HK-4225 Build and Review (by Dr. Ralph Okon, AKA PowerCroco)
© Dr. Ralph Okon 17.04.2010
translated by Pierre Mousel
The powerlimits of this motor are depending on the skill of the assembler, but if you do well it will be a lot more than the stock motor sold in the market.
8 kW peak power are possible and already tested.
Some videos of well-known German helicopter pilots with different wound HK 42xx issued from these kits:
Timo Wendland
Christian Samuelis
![](/files/HK4225 Build Review/1.jpg)
Well packed and protected.
![](/files/HK4225 Build Review/3.jpg)
Inside the packing:
![](/files/HK4225 Build Review/4.jpg)
Details of the small parts bag.
![](/files/HK4225 Build Review/5.jpg)
![](/files/HK4225 Build Review/6.jpg)
The stator plates have dimples made during fabrication process to allow a perfect positioning.
The glass fiber insulation plates are already glued on the stator package.
Unfortunately in this sample kit, the fiber plates were not correctly positioned.
![](/files/HK4225 Build Review/7.jpg)
![](/files/HK4225 Build Review/8.jpg)
First operation is to make the bounded (don’t remove the wires) stator package match the holder body.
To do this, let’s fix some sanding paper to a matching round wood and remove all the steps and remains inside the stator bore.
![](/files/HK4225 Build Review/9.jpg)
![](/files/HK4225 Build Review/10.jpg)
On the hole of the back plate where the wire will go out you need to round the edges of the machined hole. In this example a small round file is used. This is to avoid shorts during operations due to vibrations.
![](/files/HK4225 Build Review/11.jpg)
Now you can remove the wire wound around the stator package.
Handle with care from now on in order to avoid splits in the package.
If this occurs, re-glue with thin CA
![](/files/HK4225 Build Review/12.jpg)
Do this only is you experience the problem shown in 6!!
Remove the insulation by inserting a sharp knife between plate and stator package. Normally the insulation plate will split of.
As I had to remove the insulation plates, I took the occasion to remove the positioning dimples from the side there they stand out of the last stator plate.
![](/files/HK4225 Build Review/14.jpg)
The thin CA will spread very well under the insulation plate due to capillarity.
![](/files/HK4225 Build Review/15.jpg)
I tock the habit to clean the slots with a small flat file.
Take care to hold or fix the stator package firmly in order not to split off the insulation plates..
![](/files/HK4225 Build Review/16.jpg)
Then trim all the edges of the insulation plates. I use a sharp scalpel to scrap the edges at a ~45 ° angle.
![](/files/HK4225 Build Review/17.jpg)
The insulation paper for the first winding group is in place on this picture.
I recommend to newbie’s to fix the paper with a drop of thick CA in the slot ground. It will prevent the paper from slipping out of the slot during the winding.
![](/files/HK4225 Build Review/18.jpg)
The propeller blades (recycled) must be of carbon fiber reinforced plastic.
Laminated propellers are not suitable as they contain glass fibers cloth that will damage the wire insulation.
Prohibited also are any glass fiber reinforced tools.
On the screwdriver and the modified pliers all edges are sanded of with 400 -> 800 -> 1200 sanding paper and polished until there are no remaining sharp edges.
There must be no scratch on the wire insulation if you run a copper wire over them.
![](/files/HK4225 Build Review/19.jpg)
Take the first wire and fix the distance for the outside connections.
![](/files/HK4225 Build Review/20.jpg)
Bend the first radius outside of the stator block over a tapered hardwood or plastic square stick.
![](/files/HK4225 Build Review/21.jpg)
This buckle is inserted into the stator block and is the first half winding.
Before making the windings, please note that there is no difference if you work clockwise or counter-clockwise.
![](/files/HK4225 Build Review/22.jpg)
The following winding is only an example; different variants are possible and also useful.
If you realize an YY winding with identical small groups, the best is to start in the slot near the fixation groove of the stator package.
To realize a YY winding, the starting remaining wire lengths should be at least as follows:
Group 1 - Long starting wire ~5”
Group 2 - Short starting wire ~2”
Group 3 - Long starting wire ~6”
Group 4 - Short starting wire ~2 1/2”
Group 5 - Long starting wire ~6”
Group 6 - Short starting wire ~2”
I start winding from “outside”, means on the left side you see the start of the winding group and between the teeth’s the preliminary end.
![](/files/HK4225 Build Review/23.jpg)
The first tooth of the second group finished.
![](/files/HK4225 Build Review/24.jpg)
Here you see how to use the propeller blade to flatten the winding.
