Report KD 645 about 10-pole PM-generator available

[Adriaan Kragten]

The title of this report is: "Ideas about a 10-pole, 3-phase permanent magnet generator with a stator without iron in the coils, using 20 neodymium magnets size 50 * 20 * 10 mm and a housing of an asynchronous motor with frame size 80". The most important difference in between this 10-pole PM-generator and the 8-pole PM-generator as described in report KD 644 is that the stator is provided with a simpler 1-layer winding. KD 645 can be copied for free from my website: www.kdwindturbines.nl at the menu KD-reports.

[Adriaan Kragten]

A new chapter 6 "Alternative" has been added to KD 645. In this chapter it has been investigated what effect has a thin iron stator ring on the flux density in the coils. The iron ring is made out of the original stator stamping by removing the spokes. The total air gap is now much smaller and the flux density is substantially increased. As the stator has no poles, the sticking torque still won't fluctuate. So no starting problems can be expected.

[joestue]

I wonder what the cost would be to wire edm cut the teeth out of a stamping.

They could be bored out on a lathe but finish grinding and acid etch is probably needed to prevent the laminations from shorting together.

[Adriaan Kragten]

Chapter 6 of KD 645 has been completely changed. The new tittle of this chapter is: "Alternative winding with 15 coils". The original winding is a 1-layer winding with two coils per phase, so totally six coils. The alternative winding has no sleeves in the Delrin bush but thirty 8 mm holes. The armature has a length of 80 mm in stead of 100 mm and one magnet size 80 * 20 * 10 mm is used in each groove. It is expected that the reduction of the power because of the shorter armature is compensated because now per phase five coils are used in stead of two coils. To give an impression of this alternative winding I have added figure 3 of KD 645 as an attachment

[topspeed]

Could that be used in a fast spinning H-Darreus too ?

[Adriaan Kragten]

It depends on the type and dimensions of the Darrieus rotor. Most Darrieus rotors have a negative torque for low tip speed ratios, so the generator must be able to work as a motor to reach the tip speed ratio where the torque is positive. The described PM-generator can only work as a motor if it is connected to a special inverter which transforms the battery voltage into a 3-phase AC current with variable frequency. Such a device may exist but it will be very expensive. For the transformation of a 3-phase current into a DC current one only needs a 3-phase rectifier which is a cheap component.

An advantage of the described PM-generator is that there is no iron in the coils and therefore the sticking torque is low as it is only caused be the friction of the bearings and the seal on the rotor shaft. As a housing of an asynchronous motor is used, the winding and the magnets are well protected against water and dust.

The described generator has a shaft diameter of only 20 mm at the bearings and the radial load on the shaft is therefore limited. A medium size Darrieus rotor may give a too high radial load on the shaft and the front bearing if the generator is used as the bottom bearing of the Darrieus rotor. So the Darrieus rotor must probably have its own top and bottom bearing. I is certainly not allowed to use the shaft and the bearings of the generator as the shaft the and the only bearings of the Darrieus rotor.

[topspeed]

It depends on the type and dimensions of the Darrieus rotor. Most Darrieus rotors have a negative torque for low tip speed ratios, so the generator must be able to work as a motor to reach the tip speed ratio where the torque is positive. The described PM-generator can only work as a motor if it is connected to a special inverter which transforms the battery voltage into a 3-phase AC current with variable frequency. Such a device may exist but it will be very expensive. For the transformation of a 3-phase current into a DC current one only needs a 3-phase rectifier which is a cheap component.

An advantage of the described PM-generator is that there is no iron in the coils and therefore the sticking torque is low as it is only caused be the friction of the bearings and the seal on the rotor shaft. As a housing of an asynchronous motor is used, the winding and the magnets are well protected against water and dust.

The described generator has a shaft diameter of only 20 mm at the bearings and the radial load on the shaft is therefore limited. A medium size Darrieus rotor may give a too high radial load on the shaft and the front bearing if the generator is used as the bottom bearing of the Darrieus rotor. So the Darrieus rotor must probably have its own top and bottom bearing. I is certainly not allowed to use the shaft and the bearings of the generator as the shaft the and the only bearings of the Darrieus rotor.

My HD weighs less than 2 kg...for the rotating parts. Swept area is just 1 m2.

