58 | Flameproof motors
10. MOTOR ABNORMALITIES
IN SERVICE
Most of the abnormalities that interfere with the
normal operation of electric motors can be avoided
with preventive care and measures.
Sufficient ventilation, cleaning and careful
maintenance are factors of great importance.
Another important factor is to intervene
immediately when any such phenomena as
vibrations, shaft beatings, permanent decreasing
insulation resistance, evidence of smoke and fire,
abrupt temperature variations of the bearings are
noticed.
The first measure to be taken when mechanical or
electric abnormalities occur is to disconnect the
motors and examine all the mechanical and electric
parts of the installation.
In case of fire, the installation must be isolated from
the power supply. This is normally done by turning
off the respective switches. Use dry chemical
powder or CO
2
to extinguish the fire. Never use
water.
COMMOM DAMAGE TO
INDUCTION MOTORS
The motors are usually designed for insulation
class F (155 ºC), temperature rise class B (80K)
and ambient temperature of 40 ºC. Most of the
faults on the windings occur when the limit
temperatures are surpassed on all the winding or
parts of it due to current overload. This is visible in
the darkening or carbonization of the wire
insulation.
10.1.1. SHORT CIRCUIT AMONG TURNS
The short circuit among turns may be a
consequence of the casual coincidence of two
default joints in the insulation of the wires or result
from defaults caused simultaneously on two wires
placed side by side. In the three phases, different
currents appear. The difference according to the
circumstances can be so small that the protection
of the motor may not react at all. Short circuit
among turns, to the ground, or among phases due
to insulation defaults rarely occur and even so,
almost always in the first moments of operation.
10.1.2. DAMAGE CAUSED TO THE WINDING
10.1.2.1 ONE PHASE OF THE BURNT
WINDING
This damage occurs when the motor operates in
triangle and there is lack of current on the power
supply conductor. The current rises from 2 to 2.5
times on the rest of the winding at the same time
the rotation drops drastically. If the motor stops, the
current will rise from 3.5 to 4 times its rated value.
Most of the time, when this default occurs, it’s
because a protection switch has not been installed
or because this switch has received an excessively
high regulation.
10.1.2.2 TWO PHASES OF THE BURNT
WINDING
This default will occur due to lack of current on a
power supply conductor and if the motor winding is
connected in star.
One of the winding phases goes without current
while the others absorb all the power and conduct
an extremely high current. The slip may come to
double.
10.1.2.3 THREE PHASES OF THE BURNT
WINDING
POSSIBLE CAUSE 1:
The motor is protected only by fuses; overload on
the motor will be the cause of the abnormality. The
consequence will be the progressive carbonization
of the wires and insulation culminating with a short
circuit of the turns or a short circuit against the
mass. If the motor is preceded by a protection
switch this abnormality may be easily avoided.
POSSIBLE CAUSE 2:
The motor is turned on incorrectly. For example: A
motor with a winding designed for 220/380V is
connected through a star-triangle switch to a 380V
power supply. The absorbed current will be so high
that the winding will burn in a few seconds if the
fuses or a protection switch incorrectly set don’t
react immediately.
POSSIBLE CAUSE 3:
The star-triangle switch isn’t commuted and the
motor continues to rotate during some time in star
connection with excessive load. Because it
develops only 1/3 of its torque, the motor can’t
reach its rated speed. The emphasizing of the slip
means greater Ohmic losses to the motor due to
the Joule effect. Because the stator current doesn’t
surpass the rated value for the triangle connection,
depending on the load the protection switch will not
react. The motor will heat as a consequence of the
increase of losses on the winding and rotor and the
winding will burn.
POSSIBLE CAUSE 4:
Thermal overload due to an excessive number of
starts at intermittent operation rating or for an
excessively long start period will damage the
winding. The perfect operating of motors with this
rating can be assured if the following values are
considered in the specifications of the motor:
- Number of starts per hour;
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