![](/files/HK4225 Build Review/25.jpg)
If you use the screwdriver, apply some insulation paper between wire and blade in order to avoid damages on the wire insulation.
![](/files/HK4225 Build Review/26.jpg)
The modified pliers are used as shown on the picture.
This only works fine if the rework has been very meticulous and absolutely no sharp edges are remaining on the pliers.
![](/files/HK4225 Build Review/27.jpg)
The second winding group is finished now.
![](/files/HK4225 Build Review/28.jpg)
The 2 finished groups seen from front.
Please note: The fixation groove is on the left; the groove you see on the right is the groove in the insulation plate that was not well positioned during re-gluing.
The small winding groups here are realized in 6+5 windings as the finished stator is planned with 7+6 YY. Don’t be puzzled now.
The explanation will follow later.
![](/files/HK4225 Build Review/29.jpg)
The first tooth of winding group 3 is finished.
![](/files/HK4225 Build Review/30.jpg)
The winding group 3 is done.
![](/files/HK4225 Build Review/31.jpg)
If you want, you can already now add the extra top windings ( zig-zag) if the neighborhood teeth are finished with the first layer. This can also bee done at the end when all teeth's are wound with the first layer.
![](/files/HK4225 Build Review/32.jpg)
Seen from the front.
![](/files/HK4225 Build Review/33.jpg)
The 4th group is finished.
![](/files/HK4225 Build Review/34.jpg)
Picture shows group 1 to 4 finished with first tooth of group 5.
![](/files/HK4225 Build Review/35.jpg)
5 finished groups seen from front.
![](/files/HK4225 Build Review/36.jpg)
And from the back.
![](/files/HK4225 Build Review/37.jpg)
Only one tooth remaining.
The first layer is almost finished, but without the extra top winding ( zig-zag).And from the back.
![](/files/HK4225 Build Review/38.jpg)
All teeth’s finished.
From the front you can see alternating 5 and 6 wire buckles and 11 wires in every slot
![](/files/HK4225 Build Review/39.jpg)
Seen from the back, there must be a wire end in every slot.
![](/files/HK4225 Build Review/40.jpg)
If you realize now the extra top windings all together, take the end of every group and ad a winding to the tooth that has actually 5 windings.
It will fit normally between the existing winding.
Every tooth has now 6 windings, means 12 wires in every slot.
![](/files/HK4225 Build Review/41.jpg)
If you add the seventh winding on the first tooth where you have added the 6th one or on the neighbor tooth with initially 6 windings is electrically without any difference.
It is solely the personal preference of the winder, in this example the extra windings are in an 8 pattern (5+6 gives finally 6+7)
The groups are fully finished now.
![](/files/HK4225 Build Review/42.jpg)
Another possible winding: Here you see 6+7 1.32YY. Done with groups of 5 + 5 + 2 zig + 1 zag
![](/files/HK4225 Build Review/43.jpg)
To ease the induction between the teeth’s, it is strongly recommended to round the end of the wire in order to avoid damages on the already done winding.
The copper wire edges are sharp and can damage our winding insulation.
![](/files/HK4225 Build Review/44.jpg)
The needle (without sharp tip) is use as to make way for the wire as seen on the picture.
![](/files/HK4225 Build Review/45.jpg)
Here you see the preparation for the connecting of the wires.
End group 1,start group 2 and end group 3 is the first star point.;
Start group 4, end group 5 and start group 6 gives the second star point.
Start group 2, end group 2 and start group 3 are the feeding wires for partial motor 1.
End group 4, start group 5 and end group 6 are the feeding wires for partial motor 2.
Every partial motor is able to run on his own at this stage.
The 2 partial motors are connected in parallel as follows:
Start group 1 with end group 4
End group 2 with start group 5
Start group 3 with end Group 6
Every possible winding combination suitable for 12N10P stator packages can be realized and connected as Y, D , YY or DD!
![](/files/HK4225 Build Review/46.jpg)
To trim the excess insulation paper, I use the method shown in the picture.
The blade is inserted in the slot and the knife is running on it in order to protect the insulation of the windings.
![](/files/HK4225 Build Review/47.jpg)
I add some plastic coated glass fiber tube on the feeding wires (155°C resistance).
Shrinking tubes can also bee used.
The star points need to be stripped from the insulation.
I scrape the insulation of with a scalpel until the copper is shiny.
![](/files/HK4225 Build Review/48.jpg)
The shiny ends are wound together.
![](/files/HK4225 Build Review/49.jpg)
Put them in place and trim the excess.
Take care to protect the winding underneath during these operations.
