Motor voltage and RPM's

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Paul9
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Real Name: Paul Coggiola
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Motor voltage and RPM's

Post by Paul9 »

G’day people,

I have a question regarding motor voltage and RPM.

Ten years ago we completed the conversion of a 93/94 Suzuki Swift using the old 96v “Goombi” kit. In the last ten years I have had a lot of “fun” (and blood sweat and tears!) improving the performance of the car. I have made aerodynamic changes and continually added more cells. More cells meant upgrading chargers, controller and various other components. Apart from the still existing 96v motor, the vehicle bears little resemblance to the original configuration.

It now has 41 cells x 3.2v nominal = 131.2 volts though when fully charged the pack voltage normally settles down to 137v.
I normally run around the place in 3rd gear up to a speed of 60 to 70 kmh. About 65kmh I change into 4th gear. The 96v motor nameplate states a continuous RPM of 2800 rpm and a maximum RPM of 3600 rpm.

The other day I was driving where there are a sequence of 60kph and 80kph speed limits. I changed into 4th as I approached the first 80kph section and then down into 3rd when the limit changed to 60kph. The limit then went back up to 80kph but I had forgotten to change into 4th gear. A minute later I looked at the speedo and I was doing 85kph in 3rd gear. Fearing a motor meltdown, I immediately changed into 4th gear. What impressed me was that the car accelerated from 65kph to 85kph much better than it normally did in 4th gear and did not feel as if it was struggling to do it.

I thought that 85kph in 3rd gear must have over revved the motor but nothing had happened. I checked “google” to ensure my gear and diff ratios for a 93/94 Suzuki Hatch back were correct. An interesting table appeared as follows:
Per 1000 RPM speed in each gear
1st ----- 7.8 kph
2nd ---- 14.1 kph
3rd ---- 20.9 kph
4th ---- 29.2 kph
5th ---- 35.3 kph

The above table means that a continuous rpm of 2800 equals a speed of 58.52 kph and equals a max speed at 3600 rpm of 75.24 kph in 3rd gear. But I was doing 85 kph.
I believe that more torque comes from more current and more speed comes from more voltage.

So my query is, am I correct in assuming that because I now put up to 137v into my 96v motor, the rpm’s of my motor have increased? Is it true that my continuous rpm’s are no longer 2800 and my max rpm’s are no longer 3600 rpm but figures somewhat higher than these? If my “new” rpms are higher, are there any negative consequences to running at these higher rpm figures (whatever they may be)?

Hope this makes sense.
Thanks
Paul
antiscab
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Joined: Mon, 26 Nov 2007, 05:39
Real Name: Matthew Lacey
Location: Perth, WA

Re: Motor voltage and RPM's

Post by antiscab »

Motor voltage on a series would motor is a function of motor current and rpm. Increasing the battery voltage means the controller is now able to stay in motor current limit up to a higher rpm, leading to more power at high rpm.

As a side note, your motor controller is more efficient at lower motor current and higher rpm. But your gearbox is more efficient at higher torque and lower rpm.

Your motor will have max rpm, and this is a limitation of the strength of the banding on the armature windings. The bandings are stronger when cold and weaker when hot. Smaller diameter motors can go higher rpm.

Yours should be good up to 5000rpm


Edit: Increasing battery voltage allows the controller to stay in motor current limit up to higher rpm (not battery current limit)
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells
Paul9
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Posts: 293
Joined: Mon, 20 Apr 2009, 02:10
Real Name: Paul Coggiola
Location: Sydney

Re: Motor voltage and RPM's

Post by Paul9 »

Thanks for the advice Matthew
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