my charge so I missed the first drag races

When I turned up for the second series they put me next to a Ferrari! - lovely big red one

I complained but they laughed

This was the first time I had been on a proper side by side drag strip

The lights turned and I got away in front of it!

About 100 yards down the track it flew past me

But I was in front for the first bit!

He did a 14 seconds and 110mph

I did 18 seconds and 59mph - I think I had already stopped accelerating when he went past

When I came back for another go they put me next to the Tesla - Dam that thing was fast I just followed him all the way

Great fun - now I need to get my voltage back up,

Awww, you know how to calculate the rpm you will reach peak shaft power for a given peak torque, so why did you use 120V? Battery problems?

We did the event on Sunday and drove home today

I will have to look and see if Jane managed to take any usable pictures

Anyway the controller was great

But I have made a mistake!

I went down to 130v (40S) - BIG MISTAKE!

On the track it will only reach 100kph (59mph)

So I stonked out of the corners and then my tiger turned into a pussy cat and I poodled down the straights

Great fun but not fast

I did the sprints around the track and used up my charge so I missed the first drag races

When I turned up for the second series they put me next to a Ferrari! - lovely big red one

I complained but they laughed

This was the first time I had been on a proper side by side drag strip

The lights turned and I got away in front of it!

About 100 yards down the track it flew past me

But I was in front for the first bit!

He did a 14 seconds and 110mph

I did 18 seconds and 59mph - I think I had already stopped accelerating when he went past

When I came back for another go they put me next to the Tesla - Dam that thing was fast I just followed him all the way

Great fun - now I need to get my voltage back up,

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65 Posts

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6,370 Posts

Awww, you know how to calculate the rpm you will reach peak shaft power for a given peak torque, so why did you use 120V? Battery problems?

Hi Tom

I don't know - that's the problem

It's simple to calculate torque/rpm and electrical power

But calculating back EMF is more complex

My batteries should have been able to produce ~ 960amps (15C) at about 110v -

105 Kw - 140hp!

But at 59mph I was only drawing 200 amps

22Kw - 30Hp!

Because the back EMF was so high that even with the controller at 100% that was all the current that would flow

Now I know that at about 3500 motor rpm and 200 amps I must be producing a back EMF of about 100v!!!

It was not as obvious with the 500amp controller and 144v

I think I will do as you suggest and change to effectively 80S x 2P

I will need to sort out a charger and see what else I need to do first

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3,218 Posts

Back EMF is (roughly) proportional to current at any given RPM (or, conversely, roughly proportional to RPM at any given current).Hi Tom

I don't know - that's the problem

It's simple to calculate torque/rpm and electrical power

But calculating back EMF is more complex...

I use the qualifier "roughly" because BEMF is actually proportional to the square of current well below saturation, transitioning over to a linear function above saturation. For example, the WarP-9 enters into saturation around 180-200A and requires about 50V for every 1000 RPM at 1000A. A similar 9" 36/48V forklift motor with stock (neutral) brush timing would likely require 75V per 1000 RPM at 1000A (but would also put out more torque per amp - them's the tradeoffs of advancing the brush timing).

Also increases roughly linearly with rpm I expect (dB/dt)? So can extrapolate to other rpm and currents fairly well? Looking at series DC motor torque-speed graphs it does seem first order dependence on current is a good approximation at other than low rpm - which isn't of concern here. But then that would predict a bit over 50V ~ 260/5 at 200A, 3500 rpm.Back EMF is (roughly) proportional to current at any given RPM (or, conversely, roughly proportional to RPM at any given current).

I use the qualifier "roughly" because BEMF is actually proportional to the square of current well below saturation, transitioning over to a linear function above saturation. For example, the WarP-9 enters into saturation around 180-200A and requires about 50V for every 1000 RPM at 1000A. A similar 9" 36/48V forklift motor with stock (neutral) brush timing would likely require 75V per 1000 RPM at 1000A (but would also put out more torque per amp - them's the tradeoffs of advancing the brush timing).

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6,370 Posts

50v/1000 rpm at 1000 amps

That would give 10v/1000rpm at 200 amps

and 35v at 3500rpm

What I observed was that foot down (requesting 1000amps) the car accelerated (really well) and then stopped accelerating

I did half of the 1/4 mile at a constant speed - about 3500rpm and 200 amps

200 amps is about right for my unaerodynamic device

So...

