# Question about EMF



## Duncan (Dec 8, 2008)

Hi Guys,
Hoping to get some of the experts like Major to do a sanity check for me

I have just taken my car to the track with my new controller
Also first time on the track since I dropped from 44S to 40S

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

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

On the ¼ mile I could look at my instruments
After a brief surge of acceleration I was pulling a continuous 200 amps (battery)
That is about right for a constant speed of 59mph (not very aerodynamic)

I had the pedal down still
My “theory” is that the controller was at max – 100%
But there was so much Back EMF that there was not enough voltage left over to drive more current through the motor
I had about (don’t remember the actual reading) 120v
So I think BEMF = about 100v
Assuming that the motor was “saturated” and BEMF was proportional to current and speed that gives me a point on the line
3500rpm – 200Amps = 100v
Making up a quick spreadsheet I come up with

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

Specifically - Is that a reasonable amount of BEMF?

From that my 11 inch Hitachi motor is producing about three times as much BEMF at a given rpm/amps as the tables for a Warp9
I expected more – but not that much more


_For example, the WarP-9 enters into saturation around 180-200A and requires about 50V for every 1000 RPM at 1000A._


Which at 200amps and 3500rpm is 35v 


Tesseract has told me to measure the actual motor voltage and the stall current and voltage
I do intend to do that - but I won't be able to get it done for a while


So meantime - sanity check - 

Is it reasonable for the 11inch Hitachi to have three times the EMF of a Warp9 ??

I have 8 degrees of brush advance


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## major (Apr 4, 2008)

Hi Dunc,

Not sure why you're hung up on BEMF and saturation and all that. You have a different motor than a WarP9 and it has a different speed torque characteristic. The BEMF is always going to be a few volts lower than the applied voltage (on the motor terminals) otherwise you'll have enormous current. For instance, at 200A, BEMF is 2 volts lower than motor terminal voltage, at 400A, BEMF is 4V lower, at 600A, BEMF is 6V lower and so on. And that isn't too far away (compared to battery voltage) from the BEMF vs current for the WarP9. What's different is the RPM vs current.

And the RPM characteristic varies due to the difference in flux (primarily a function of rotor size) and the number of armature turns (proportional to # of comm segments (bars)). WarP9 has 49 bars IIRC. How many does your baby have? 

It shouldn't have been a surprise to go slower with a lower voltage battery. So if you want to increase motor RPM vs load, increase voltage. If you want to increase vehicle speed, decrease the gear ratio. Obviously the taller gear will slow acceleration and load the motor more at high speed (increase current). So the higher voltage would be the better way to increase vehicle speed. But that will also come with the expense of increased current due to the nature of the aerodynamic load.

Regards,

major


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## Duncan (Dec 8, 2008)

Hi Major
I didn't realize that I was hitting the battery voltage limit until the slightly lower battery voltage and much more powerful controller rubbed my nose in it.

The com has about 52 bars - 

I was expecting the motor to (very roughly) produce about one and a half times as much voltage as the Warp9

Which would have given
1000 amps to about 1500rpm - 40Kph
800 amps to about 2000rpm - 54Kph
600 amps to about 2500rpm - 67Kph
500 amps to about 3000rpm - 81Kph
400 amps to about 3500rpm - 95Kph
200 amps to about 4500rpm - 110Kph - back off before I hit overspeed

If it is producing three times the EMF - which would make sense with the performance I'm getting then I will need to jack the voltage up a good bit

Paul's new controller is a high voltage one so I'm thinking about changing my four strings of 40S to two strings of 80S

Just got to make sure everything else will be OK at 260v

Just checking with you that roughly 100v EMF at 3500rpm and 200amps 
is not a silly/nonsense number


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## major (Apr 4, 2008)

Do you have a photo of your armature?


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## Duncan (Dec 8, 2008)

major said:


> Do you have a photo of your armature?


This any good?


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## major (Apr 4, 2008)

Yep, it helps. Not as many bars as I thought maybe. But looks quite a bit larger armature than the WarP9. Also, I had a thought. Imagine that. Are you sure this new control was going to full-on? Also, what was your top speed before the control swap/battery alt?


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## Duncan (Dec 8, 2008)

Hi Major

I think the controller is full on - the controller designer (Paul) agrees

I don't know what the old 44S top speed was - 
I did have it up to 105Kph before (on the speedometer)
but I don't know if that was the top speed,

Now it will only reach 95Kph (100Kph on the speedometer)

This was the first time I had done a 1/4 mile drag,
On the 1/8th mile with the 500amp controller and 44S I thought that it was still accelerating at the 1/8th mile when I lifted off - but It could have just reached top speed


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## Duncan (Dec 8, 2008)

I did think about the controller de-rate with temperature but with the current software it just shuts you down completely if you go over temperature,

On the track it would accelerate hard - turn into a pussy cat - poodle along the straight - then power me out of the next corner
If I slowed down there was lots of torque waiting for me


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## major (Apr 4, 2008)

Duncan said:


> I think the controller is full on -


It'd be wise to verify that before doing much else, like battery reconfigure or regear. Check motor voltage (at motor terminals or controller output) vs battery voltage at full speed WOT.


