How to Choose the Bushless Motor Magnets?
After you choose the slots and poles, you need to choose the magnets. This is not very easy task because you need high temperature magnets and are not so cheap and easy to find for your needed size. I bought custom made magnets from a Chinese SUPPLIER MPCO MAGNETICS.
The temperature rating for Neodymium is only a guide value. The actual temperature where magnet start to loose strength is size, shape and magnetic circuit dependent. If you have a magnet attached to a piece of steel i will demagnetize at higher magnetic flux than in a free space. On the other hand demagnetization temperature will be lower if you subject the magnet to a strong opposite magnetic field such in a motor.
If the thickness of the magnet is bigger and you will need a bigger magnetic field to start demagnetize it.
Temperature classification for neodymium magnets, N stays from Neo MAGNET.
Magnet grade Temperature
Neodymium magnets needs a coating otherwise they will rust in contact with air.
The coatings can be: Nickel, Zinc, Phosphate, Epoxy, Gold and others.
Bushless motor design – Choose the Bushless Motor Magnets
After successfully building a brushless controller i have decided also to build my own brushless motor. Autocad Inventor 3D cad software was used by me to design 3D model of the motor.
Before start to design something you need to know the RPM needed,torque needed,running voltage, max Amps. This formula is for calculating the torque if you have the power and the speed. motor Torque foruma After reading the gear ratio on Opel Agila it resulted that i need a speed of 4000RPM to reach 73.4Km/h in 3-rd gear ( i do not need more than that in a city). I decided to make a reverse outrunner motor design because is more easy to cool the stator if is outside. If the sator is inside is more difficult to cool down. The draw-back is that you loose torque because you have a smaller diameter of the rotor. I opted for a 48 slots (teeth) and 40 magnets design and i will have 142.5Nm of torque.
Nominal Power: 45KW
Nominal Voltage: 230v
Winding configuration : Delta
Construction : 48 slot, 40 Magnets Neodymium
Lamination Grade M330
Cooling : Glycol
Copper weight ~ 2.7kg
Can be : delta or Star ( WYE)
Delta connection will give you higher power per cooper amount, higher RPM, higher current, lower phase voltage.
STAR will give you lower RPM, higher torque (1,73 more than dela) , higher the voltage, lower the current.
Can be: concentrated or fractional slot type.
If there are concentrated can be: LRK, distributed LRK etc.
A good winding scheme calculator can be found here.
Mathematical magnetic flux density analysis.
Next step is to run mathematical analysis in magnetic field to see if i have some areas with saturated magnetic field. We want to avoid core saturation. For this i used Finite Element Method Magnetics Tool A Windows finite element solver for 2D and axisymmetric magnetic, electrostatic, heat flow, and current flow problems with graphical pre- and post-processors.
We can observe that i have areas in pink color with to much magnetic flux, above 2 Teslas, so i need to increase the thickness of the tooth to stay under 2 Teslas because the saturation of the laminations.
Multiple factors can have a huge difference in motor performance and efficiency:
This factors can be:
1. Maximum working frequency (depending on RPM and no. of poles). The frequency is calculated by next formula:
f= rps (motor rotation per second) x (nr. of poles/2). no.of poles is equal with no. of magnets.
or: f[hz]= Magnets nr. x rpm / 120
Example for 1000Rpm: the rps will be 1000rpm/60s = 16,66 then f=16,22 x 40poles/2 will result in: f= 333.2Hz
Because the losses in the core lamination are increasing with increase (non linear) in frequency we want to have a frequency as low as possible for max motor RPM. For exampe for lamination grade M330-50 the losses at 50Hz and 1Tesla are 1,29W/kg but 132W/kg at 1000Hz.
2. Proper combination between slots and poles count.
3. Material properties and thickness of stator and rotor laminations.
4. Air gap thickness.
5. Magnets grade.
6. Current density.
7. Slot fill factor.
8. Cogging torque. A summary of techniques used for reducing cogging torque:
Skewing stator stack or magnets
Using fractional slots per pole
Modulating drive current waveform
Optimizing the magnet pole arc or width
The final motor without the caps.
The motor with the caps and without the ball bearings.
I have also an YouTube video to present the 3D model.
Design stage Brushless motor 60Kw
Brushless motor 50Kw – Choose the Bushless Motor Magnets
Close up view of the coils, magnets.
Brushless 40 poles 48 slotes 60kw
Air gap 0.75mm
Custom Magnets received and tested by heating up to 120 Celsius to see if any drop in magnetic field occurs.
Custom cut, N35UH grade, Phosphate coating magnets
Because the laser shop in Romania only had M330-50 grade, i was forced to use this material with higher losses for high RPM (frequency)
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