# Precharge Resistor info needed



## madderscience (Jun 28, 2008)

Contactor control systems were used on older, low voltage golf carts and burden carriers and some EVs like the comuta-car. Usually there were 3 or 4 steps to get from 0 to full 'throttle'. The lowest included a resistive element and only switched in part of the battery pack, then depending on the arrangement and number of steps, the batteries were hooked in a series/parallel arrangements going from more parallel at low speeds to all in series at the highest speed.

with 36V and 6V batteries, and full series/parallel switching you could step from 12V to 18V to 36V. That is a pretty complicated mechanism however and you would probably have to build it from scratch.

For simplicity's sake if you figure you will only use the lower throttle settings for startup, there is probably no need to worry about series/parallel switching, and a 2 step mechanism you are considering would probably be OK though but you would need to expirement to figure out what the appropriate series resistance for the first stage was; start with something around 1/4 to 1/2 ohm and a rating at least several hundred watts. (maybe a 1/4 by 10" bolt would be a good place to start for the cheapskates amongst us) Even at that high wattage rating the resistor will only be able to handle intermittent startup load. Hopefully also you can find a way to reduce load on the motor until it is fully wound up. 

FWIW 36V controllers are cheap and plentiful on ebay. That would give you 100% control and won't dramatically increase the cost of the project.


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## mizlplix (May 1, 2011)

I am a Hydraulic equipment mechanic. 

Your unloaded RPM would be limited by the friction of the deck, grass and those two hydraulic pumps. It would vary warm to cold. They have a definite "happy zone".

Use a controller.

Miz


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## repairman6 (Dec 6, 2011)

Hi, Thanks for the info. I will be doing my first live tests shortly. For a resistor bank im going to try an enclosed ni-crome resistor that was part of a cheap battery load tester. I am thinking of using a start button that will latch in a bypass relay when released. I am privy to a wealth of industrial contactors and switches in my surplus business. Besides the cost of a controller i have several reasons to stay with contactors. One is if i can make this upgrade work it will adhere to the swiss army k(eep)i(t)s(simple)s(stupid) theory of operation. With a diagram and a few spare parts in the waterproof electrics console anybody with a voltmeter and a screwdriver can repair it in a short time (no programming or ebay searches needed) . Also with so many manufacturers building in planned obsolescence and the millions of electronic circuits that were made with bad capacitors (google capacitor plague) it is stilll unknown how many bad capacitor circuits are still on the shelves unsold. Computers that run 24/7 fail after a year or so but we may see an overall reliability drop in everything electronic. My 40 year old Ge electric tractor needed $60 worth of contactors and a new mov to restore it to original operating condition but it is way too complicated. I am trying to do an electric upgrade that the average mechanic can do simply and inexpensively.
Thank you for your help
Glenn


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## Guest (Dec 11, 2011)

Set it up for 48 volts and get a good Kelly controller for your motor. Then you don't need clunky contactor speed control. You get full smooth throttle action and you don't have to worry about hot wasted electricity through your resistors. I am upgrading my Cushman Truckster with a D&D SepEx motor with a 72 volt controller. I will more than likely only use like 48 volts but I can go up to 72 if I want. More than plenty for the trickster. The original contactor speed control has three speeds and long lanky resistors that get super hot when driving. Kept under the deck they won't harm anything but if I ever drove it in some tall dry weeds it would be hot enough to ignite. So if you do go the contactor route you need to be sure that your resistors can't touch any dry grass. You don't want to start a fire on your machine or the yard. 

Pete 

You can get controllers for your motor. Is your motor strong enough for the task? Is your voltage enough. Get a controller with enough amperage output if you need it. Don't get a controller with like 100 amp output. You should have one that can do at least 400 for your application. You most likely won't ever get into the 400 amp range even on startup but its nice if you get in a jam and heavy grass. Keep your blades sharp. Be safe. Have fun and by all means send photos.


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## repairman6 (Dec 6, 2011)

Hi Pete,
The hydro packs on the zero turn will do all the speed control and the reverse also. The resistor will only be used for momentary soft start for the motor and only used for 1 second while the motor is revving up to keep the inrush in check. I will install a 100 amp self reset breaker on the resistor in case someone holds the start button too long. I hope to stay with 36 volts because i have a 36 volt dc mower deck (3 x 36 volt motors) to mount underneath. I will also be installing a nice stereo, drink holder & fishing pole holder 
tnx
Glenn


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