Induction motors are simple in construction , economical and reliable in operation , owing to which they are widely used in all industries . On board ships they are afforded to drive various machines , mechanisms and devices ; they make 80-90 % of the total number of the electric motors installed .
Induction motors are the machines the rotor of which rotates asynchronously with the magnetic field . The rotor speed of rotation varies with the change in the load on the shaft of the asynchronous motor . The ratio between the speed of rotation of the magnetic field n1 and the rotor speed of rotation n2 is defined by the slip S =(n1-n2)/n1 .
The construction of induction motors provides for two main parts : the stator which is the stationary part and the rotor which is moving part . The stator is a steel frame with the core mounted inside and assembled of sheet steel laminations needed to reduce eddy currents . A three-phase winding is laid in the core slots and made either one-layered (with low powers) , or double-layered .
The rotor is a steel shaft on which the core also made of sheet steel laminations is mounted . In the core slots , the copper and aluminium legs can be found being short-circuited from their ends by rings . Such rotor is called a (squirrel-) cage type .
The operation of the induction motor is principally based on the electromagnetic interaction of its rotation magnetic field with the currents in the rotor winding . The way in which the mentioned rotating field is established by means of a set of stationary windings may be briefly described as follows . Across three stator windings (AX , BY , CZ) shifted by 120 deg relative to one another a three-phase current is flowing . The direction of the current from the beginnings of phase windings (A,B,C) towards their ends (X,Y,Z) is presumably regarded as opposite . At the time t1 the current in winding AX is’t flowing , but it obtains a negative direction in winding BY and a positive direction in the winding CZ . The resultant magnetic field inside the stator is , therefore , directed downwards . Since the N and S poles of the field are being permanently displaced across the inside of the stator , the revolving magnetic field is produced . The above mentioned stator winding has only one pair of poles , owing to which the magneti field does one revolution during one cycle of the current reversal . If the winding has (p) pairs of poles and is supplied by the current of frequency f1 , the speed of rotation of the revolving magnetic field n1=60f/р
This speed of rotation is called asynchronous one . The rotor speed of rotation is asynchronous . It is smaller than the stator speed of rotation by the value of the slip S .
If the rotor is stationary (n2=0) , S=1 .If theoretically n2=n1 , S=0 , i.e. the slip may vary between 0 and 1 . With the rated load ST = 2:6% . The rotor speed of rotation may be detected from the formula n2=n1(1-S). The larger is the rotor speed of rotation , the less is its slip and the smaller is the rotor current frequency f2=f1 S (where f1 is the stator current frequency).
On board ships three-phase cage motors are used for driving ship’s auxiliaries , anchor-mooring mechanisms , cranes , winches etc.
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