2. INDEX
Classification
Laws of electromagnetism
Rotating Magnetic Field
AC Motor
Induction Motor
Synchronous Motor
Maintenance Practices
8. Faraday’s Law of Electromagnetic
Induction
When the magnetic flux
through a circuit is
changing an induced EMF
is setup in that circuit
and its magnitude is
proportional to the rate
of change of flux”
Simulation
9. Lenz’s Law
“ The direction of an
induced EMF is such
that its effect tends to
oppose the change
producing it”
Simulation
10. Fleming’s Right Hand rule
Used to measure the
direction of induced
current in a conductor
when cut by a magnetic
field.
11. Fleming’s Left Hand rule
Used to measure the
direction of motion of a
current carrying
conductor when placed
in magnetic field.
13. + When Current positive and going into
· When Current negative and coming from
14. Speed of RMF
The magnetic field established rotates at a speed given by
N = 60* f / P
where f = frequency of stator current
P = Number of pair of poles
16. Induction Motor-Intro.
The induction motor is the most commonly used
type of ac motor. It is simple, low cost and rugged in
construction.
The induction motor derives its name from the fact
that ac voltages are induced in the rotor circuit by
the rotating magnetic field of the stator.
21. Slip in Induction Motor
slip speed = synchronous speed - rotor speed
measured in RPM
Slip = (synchronous speed - rotor speed ) /synchronous speed
expressed as a percentage
The greater the slip speed, the greater is the force on
each conductor and the torque exerted by the whole.
22. Starting Current
The starting current is very high which may damage
the stator winding.
To reduce this heavy starting current, star-delta
starting switch is used.
For starting, the stator winding are connected up in
star via the switch to the supply so that the phase
voltage is 1/√3 of the normal voltage. This reduced
voltage limits the starting current.
23. Phase voltage is 1/√3 of the
normal voltage
phase voltage is equal to the line
voltage.
24. Torque- Starting
The resistance of the squirrel cage rotor
is small and inductance high.
Thus on starting rotor current and the rotor
emf are nearly 90 degrees out of phase.
The lagging rotor current interacts little with
stator current and therefore the starting
torque is poor.Torque- Running
As the rotor current come into phase with the rotor
emf with increased rotor speed (decreased slip and
inductive reactance) the rotor and the stator flux
comes more into phase and the torque increases.
25. Methods Of Improving Starting
Torque
In creasing the resistance of the rotor conductors
Using a combination of high and low resistance
conductors
Using a wire wound rotor connected to variable resistor
29. Single Phase Induction
Motor
The single phase induction motor produces a
pulsating field.
However, if the rotor is rotated forward at a bit less
than the synchronous speed, It will develop some
torque.
If the rotor is started in the reverse direction, it will
develop a same torque in other direction
30. Split Phase Induction Motor
Two phases are produced by
splitting a single phase.
A capacitor is inserted in one of
the windings and is called a
permanent-split capacitor motor.
The direction of the motor is
easily reversed by switching the
capacitor in series with the other
winding.
31. Summary
The three phase induction motor
Is very robust in construction
No need for slip rings and therefore less maintenance.
Has a high starting current reduced by star-delta switch.
Has a poor starting torque.
Runs at a speed less than synchronous speed.
Direction of rotation can be reversed by interchanging any
two stator phases.
Is of two types depending on motor construction: Squirrel
Cage or Slip Ring
32. Uses of Various Type Motors
1. Constant speed with varying loads and require
smoother torque e.g. fuel booster pumps, hydraulic
system’s Electric Motor Driven pumps.
2. Systems which need high torque and reversing e.g.
Flap Power units (for alternate flap drives),
Stabilizer Trim Actuator.
3. Two phase induction motors also used in aircraft
such as aileron trim actuators and in reversible
valve actuators in Fuel, hydraulic, oil, and
pneumatic systems etc.
33. Synchronous Motor
Synchronous Motor-Intro
Synchronous Motor-principle
Changing the Load
Starting Torque
Improvement of starting torque
Synchronous Machine Construction
V curves
Torque versus Speed
Summary
34. Synchronous Motor- Intro
• The synchronous motor rotates at the
synchronous speed i.e. the speed of the RMF.
• Stator is similar in construction to that of an
induction motor, so same principle is applied to
the synchronous motor rotor.
• Field excitation is provided on the rotor by either
permanent or electromagnets with number of
poles equal to the poles of the RMF caused by
stator
35. Synchronous Motor-Principle
The rotor acting as a bar magnet will turn to line up
with the rotating magnet field. The rotor gets locked to the
RMF and rotates unlike induction motor at synchronous speed
under all load condition
36. Starting Torque
It cannot be started from a standstill by applying ac to
the stator. When ac is applied to the stator a high
speed RMF appears around the stator. This RMF
rushes past the rotor poles so quickly that the rotor is
unable to get started. It is attracted first in one
direction and then in the other and hence no starting
torque.Improvement of starting torque
It is started by using a squirrel cage within a rotor
construction and therefore starts as an induction
motor.
