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