KTM RFS Stator Rewind: Floating the Ground and Upgrading to Heavier Wire for a Pure DC System.
John Davies 12/12/05 Revision 12/26/05 added Stator Output Chart and additional procedural comments.
Revision 02/11/07 added a pic at the top of the page.
Revision 07/23/09 added some motor epoxy notes
There are many options for improving the marginal performance of the stock KTM charging system.
1: “Float the ground” and add a bigger regulator/ rectifier: eliminates the 55 watt AC portion that supplies the stock lighting, and converts the stator output to pure DC, but it will not increase the total current capacity. 75-80 watts, $40 for the R/R
2: Float the ground and add additional 20 gage wire to completely fill the “skimpy” poles: increases wattage slightly. 85 watts ?? $6.00 for motor wire (or $0.00 if you can scrounge 20 or 25 feet), $40 for the regulator/ rectifier.
3: Upgrade to 18 gage wire, completely rewiring the entire stator and maxing out every pole: increases wattage slightly (20%) and allows extra current capability with stronger magnets. 16 gage wire will simply not fit onto the poles in sufficient amount. 95 watts +, $6 for motor wire, $40 for the R/R
4: Have your stator professionally rewound: 95 watts +, $90 plus shipping, $40 for the R/R
5: Install a TrailTech flywheel with stronger magnets: 110 watts with a rewound stator, $120 plus shipping.
6: Install a 3 phase stator from ElectroSport: 273 watts, $196 for the stator plus shipping, $40 for the R/R
7: Combine 5 and 6: 285 watts, About $380.
I chose number 3 because it was cheap, I could easily install the TrailTech flywheel later if needed, and I wanted to try to rewind a stator myself. I planned on retaining the stock KTM lights, other than changing the tail light to an LED, and all I wanted was to be able to heat an electric vest while riding down the highway, and keep the battery from discharging when trail riding in very tight terrain.
Keep in mind that the stator produces a continuous output, depending only on rpm. It will supply any load up to the maximum amount of current available. If the load is light, excess current is dissipated as heat from the regulator/ rectifier (the reason a cool location with good airflow is desirable) Here is an excellent thread on modifying stators (you need to be logged in at KTMTalk to view it): KTMTalk Home Page -> Electrical Output Chart. The following chart from that thread is very helpful in visualizing what is going on inside these windings. (Note, especially, the nearly horizontal curve of the stock stator - additional revs don't do much to increase output. Also note that the chart shows a higher output for the stock stator than reported by many other sources, and from my own measurements - possibly due to the way the chart measurements were made in a lab environment?)
Stock Configuration: For reference you can draw in your original wire routing, winding direction and notes before proceeding. It isn’t at all necessary unless you are going to rewind with the stock configuration.
My 2005 525 EXC stator:
Wire length by pole (1 to 7): 122”, 146”, 148”, 145”, 147”, 145”, 45”
Total wire length all windings: 74.8 ft of 20 gage wire (0.033” OD)
Average wire length on the five “fully wound” poles: 12.2 ft
The ground lug is NOT used for floated ground, series winding. (Values in quotes are observed numbers from my own stator – yours may be a little different.)
Resistance Yellow to White, before soldering: ____________ (about 0.5 ohms)
Resistance Yellow or White to Ground, before soldering ____________ (infinity – open)
Total wire length: ____________ (93 ft, 18 gage, 0.044" O.D.)
Operational Checks: Ensure the battery is in good condition and is fully charged.
Resistance: Regulator/ Rectifier ground wire, end to end, with wire completely disconnected _____________ (ZERO ohms)
Battery voltage, engine off: _____________ (12.3 volts)
Battery voltage at idle, no load: _____________ (13.8 volts)
Battery voltage at mid-rpm, no load: _____________ (13.8 volts)
Battery voltage at idle, headlight on and fan running _____________ (12.6 volts steady)
Battery voltage at mid-rpm, heavy load (____watts) _____________ (13.0 volts, 95 watts)
Max current output from stator red wire _____________ (7.3 amps)
1: The KTM radiator fan uses 2.7 amps at 13.8 volts = 37 watts
2: Stock ‘05 headlight is 35 watts High, 35 watts Low, 5 watts city light (ON with Low). So my bike draws 5 watts more on Low than High!