At synchronous speed the squirrel cage has no part to
play.
40. Summary
The synchronous motor:
1. requires to be started by an external prime mover.
2. Runs only at synchronous speed, this is an advantage
where continuous speed is required but a disadvantage
where a variable speed is required.
3. Can be used to adjust the power factor of a system at
the same time it is driving a mechanical load.
41. Electric Motor Specifications
Inpro/seal on Drive end Only 25 HP & Above.
Oversized J-Box per specifications.
Blue Chip Quality. 100% cast iron construction for rigidity and reduced vibration.
Internal and external epoxy paint.
MAX GUARD insulation system
1.15 Service Factor.
Extended grease tubes, regreasable in service.
Brass drain and breather
Meets IEEE45 USCG Marine Duty IP54 Construction.
Actual test and vibration data supplied with each motor
CSA Certified
Division 2 CSA certification nameplate, for hazardous locations, Class I Groups A, B, C, and
D.
Temperature code T2B
Three Year warranty.
42. Electric Motor Acceptance Test
All motors for a plant should go through an acceptance test prior to be
put into service or storage
The purpose would be to insure:
1. Not damaged during shipping and handling
2. No obvious manufacturing defects
3. Motor has been repaired properly
Incoming visual inspection
Electrical – Megger – PDMA
Mechanical – Vibration Test
43. Electric Motor Storage Guidelines
Pick a location:
Clean and dry area indoors if possible
Avoid heat, humidity, and vibration
Store in position for the intended use- horizontal –
horizontal and vertical - vertical
Outdoor storage of large motors:
Cover – allow for breathing at the bottom
Energize space heaters if they exist – 10–20 degrees F >
ambient
Prevent rodents, snakes, birds, and small animals from
nesting inside
44. Electric Motor Storage Guidelines
Apply rust preventative coating to shaft and other exposed
machine surfaces
Bearing damage is possible in storage – avoid humidity and
vibration
False brinelling of ball and race
Fretting from corrosion
Recommend to rotate shafts at regular
intervals – Monthly
Redistributes lubrication to prevent corrosion
Minimize brinelling by relocating the balls within the
races
45. Electric Motor Storage Guidelines
Tip
Leave all keyways the same, and in a different position each time
This provides an easy visual indication
Periodic shaft rotation is more critical on:
Larger 2 pole (3600 rpm) machines
Machines with long shafts and heavy rotors
Critical to avoid shaft distortion due to rotor sag
46. Electric Motor Storage Guidelines
Oil Lubed Bearings
These motors are always shipped without oil
Fill to capacity as soon as set into storage
Do not move motor with oil in the reservoirs
Drain it – Move it – Refill it
Tiered Maintenance
Define motor population
Apply appropriate maintenance and predictive tools
according to criticality, safety significance , and
economic significance of each motor
47. Categorize level of Maintenance
Minimum Maintenance
Moderate Maintenance
Trend able Maintenance
Extensive Maintenance
48. Minimum Maintenance
Category
Non-critical motors less than 50 HP
Motors having low safety and economic significance
Motors not of special design and normally readily
available
Unexpected failures are tolerable
Typically not repaired, but replaced with new
50. Trend able Maintenance
Category
Mid sized low and medium voltage
motors
50 -200 Hp – 460 volt
200 – 1000 Hp – 2300/4160 volt
Larger DC motors - > 50 hp
51. Extensive Maintenance
Category
Mission Critical Motors
Require comprehensive electrical and mechanical
monitoring
Usually the larger and medium voltage motors
Motors that have highest safety and economic
significance
52. Testing Motor Windings
Motor Winding Failures
Grounded winding
Turn to turn short
Single phased condition
Roasted winding due to overload
Locked rotor condition
Shorted connection
Winding damaged by voltage surge
53. Tests for Winding Condition
Insulation Resistance – megger test
Spot Check and Trend able
Indicates condition between the conductors and ground
Low readings indicate moisture, dirt, or damaged insulation
Minimum 1 meg ohm/1000 volts
54. Tests for Winding Condition
Polarization Index
Further indicates condition between the conductors and
ground
It’s the ratio of 10 min/1 min reading
A PI > 2 or 1 min reading > 5 giga ohms indicates motor is
suitable for service
PI > 7 could indicate brittle or aged insulation
PI can also help determine if a winding is wet or
contaminated
55. Tests for Winding Condition
DC Hipot
DC test voltage is applied to entire winding to verify the
insulation to ground
[ (2 x nameplate volts + 1000) x 1.7 x .60
Common on motors rated 4000 volts and higher
Done on low voltage motors to verify that its safe to perform a
surge comparison test
23
56. Tests for Winding Condition
Surge Comparison Test
Normally not performed in the field
Indicates presence of phase to phase and
turn to turn shorts within a winding
57. Tests for Winding Condition
Rotor Current Analysis
Indicates the presence of cracked and
broken rotor bars or voids in cast rotors
These could be the cause for vibration especially under load
58. Electric Motor Lubrication
According to EASA the motor component with the
highest failure rate is the bearing.
