Motor theory, ac vs bldc

From the perspective of best efficiency over a wider range of speed and torque the "BLDC" or permanent magnet synchronous motor (PMSM) is best. If however you want to brake speed records with your 48V small electric car you might be better off with an AC-induction motor :-)

I agree that synchronous motors are the best, but you also have to take in account the way they are driven.
The most efficient is using a servo drive, that keeps the magnetic flux perpendicular to the rotor windings (zero torque ripple). The problem is that a servo drive needs to know in every moment the angular position of the rotor and to do so it needs a hi-resolution feedback device (resolver, encoder)
BLDC is not a particularly efficient system of driving a brushless motor, since it is a simplicity-oriented tradeoff that uses only the hall sensors (with a 120° resolution). This, in a 3 phase system, gives about 50% torque ripple since the flux is not always perpendicular to the rotor.
In between you can find 6 phase motors (efun) that use 6 windings 3 by 3 like two smaller motors tied together. In this case the torque ripple is reduced since the maximum phase lag is halved.

Modern asynchronous drives use a mathematical model to calculate actual rotor position and are replacing, in industrial environment, the synchronous brushless motors where there is no need of extreme performance/ high torque at low-zero speed.

Modern asynchronous drives use a mathematical model to calculate actual rotor position and are replacing, in industrial environment, the synchronous brushless motors where there is no need of extreme performance/ high torque at low-zero speed.

This can also be done for synchronous motors

Sevcon, for instance, do this for vehicle motors

it still uses the 120 deg hall sensors, but calculates actual rotor position to a far greater accuracy, applies a sine wave and does field weakening. It may even be setup to do field strengthening.

it's a real pain to setup - but the results are outstanding

The D.C.machine has high efficiency, so with the same weight or size conditions,it is better than the AC motor, so it is widely used in portable electric tools
The A.D motor: simple structure,easy for repair and maintenance

The D.C.machine has high efficiency, so with the same weight or size conditions,it is better than the AC motor, so it is widely used in portable electric tools
The A.D motor: simple structure,easy for repair and maintenance