İf you mean trapezodial back EMF pmsm,
İt has harmonic content in back EMF so some torque ripple will occur at motor torque.
İt is expensive because of the permanent magnets.
There is temperature limit on rotor because of the magnets
Demagnetization possibility also limits the input current of BLDC.
Dear Rajiv,
In addition to what Yousuf has quoted, one more disadvantage is that the rotor is a permanent magnet structure. A permanent magnet is inferior to an electro magnet ( a wound rotor) in terms the flux density it can produce. One of the factors which directly influence the torque and rating of the machine is the magnitude of rotor flux. I think the possibility of very high rating machines is remote. There are some companies which are supplying even 10kW BLDC motors for the electric vehicles. I think beyond this rating, the construction of the rotor as well as the motor itself will not be economical.
Dear Prakash,
Thanks! Can you share the list of manufacture those are providing the 10kW BLDC motor for vehicles. I will be thankful to you for this act of kindness.
Try the following links. Hope you find them useful.
http://www.made-in-china.com/products-search/hot-china-products/Electric_Car_Motor.html
http://www.goldenmotor.com/
The maximum power that can be applied to a BLDC motor is exceptionally high, limited almost exclusively by heat, which can weaken the magnets. BLDC motor's main disadvantage is
http://pediain.com/seminar/Brushless-Dc-Motor.pdf
1.Limitation of the BLDC is mainly in power rating.Generally PMSM with trapezoidal winding distribution are referred to as BLDC,the ones with sinusoidal winding distribution are referred to as PMSM.I assume that you are referring to PMSM (trap as well as sine).PMSM are used for power outputs upto 20 KW.Above 20 KW vector controlled induction motor are predominating for specialist applications.
2.other major limitation is thermal withstanding of the permanent magnets.Samarium cobalt magnets are assumed to have the least temp effect(2-3% per 100deg celsius),neodymium magnets have 12 to 13% per 100 der rise in temp,ceramic mag have 19% per 100 deg rise in temp.
3.magnets are in the air gap of the machine and therefore are exposed to all of the time and space harmonics of the stator winding mmf(some mag have elec conductivity),any asynchr fields will result in eddy currents and consequently loss in the magnet
I agree with the views of Mr Prakash kumar "
A permanent magnet is inferior to an electro magnet ( a wound rotor) in terms the flux density it can produce. One of the factors which directly influence the torque and rating of the machine is the magnitude of rotor flux''
flux concentrating type PMSM are designed to overcome this above mentioned limitation.
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