Page 285 - Industrial Power Engineering and Applications Handbook

11/264 Industrial Power Engineering and Applications Handbook
exceed the permissible value for a given class of dynamometer or other methods it may be accurately
insulation. determined as follows:
974
The readings at any point should be taken within 6 seconds Torque = - 0.9 x (input in kW - Stator 1; R1 loss)
x
for motors of output 7.5 kW and below and 10 seconds Ns
for motors above 7.5 kW. The factor 0.9 accounts for a 10% reduction in the torque
as an arbitrary allowance for harmonic losses.
Measurement of starting torque, pull-out torque When the torque is determined by the above method,
and pull-in torque the voltage during the test should be so adjusted that the
locked rotor current is approximately equal to the full
Any of the methods described in Section 11.4.5 may be load current. After the locked rotor test, the resistance of
adopted to measure the torques developed. The torque the stator windings should be measured and may also be
should be measured with the rotor in various positions, considered for calculating the 12R losses.
wherever possible. The minimum value should be taken
as the starting torque. Readings of voltage, current, 11.5.2 Open-circuit voltage ratio test for slip-
frequency and power input will also be taken. The starting ring motors
torque and starting current may be extrapolated when
the test is carried out at a reduced voltage. For extrapolation In slip-ring motors normal voltage and frequency are
of the test results at rated voltage the test must be applied to the stator with the connection of the rotor
performed at least at three test voltages. At each test brush gear open-circuited. The phase voltages induced
voltage, readings of voltage, current, torque, frequency in the rotor are then measured across the slip-rings.
and power input must be taken. The values of starting
current and starting torque may then be extrapolated from 11.5.3 Verification of degree of protection
these curves. The effect of magnetic saturation is not
considered in this test method. We have defined the various types of enclosures adopted
by various manufacturers to suit different locations and
environmental conditions in Tables 1.10 and 1.11. Here
Alternate method we briefly discuss methods for testing these enclosures
When the locked rotor torque cannot be measured by the to check their compliance with defined requirements.

Table 11.7 Protection against contact with live or moving parts

First characteristic Test requirements
number
0 No test is required
1 The test is carried out with a sphere of SO mm diameter. The sphere should not touch live or moving parts inside the
enclosure.
2 (a) Finger test
In LT motors, a standard test finger as shown in Figure 11.7 is used, connected by an incandescent lamp to one pole
of a supply of at least 40 V, the other pole of the supply being connected to the parts intended to be live in normal
service. All parts must be connected electrically. The lamp should not glow when an attempt is made to touch the bare
live parts or insufficiently insulated parts. Insufficiently insulated parts may be covered with a metal foil connected
to those parts that are live in normal service. Conducting parts covered with varnish or enamel only or protected by
oxidation or by a similar process may be considered as insufficiently insulated. In HT motors, the clearance is verified
with the minimum clearance required to withstand the dielectric test as in Section 11.4.8.
(b) Sphere test
The enclosure should not permit a ball of 12 mm diameter to enter the enclosure.
3 The test is carried out with a steel wire of 2.5 mm diameter. The wire should not go through the enclosure.
4 The test is carried out with a steel wire of 1 mm diameter. The wire should not go through the enclosure.
5 The test is carried out by using an apparatus shown in Figure 11.8, consisting of a closed test chamber in which talcum
powder can pass through a sieve having square openings of 75 pm, and is held in suspension by an air current. The
amount of talcum powder is supplied at 2 kg per cubic metre size of the test chamber. The enclosure under test is
placed inside the chamber and is connected to a vacuum pump which maintains, inside the enclosure, a differential
pressure equivalent to not more than a 200 mm column of water.
The test is stopped at the end of two hours if the volume of the air drawn in during this period is from 80 to 120 times
the volume of air in the enclosure under test.
If, with a vacuum equivalent to a 200 mm column of water, it is not possible to draw air 80 times the volume of the
enclosure under test, the test must be continued until the value is attained. In no case must the test be carried out for
more than 8 hours.
The permissible amount of talcum powder penetrating the enclosure should not affect operation of the equipment.
6 The test is similar as for number S but now no deposit of dust should be observed at the end of the test.