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## rwaudio (May 22, 2008)

Voltage plays a massive roll in speed. When I first built my car with the Warp 11HV/Soliton1 I built the battery in sections (started with A123 20Ah cells). At 120v I topped out at about 2000rpm with a bit of power remaining under minimal load I could reach 3000rpm. On 150v it was much better, the torque didn't taper off as quickly and I had power to 3000 and could rev to 4000. Somewhere around there I switched to Calb's/A123's and had 300v, that voltage was incredible and the torque didn't feel like it tapered off I would simply hit my red line rpm 5500rpm (bumped it to 6000 a few times for 0-60mph runs, which my best was 4.8seconds with severe traction issues off the line).

Find out what your motor will handle and try to get close without blowing anything up.


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## Duncan (Dec 8, 2008)

rwaudio said:


> Voltage plays a massive roll in speed. When I first built my car with the Warp 11HV/Soliton1 I built the battery in sections (started with A123 20Ah cells). At 120v I topped out at about 2000rpm with a bit of power remaining under minimal load I could reach 3000rpm. On 150v it was much better, the torque didn't taper off as quickly and I had power to 3000 and could rev to 4000. Somewhere around there I switched to Calb's/A123's and had 300v, that voltage was incredible and the torque didn't feel like it tapered off I would simply hit my red line rpm 5500rpm (bumped it to 6000 a few times for 0-60mph runs, which my best was 4.8seconds with severe traction issues off the line).
> 
> Find out what your motor will handle and try to get close without blowing anything up.


Thanks Rwaudio
So the Warp11Hv is actually a little bit "worse" than my 11 inch Hitachi
_At 120v I topped out at about 2000rpm with a bit of power remaining under minimal load I could reach 3000rpm._
At 130v I can reach 3500rpm against the aerodynamic load (200amps)

I will check (as Major suggested) that I am definitely on 100% from the controller

But I am grateful for your input that the EMF that I am getting is "reasonable"
It looks as if the sensible thing to do is to change to a higher voltage,

If I just change from 4P40S to 2P80S
And keep the 1000amps
I will go over the 15C for a couple of seconds - I may just try that and see if my Headways survive


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## major (Apr 4, 2008)

Hey Duncan,

There is talk of this on the EVTech list. Someone suggested field weakening. Holmes said he'd tell you. I tried to post there. I have before. But it isn't showing up. So I thought I'd relay it to you here. 


> Field weakening can work with series wound DC motors. Forklifts used it for decades. You have to divert current around the field with a contactor and power resistor. This must be coordinated with the PWM controller so it does not chop when in FW. Long story short, it is a good way to damage the motor and/or controller unless the system is engineered to handle it. And then, it can buy another 10% top speed or so.


I've seen some guys damage perfectly good motors trying this. It won't get you what you're looking for and can easily toast your comm.


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## tomofreno (Mar 3, 2009)

evsource has the Netgain data for the Warp11 at 72V posted at their site:
http://www.go-ev.com/images/003_20_WarP_11_SpreadSheet.jpg

Current Torque RPM
97 10 3919 
122 15 3412 
141 20 3051 
176 30 2580 
211 40 2268 
236 50 2094 
257 60 1979 
291 70 1842 
319 80 1749 
345 90 1673 
369 100 1606 
405 110 1534 
442 125 1466 
453 135 1442 

Then I would expect if you apply 442A you will have 125 ft-lb until the motor reaches 1466 rpm then the torque will decrease as in the table for higher rpm. 

120V is 1.67 times 72, so you might roughly extrapolate that with 120V you would have 125 ft-lb to around 2450 rpm then decreasing after, following a similar curve to the above but translated to higher rpm.


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## tomofreno (Mar 3, 2009)

How much motor torque is required to move your device at the top speed you could reach? I would expect a torque-speed curve for your motor wouldn't differ greatly from the one for the Warp11, so if the Warp data indicates you should have had significantly higher speed, I'd suspect an issue with the controller or pack sagging more than you thought.


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## Duncan (Dec 8, 2008)

Hi Tom

I use 200amps at 59mph - which from the Warp11 figures is 40ft lbs 
With effectively 2ftDia tires and a 4.1:1 diff is just over 160lbs force and 20Kw mechanical 
Sounds about right - its a small car but not very (understated) aerodynamic


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## tomofreno (Mar 3, 2009)

About 828 wheel rpm, 3390 motor rpm. Then 3390/1.67 = 2030 rpm, or around 50 ft-lb and 14.4 kW shaft power extrapolated for a Warp11 at 120V, so yeah, seems reasonable.


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