22 l
Installation, operation and maintenance manual – Squirrel cage motor – W Line - Horizontal and vertical 11239449
4.4.7
Insulation assessment
Table 4.2 and Table 4.3 present guiding limits of insulation
resistance and polarization index for the assessment of
the motor insulation conditions.
Table 4.2: Insulation resistance guiding limits on electrical
machines
Insulation resistance value
Insulation assessment
2 M
or lower
Unacceptable
< 50 M
Dangerous
50...100 M
Fair
100...500 M
Good
500...1000 M
Very Good
> 1000 M
Excellent
Table 4.3: Polarization index (ratio between 10 minutes and 1
minute)
Polarization Index
Insulation assessment
1 or lower
Unacceptable
< 1.5
Dangerous
1.5 to 2.0
Fair
2.0 to 3.0
Good
3.0 to 4.0
Very Good
> 4.0
Excellent
ATTENTION
If the measured insulation resistance,
referred to 40 ºC, is below 100 M
or the
polarization index is below 2, than before
putting the motor into operation, consult
WEG.
4.5
PROTECTIONS
Motors used in continuous duty must be protected
against overloads by means of a motor integral device, or
an independent protection device, which is generally a
thermal relay with rated or adjustable current equal to or
below the value obtained by multiplying the full load motor
supply current by:
1.25 for motors with service factor equal to or above
1.15;
1.15 for motors with service factor equal to 1.0.
Motors also have protection devices against overheating
(in cases of overloads, locked rotor, low voltage, lack of
motor ventilation).
4.5.1
Thermal protections
The temperature sensors are installed on the main stator,
bearings and other parts that require temperature
monitoring and thermal protection.
These sensors must be connected to an external
temperature monitoring and protection system.
4.5.1.1
Temperature sensors
Thermostats – Are bimetallic thermal detectors with
normally closed silver contacts. They open at a certain
temperature. The thermostats are connected in series or
independently according to the connection diagram.
Thermistors (PTC or NTC) – Are thermal detectors
composed of semiconductors that vary their resistance
sharply when they reach a certain temperature.
Thermistors are connected in series or independently
according to the connection diagram.
NOTE
Thermostats and thermistors must be
connected to a control unit in order to interrupt
the power supply to the motor or activate a
signaling device.
RTDs (Pt100) - Are calibrated resistance elements. Their
operation is based on the principle that the electrical
resistance of a metallic conductor varies linearly with the
temperature. The detector terminals must be connected
to a control panel which includes a temperature meter.
NOTE
Resistance temperature detectors (RTD) allow
the monitoring via the absolute temperature
informed through their instantaneous
resistance value. With this information the relay,
which may be programmed for alarm and trip
according to pre-defined values, is able to
perform the temperature reading.
4.5.1.2
Temperature limits for the windings
The temperature of the hottest spot of the winding must
be kept below the limit of the insulation thermal class. The
total temperature is obtained by the sum of the ambient
temperature and the temperature rise (T), plus the
difference between the average temperature of the
winding and the hottest spot of the winding.
The ambient temperature must not exceed 40 °C, in
accordance with NBR IEC60034-1 standard. Above this
temperature, the working conditions are considered
special and the motor specific documentation must be
consulted.
Table 4.4 shows the numerical values and the
composition of the acceptable temperature at the hottest
spot on the winding.
Table 4.4: Insulation class
Insulation class
F
H
Ambient temperature
°C
40
40
T = temperature rise (temperature
measurement method by resistance variation)
°C 105 125
Difference between the hottest spot and the
average temperature
°C 10 15
Total: temperature of the hottest point
°C 155 180
ATTENTION
If the motor operates with winding
temperatures above the limits of the insulation
thermal class, the lifespan of the insulation, and
hence that of the motor, will be significantly
reduced, or it may even result in the motor
burnout.
