Induction (Asynchronous) Motors , a Three-phase Cage Motor

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.

Marine Alternating current machines , Synchronous Alternators

     This term refers to a particular type of generator the number of revolutions of which standard frequency is coordinated with the number of pairs of poles . The synchronous number of revolutions n1=60(f/p) . The standard frequency f=50 Hz .

     Synchronous alternators of small power are sometimes built like d.c. generators equipped with stationary poles and a rotating armature . The difference is that not the commutator but the armature winding is connected to the slip-rings causing its alternating current to flow into the external circuit .

     Taking into account that it is difficult to collect large power by means of sliding contacts , synchronous alternators are usually provided with rotating poles (the rotor) and a stationary armature (the stator). The power of such alternators is practically unlimited (1,400,000 kVa).

     The stator is found in a steel frame with the core mounted inside and assembled of 0,35-0,5 mm electrical steel laminations . The stator winding is laid the core slots , its leads being brought out the terminal box . Synchronous alternators may be either one-phased or three-phased . A three-phase alternator has the stator windings either star-  or delta-connected in the terminal box .

     The greater part of shipboard alternators operated at the speed of rotation ranging from 500 to 1500 rev/min and has the rotor equipped with salient poles .

     The rotor is a steel shaft on which the cores of the poles with mounted on them excitation windings are fixed , their supply being obtained through slip-rings and and brushes . The source of direct current is usually a shunt-wound generator (the exciter) which is mounted with a synchronous alternator on the same shaft .

     The principal characteristics for synchronous alternators are the following :

      No-load characteristic , which is the relationship between the alternator e.m.f. and the excitation current E=f(Iex) with the speed of rotation n=0 . This characteristic makes it possible to judge the degree of steel saturation .

     External characteristic U=f(I) which detects the relationship between the alternator voltage and the load current with the speed of rotation n=const , the excitation current Iex =const AND POWER FACTOR COS f=const . The external characteristics show the change of the voltage when the load current value and character are varied .

    Regulation characteristic Iex=f(I) which reveals the relationship between the excitation current and the load current with the alternator voltage U=const , the speed of rotation n=const , and the power factor COS f=const . The regulation characteristics indicate in what way the excitation should be varied with the alteration of the load current value and character in order to keep the voltage at the alternator terminals unchangeable .

     Synchronous alternators in marine application are those with silicon rectifier in the excitation system , those with static excitation system , brushless alternators ets . Self-excited brushless alternators are the most widespread because they don’t have either a commutator , slip-rings or brushes , which makes them more reliable and easier to operate .   

Application of Direct Current generators and their parallel operation

   Practically all d.c. generator installed on vessels for supplying auxiliary power are of the self-excited type .

   Shunt-wound generators are usually used as the exciters of separately excited generators and have the added feature of charging accumulative batteries . The latter is associated with the fact that with a reverse current they aren’t demagnetized in view of the unchangeable current direction in the field winding .

   Series-wound generators have the voltage which varies abruptly with the load and , therefore , this type of d.c. generators is not used on board ships.

   Compound-wound generators are not subject to frequent overloads and short-circuits , for the series field winding demagnetizes them . they are employed in welders as well as in some electric drives on the voltage-control system .

    Separately excited generators find application where a wide-range voltage regulation is required , thet is in the electric drives of steering gears , windlasses , winches etc. as well as in the electric propulsion plants as main generators and exciters .

    When putting d.c. generators in parallel operation , it is necessary that two condition be met : 1) their polarity should be the same as the one in the mains to which they are connected ;  2) their e.m.f. is required to be equal to the mains voltage . The device responsible for fulfilling the indicated conditions is a voltmeter of a magnetic electric system . In order to transmit the power from one generator operating in parallel to another , the excitation currents of the generator to which the power is transferred , should be increased but those of the generator from which it is collected , should be reduced . The generators are required to be loaded as uniformly and proportionally to their rated capacities as possible . If this condition is not fulfilled , the efficiency will be lower .

Generators and their Characteristics

    Direct Current generators are provided with the armature winding ang one or two field windings . Depending on the type of the armature winding and field winding interconnection d.c. generators may be recognized as those of separate excitation , of shunt excitation , of series excitation , and of compound excitation .  