Before you start:
Tools and supplies needed:
High temp silicone sealant
New stator cover gasket
Black Sharpie or heavy felt tipped pen
Digital multi-meter – it needs to have a low range resistance scale to measure below 1.0 ohms. A 10 DC amps current scale is desirable, but not necessary. Anything below a 10 amp rating will overload and possibly ruin the meter.
Small needle nosed pliers
Tap - 6 x 1.0 mm – to clean out the threads in the stator mounting holes
Soldering iron or gun, 30 watt or larger
Rosin core electrical solder – acid core is NOT advised, as it will corrode your connections and render them useless in a few months. Silver solder is not necessary.
Solder Wick or equivalent - used to soak up excess old solder from terminals
Electric motor (contact) cleaner (brake cleaner is not advised)
Heat shrink tubing, 1/8 inch or larger x 6 inches. NOT the pvc variety, which will not take the heat.
18-gage magnet (motor) wire. Allied Electronics 800-433-5700, P/N 214-3574 or equivalent. 1-lb spool is $10 and enough for 3 or more rewinds. Or wire may be bought in bulk for even less from your local electric motor repair shop, if they are feeling friendly. Be polite and offer up front to pay for their time.
Epoxy spray: 3M 2216 or 3M 1838. I used a generic equivalent from my local motor repair shop. Use only an industrial motor epoxy that will withstand the hot oily environment inside your engine.
Update 07/23/09: Some folks have had a hard time finding this epoxy. The 3M numbers don't come up anymore - maybe their product has been discontinued.... I really do think a local shop is the best source. The spray I used was Aervoe Industries Inc "Insulating Epoxy 403" Clear
Product Details: Aervoe’s Insulating Epoxy is a tough, flexible coating that protects and insulates exposed wires and electrical windings. Effective for insulating coils, transformers and other electrical components.
Uses: Field coils, motor windings, taped coils, exposed metal, frayed insulation, pitted motor windings, bus bars and switchboard parts, transformers, commutator ends, sealing electrical and electronic components, collector rings, transformers, armatures, and inside walls of control boxes.
Remove the gas tank to access the stator connectors. Disconnect the two Yellow and White flat spade connectors, and the blue plastic connector (Red/ Black wire and Red/ White wire). Drain the engine oil or lay the bike on its right side before removing the stator. Since it will take a while to do the job, I suggest you drain the oil and leave the bike upright so you can move it if necessary. Remove the skid plate, if installed, the shift lever and the four cover screws, as well as the wire ties holding the stator wires to the oil line. Save the little wire tie stand-offs for re-use!
The stock yellow and white stranded wires coming off the stator are 18 AWG and are fine as-is for this rewind, though if extending the wires to a different regulator/ rectifier location you could use 16AWG for less resistance.
Flywheel to stator clearance is very generous – the limiting factor in over-winding is the wire colliding with adjacent coils near the bases of the poles.
Be careful – the plastic is breakable, especially the plastic wire clip on the main harness support. In order to support the stator while you are pulling on the winding wire, do so by the center using a couple of large sockets in a 6 inch or larger bench vise. It is advisable to always work over a bench padded with cardboard, and place a box or another piece of cardboard on the floor under the vise, in case you drop the stator. Rotate the stator as needed during the job to keep the working pole easily accessible.
Support the spool of wire horizontally on a free-running dowel – I used a Park bicycle wheel truing stand clamped to a nearby drill press. Try not to get any kinks in the wire as you work, as that makes it more difficult to get it to lay flat and tight on the poles.
You can make a nifty tool by gutting a Bic pen, drilling the end plug, putting duct tape around the outside, and threading the wire through the pen before you start. This will allow you to work the wire in areas too tight to get your fingers. Be careful not to kink the wire when using this tool!
Another useful tool is a short length of ¼ inch brass rod (heated, shaped like a spoon, and polished) for pushing around the wire as you lay it onto the poles. I have a set of home-made brass o-ring tools, one of which worked perfectly for this.
Pliers: take a couple of 1 inch sections of heat shrink tubing, or use duct tape, to pad the ends of the tip. You will use this tool as needed to compact the initial layer of wire on each pole. Do NOT use an unpadded pliers or you risk nicking the insulation and creating a short, which will reduce the output of that winding slightly.
