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Philosophy of  quality systems and testing of electrical machines  11/253

       The bridge method                              to allow for free circulation of air. In normal conditions,
                                                      a  distance of  2 m  is  sufficient.  The  duration  of  the
       Here, the unknown resistance is compared with a known   temperature rise test is dependent on the type and rating
       resistance using a suitable bridge. Resistance above 1 R   of the motor. For motors with continuous rating, the test
       can be measured by Wheatstone bridge. Resistance less   should be continued until thermal equilibrium has reached.
       than  1 52  can be measured by a Kelvin double bridge,   Whenever possible, the temperature should be measured
       where the lead resistance must also be compensated.   both while running and after the shutdown.
         All precautions must be taken to obtain the temperatures
       of the windings when measuring cold resistance. These   1  While conducting the test the negative sequence voltage
       temperatures, when measuring the cold resistance,  may   should be less  than  0.5% of  the  positive  sequence
       be obtained by a thermometer placed in contact with the   component. In terms of current, the negative sequence
       windings or by resistance temperature detectors, if they   current should not exceed 2.5% of the positive sequence
       are provided  in  the  windings.  The temperature  of  the   component  of  the  system current.  Measurement of
       surrounding air will not be regarded as the temperature   any one of the components will be adequate.
       of  the  windings, unless  the  motor  has  been  idle for a   2  For continuously rated machines, readings should be
       long  period  under  similar  atmospheric  temperature   taken  at  intervals  of  one  hour  or  less.  For  non-
       condition. If the resistance of a copper conductor is known   continuously rated machines, readings should be taken
       at one temperature, it may be calculated for any other   at  intervals  consistent  with  the  time  rating  of  the
       temperature by using the following formula:
                                                        machine.  The temperature rise test  should continue
            (235 + t2)                                  until there is a variation  of  1°C or less between the
        R,'  =       .R1                                two consecutive measurements  of temperature.
            (235 + tl )
                                                          For motors with a short-time rating, the duration of
       where                                            the test should correspond to the specified short-time
        R1 = resistance measured at tl"C, and           rating. At the end of the test the specified  temperature
        R,' = resistance at t20C                        rise limits should not exceed. At the beginning of the
       If R1, R,'  and tl are known, t2 can be calculated.   test, the temperature of  the motor  should be within
         In slip-ring motors  the rotor winding resistance will   5°C of that of the cooling air.
       be measured at the point of connection of the rotor winding   In  motors  for  periodic  duty,  the  test  should  be
       to  the  slip-rings,  so that  the  slip-ring  resistance  is   continued until thermal equilibrium has been reached.
       eliminated from the measurement of the true rotor winding   Unless  otherwise agreed, the duration  of  one cycle
                                                        should be  10 minutes  for  the  purpose  of  this  test.
       resistance.
                                                        Temperature  measurements  should  be  made  at  the
                                                        end of a cycle to establish thermal equilibrium.
       11.3.2  Temperature rise
       This test is intended to determine the temperature rise in   Measurement of temperature
       different  parts of the motor  windings  while running at   When thermal equilibrium is reached, the motor must be
       rated  conditions  and  the  permissible  temperature  rise   stopped as quickly  as possible. Measurements must be
       limits are specified in Table  11.1. While preparing  for   taken both while the motor was running and after shutdown
       the temperature rise test  the motor  should be shielded   (wherever  possible).  No  corrections  for  observed
       from currents of air coming from adjacent pulleys, belts   temperatures  are necessary  if the stopping period  does
       and  other  components  to  avoid  inaccurate  results.   not exceed the values given in Table 11.2.
       Sufficient floor space should be left between the machines   Where  successive  measurements  show  increasing


       Table 11.1  Permissible temperature rise for naturally (indirectly) cooled motors

       Method  of measurement   Insulation  class
                             A             E             B              F             H

                             a      b      U      b      a      b       U      b      a      b
       Resistance     "C     60     60     75     -      80      80     105    100    125    125
       ETD            "C     65     65     -      -      90      85     110    105    130    130
       ~~
       On the basis of IEC 60034-1
       a - < 50OO kW
       b - 2 5000 kW
       Notes
       1  The temperature rise limits are based above an ambient temperature of 40%  For temperatures other than 40°C see Section 1.6.2(C).
       2  For exact details refer to IEC 60034-1.
       3 For other types of cooling methods refer to IEC 60034-1.
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