Commissioning; Preparations - Siemens 1LA8 Series Operating Instructions Manual

4

Commissioning

4.1 Preparations

Safety instruction
Warning
Only expert persons should be entrusted with
work on power installations. All covers which
are designed to prevent active or rotating parts
from being touched, or which are necessary to
ensure correct air guidance and thus effective
cooling, must be installed prior to
commissioning.
Commissioning
Note
Where the torque is very uneven (the drive of a
reciprocating type compressor, for example), the
inevitable result is a non-sinusoidal motor current,
whose harmonics can lead to excessive system
perturbation or excessive electromagnetic interference.
In case of converter-fed motors, high-frequency
current or voltage harmonics in the motor cables can
give rise to electromagnetic interference. This is why
the use of shielded cables is recommended.
Checking the insulation resistance
Caution
The insulation resistance needs to be checked
prior to commissioning and again after any
extended periods of storage or periods during
which the equipment is not in operation. Before
you begin measuring the insulation resistance,
please read the manual of the insulation
resistance meter you are going to use. Any
cables of the main circuit, which are already
connected should be disconnected from the
terminals in order to carry out the insulation
measurements.
Warning
During the measurement and immediately
afterwards, some of the terminals are at
hazardous voltage levels and must not be
touched.
Preferably measure the insulation resistance of
the winding with reference to the motor
enclosure when the winding temperature is
between 20 and 45°C. When making the
measurement, you must wait until the final
resistance value is reached (approx. 1 minute).
14
Limit values
The table below shows the measuring voltage and the
threshold values for minimum insulation resistance
and critical insulation resistance.
Insulation resistance
Measuring-circuit voltage
Minimum insulation resistance with
new, cleaned or repaired windings
Critical specific insulation resistance
after a long operating time
(values apply at a winding temperature of 35°C)
Note the following points:
• If the measurements are performed when the coil
temperature is not at 35°C, the measured value
will need to be converted to the reference
temperature of 35°C in order to be able to
compare the values with the table above. The
insulation resistance is reduced by a factor of a
half for every 10 K increase in temperature, and it
is increased by a factor of two for every 10 K
decrease in temperature.
• Dry, new windings have insulation resistances of
between 100 and 2000 MΩ, or possibly even
higher values. If the insulation resistance is close to
the minimum value, then the cause could either be
humidity and/or dirt accumulation.
• Over its operating lifetime, the motor winding
insulation resistance can drop due to ambient and
operational influences. The critical insulation
resistance for a 35°C winding temperature can be
calculated, depending on the rated voltage, by
multiplying the rated voltage (kV) by the specific
critical resistance value (0.5 MΩ/kV); e.g. critical
resistance for a rated voltage (UN) of 690 V:
690 V x 0.5 MΩ/kV = 0.345 MΩ
Notice
If the critical insulation resistance is reached or
undershot, the windings must be dried or, if the fan is
removed, cleaned thoroughly and dried. After drying
a cleaned winding, please note that the insulation
resistance is lower for windings in the warm
condition. The insulation resistance can only be
properly assessed after conversion to the reference
temperature of 35°C.
Notice
If the measured value is close to the critical value, the
insulation resistance should be subsequently checked
at appropriately shorter intervals.
Rated voltage
U < 2 kV
N
500 V DC
5 MΩ
0.5 MΩ/kV