It is VERY important that you wind each successive pole in a different direction. If the poles are all wound in the same direction they will cancel each other out, and the net result will me nearly zero A/C output. You won’t damage anything, but the stator will have to be rewound correctly. To avoid confusion you can mark each pole with the Sharpie or just keep your wits about you as you work. Since my wits left years ago, I used the Sharpie….
Motor wire is insulated with a semi-clear coating that must be carefully scraped away using the flat of a sharp knife blade, followed by polishing to bare copper with sandpaper prior to soldering. Do not disturb the insulation in any areas other than 1/8 inch or so at the electrical solder points.
Removing the old windings:
Remove the stator and wire clamp from the magnesium housing and thoroughly degrease everything using electric motor (contact) spray cleaner. (NOTE: brake cleaner may harm paint, some plastics and rubber – if you must use this, rinse promptly with alcohol and blow dry.) Protect your eyes! Degrease again after removing the old windings (there will be a lot of trapped oil underneath), and again before applying the epoxy.
Heat the Yellow wire solder joint and carefully remove the wire (DO NOT CUT!!), and also the motor wire, from the terminal. Repeat for the White wire and its two “center tap” motor wires, and the Ground terminal. Use your iron and the Solder Wick to soak up the excess old solder to prepare the terminals for the new joint.
Unclip the hook on the plastic cable clamp and carefully peel out the four wires. Work them out and over the surrounding poles without overstressing them. DO NOT disturb the ignition coil or its two red wires, other than laying the wires carefully out of the way!! Secure these two wires with a nylon wire tie to prevent over-stressing their solder joints. Wrap the wire clamp in tape to prevent it from breaking off, as mine did near the end of the job. Note the incomplete winding of Poles 1, and 7 especially, in this pic:
Inspect the loose ends of the windings to see which wire lays over the others. Grab it with your pliers and pull reasonably hard, and it should pull loose from the epoxy and start to un-lay. I have read stories of epoxy that is so stubborn that the first layers have to be removed with a hacksaw. If yours falls into this category, feel free to swear before you grab a saw and be very careful not to hurt the core or ends of the poles. My windings came off very easily.
If you want to measure your original winding lengths – clip each wire as you finish un-laying it, and lay it on the floor out of the way. When you are finished removing all the wires, take them one at a time. It will be all crinkled into a zig-zag pattern, so clamp one end in the vise and the other in a Vise Grip plier, and snap the wire gently to straighten it out. Measure and record the amount. Add all the wires up for a total stock winding length, and record that number if you like on the worksheet Page 1. My stator had just under 75 feet.
1: Begin by spooling off 6 inches of wire and making a heavy index mark with a black Sharpie pen. This marks the starting point at the base of the first pole. Slide on your Bic winding tool at this point, if you want to use it, and just let it hang until needed. Spool off an additional 12 feet and make another heavy mark, and put a flag of masking tape on it as well, which will make it much easier to see. This represents the minimum amount you want to wind around each pole (the stock “full pole” equivalent). Ideally you want to add more than this to maximize the voltage. You also want to make each pole as close as possible to all the others in terms of wire length.
2: Clamp the stator firmly in the vice with the terminals visible and Pole 1 within easy reach. Lay the first wire index mark at the base of Pole 1 on the side facing you, and route the 6 inch tail snugly under Pole 2 and across to the White terminal. DO NOT solder the wire yet.
3: Wind Pole 1 clockwise starting at its base and working the wire coils tightly together, all the way to the tip. At each HALF turn on this initial layer you can use your padded pliers to gently squeeze the wire tight against the plastic pole. Subsequent layers don’t need this to lay flat. Try to minimize gaps so that the next layer doesn’t fall into them. A minimum number of gaps also means that you are getting the most wire possible onto the pole, though on this initial layer the length of each turn is pretty small. I used my brass o-ring tool a LOT to push the wires tightly against each other and to manipulate the wire in tight spots. I strongly recommend this gadget!
4: Wind each successive layer until you reach your 12 ft index mark/ tape flag. Anything more than this that you can squeeze onto the pole is icing on the cake, giving you a little bit more voltage and current! This length depends entirely on the outside diameter of your particular batch of wire. (Mine measured 0.044 inches.) Measure out another 5 feet and make another index mark and flag.
