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Author Topic: Current measurement on positive or negative side?  (Read 1283 times)
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texas
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 smiley-red Oops, I saw the hall effect verbiage and assumed it to be an inductive pickup.  I see that it is actually in series with the circuit, sorry about that.  I think I'd look for an inductive pickup sensor if it were me.
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Denmark
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The motor is 133 amp 24 volt series wound dc. I don´t know what they where intended for (maybe forklift), but i managed to get hold of 6 of them, maybe 10 years old, but unused. I´m converting a lawn tractor to electric with a motor for drive, and a motor for grass cutters. Arduino is at the heart of the operation. I promise to make a ”bragging” post when the project is finished smiley
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Recently I bought 15 LEM HAL 300 current transformers as surplus from Ebay (the price was Very Right) and one of these devices would be perfect for your application. The device has a 30? mm (approx) hole in it to pass through the main motor current carrying wire. Due to the inherent losses involved in getting the Allegro  device soldered in a suitable PCB (4 Oz Cu?) and connections to the controller and battery, a through hole or pass through device is really called for. The HAL 300's can be found for about $50.00... Mine for considerably less (New on Mfr's shipping material). But I see no easy way to get one to you.
The device can be placed anywhere in the motor power leads, hot or ground as it is totally isolated.The LEM device is spec'd from DC to 50 KHz and has an output similar to the Allegro device however it requires a +/- 12 - 15V split supply to bias the Hall effect circuitry. There are available 12 to 15 V isolated switchers that will supply those voltages. Or in short.. I personally don't think that the Allegro device is ideal for your application. My intent here is NOT to sell current transformers but to point out a more suitable device. I intend to make DC transformers by winding a multi-turn primary so I can use them for more reasonable currents... +/- 30 A = 10T primary. Unfortunately.. I had to buy 15 (at less than the single unit surplus price, much less). It just occurred to me that the motor current is likely going to be considerably less than 130 A since your load will be just the mower and batteries, I should think more like 10 - 20 A which is easier to deal with than the rated full load current and entirely do-able with FR4 PCB material (I would use double sided FR4 laminate and have it plated to 4oz/in with a lot of via's stitching the top and copper together).

Bob
« Last Edit: February 15, 2013, 04:17:44 pm by Docedison » Logged

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smiley-red Oops, I saw the hall effect verbiage and assumed it to be an inductive pickup.  I see that it is actually in series with the circuit, sorry about that.  I think I'd look for an inductive pickup sensor if it were me.
hay, dont be sorry just pointing it out
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Anaheim CA.
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It is still an inductive pickup, just different physics.

Bob
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SW Scotland
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Don't wish to put a dampener on your project but I think some maths need working on.

Your motor rating is only 3kW (4HP) assuming 100% efficiency.  This seems far too small for a lawn tractor.  Series motors are not constant speed, their speed being dependant on load applied which in turn increases current demand.  (theoretical speed, ignoring physical restraints, at zero load is infinite)  If from a fork-lift, they were probably used to drive the hydraulic systems which is only intermittent use.  In a tractor they will be subject to continuous use and therefore may be prone to overheating. On basis that battery capacity is a pair of 100AH, 12volt units, with an average load of say 100A and limiting depth of discharge to 40% (too much some will say) the duration of running (or should that be crawling) will be a maximum of 24 minutes.  Decent batteries are gone to weigh about 25+kg each so there will be a weight change in the tractor of +50kg for batteries and say -15kg for engine, so a net increase in weight of around 30kg.

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Denmark
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I bought the Allegro sensors on ”breakout” boards with large double sided conductive path (looks gold-plated, not sure about that though) with 6mm bolt holes. I can´t find them on eBay anymore, so i don’t have a link. As I already have them, I’ll try them out, but if it proofs too inefficient, I’ll give you a call Docedison, thanks.

The motors are replacing a 13hp ice motor (maybe the right term is ride-on lawnmower rather than tractor). So about 4 hp for drive, which I think is more than enough, and about 4 hp for the blades which might be too little, but if it works I’ll probably be happy to cut the grass before it gets to long. With this setup, I can also eliminate some pulleys and the clutch, which hopefully will reduce the power requirements a bit. I’m going to use four 85ah marine batteries, and I’m hoping to get a couple of hours juice for just driving, and maybe 40 minutes of grass cutting.

The Arduino will try do reduce power to 130 amps per motor, and will allow 150-200 amps for max 5 seconds, and will shut of power completely if consumption is more than 200 amps for one of the motors. The Arduino also handles speed control, rpm-safety-shutdown, general safety, voltage monitoring, battery protection etc., but more on that when my code fails, and I´ll be looking for help in the programming sector smiley-grin

I bought an small electric forklift a while back for a different project, and the drive motor seems very similar in size and weight to the motors I'm going to use. It’s also 24 volts and series wound, but there no dataplate, so i cant be sure about the specs. I think it’s popular to use series wound motors for forklift, because its easy to reverse rotation by reversing power to either the stator or the rotor?
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Dubuque, Iowa, USA
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Your motor rating is only 3kW (4HP) assuming 100% efficiency.  This seems far too small for a lawn tractor.

For a comparison you could look at the old GE "Elek-Trak" models -- the most popular model 12 was 1.5HP for the drive motor. They carried six batteries and ran at 36V.

The motors are replacing a 13hp ice motor (maybe the right term is ride-on lawnmower rather than tractor). So about 4 hp for drive, which I think is more than enough, and about 4 hp for the blades which might be too little, but if it works I’ll probably be happy to cut the grass before it gets to long. With this setup, I can also eliminate some pulleys and the clutch, which hopefully will reduce the power requirements a bit. I’m going to use four 85ah marine batteries, and I’m hoping to get a couple of hours juice for just driving, and maybe 40 minutes of grass cutting.

That sounds about right to me.

The Arduino will try do reduce power to 130 amps per motor, and will allow 150-200 amps for max 5 seconds, and will shut of power completely if consumption is more than 200 amps for one of the motors.

You should still use a breaker or fuse for overcurrent protection.
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