My "theory" is that I was getting enough back EMF to limit my current

About 100v of back EMF at 200 amps and 3500rpm

I have 8 degrees of advance and an 11 inch forklift motor (Hitachi)

I should have more back EMF at a given rpm and current than a 9 inch motor - but is three times as much (35v to 100v) reasonable??

The controller has no speed input - just 12v power and throttle signal (0-5K pot)

I was not expecting this much back EMF - or am I doing something else wrong??

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3,218 Posts

I addressed both of those questions in my initial reply... but because both you and Duncan missed it, I will summarize:Also increases roughly linearly with rpm I expect (dB/dt)?

...

So can extrapolate to other rpm and currents fairly well?

1. Back EMF is roughly proportional to current at a constant RPM, or to RPM at a constant current.

2. The actual relationships between BEMF and current (or RPM) is square law below "saturation" of the field (roughly 200A for a WarP-9) and linear above saturation. There are also additive terms for the various resistive voltage drops in the motor as well.

Maybe, maybe not. First off, you are repeating the data for the WarP-9 which may not apply to your Hitachi motor. Furthermore, we don't know the value of current it takes to fully saturate your motor, and the linear relationship between BEMF and current and RPM only holds in saturation.50v/1000 rpm at 1000 amps

That would give 10v/1000rpm at 200 amps

and 35v at 3500rpm...

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6,370 Posts

The Hitachi is "rated" at 10Kw and 48v - about 200amps - I kind of assume that it will be saturated about there

What I really need to know is am I correct in my "theory" that I'm getting about 100v of back EMF at 200amps and 3500rpm

Is that reasonable??

Or is there something else going on?

I had initially assumed that the back EMF would be "similar" to the warp 9 - maybe 50% more

Which would have enabled me to spin to to 4000rpm - and then back off to avoid overspeed

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6,370 Posts

(and assuming linear)

I get

1000 amps to about 700rpm - 19Kph

800 amps to about 900rpm - 25Kph

600 amps to about 1100rpm - 30Kph

500 amps to about 1500rpm - 40Kph

400 amps to about 1900rpm - 50Kph

200 amps to about 3500rpm - 95Kph

And that is about how it felt - really fast off the line - then disappointing

So is that "reasonable" - or what am I missing

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Saturation is not sharply defined in a motor (or in any magnetic structure with an air gap) and so, e.g., the WarP-9 will begin to saturate at around 200A but won't be fully saturated until over 400A (NB - I did these tests 4+ years ago and the range is kind of squishy so the numbers maybe squishy as well).The Hitachi is "rated" at 10Kw and 48v - about 200amps - I kind of assume that it will be saturated about there

Where did you get this BEMF value? The proper way to determine BEMF is to calculate or measure the average voltage applied to the motor and then subtract out the fixed ohmic losses (which can be obtained by stalling the motor and measuring the voltage drop across it at a reasonably high current, say 20-50A; needless to say, don't take too long to make this measurement as it is hard on the brushes/commutator).What I really need to know is am I correct in my "theory" that I'm getting about 100v of back EMF at 200amps and 3500rpm

Is that reasonable??

If you can't obtain the average voltage applied to the motor via the controller (ie - duty cycle * battery voltage) then note that an old fashioned analog meter will usually do an excellent job of directly reading the average value of PWM DC.

Yeah, I missed your parenthetical comment about rpm Jeffrey, but not the one on current. My statement on that was just providing some verification of your comment.I addressed both of those questions in my initial reply... but because both you and Duncan missed it, I will summarize:

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6,370 Posts

In the last year I have built a new house and moved into it - unfortunately my shed is still not finished

This lack of a shed has resulted in the Device spending the whole of last winter under a tarp

I took the tarp off last week and discovered that it had leaked AND I had had a blocked drainage hole

My battery boxes had two inches of water in them!

32 of my 160 cells are rusty and horrible!

I have stripped the battery boxes down but not yet tested any cells

On the other side I have purchased a Volt battery pack - should be here soon

So I intend doing a major rebuild on the Device

But first I need to get my damn shed built!

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6,370 Posts

Shed now built,

Volt battery has arrived and has been reconfigured to fit

I also took advantage of the fact that the Device is in bits to lower the rear a bit - not for performance just for aesthetics

Photographs soon

This is one of my favorite builds, so I am looking forward to the pics and updates.

Shed now built,

Volt battery has arrived and has been reconfigured to fit

I also took advantage of the fact that the Device is in bits to lower the rear a bit - not for performance just for aesthetics

Photographs soon

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157 Posts

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