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Old 12-03-2014, 07:32 PM   #6951 (permalink)
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Quote:
Originally Posted by MPaulHolmes View Post
. I remember you saying that at one point the battery amps were over 800 while testing it out? How fast was the motor turning when that happened?
That was just along the street maybe 25 - 30Kph about 1000rpm

Thanks for confirming about the controller - I can't think of anything else other than the motor EMF that would have the effects I'm seeing

Duncan

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Old 12-03-2014, 07:35 PM   #6952 (permalink)
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Question posted! Now I'm waiting for replies... There are some crazy smart people on there, so we should get some good insight.
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Old 12-04-2014, 08:24 AM   #6953 (permalink)
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Here's what Greg Fordyce had to say:
The way I understand it, amps gives you torque and voltage gives you rpm. 500 amps through a motor will give the same torque at 12 volts as it will at 120 volts but the rpm will be 10x higher at 120 volts. So if you want ~500 amps through the motor at 3000 rpm then you need to double pack voltage. Or if this is not possible shift to a higher gear so motor rpms go down and allow more torque (amps) through motor.

So your choices are more batteries or a gearbox.
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Old 12-04-2014, 03:47 PM   #6954 (permalink)
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Here's a very interesting suggestion Duncan fro Cor van de Water:
BTW, you *can* change (weaken, but also increase) the field on a series motor
because most motors have external contacts for field and armature (which are
usually connected together with a strap to send the same current through both)
The simple solution to field weakening is to add a contactor and a low value,
high power resistor like a piece of steel rod and wire this across the field
contacts. When yo uwant field weakening, close that contactor and part of
the current bypasses the field.
The original issue of the 130V drag racer that was maxing out at 59MPH
can easily be solved this way: closing the field weakening is like engaging
overdrive on the series motor, it will run faster at the same voltage
(note that it canít produce the same torque due to the weaker field,
so it is best to make it controllable and you only enagage it at a certain speed)
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Old 12-04-2014, 04:07 PM   #6955 (permalink)
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Here's a warning though:
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.

So 65 instead of 59?
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Old 12-04-2014, 06:38 PM   #6956 (permalink)
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If messing with the field will only give 10% and risk damage to the motor i would shy away from that if it were me.
I would play around with the battery pack configuration.
Either adding more cells to raise the voltage or rewiring the pack to be 2 strings of 230v rather than 4 strings of 130v.

If redoing the pack configuration caused too much reduction in torque at low speeds then maybe make the configuration adaptive.

At low speeds 4 strings of 130v and high current then at some speed flick a switch and have contactors in the pack that change it to 2 strings of 230v. Sort of like a 2 speed gearbox. It would give you the high current at low speeds and high voltage at high speeds.
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Old 12-05-2014, 09:40 AM   #6957 (permalink)
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Just got done reading this thread - for the second time. Well at least the part since March. For me this controller is the most interesting thing I've come across lately on the homemade EV front and I would like to get involved. I'm good with a soldering iron but my electronic design skills are fairly basic. Like Flores I know a lot about programming, both microprocessors and higher level machines.

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Old 12-05-2014, 12:52 PM   #6958 (permalink)
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Duncan, I think you're right on when thinking you ran out of volts (PWM maxed out). I wrote a post on my blog about that not too long ago with calcs for a Warp 11 motor and 100v system if you're interested. Just look about half way down. There's a guy from the Ukraine who keeps asking me about that setup in a Porsche 924 like mine. That's why I decided to run 230 volts in my car.

Porsche 924 Electric Conversion: The Need for Speed
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Old 12-05-2014, 09:10 PM   #6959 (permalink)
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Quote:
Originally Posted by Astro View Post
At low speeds 4 strings of 130v and high current then at some speed flick a switch and have contactors in the pack that change it to 2 strings of 230v. Sort of like a 2 speed gearbox. It would give you the high current at low speeds and high voltage at high speeds.
My warp 9 and warp 11 are rated for 160V, maybe 170V. I read somewhere .. perhaps over at EVTV? .. that you start to damage the brushes above 170V. I don't remember if it's just the brushes that wear or if the commutator starts to pit as well. I can't locate it right now but I remember that I don't want to experience it.

As for changing battery pack configurations while moving, it sounds risky to me. Contactors take a certain time to open and a different time to close. All things fail eventually. If one set of contacts fails to move at the correct time, the failure will be catastrophic, in my opinion.

In my experience, a battery pack at 50V and 100A will deliver about the same V*A at whatever voltage you select. So at 50V you get 100A, at 25V you get 200A, at 15V my Curtis controller will limit to 300A, so I get max current and max torque up to maybe 17V.

I think that 3 strings of the max voltage the motor can safely handle may give the most current without damaging the motor.
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Old 12-05-2014, 09:39 PM   #6960 (permalink)
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I've got a standard Warp 9 and it works fine because the PWM never gets high enough for the motor voltage to reach 170v. In fact if I was at 100% duty cycle I think the pack voltage would sag to around 170v anyways as long as the rpm is less than about 5k. Seems like I calculated that at some point in the past.

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