MALFUNCTIONS in Electric Motors
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Installation and Maintenance ManualThe greater part of the malfunctions affecting the normal running of electric motors can be avoided by maintenance and precautions of a preventive nature.
Wide ventilation, cleanliness and careful maintenance are the main factors. A further essential factor is the prompt attention to any malfunctioning as signaled by vibrations, shaft knock, declining insulation resistance, smoke or fire, sparking or unusual slip ring or brush wear, sudden changes of bearing temperatures.
When failures of an electric or mechanical nature arise, the first step to be taken is to stop the motor and subsequent examination of all mechanical and electrical parts of the installation.
In the event of fire, the installation should be isolated from the mains supply, which is normally done by turning off the respective switches.
In the event of fire within the motor itself, steps should be taken to restrain and suffocate it by covering the ventilation vents.
To extinguish a fire, dry chemical or C02 extinguishers should be used - never water.
1 - STANDARD THREE-PHASE MOTOR FAILURES
Owing to the widespread usage of asynchronous three-phase motors in industry which are more often repaired in the plant workshops, there follows a summary of possible failures and their probable causes, detection and remedies.
Motors are generally designed to Class B or F insulation and for ambient temperatures up to 40°C.
Most winding defects arise when temperature limits, due to current overload, are surpassed throughout the winding or even in only portions thereof. These defects are identified by the darkening or carbonizing of wire insulation.
1.1 - SHORT CIRCUITS BETWEEN TURNS
A short circuit between turns can be a consequent of two coincident insulation defects, or the result of defects arising simultaneously on two adjacent wires.
As wires are random tested, even the best quality wires can have weak spots. Weak spots can, on occasion, tolerate a voltage surge of 30 % at the time of testing for shorting between turns, and later fail due to humidity, dust or vibration.
Depending on the intensity of the short, a magnetic hum becomes audible.
In some cases, the three-phase current imbalance can be so insignificant that the motor protective device fails to react. A short circuit between turns, and phases to ground due to insulation failure is rare, and even so, it nearly always occurs during the early stages of operation.
1.2 - WINDING FAILURES
a) One burnt winding phase
This failure arises when a motor runs wired in delta and current fails in one main conductor.
Current rises from 2 to 2.5 times in the remaining winding with a simultaneous marked fall in speed, If the motor stops, the current will increase from 3.5 to 4 times its rated value.
In most instances, this defect is due to the absence of a protective switch, or else, the switch has been set too high.
b) Two burnt winding phases
This failure arises when current fails in one main conductor and the motor winding is star-connected. One of the winding phases remains currentless whilst the others absorb the full voltage and carry an excessive current. The slip almost doubles.
c) Three burnt winding phases
Probable cause 1: Motor only protected by fuses; an overload on the motor will be the cause of the trouble. Consequently, progressive carbonizing of the wires and insulation culminate in a short circuit between turns, or a short against the frame occurs.
A protective switch placed before the motor would easily solve this problem.
Probable cause 2: Motor incorrectly connected.
For example: A motor with windings designed for 220/38OV is connected through a star-delta switch to 38OV. The absorbed current will be so high that the winding will burn out in a few seconds if the fuses or a wrongly set protective switch fail to react promptly.
Probable cause 3: The star-delta switch is not commutated and the motor continues to run for a time connected to the star under overload conditions.
As it only develops 1/3 of its torque, the motor cannot reach rated speed. The increased slip results in higher ohmic losses arising from the Joule effect. As the stator current, consistent with the load, may not exceed the rated value for the delta connection, the protective switch will not react.
Consequent to increased winding and rotor losses the motor will overheat and the winding burn out.
Probable cause 4: Failures from this cause arise from thermal overload, due to too many starts under intermittent operation or to an overly long starting cycle.
The perfect functioning of motor operating under these conditions is only assured when the following values are heeded:
a) number of starts per hour;
b) starting with or without load;
c) mechanical brake or current inversion;
d) acceleration of rotating masses connected to motor shaft;
e) load torque vs. speed during acceleration and braking.The continuous effort exerted by the rotor during intermit- tent starting brings about heavier losses which provoke overheating.
