Sparta Ion front wheel hub motor

[Adriaan Kragten]

The new public report KD 745 has been placed on my website: https://www.kdwindturbines.nl at the menu KD-reports. The title of this report is: "Investigation of the Sparta Ion front wheel hub motor as generator for a small wind turbine". The cogging torque and the open DC voltage have been measured. The conclusion is that the peak on the cogging torque is too high to use this motor as a generator. Figure 3 out of this report is added as an attachment to show the test rig.

[mbouwer]

Certainly enough space to mount a new axle.

[Adriaan Kragten]

Certainly enough space to mount a new axle.

(Attachment Link)

No, there is not more space if the original bearings are used. The bearings have an inside diameter of 12 mm. There is a 6 mm wide and about 4 mm deep groove in the shaft through which the cable is guided. This groove is weakening the shaft seriously. There is no other way to get the cable from the inside stator to the outside than through a groove in the shaft. In the bicycle, the shaft is supported at two sides but for a wind turbine only the back shaft end is used. This strongly limits the maximum rotor diameter which can be used. Using a bigger bearing at the back side is also not possible because the bearing cover can't have a bigger bearing. But the biggest problem is the very large peak on the cogging torque.

[mbouwer]

If you remove the original axle there is about 80 mm space.
Just say that with ball bearings you then can apply a (hollow) shaft of roughly 40 mm.

Can we not assume that with the blades mounted the cogging is negligible?

[Adriaan Kragten]

If you remove the original axle there is about 80 mm space.
Just say that with ball bearings you then can apply a (hollow) shaft of roughly 40 mm.

Can we not assume that with the blades mounted the cogging is negligible?

(Attachment Link)

The peak on the cogging torque is only negligible once the rotor is rotating because then the rotor is working as a fly wheel. But to start from stand still position, the rotor has to supply the peak torque which is 0.692 Nm. I have also measured the average cogging torque if the hub is running slowly and it is 0.315 Nm (see chapter 3). The peak torque results in a very high starting wind speed of more than 5 m/s if a rotor with a diameter of 1.5 m is used. I have made some provisional calculations of such a rotor and it is impossible to give it a starting torque coefficient which is high enough without getting a too low design tip speed ratio.

Using a bigger bearing at the cable side means that you have to make a new bearing cover and a new shaft and this cancels the whole idea of using a cheap generator. So that will cost a lot of money and still you end up with a generator with a very high peak on the cogging torque. So this confirms my conclusion that this wheel motor is useless. I think wheel motors of other manufacture will have the same problem. You need a generator with a much thicker shaft like the generators supplied by Hefei Top Grand.

[ruddycrazy]

That shaft does look like it would press out quite easily then a more thicker shaft can be machined to provide stability, now the F&P washing motor does have cogging but with a decent sized blade it isn't a problem.

Nothing ventured nothing gained

[Adriaan Kragten]

That shaft does look like it would press out quite easily then a more thicker shaft can be machined to provide stability, now the F&P washing motor does have cogging but with a decent sized blade it isn't a problem.

Nothing ventured nothing gained

I thicker shaft requires a bigger bearing and a bigger bearing requires a totally new bearing cover. This is a cast aluminium component and who can make it? Such a modification isn't as simple as it sounds. I have made calculations for a 4-bladed rotor with stainless steel 7.14 % cambered blades, a design tip speed ratio of 4 and a rotor diameter of 1,78 m and even with a rotor that big, the starting wind speed is 4.3 m/s. So a peak on the cogging torque of almost 0.7 Nm is really a big problem. I think you can better try to find a generator with a lower peak on the cogging torque. But if someone wants to use this hub motor, he should do it but then he should not be supprised to find a bad starting behaviour. Direct drive washing machine motors have similar problems.

[Adriaan Kragten]

There might be a way to use a stronger shaft without modification of the bearing cover. I assume that the used bearing has size 12 * 32 * 10 mm. Assume that the diameter of the shaft is increased from 12 mm up to 15 mm and that a bearing 15 * 32 * 9 mm is used. The bearing code of a sealed bearing is 6002-2RS1. The static and dynamic load factor of this bearing are about a factor 0.8 lower than that of the original bearing but the bearing might still be strong enough. A 1 mm thick washer has to be added to get the same total thickness of 10 mm. Now it is possible to give the shaft a central hole of 5 mm in stead of a 6 mm wide and 4 mm deep outside groove. There must be an about 45° radial entrance to this central hole as close as possible to the front bearing. A central hole weakens the shaft much less than an outside groove. No thread should be made in the shaft and the shaft should be clamped in the head frame. A 15 mm shaft might be strong enough for a rotor diameter of about 1.8 m if the windmill is equipped with a proper safety system which limits the rotational speed and the yawing speed.

Another option might be to use an INA needle bus HK 2512 which can be pressed into the front bearing cover as it has an outside diameter of 32 mm. If this bearing is used in combinationn with a needle bush size 20 * 25, a 20 mm shaft can be used. But the problem of the chosen needle bush is that it has no seal. Needle bushes with seals are too thick.