![](/files/HK4225 Build Review/50.jpg)
Proceed the same way with the second star point.
![](/files/HK4225 Build Review/51.jpg)
Solder the wires of the star points together.
The picture shows the results if stripping and soldering are well done.
![](/files/HK4225 Build Review/52.jpg)
Both star points are protected the same way as the feeding wires.
![](/files/HK4225 Build Review/53.jpg)
Let’s assemble now the stator on the stator holder and fix it with the anti rotating pin.
![](/files/HK4225 Build Review/54.jpg)
I used a 2mm piano wire and a small hammer to drive in the anti rotating pin.
The pin is fitting tightly into the groove.
![](/files/HK4225 Build Review/55.jpg)
The anti rotating pin is fixed now.
![](/files/HK4225 Build Review/56.jpg)
Alternatively you can make the fixation of the stator package removable.
To do this, you need make a M2 treats in the 1.5mm hole.
![](/files/HK4225 Build Review/57.jpg)
Then you add a M2x12 hexagonal head set screw.
![](/files/HK4225 Build Review/58.jpg)
The crew needs to be inserted until there is no remaining over the bearing seat.
![](/files/HK4225 Build Review/59.jpg)
To fix the windings I use the bi-component and 200°C resistant Scorpion glue system.
Alternatively "ENDFEST 300" can be used.
![](/files/HK4225 Build Review/60.jpg)
After a short time the glue will run into the slots.
Now lets wait 24 hours until the glue is fully cured.
![](/files/HK4225 Build Review/61.jpg)
The bearing is inserted into the stator holder backplate.
Never push on the inner Ring to drive it into the bearing seat. Always use the outer bearing ring. Secure with some Loctite (638,620 or 648)
![](/files/HK4225 Build Review/62.jpg)
Then add some Loctite (638,620 or 648) or similar to the stator package hole.
![](/files/HK4225 Build Review/63.jpg)
Always use a support on the outer bearing ring to drive the bearing into the stator bore.
![](/files/HK4225 Build Review/64.jpg)
The bearings are mounted now.
![](/files/HK4225 Build Review/65.jpg)
The bell is completed and ready to use. The only thing that needs to be added is a bearing trust washer on the shaft between bearing shoulder and bell-bottom.
This washer is not part of the kit. It is strongly recommended to put it in as it will prevent the break in of the hard steel bearing shoulder into the weaker aluminum of the bell-bottom.
An additional check can be performed on the magnets inside the bell. Use a small magnet to check the polarity.
The test magnet must be attracted and pushed away alternatively on every magnet of the bell.
![](/files/HK4225 Build Review/66.jpg)
Then fist assembly is done to check mechanical functions.
![](/files/HK4225 Build Review/67.jpg)
The shaft is pushed in to allow the circlips and the spring ring to secure the position of the bell free from axial play.
I had to push my shaft approximately 3 mm out of the bell-bottom plate.
![](/files/HK4225 Build Review/68.jpg)
Circlip and the springring shown standalone on this picture.
Circlip is handled ideally with the appropriated pliers.
![](/files/HK4225 Build Review/69.jpg)
Here you see both mounted on the finished Motor.
![](/files/HK4225 Build Review/70.jpg)
The setscrews on the bell-bottom are tightened now. You should secure them with some threat-lock.
![](/files/HK4225 Build Review/71.jpg)
Motor ready to run.
For security, check if there is no short circuit between winding and casing.
If the motor turn quite easy and there is no strong cogging, everything should be ok.
If the restistence between the cogging is strong, in general there is an error in the connections of the windings.
Some hinds for error chasing can be found here!
After the first extended run, recheck the axial play and correct it if necessary.
First run will break-in the mechanics and can result in some increased axial play.
![](/files/HK4225 Build Review/72.jpg)
Test run is performed with an YGE controller and a Kontronik JIVE120HV.
The scratches on the back plate and the removed anodizing in the screwholes are a result of an extended test run and are normal and are in no case altering the quality of the motor.
![](/files/HK4225 Build Review/73.jpg)
Finally a small performance diagram of another motor from the same series.
Test was performed with 5x1.5YY winding driving a 10x20 speed propeller.
Peak performance was 9500 W during 2 seconds; current 250 A.
Over a period of 10 seconds, the measured power input was over 5000W.
The following windings are tested and can be realized by a skilled winder with some training:
4+4 x(2x1,25)
4+5 x 1,6
5+6 x 1,5
6+6 x 1,4
6+7 x 1,32
7+7 x 1,25
8+8 x 1,18
.....