8.1 GENERAL
The general requirements for the testing of all rotating
electrical machines, including motors, are laid down in BS 4999 :1976, Part 60.
As far as induction motors are concerned, the principal
requirements are:
·
Manufacturer’s
Tests
·
On-site
Test
·
Insulation
Resistance Testing.
8.2 MANUFACTURER’S TESTS
Manufacturer’s tests are given three classifications:
‘Basic’ (formerly called ‘Type Tests’), ‘Duplicate’ and ‘Routine Checks’. Basic tests are mainly to prove a new
design. They include exhaustive tests to
ensure that the design meets the specification and all other performance
requirements. They are normally carried
out only on the ‘first of class’ motor, and a test certificate is usually
provided to confirm the tests. Basic
tests may, on special request, be repeated on the first machine of a new, large
order, but this is not usual.
Duplicate tests are for performance. They are applied to a motor that is of the
same design and construction as one previously made (and in no way altered) and
which has already undergone basic tests.
The duplicate tests are to ensure that the motor is still in accordance
with the original design.
Routine checks are tests to show
that each motor has been assembled correctly, is able to withstand the
appropriate high-voltage tests and is in sound working order both electrically
and mechanically.
The three
classes of test are listed in Table 1.
TABLE 1 - MANUFACTURER’S TESTS
|
Test
|
Basic
|
Duplicate
|
Routine
|
|
Resistance of windings (cold)
|
X
|
X
|
-
|
|
No-load losses and current
|
X
|
X
|
X
|
|
Locked rotor - current
|
X
|
X
|
-
|
|
- torque
|
X
|
-
|
-
|
|
Secondary induced voltage
(wound-rotor type only)
|
X
|
X
|
X
|
|
Temperature rise
|
X
|
-
|
-
|
|
Power factor and efficiency
|
X
|
-
|
-
|
|
Momentary overload
|
X
|
-
|
-
|
|
High voltage
|
X
|
X
|
X
|
|
Vibration
|
X
|
-
|
-
|
Most of these tests are
self-evident, but two are further explained overleaf.
8.2.1 Locked Rotor Test
This test is carried out with the
rotor locked and at a voltage reduced so that the stator and rotor currents do
not exceed their normal full-load values.
It simulates the standstill situation at the moment of starting without
motion actually taking place. By
measuring the current and torque at this reduced voltage, the actual standstill
current and torque when starting at full rated voltage may be calculated.
8.2.2 High Voltage Test
(also called a ‘Withstand’ test)
At this test a high voltage is
applied between the frame and all the motor windings (the stator windings only
in the case of squirrel-cage motors), with all other conductors, metal and
auxiliary (i.e. heater) circuits bonded to the frame. The actual voltage applied is in accordance
with Table 2 and is sustained for one minute.
It may be at any frequency between 25Hz and 100Hz. It is primarily an insulation test for the
motor’s windings and is included also in the routine checks to ensure that
there has been no fault during assembly of any individual motor.
TABLE 2 - HIGH
VOLTAGE TESTS
|
Windings
|
Test Voltage (rms)
|
|
Motor stator windings:
|
|
|
|
|
|
|
500V + twice
rated voltage
|
|
>1 000V, <1kva span="span">
|
1 000V + twice
rated voltage
|
|
1 – 10 000kVA
|
1 000V + twice
rated voltage, min 1 500V
|
|
>10 000 kVA and
|
|
|
<2 000v="000v" span="span">
|
1 000V + twice
rated voltage
|
|
2 000 — 6 000V
|
2.5 times rated
voltage
|
|
6 000 — 17 000V
|
3 000V + twice
rated voltage
|
|
>17 000V
|
Special
agreement
|
|
|
|
|
Rotors of wound-rotor machines:
(non-reversing
motors)
|
1 000V + twice open-circuit standstill
voltage with normal stator voltage applied.
Min 1 500V
|
8.3 ON SITE TESTS
Any motor installed on an offshore or onshore
installation may be assumed to have undergone its full routine check tests, and
its prototype a full basic or duplicate test.
On-site tests are therefore only needed to check the original
installation and commissioning, and thereafter to ensure that no deterioration
has taken place. The remainder of this
chapter deals only with tests for deterioration.
Deterioration can occur for many reasons: among them are
entry of dampness or water leakage in the motor or cable-entry box, overheating
of the windings due to overloading or prolonged stalling, or mechanical faults
such as vibration or bearing failure.
Both dampness and overheated windings can cause reduced
insulation resistance of the windings.
After drying out, the motor should be insulation-tested to ensure that
insulation resistance has been restored.
Deterioration can be progressive, especially when a motor is little
used, and a regular programme of insulation resistance testing every motor
should be drawn up and the results logged.
After temperature correction (see para 8.4) the resistance levels should
be plotted, and, if there is progressive deterioration, this will be
immediately apparent.
After repairs to a motor set an insulation resistance
test should always be carried out before reconnection if the motor or its
connections have in any way been interfered with.
High voltage withstand tests should never be needed on
site unless a major overhaul has been carried out, in which case it would be an
engineering or manufacturer’s concern.
8.4 INSULATION RESISTANCE TESTING
Instruments such as a ‘Megger’ are provided to operate
at 250V, 500V or 2 500V, and the correct one must be used depending on the
rated voltage of the motor to be tested.
Normally motors over 415V and high-voltage motors would require a
2 500V megger. Motors in the 100V
range would need the 500V instrument; a higher-voltage instrument might itself
damage the insulation.
When the tester is connected and the handle wound up,
the voltage should continue to be applied until the needle settles down to a
steady value; this might take one minute or more.
When testing the insulation resistance of a winding, all
other conductors, metalwork (stator and rotor) and auxiliary circuits such as
for temperature protection and heaters should be connected to the frame with
light wire (fuse wire will do), and the test voltage applied between winding
and frame. Where the three phase windings
are independent and
FIGURE
8.1
INSULATION
RESISTANCE TEMPERATURE COEFFICIENT
brought out to six terminals (fairly rare), it is
advisable to make a test also between pairs of windings. However, most motors are star-connected with
their star-point internal and permanently made, so inter-phase tests are not
possible.
Insulation resistance is very dependent on temperature,
and, in order to compare one reading with another, it is necessary to reduce
the value to a common temperature. This
is usually 40°C. Unlike the resistance of a
conductor, which rises with temperature, the resistance of insulation falls
rapidly with increase of temperature.
The graph of Figure 8.1 is used to
make this correction by means of a ‘temperature coefficient’.
For example, if the observed reading (Rt) is 10 megohms when taken
at 70°C, then, using the graph, the temperature coefficient (Kt) is 8.0, and the corrected
reading (R40) at 40°C is then 10 x
8.0 = 80 megohms. It can be seen from
this example that the correction is considerable when the winding is hot at
normal working temperatures.
The recommended minimum value of insulation resistance
for items of plant is given in manufacturers’ literature. As a guide where precise information is not
available, the minimum acceptable value (Rm)
for a motor stator winding is given by
Rm = (kV + 1) megohms when corrected to 40°C,
where kV is the motor’s rated voltage in kilovolts.
Thus:
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