[mbouwer]

A while ago for a windmillfriend I made a setup with a new heavy shaft for a direct drive washing machine motor.
But whilst doing that I wasn't really motivated because he didn't intend to apply blade adjustment.

[ruddycrazy]

If one made a jig up so one could twist each pole this would reduce the cogging by a big margin, even just twisting so each pole touch's the one next to it then go right around to finish. Then go do the same tests and see if twisting the poles work as they sure do using a F&P Washing machine motor.

[Adriaan Kragten]

In the photo of mbouwer it can be seen that the armature has 20 poles and that the stator has 24 poles. So the armature pole angle is 360 / 20 = 18° and the stator pole angle is 360 / 24 = 15°. So the difference in pole angle is 3°. Assume that the armature has a preference position if an armature pole is just opposite to a stator pole. This changes every 3° and so the armature should have 360 / 3 = 120 preference positions per revolution. However, in practice it appears that it has only 20 preference positions per revolution and the peak on the cogging torque is therefore much higher than what if should be theoretically. I found the following reason for this phenomenon.

The flat magnets are glued in the armature at a certain distance of each other. There are no machined flat grooves in the steel armature ring which position the magnets at exactly the correct place. So it might be that the magnets arn't positioned exactly at an angle of 18°. It might also be that the thickness of the glue layer isn't the same for all magnets. This can make that there is one of six preference postions for which the magnetic flux flows easier from armature to stator than for the other five preference positions. This makes one of six preference positions stronger than the other five. If you rotate the armature, you only feel the strongest preference positions. I have tried to find the five less stronger preference positions in between the strongest one and they can be felt. So inaccurate manufacture is part of the problem.

[MattM]

I would assume once you overlap magnet field they will work on the macro scale together to self assemble into alternating zones around a singular central zone.  Magnetic fields appear to form circuits as a composite when they interact.  Basically some cancel each other out and what you have left over is a pattern of magnetic zones that alternate direction.  Its called chirality.

[Adriaan Kragten]

I would assume once you overlap magnet field they will work on the macro scale together to self assemble into alternating zones around a singular central zone.  Magnetic fields appear to form circuits as a composite when they interact.  Basically some cancel each other out and what you have left over is a pattern of magnetic zones that alternate direction.  Its called chirality.

Is this another explanation for the fact that it has 20 preference positions in stead of 120?

[MattM]

It would explain why, unless you put great detail into size and shape of your fields, they may not pan out on the micro scale of simply accounting for individual magnets.  The more I look into magnetism the more it gets complicated.  It would explain there is more going on than chocking it up to build quality.  It has this chiral nature that probably explains what you are seeing.

[Adriaan Kragten]

A new chapter 5 has been added to report KD 745 about the Sparta Ion hub motor. The title of this chapter is: "Designing a small rotor with a high starting torque coefficient". It appeared that the peak on the cogging torque of this generator is very high (about 0.7 Nm) and so in the first instance it was thought that designing of a windmill rotor which has a sufficiently low starting wind speed was too difficult. However, the generator is rather cheap and it would be a pity if it couldn't be used. So in chapter 5, I have designed a rotor for which I think that it can be used. The rotor has a diameter of 1.4 m, four steel blades with 15° bent edges and a rather low design tip speed ratio of 2.5. The starting wind speed is 3.2 m/s. The stopping wind speed is 1.5 m/s. So the starting behaviour seems acceptable. The cut in wind speed for 12 V battery charging is 2.5 m/s (if the rotor has started). The mass of the rotor is about 6.5 kg. As the generator hasn't been measured for a 12 V battery load, it wasn't possible to determine the Pel-V curve but I expect that the maximum electrical power is about 75 W for a wind speed of 9 m/s and higher. A sketch of the rotor is given in figure 5 which has been added as an attachment.

[Adriaan Kragten]

A new chapter 6 is added to report KD 745. The title of this chapter is: "Design of the 3-bladed VIRYA-1.25B3 rotor with lambda design = 2.25". This rotor is smaller and lighter than the 4-bladed rotor as described in chapter 5. Every blade now has its own spoke and this result in less sawing and less material waste. The starting wind speed is somewhat higher than that of the 4-bladed rotor (3.7 m/s instead of 3,2 m/s) but I think that this is allowed because the rotor stays turning up to a very low wind speed once started. A picture of this new rotor is given in figure 10 which is added as an attachment.

[Adriaan Kragten]

Chapter 6 of KD 745 has been modified completely. The 3-bladed VIRYA-1.25B3 has been replaced bij the 3-bladed VIRYA-1.38 because the starting wind speed of the VIRYA-1.25B3 was too high. The P-n curves of the 3-bladed VIRYA-1.38 are about the same as the P-n curves of the 4-bladed VIRYA-1.4 as described in chapter 5 but less material is needed for the VIRYA-1.38. Manufacture of three separate spokes is also easier than manufacture of the hub plate of the VIRYA-1.4. A sketch of the VIRYA-1.38 is added as an attachment.