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                    ci  \                                   ampere  meter  would  show  almost  a  constant
        f ai    bi                                          starting current. The operator, possibly also judging
                                                            the speed of the ball mill, may have paused in the
                                                            star position before changing it over to delta but
                                                            by then the motor had failed. Thus, it was a wrong
                                                            application of the switching device for the type of
                                                            load.
                                                        7  Motor trips and/or fuses blow  during start
                                                           0  Check selection of fuses for the type of load and
        (a) Terminal box            (b) A connected winding   switching.  Perhaps  they  are  under-rated  for the
                                                            starting  inrush  current  and  its  duration.  For
                                                            selection of fuse ratings see Section 12.10.4.
         Figure 10.9  Terminals inside a terminal  box
                                                           0  Supply voltage may be low, causing the motor to
                                                            take longer to start.
                                                            Starter relay selection or relay setting may not be
            and 4  above have been  checked but  this problem   matching  the  starting  requirements.  The  relay
            still  arises,  then  motor  selection  may  not  be   thermal  characteristics  must  match  motor
            appropriate for the  type of  load  or the  method  of   characteristics.  In  heavy  drives  with  prolonged
            switching.  For  example,  a  ball  mill  at  a  thermal   start times the relay  may not operate if  it is not
            power station required a 75 kW motor employing a   properly  selected.  For  heavy  loads,  with  large
            Y/A manual  switching  (an  old  installation).  The   moments of inertia and requiring a longer starting
            method of switching was overlooked at the time of   time,  a  timer  can  be  introduced  into  the  relay
            motor  selection. The motor burnt  during  the  start   circuit, to bypass this until the motor accelerates
            while the starter was still in  the star position. The   to  a  reasonable  speed. Then  the relay  circuit  is
            stator was found to be completely charred  and the   reactivated.  The  most  common  practice  is  to
            rotor damaged to the extent that its short-circuiting   provide  a  CT-operated  thermal  overload  relay
            rings also melted due to excessive starting heat and   (Figure 13.53; see also Section 12.4.1). The CTs
            some molten  metal  was  scattered  over  the  stator   have a low saturation point. In the event of a high
            overhangs causing extensive damage. The reasons   starting current, they become saturated at two to
            for such a failure can be as follows:            three times the full load current and bypass  the
              The starter was manually  operated and thus the   excessive starting kick through the relay and thus
              changeover  time,  from  star  to  delta,  largely   avoid a false trip during a prolonged start.
              depended upon the skill of the operator, who may   8  Motor vibrates. A motor already tested at the manu-
              have been unaware of the implications of a longer   facturer’s works  for  vibration  limits,  as  shown  in
              starting period in the star position. Moreover, on   Table 1 1.3, may still appear to have more vibrations
              load, the starting heat in  a  Y/A starting is much   at  site  during operation.  This  may  be  due to  the
              more than on a DOL starting as discussed in Section   following reasons:
              2.7.3.                                         The foundation bed or the structure on which the
                                                             motor is mounted is not rigid, is tilted or is uneven.
                                                             Tighten  foundation  bolts  and  check  for proper
              Note  A  similar situation  may be found in an  automatic
              YlA starter when  the  setting of  the  timer,  to changeover   alignment of  the motor and the drive.  Make the
              from star to delta, is  at  a higher  value  to allow a longer   foundation or structure as rigid as possible.
              starting time to pick up speed. Then also the motor would   0  Single phasing, in which case the motor may make
              meet the same fate if the starting time exceeded the thermal   a  humming  noise  and  cause  vibrations  due to
              withstand time of the motor.                   uneven flux distribution.
                                                             Bearing-end play may also cause such vibrations.
              The fuse rating,  if  used  before  the  starter, may   It is possible that the driven equipment to which
              have been inadvertently high, inconsistent with the   the motor is coupled has a higher vibration level
              thermal withstand capacity of the motor. Thus the   than the motor, resulting  in quantum imbalance
              fuse did not blow which would have protected the   and  more  vibrations  than  when  the  motor  was
              motor from such severe and persistent overloading.   tested. All  attempts  must be  made,  to bring  the
              The over-current relay selection or its setting may   vibration level of the drive and the driven system
              not have  been  appropriate, otherwise the starter   within the limits as prescribed in Table 1 1.3.
              would have tripped under such a faulty condition.   9  The  motor  makes  rumbling  noise  and  the  stator
              Selection  and  over-current  setting  of  the  relay   current fluctuates. The rotor circuit may be broken
              should have  been  consistent  with  the  motor’s   and should be repaired. If the motor also overheats,
              thermal capacity.                            there  may  be an  inter-turn fault  or a  short-circuit
              A ball mill requires a high starting torque (Section   between  the  phases.  Detect  these  and  rectify  if
              2.5 and Table 2.3) to accelerate its heavy masses.   possible, otherwise rewind the motor.
              In the star position, the motor torque diminishes   10  If  heavy  rain  causes  a  flash  flood  at  the  site  of
              to one third the starting torque  as on DOL. The   installation, the motor may be submerged in water
              mill would take a long time to accelerate and the   for some time. A reasonably good motor, with good
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