Because an induction motor creates its rotor field by inducing current in the rotor, and that current generates heat as an inherent part of how the machine works. A permanent-magnet rotor provides its field without any current, so those losses are absent. The gap widens at part load, which matters because fans, pumps and compressors spend most of their operating hours well below full duty.

Less than the terminology suggests. All are permanent-magnet synchronous machines. Brushless DC generally refers to trapezoidal commutation with simple position sensing, permanent-magnet synchronous to sinusoidal commutation with higher-resolution feedback and smoother torque, and electronically commutated is a commercial term for a motor with its drive electronics built in, common in ventilation and pumps. The distinction is control, not construction.

Generally no. They need electronic commutation to rotate at all, so a drive is not optional. An electronically commutated motor contains that electronics and accepts a mains supply plus a control signal, which makes it look like a direct replacement. A permanent-magnet synchronous motor normally requires a separate matched drive. Line-start permanent-magnet motors exist but are a specialised subset rather than the norm.

Ventilation and air handling fans, refrigeration evaporator and condenser fans, circulation and heating pumps, fan coil units and air curtains. The common factor is continuous or near-continuous running at varying duty, where part-load efficiency dominates lifetime cost, and where the ability to control speed smoothly replaces cruder methods such as throttling, dampers or multi-tap windings.

Mechanically often yes, electrically not automatically. The commutation electronics must be accounted for, whether integral or external, and the control method usually changes: an electronically commutated fan expects a speed signal rather than a contactor. Existing controls, wiring and any speed regulation scheme need reviewing. Frame and shaft compatibility should also be confirmed rather than assumed from nominal power.

Permanent-magnet machines use rare earth magnets in most designs, which makes them more expensive than an equivalent induction motor and exposes them to magnet material pricing. The case is made on running cost over the operating life rather than on purchase price, so it depends on running hours and duty. For intermittent or lightly loaded applications the payback may not materialise.

Both give variable speed, but the permanent-magnet machine is more efficient at every point, particularly at low load, and generally more compact for the same output. A variable frequency drive on an existing induction motor is cheaper to implement and uses equipment already installed. For new plant running long hours, the permanent-magnet option usually wins on lifetime cost; for retrofit, the drive often does.