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Condition assessment of DC motors requires a basic understanding of the design and operating characteristics of the various types available: the series motor, the shunt motor, and the compound motor. Each type has unique operating characteristics and applications. These characteristics enable the operator to perform a wide variety of tasks.
DC motor
fault zone analysis is a vital part of any DC motor maintenance program. Visual
inspection and electrical testing of the armature and fields give the
maintenance personnel an understanding of the condition of the motor.
Implementing a predictive maintenance program takes a PM program to the
next level. We will review some case studies that will illustrate the
utilization of an effective predictive program.
Components of a series motor include the armature, labeled A1 and A2, and the field, S1 and S2. The same current is impressed upon the armature and the series field. The coils in the series field are made of a few turns of large gauge wire, to facilitate large current flow. This provides high starting torque, approximately 2 ¼ times the rated load torque. Series motor armatures are usually lap wound. Lap windings are good for high current, low voltage applications because they have additional parallel paths for current flow. Series motors have very poor speed control, running slowly with heavy loads and quickly with light loads. A series motor should never drive machines with a belt. If the belt breaks, the load would be removed and cause the motor to overspeed and destroy itself in a matter of seconds.
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Common uses of the
series motor include crane hoists, where large heavy loads will be raised and
lowered and bridge and trolley drives on large overhead cranes. The series motor
provides the starting torque required for moving large loads.
Traction motors used to drive trains are series motors that provide the
required torque and horsepower to get massive amounts of weight moving.
On the coldest days of winter the series motor that starts your car
overcomes the extreme cold temperatures and thick lubricant to get your car
going.
The shunt motor is probably the most common dc motor used in industry today. Components of the shunt motor are the armature, labeled A1 and A2, and the field, labeled F1 and F2. The coils in the shunt field are composed of many turns of small wire, resulting in low shunt field current and moderate armature current. This motor provides starting torque that varies with the load applied and good speed regulation by controlling the shunt field voltage. If the shunt motor loses it’s field it will accelerate slightly until CEMF rises to a value sufficient to shut off the torque producing current. In other words, the shunt motor will not destroy itself if it loses its field, but it won’t have the torque required to do the job it was designed for.
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Some of the common uses of the shunt motor are machine shop lathes, and industry process lines where speed and tension control are critical.
When comparing the advantages of the series and shunt
motors, the series motor has greater torque capabilities while the shunt motor
has more constant and controllable speed over various loads. These two desirable
characteristics can be found in the same motor by placing both a series field
and shunt field winding on the same pole. Thus,
we have the compound motor.
The compound motor
responds better to heavy load changes than a shunt motor because of the
increased current through the series field coils. This boosts the field strength, providing added torque and
speed.
If a shunt coil is added to a series motor at light loads (when a series motor tends to overspeed) the added shunt field flux limits the top speed, eliminating self-destruction.
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Common uses of the compound motor include elevators, air compressors, conveyors, presses and shears. Compound motors can be operated as shunt motors by disconnecting the series field. Many manufacturing process lines are designed this way. The reason being that, most off the shelf motors are compound motors, and the series field can always be connected later to provide additional torque, if needed.
Compound motors can be connected two ways,
cumulatively and differentially. When connected cumulatively, the series field
is connected to aid the shunt
field, providing faster response than a straight shunt motor.
When connected differentially, the series field opposes the shunt field.
Differentially connected compound motors are sometimes referred to as
“suicide motors,” because of their penchant for self-destruction.
If perhaps, the shunt field circuit were to suddenly open during loading,
the series field would then assume control and the polarity of all fields would
reverse. This results in the motor stopping, and then restarting in the opposite
direction. It then operates as an unloaded series motor and will destroy itself.
Differentially connected motors can also start in the opposite direction if the
load is too heavy. Therefore, it is seldom used in industry.
Fault zone
preventative maintenance on dc motors includes electrical testing and visual
inspection of the armature, commutator, brushes and fields. Over the years,
people have been performing insulation to ground tests on DC equipment to
evaluate the condition of insulation, particularly with regard to moisture and
dirt. These parameters are valuable readings when taken under similar conditions
at various times. High insulation resistance values do not necessarily indicate
high dielectric strength. Insulation that is mechanically damaged may show high
resistance values but fail at relatively low dielectric test voltages.
Insulation resistance varies inversely to the temperature of the motor.
As the temperature increases, resistance will decrease. Approximately 8
to 15°C
temperature rise will half the resistance.
Checking brushes and commutator condition are very important parts of an effective PM program. The brush face condition can provide valuable insight into the motor operation. The commutator surface condition can also indicate how the motor is reacting to various load and atmosphere conditions.
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Brush face from burned commutator |
Grooved commutator |
Armature
Visual inspection
of the armature should include the search for cracked or brittle insulation,
loose or broken banding, and any dirt or oil contamination.
Leakage to ground testing of the armature indicates the relative
condition of the insulation. Performing a bar-to-bar resistance check will indicate any
shorted windings or defective solder joints at the risers.
Infrared inspection of the armature can reveal overheating of the
brushes, commutator, as well as loose or hot connections on the risers. The
ideal temperature for proper commutation is between 120-140 °F.
Figure below shows a typical leakage to ground profile graph
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Fields
Visual inspection
of the field coils will reveal cracked or brittle insulation. Leakage to ground
testing provides a general assessment of the insulation condition.
The most common
method used to check for shorted windings is to perform a drop test. In this
test an ac voltage, normally 110 volts, is applied to the field leads.
The voltage drop across each field pole is measured with a voltmeter. In
a healthy motor, all voltage drops should be equal.
Commutator
Commutator film is developed by the chemical reaction that takes place between the copper surface of the commutator, the graphite surface of the brush and the air surrounding both. This film is very delicate and any variation in load or atmosphere can destroy it. Where poor commutation is present excessive physical wear will result on both the commutator and the brushes. The following pages indicate common conditions that contribute to poor commutation.
High Mica: Mica is
the insulation material used between each segment in a commutator. The mica is
approximately 1/16” lower than the adjacent commutator bars. If the commutator
bars are worn or if the mica becomes loose and extends higher than the
commutator bars the result will be brush chatter. Putting the dumb end of a lead
pencil, or other insulated device, on a brush while the motor is rotating and
feeling for vibration can identify this.
High Commutator
Bars: This condition is usually caused by the wedge or wedge ring that hold the
bars in place coming loose. Normally you can hear the brush chatter caused by
this condition or you can use the pencil method described above.
DC motors have definite applications in todays
industry. Each type offers specific characteristics and strengths, depending on
the task at hand.
Because they are
expensive to replace and repair, proper maintenance is a necessity.
Utilizing fault zone analysis should be an integral part of any DC
predictive maintenance program. Utilizing both electrical testing and visual
inspection of the armature, commutator, brushes and fields will ensure a
thorough understanding of overall motor condition, giving you the knowledge
necessary to keep these motors operating.