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you will find the central part of christchurch like a building site/sheep farm
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
 

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Discussion Starter · #85 ·
Just got back in
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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Just got back in
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?
 

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LOL duncan did a demo 1/4 mile in pukekohe 1974 changed my hybrid ( to get to the race) from vw motor to twin electric and found out the two motors were great for the first 20 meters but fought each other after 30 mph so just dribbled the rest of the 1/4 mile --- oh well 25 seconds still a record for me

 

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Discussion Starter · #88 ·
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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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...
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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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).
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.
 

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Discussion Starter · #91 ·
Hi Guys

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

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.

50v/1000 rpm at 1000 amps
That would give 10v/1000rpm at 200 amps
and 35v at 3500rpm...
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.
 

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Discussion Starter · #93 ·
Hi Tess

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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Discussion Starter · #94 ·
Using the assumed 100v at 3500rpm and 200 amps
(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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The Hitachi is "rated" at 10Kw and 48v - about 200amps - I kind of assume that it will be saturated about there
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).


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??
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).

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.
 

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I don't know if it is reasonable or not. I've always just used the published torque-speed curves for motors, which of course you don't have for yours. Perhaps you can find one for a 13" series DC motor and estimate based on that?

I addressed both of those questions in my initial reply... but because both you and Duncan missed it, I will summarize:
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.
 

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Discussion Starter · #97 ·
Update on the Device

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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Discussion Starter · #98 ·
Update
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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Update
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.
 

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I need to send you an updated .hex file. thingstodo on ecomodder is using a controller like yours, fighting against an AC controller for testing, and found that you can accidentally hit a little over 100% duty, which makes the controller shut down. It's just one line of code different, but still I should send it to you. I"ll email it now.
 
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