[Adriaan Kragten]

It depends on the construction of the rotor if the bending moment in the generator shaft becomes too large at high wind speeds. If the rotor has a diameter and height of 1 m, the resulting thrust on all blades will exert 0.5 m from the bottom side of the rotor. If the whole Darrieus rotor is mounted above the generator, this long distance will certainly give a too high bending moment in the shaft at high wind speeds. It might be that your Darrieus rotor has a horizontal frame half way the rotor height and if this frame is mounted on the generator hub, the bending moment will be much smaller. But now the generator and the tower pipe will give a lot of turbulence at the lowest halve of a blade when the blade is at the backside of the rotor.

A Darrieus rotor with a swept area of only 1 m^2 will run at very low Reynolds values when it is starting. So the aerodynamic drag on the airfoil will be large as the blade will stall at a large part of a revolution. I expect that the sticking torque of the generator of only the bearings and the shaft seal will be too high to let the rotor start, whatever construction the rotor may have. The generator is meant for a HAWT with a rotordiameter of about 2.2 m and the starting torque of such a rotor is large enough at low wind speeds.

[topspeed]

It depends on the construction of the rotor if the bending moment in the generator shaft becomes too large at high wind speeds. If the rotor has a diameter and height of 1 m, the resulting thrust on all blades will exert 0.5 m from the bottom side of the rotor. If the whole Darrieus rotor is mounted above the generator, this long distance will certainly give a too high bending moment in the shaft at high wind speeds. It might be that your Darrieus rotor has a horizontal frame half way the rotor height and if this frame is mounted on the generator hub, the bending moment will be much smaller. But now the generator and the tower pipe will give a lot of turbulence at the lowest halve of a blade when the blade is at the backside of the rotor.

A Darrieus rotor with a swept area of only 1 m^2 will run at very low Reynolds values when it is starting. So the aerodynamic drag on the airfoil will be large as the blade will stall at a large part of a revolution. I expect that the sticking torque of the generator of only the bearings and the shaft seal will be too high to let the rotor start, whatever construction the rotor may have. The generator is meant for a HAWT with a rotordiameter of about 2.2 m and the starting torque of such a rotor is large enough at low wind speeds.

What is the max teorethical out put of it ?

[Adriaan Kragten]

As a prototype of the generator has not yet been built and measured, I don't know the maximum output for a certain load and rotational speed. The maximum output of any PM-generator depends very much on the rotational speed and the loaded voltage. The higher the rotational speed, the higher the maximum power. But more power also means more heat losses and finally the maximum power is limited by the allowable temperature of the winding. The power is about maximal if the loaded voltage is 2/3 of the open voltage.

In my public report KD 78, I give measurements for a PM-generator with frame size 90 (and a lengthened stator stamping) for different load conditions. The generator described in KD 645 has frame size 80 and I expect that the power will be in between 1/3 and 1/2 of the power measured in KD 78 for similar load conditions. In chapter 5 of KD 645 it is described how the winding should be determined for a certain battery voltage but for that procedure you need the optimumum cubic line of the wind turbine which is used and I expect that you don't have it for your Darrieus rotor.

[topspeed]

As a prototype of the generator has not yet been built and measured, I don't know the maximum output for a certain load and rotational speed. The maximum output of any PM-generator depends very much on the rotational speed and the loaded voltage. The higher the rotational speed, the higher the maximum power. But more power also means more heat losses and finally the maximum power is limited by the allowable temperature of the winding. The power is about maximal if the loaded voltage is 2/3 of the open voltage.

In my public report KD 78, I give measurements for a PM-generator with frame size 90 (and a lengthened stator stamping) for different load conditions. The generator described in KD 645 has frame size 80 and I expect that the power will be in between 1/3 and 1/2 of the power measured in KD 78 for similar load conditions. In chapter 5 of KD 645 it is described how the winding should be determined for a certain battery voltage but for that procedure you need the optimumum cubic line of the wind turbine which is used and I expect that you don't have it for your Darrieus rotor.

Yes okay...I think I might recoil and remagnetise one of these. It ought to deliver 2 KW at 800 rpm.

That is a direct drive LG washing maschine motor.

It looks a lot like a Fisher & Paukel.

[Adriaan Kragten]

The disadvantage of this kind of PM-generators is that they have a rather large peak on the sticking torque. So you need a rotor with a rather large starting torque coefficient to make the rotor start at low wind speeds. At the bottom of the menu KD-reports on my website you find the note: "Modification of the winding of the PM-generator FP-640". This note explains the winding of a similar generator made from the motor of a washing machine.