    In a separately excited generator , the field winding obtains its supply from a separate current source . It is connected in series with the armature winding in a series-wound generator in parallel - in a shunt-wound generator . A compound-wound generator has two field windings positioned on the main poles , one of them being connected in shunt , the other - in series with the armature winding . The parallel field winding concentrates the current ranging from 1 to 6 % of the rated armature current . It is made from copper conductors provided wity a large number of turns of relatively small section . The series field winding carries the entire armature current and hence its conductors are of large section .

     Shunt-wound generators , series-wound generators and compound-wound generators are self-excited ; that is to say they don't require a separate current source for their excitation . The  current supplying the winding is derived from the generator armature .

     The generator properties depend upon the method of the generator excitation . They may be expressed by definite characteristics , that is , the relationships between the e.m.f. , the voltage , the armature current and the excitation , all of them being responsible for the operation of a d.c. machine on different loads . The most important of the indicated magnitudes is the voltage which is dependent on the above mentioned excitation current , the armature current and the rotational speed .The main characteristics which reveal the properties of d.c. generators may be described as follows .  

     No-load characteristic is defined as the relationship between the generator e.m.f. E and the excitation current  Iex  with the armature current  I=0 : E=f(Iex) . This characteristic lets us judge the saturation of the magnetic circuit and may be applied for plotting the other characteristics .

     Short-circuit characteristic shows the relationship between the armature current Ish  and the excitation current with the short-circuited armature and the voltage equal to zero : Ish = F(Iex) . This characteristic is plotted as a straight line since under short-circuit conditions the generator magnetic circuit is practically not saturated .

     External characteristic is the relationship between the voltage U and the armature current  I with resistance in the excitation circuit Rex =const. The equation is U=f(I) .

     Regulation characteristic makes up the relationship between the excitation current and the armature current with the generator voltage U=const : Iex=f(I).

     Load characteristic is the relationship between the voltage U and the excitation current  Iex:U=f(Iex) with I=const.

Ship's Direct Current Electric Machines construction

Direct Current machines converting mechanical energy inyo electrical one are referred to as d.c. generators . Those , on the other hand , which convert electrical energy into mechanical one , are spoken of as d.c. motors . D.c. machines are reversible and , therefore identical in construction . They are composed of two main parts - the stationary part which is called the frame provided with the main poles and the commutating poles , and the rotating part which is said to be the armature , its windings and commutator being positioned on it .
    The frame is made of steel in the form of a closed magnetic conductor . The main poles for the purposes of decreasing the losses are assembled of sheet steel laminations . Mounted on them , the field windings are built of copper isolated conductors . The main poles are intended to produce the main magnetic flux .
    The commutating poles are also made of steel and arranged midway between the main poles . Their windings as well as of the main poles are built of copper conductors . The commutating poles with the windings are designed to ensure non-sparking operation of an electric machine .
    The armature makes up a cylindrical core made of sheet steel laminations , a two-layer winding being fixed in their slots . The former is built of sections made of isolated copper conductors . The commutator is constructed of separate copper bars isolated from each other and from the frame . The section made of isolated copper conductors .
    The commutator is constructed of separate copper bars isolated from each other and from the frame . The section leads of the armature winding are connected to the bars . The commutator is designed for the conversion of an alternating emf induced in the armature winding into a direct one .
    The brushgear is an element designed for collecting the current from the armature winging and leading it to the latter . It is composed of brushes , brush-holders , brush-studs , brush-rockers and current collecting bars. 
The commutator and brushgear are the most essential parts of a d.c. machine , its reliable operation being dependent on their condition .
    The operative principle of a d.c. machine is built upon the laws of electromagnetic induction and electromagnetic force . As the armature is rotated in the magnetic flux of the main poles by a drive motor , the e.m.f. is induced in the armature winding . In the loaded machine , the e.m.f. brings into existince the current with which it coincides in direction . This current interacting with the magnetic flux produces the electromagnetic torque directed in opposition to that produced by a prime mover . As this takes place , the machine operates as a generator . The mechanical power consumed from the drive motor is converted into the electrical one and is given up to the mains .
    When a d.c. machine is connected to the electrical source of supply , the current is generated in the armature winding . INteracting with the magnetic flux of the poles , it produces the electromagnetic torque ehich brings into rotation the armature . The e.m.f. directed in opposition to the current direction is induced in the armature winding . in this case the machine works as a motor . The electrical power consumed from the mains is converted into the mechanical one .