Motor Used In Fans

Go and read your textbooks.

Or give some kind of criteria to base a decision on.

Are you talking about fan in your home OR fan in a large industry ? The type of motor also depends on the type of voltage available.

Usually asynchronous.

(There normally is no need to run a fan at an exact speed.)

I guess all induction motors are asynchronous.

No. Induction motors are of two types. Asynchronous and Synchronous.

The common induction motor is synchronous when there is no load.

When some load is applied, it 'slips magnetically' to an asynchronous speed.

E.g 4 pole on 50Hz will run unloaded at ~1500 RPM. "Rated rpm" is ~1440.

Meaning ~60 rpm of "asynchronous" is expected at rated load.

When you say induction motor it is usually asynchronous.It can be a cage or wound rotor motor.A synchronous motor on the other hand is usually used in large rating with very slow speeds like 18 0r 20 pole where the power factor becomes low if induction motor is used and in synchronous Pf can be made high by varying the excitation.Induction motor terminology normally refers to cage or wound rotor (slip ring) motor and does not include synchronous motors.In most fan applications Cage induction motors are normally used with polarity of 4 ,6 or 8 as required.The above does not cover fans used at home where single phase motor of capacitor type or other designs are used.

I'd buy the one that was on sale.

The way that induction motors work is that there is a rotating magnetic field which "pulls" the rotor. (In single phase, this rotation is achieved by a shaded pole, which is a copper loop around part of the winding and the delay caused by the induced currents causes the field to rotate rather than just alternate back and forth.) The rotor has slots cut in it that provide conduction loops. The currents induced in the rotor interact with the rotating field to cause rotation. If the rotor is rotating at the same speed as the rotating field, it "sees" only a static field and no torque is produced. To produce any torque, the rotor has to slip behind the field (rotate slower), and is therefore asynchronous.

There are mainly 2 types of motors-ac motors & dc motors

for ac motors are further classified to

1.Synchronous motors-they run at only one speed

2.Induction Motors-their speeds vary based on load

ALL INDUCTION MOTORS ARE ASYNCHORONOUS MOTORS & if you are talking about fans used for cooling purpose, then it is definitely belong to Induction motors-may be it house hold or industrial.This is because Fans do not need constant speed & Induction motors are rugged & better suited for fans

Assuming that your question is about motors for industrial fans/blowers, normally I would go for any motor that gives a very good starting torque - as the inertia of fans/blowers and thus the starting torque requirements would be high. Mind you! Amongst the many possibilities, I would go for the most cost effective one - not only capital cost but also the operating cost.