Under certain circumstances, there is a possibility that the stator winding be subjected to damage with the motor idle as a result of the heating of the motor. In such a case, a slip ring motor is recommended as a large portion of the heat (due to rotor losses) is dissipated in the rheostat.
1.3 - ROTOR FAILURES
If a motor running under load conditions produces a noise of varying intensity and decreasing frequency while the load is increased, the reason, in most cases, will be an unsymmetrical rotor winding.
In squirrel-cage motors the cause will nearly always be a break in one or more of the rotor bars; simultaneously, periodical stator current fluctuations may be recorded. As a rule, this defect appears only in molded or die cast aluminum cages. Failures due to spot heating in one or another of the bars in the rotor stack are identified by the blue coloration at the affected points.
Should there be failures in various contiguous bars, vibrations and shuddering can occur as if due to an unbalance, and are often interpreted as such. When the rotor stack acquires a blue or violet coloration, it is a sign of overloading.
This can be caused by overly high slip, by too many starts or overlong starting cycles. This failure can also arise from insufficient main voltage.
1.4 - SLIP RING ROTOR FAILURES
A break in one phase of the rotor winding is revealed by a strong vibratory noise that varies according to the slip and, in addition, stronger periodical stator current fluctuations.
Assuming that two slip rings have been mottled by sparking of the brushes and that the third remains unharmed, the cause can more often arise from a weld failure brought about by an overload carried by a connection between the coils of the rotor winding.
It is possible, but rarely so, that a rupture could have occurred in the connection between the winding and the slip ring. However, it is advisable to first check if there is a break in the rheostat starter connection, or even in the part itself.
1.5 - SHORT CIRCUITS BETWEEN TURNS IN SLIP RING MOTORS
This malfunction occurs only under extremely rare circumstances. Depending upon the magnitude of the short circuit the start can be violent even though the rheostat is at the first tap of its starting position.
In this case heavy starting currents are not carried through the rings and so no burn marks will be observed on them.1.6 - BEARING FAILURES
Bearing damage is a result of overloading brought about by an overly taut belt or axial impacts and stresses. Underestimating the distance between the drive pulley and the driven pulley is a common occurrence.
The are of contact of the belt on the drive pulley thus becomes inadmissibly small and thereby belt tension is insufficient for torque transmission.
In spite of this it is quite usual to increase belt tension in order to attain sufficient drive.
Admittably, this is feasible with the latest belt types reinforced by synthetic materials.
However, this practice fails to consider the load on the bearing and the result is bearing failure within a short time. There is yet the possibility of the shaft being subjected to unacceptably high loads when the motor is fitted with a pulley that is too wide.
1.7 - SHAFT FRACTURES
Although bearings traditionally constitute the weaker part, and the shafts are designed with wide safety margins, it is not beyond the realms of possibility that a shaft may fracture by fatigue from bending stress brought about by excessive belt tension.
In most cases, fractures occur right behind the drive end bearing.
As a consequence of alternating bending stress induced by a rotating shaft, fractures travel inwards from the outside of the shaft until the point of rupture is reached when resistance of the remaining shaft cross-section no longer suffices. Avoid additional drilling the shaft (fastening screw holes) as such operations tend to cause stress concentration.
1.8 - UNBALANCED V-BELT DRIVES
The substitution of only one or other of various parallel belts of a drive is frequently the cause of shaft fractures, as well as being malpractice.
Any used, and consequently stretched belts retained on the drive, especially those closest to the motor, whilst new and unstretched belts are placed on the same drive turning farther from the bearing can augment shaft stress.
1.9 - DAMAGE ARISING FROM POORLY FITTED TRANSMISSION PARTS OR IMPROPER MOTOR ALIGNMENT
Damage to bearing and fracture in shafts often ensue from inadequate fitting of pulleys, couplings or pinions. These parts "knock" when rotating. The defect is recognized by the scratches that appear on the shaft or the eventual scale like flaking of the shaft end.
Keyways with edges pitted by loosely fitted keys can also bring about shaft failures.
Poorly aligned couplings cause knocks and radial and axial shaking to shaft and bearings.
Within a short while these malpractices cause the deterioration of the bearings and the enlargement of the bearing cover bracket located on the drive end side.
Shaft fracture can occur in more serious cases.
