HESB Asynchronous-Machines-TEST-2

Phrases

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Theory

What is asynchronous machine?

• Definition: “Asynchronous (=induction) motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.”

• Asynchronous machines enable electromechanical conversion of energy.

• An induction motor's rotor can be either wound type or squirrel-cage type

• Energy flow in between the main parts of the motor (stator and rotor) is implemented exclusively using electromagnetic induction (asynchronous motor = induction motor).

• Usually is asynchronous motor power from common AC network. Supply can be single-phase or three-phase.

• Asynchronous machine is connected to directly mains and can operate in limited speed range, or its working is non-effective. Today it is possible to operate within a wide speed range thanks to frequency converters.

Principles of operation

The basis of asynchronous machine operation is creation of stator rotating magnetic field, which is produced by an alternating current three-phase through passing the stator winding. This magnetic field induces voltages in rotor and generated rotor current of rotor induces magnetic flux, which is coupled to the stator. Coupled magnetic flux (= spřažený magnetický tok) induces force acting on rotor and therefore the rotation of the rotor.

The speed of the rotating field of the stator (= synchronous speed) are given by frequency of voltage supply and by numbers of poles of three-phase motor.

f… current frequency
p… the number of pole pairs of the stator

Asynchronous machine can induce torque on the shaft only when, the rotational speed of stator magnetic pole is different compared to mechanical rotor speed, of the slip. At zero slip in the rotor voltage is not induces, current doesn't flow and there is no rotor part of coupled magnetic field machine. At synchronous speed (= synchronní otáčky), stator magnetic field against rotor is not moving at all, thus there is no voltage induction and no torque.

The rate of rotor speed difference is called slip,it is given in percentage and defined as following:

ns… synchronous speed of the magnetic field in the stator

n… mechanical rotor speed

According to values of slip we can divide asynchronous machine into regions of operation:

generator, normal limits:

motor

brake, normal limits: