Examens De Fin Formation Électromécanique des Systèmes Automatisées
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Characteristics of three-phase asynchronous motor
The three-phase asynchronous motor is the most used in the industry. It has
resulted in more standards, studies and records than any other. This is why
special consideration will be given to it.
In asynchronous or rotating field motors, only the stator is connected to
the mains, the current flowing in the rotor being created by standard
acceptance gives rise to a rotating electromagnetic field which drives the
rotor. To start it requires a launch device. It is a basic and robust motor.
There are three-phase asynchronous motors whose rotor is supplied with
standard bushings.
It is a straightforward and robust motor widely used in the industry.
The speed of a three-phase asynchronous motor is given (to the nearest
slip) by the formula:
|
n=60*f/nb |
F : fréquence du courant alternatif nb : nombre de paire pôles
magnétiques du bobinage inducteur (rotor) N = vitesse en tours/minute |
Benefits
:
•
Low price.
• It
does not necessarily require an auxiliary starting device for low and medium
powers
Disadvantages:
•
Low starting torque.
•
Does not rotate at the speed of the stator field.
•
Use of a three-phase network
The
rotor of the asynchronous motor
Three-phase
asynchronous AC motors with squirrel-shaped rotor.
Their
cylindrical rotor carries copper or aluminum conductors which seem to represent
the generatrices of the cylinder, and the ends of which are joined by two
conductive rings. This outfit draws a border of squirrel on the surface of the
rotor as shown in the classless figure below. This confinement of squirrel is
masked by the sheets of the rotor.
Three-phase
asynchronous AC motors with wound rotor.
It allows you to control the current especially at start-up. On a metal rotor, coils are arranged so as in the previous three-phase circuit having the same number of poles as the stator; they are joined to three rings. Brushes are used to connect this rotor circuit to resistors which are then short-circuited when the motor has started.