5: Continue to wind Pole 1 until the wire is level with, or just below, the edge of the plastic end piece facing you. Don’t go past the diameter of the end piece or you risk having the wire fall off – you’ll know when you can’t fit any more on. On my stator, the pole was filled with the wire ending near the outside, so I ran it down the backside at an angle over half a turn to get back to the base. Be sure to check for clearance with the next pole – study the size of this first winding and see if there will be enough room for the next one with an equal amount of wire. If not, you will have to backtrack and remove one or two of the windings near the base. DON'T REMOVE AN ENTIRE LAYER or you will end up with too little wire on the poles!
Warning!: If this doesn't fix the problem, the wire diameter is too big and you need to find some smaller O.D. 18 gage wire, or go back down to the original 20 gage size. If your wire measures near 0.044" O.D. you won't have any problems, IF you wrap as tightly as possible.
6: Measure the remaining wire to your newest index mark and record that distance (my numbers are in quotes):
A = ________ ft (3.75 ft)
Let’s see how much wire you added
B = 5 ft - A + 12 ft = ________ ft (13.25 ft)
C = B x 7 poles = ________ ft (92.75 ft)
D = C – 75 = ________ ft (17.75 ft)
“B” is the new “full pole” length, “C” is the total wire length, and “D” is the total extra wire for all seven poles you will have added, over and beyond the factory amount of 75 feet. To calculate a purely theoretical improvement in the stock 80 watt (approximate) combined AC DC output:
C / 75 x 80 = _______ max watts DC output (99 watts)
C / 75 x 100 = _______ % change (123%)
Doesn’t that make you feel better? If you have added an LED taillight, saving up to 18 watts, that figure looks even better. Remember, at 14 volts, 100 watts is 7.1 amps. This calculation doesn’t take into account loses or inefficiencies, so your actual stator performance WILL be lower, but not by a lot. (Mine actually came out later to be 95 watts, calculated from measured amperage and voltage.)
7: Remove the extra flag, and from the base of Pole 1, measure out another “B” feet, make an index and flag, and proceed to wind Pole 2, but in a CCW direction.
8: Wind the remaining poles, alternating directions. When you have ended up at the base of Pole 7, wind the wire zig-zagging around the bases of Poles 6, 5 and 4 in whatever path keeps the wire neat and tight, ending up at the Yellow terminal. Cut the wire here, leaving enough slack to work with. Go take a break and rest your hands. Mine were cramping at this point.
9: Place each motor wire across its respective terminal and clip it to the correct length. Scrape and sandpaper the insulation off the last bit of both tails and measure from end to end – resistance should be somewhere around 0.5 ohms. Measure from one end to ground (the stator core); the resistance should be infinite (open circuit). If you find anything different, there is a problem and you need to correct it now rather than later! Lay each wire into its proper terminal, bend and crimp the wire back lightly at an angle to keep it in place.
10: Bring the ignition wires and two charging wires back around into their original positions, routing the red wires carefully around the bases of the poles and into their proper retaining notches in the plastic frame. Because of the new, larger diameter of the windings, these wires won’t fit into their retainers in back – that’s ok. (See step 13.) Work the four wires loosely into the plastic clamp, leaving it unclipped. Position and solder the Yellow and White wires to their terminals and the awaiting winding wire. Perform any adjustments to the wires and clip the latch on the clamp.
11: Thoroughly degrease the entire stator one last time, blow it dry if you have a compressor, wash your hands, and mask off any areas other than the windings. Be especially careful to mask the mounting areas in the center. Spray a couple of heavy, dripping coats of epoxy onto the windings and let the stator dry overnight before installing it into the cover. The epoxy is thin and will penetrate deep into the windings if applied heavily.
12: Run the 6 mm tap loosely into the two cover holes to loosen the old thread locker, and blow out the junk with compressed air. Wire brush the mounting bolts and check that they run freely into the cover holes. Clean the wire retainer screw and hole also.
13: Remove all masking tape from the stator. I felt that the Red/ White and Red/ Black ignition wires need a little extra securing in back where they don’t go under the retainers, so I placed a small dab of high temp silicone sealer at each wire, and also one at the harness because my clip broke. Reinstall the stator using blue Loctite and torque to 89 inch pounds. Install the retainer screw with blue Loctite and hand-tighten snugly.
The Yellow and White wires may be extended with 18 AWG or 16 AWG wire if necessary, long enough to reach the regulator/ rectifier. (Color and polarity do not really matter, since it is AC current to this point.) Make any splices before installing the cover/ stator assembly onto the engine. It is best to make clean solder splices, covered with heat shrink tubing and staggered to minimize bulk, and hide them inside the factory plastic wire sleeve. Inject a little silicone sealer into the top of the sleeve when finished to keep out water.
Clean the gasket surfaces and use a new gasket. Install the stator cover and torque the screws evenly to 89 inch pounds. Be careful when installing the cover – the magnets in the flywheel will try to jerk the cover hard against the crankcase. Clean the shifter hole and bolt and install with blue Loctite at 89 inch pounds.
Refill engine with oil if drained! Do it NOW so that you don’t forget.
This mod changes the stock AC/ DC split configuration to a single series wound DC output and requires the use of a new 150 watt regulator/ rectifier. The stock unit is not sufficient. Plug in the blue ignition connector and connect the Yellow and White stator wires to the new regulator/ rectifier using the wiring diagram on the reg/ rec housing. Use new insulated bullet connectors if needed and use dielectric grease in all connections. I used insulated 1/4 inch spade connectors, which fit very nicely into the stock connectors with a little trimming of the plastic sleeve on the stator wires.
I used a $40TrailTech unit, and had some difficulties installing it to my satisfaction. There are several possible mounting points:
1: On the front of the battery, secured under the rubber straps – I simply did not care for this method, though it works. If you lengthen the AC output wires, the power and ground wires are kept very short.
2: On the right side of the steering head: not an option for me – the unit will not clear the KTM steel radiator guards. Mounting here puts the regulator in nice cool air, but it is vulnerable to damage from debris, or from contact with the fork, if the steering stop should bend in a crash. It also requires drilling the frame and moving the gas tank vent.
3: The stock location under the tank: I chose this as the most satisfactory, though there is very little room and it is a hot environment.
Because the tank fits so closely along the frame it is very important to tuck the harnesses and connectors under the “overhang” of the frame tubes, or installing the tank will be difficult and the wires can chafe hard. I trimmed and filed one tab from the regulator to give extra room underneath for wires. The final position I ended up with, was with the wires exiting to the right side of the bike, and 3 thick washers between the housing and the mounting tab. This gave enough room to fit everything – the TrailTech unit is a lot bigger than the KTM unit. There is still very little clearance all around – the ignition coil terminal and thermostat are both very close, but not chafing. I think a smaller unit would be MUCH easier to mount in this location.
Ground wire: Run a separate 16 AWG brown or black ground wire with soldered terminals, in addition to the existing frame ground, between the regulator/ rectifier and the battery negative terminal. Do NOT trust frame ground for the charging circuit. A bad ground can cause poor charging or even component failure.
Power wire: Run a separate 16 AWG red wire directly to battery positive. The existing 20 AWG main harness wires are really not good enough. I installed a 10 amp mini-blade fuse in a hooded in-line fuse holder right at the regulator to protect this wire.
Secure all wires with nylon tie-wraps and install the gas tank. Be sure to use stand-offs on the oil line. Perform the operational checks on the chart above You can add additional heavy loads in many ways:
jumper the radiator fan thermoswitch: 37 watts
toggle between high beam 35 watts and low beam with city light 40 watts
clip on an automotive headlight bulb to the battery with heavy gauge test cables: 55 watts or whatever.
connect or disconnect the tail light at the airbox harness connector.
I experimented with load while observing two digital voltmeters: one measured amperage coming out of the regulator red wire, and the other monitored battery voltage. By adding progressively heavier loads I could easily tell when the current maxed out – any additional load just causes the battery to begin to discharge and the voltage would start to decline. My stator managed 7.3 amps and about 95 watts.
Remember to recheck your oil level. Enjoy your bright lights and happy battery. And please, email me with comments or corrections!
Here is one of several available articles on Floating the Ground: TrailTech: AC to DC Stator Conversion-KTM.pdf
The following shops offer rewind services or exchange high output stators and accessories:
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