Page 280 - Industrial Power Engineering and Applications Handbook

Philosophy of quality systems and testing of electrical machines 111259
where that the insulation resistance, measured as noted in Section
a = double amplitude of vibration displacement, peak 9.5.3 is acceptable. The test should be performed
to peak (mm) immediately after the temperature-rise test on those
Vr,,,8. = r.m.s. value of velocity of vibration (mds) occasions when the latter test is also to be carried out.
f = frequency of vibration, which is approximately This test reveals any weakness of the insulation or
equal to the supply frequency in Hz. insufficient clearance between coils or between winding
and core. It consists of the application of a high voltage,
Both these levels are indicated in Table 11.3. For more as shown in Table 11.4, between the windings and the
details and for conducting the vibration test, reference frame (or cores).
may be made to IEC 60034-14. Windings that are not under test and all other metal
parts must be connected to the frame during the test. The
windings under test should be completely assembled.
11.4.7 Measurement of noise level
The test voltage should be of power frequency and as
When measuring the noise level, i.e. the limiting mean near to the sine waveform as possible. Component parts,
sound power level in dB for airborne noise emitted by a such as space heaters, thermostats and resistance tempera-
machine, reference may be made to the following IEC ture detectors, which are connected to parts other than
and IS0 publications: the power line, must be tested at twice their rated voltage,
plus 1000 volts, with all other windings and components
IEC 60034-9 - For recommended values of noise limits connected together and then connected to the frame (core).
in dB. A decibel (dB) is the unit of sound power level Insulation breakdown during the application of a high
and is derived from the unit of sound measurement (‘bel’), voltage should be considered as a dielectric failure. The
so called after the American inventor Alexander Graham test should commence at a voltage not more than one-
Bell and half of the full test voltage. The voltage should be increased
to the full value in not less than 10 seconds and this
1 voltage will then be maintained for one minute. At the
ldB=-bel
10 end of this period, the test voltage will be rapidly
IS0 1680-1, Acoustics - Test code for the measurement diminished to one-third of its value before switching
of airborne noise emitted by rotating electrical machinery off.
- Part 1: Engineering method for free-field conditions The test voltages for wound rotors, reversing and brake
over a reflecting plane. motors are also indicated in Table 11.4. Repetition of
IS0 1680-2, Acoustics - Test code for the measurement this test is not recommended to avoid excessive stresses
of airborne noise emitted by rotating electrical machinery on the insulation. However, when this becomes necessary
- Part 2: Survey method. such as at site before commissioning, the test voltage
IS0 3740, Acoustics - Determination of sound power must be limited to only 80% of the actual test voltage.
levels of noise sources - Guidelines for the use of basic After the test, the insulation resistance must be checked
standards and for the preparation of noise test codes. again, to make sure that no damage has been caused to
the windings.
11.4.8 Verification of dielectric properties
Dielectric loss factor or dissipation factor tan S
Power frequency withstand or HV test
This is a test to monitor the quality and dielectric behaviour
This test is conducted only when it has been determined of the insulating system of high-voltage machines, 5 kV

Table 11.3 Limits of vibration levels when measured in a state of free suspension

Vibration gradea Speed NP (r.p.m. j Maximum cm.s. value of the vibration velocity (mdsj for shajl heighf H (mm)
56 S H I 132 132 < H 1225 225 < H I400 H > 400
N >600 I3600 1.8 2.8 3.5 2.8
R > 600 5 1800 0.71 1.12 1.8 1.8
> 1800 I 3600 1.12 1.8 2.8 1.8
S >600 6 1800 0.45 0.71 1.12
N.A
> 18001 3600 0.71 1.12 1.8
Based on IEC 60034-14
“Vibration grade signifies the accuracy of rotor balancing, e.g. N = normal, R = reduced, S = special. This may be based on the type of
installation and the accuracy of the function the motor may have to perform.
%or both 50 or 60 Hz systems.
Note The level of vibration at site may be higher than that mentioned above, perhaps due to the foundation or the coupling of the load.
This must be checked and adequate precautions taken to avoid excessive vibrations.