51% of all motor failures are due a bearing failure.
Bearing lubrication is one of the many aspects of
motor care and one of major importance to the life of
a motor.
59. Preventive/Predictive Maintenance
The establishment of an effective predictive
maintenance system will significantly affect the life of a
motor.
Lubricating bearings at arbitrary intervals can result in
bearings that are under lubricated or over lubricated.
Either of these conditions can reduce the expected life of
a bearing.Bearing Protection
Shaft slinger
Inpro/Seal Bearing Isolator
60. Bearing Types
Motor bearings are manufactured in various types of
configurations.
Shielded (2Z), shielded bearings have a metallic shield on
both sides of the bearing that is open on the ID or inner
race side.
Single Shield (1Z), same as above except one side of the
bearing is open.Sealed (2RS) sealed bearings have a seal arrangement on
both sides of the bearing that will not allow any
contaminants to enter the bearing. These bearings are
lubricated at the factory and do not require any additional
grease.
Single Seal (RS) same as above, but sealed on one side
only.
61. Determining Frequency of
Lubrication
Determining what frequency at which a particular
bearing needs to be lubricated requires consideration
of many criteria.
1. Type of grease
2. Type of bearing
3. Motor operating temperature
4. Motor speed
5. Environmental conditions
6. Duty Cycle
62. Lubricant Compatibility
If two lubricants that are incompatible are
mixed they will lose their lubrication ability.
If in doubt check with your motor
manufacturer or lubrication supplier.
The majority of motor manufacturers use a
polyurea based grease that meets EP-2
standards such as Mobil Polyrex-EM
Motor Operating Temperature
Motors that operate in elevated ambient temperatures need
to be lubricated more frequently.
Motors operating in a temperature controlled environment
can be lubricated less frequently.
63. Motor Speed
Motors operating at 3600 RPM need to be lubricated more
frequently.
Motors operating at 900 RPM need to be lubricated less
frequently.
Roller bearings require more frequent lubrication than ball
bearings.Environmental Conditions
Motors operating in a cement plant need to be
lubricated more frequently.
Motors operating in a clean room need to be lubricated
less frequently.
64. Duty Cycle
Motors operating 24/7 need to be lubricated more
frequently.
Motors operating 8 hours/day 5 days/week need to be
lubricated less frequently.
Bearing Size
The size of a particular bearing will determine the
amount of lubricant the bearing needs.
Most motor manufacturers provide instruction
manuals detailing the correct procedures and the
amount of lubricant required to re-lubricate a bearing.
65. Maintenance Practices-A.C. Motors
Clean, but don’t forget to inspect before and after cleaning
Check electrical connections for security, the insulation to be in
satisfactory condition.
Examine for signs of over heating
Check that the motor is secure
Do an audible check
Ensure that the motor is not over heating when operating, a
rule of thumb is that if it is too hot for the hand, it is too high.
When replacing a motor always ensure that the load, valve has
not seized.
Also ensure that the motor operates in the correct direction
67. Why do Motors Fail?
Failed in service
Motor stored in preparation for service
Regularly scheduled maintenance
Predictive maintenance testing reveals potential
concern regarding reliability
Motor requires upgrading
Modifications or addition of accessories
for new process
Failed or damaged accessories, i.e. brakes, tachs,
encoders, thermal devices
68. Why do Motors Fail?
Motors don't fail just because of age or operating hours.
Typical failures are caused by:
Heat
Power Supply Anomalies
Humidity
Contamination
Improper Lubrication
Unusual Mechanical Loads
Motors have survived for several hundred
thousand operating hours when these stresses
have been minimized.
69. Common Causes For Motor Failures
Failure distribution statistics, like these
from IEEE Petro-Chemical Paper PCIC-
94-01, are helpful, but still necessary to
conduct a thorough root cause analysis
when determining modes of failure.
70. Why do motors fail?
Heat
Temperatures over the design rating take their toll in various ways. Electrical
insulation deteriorates at a rate that may double for every
10 ºC. Excessive temperature also causes separation of greases and
breakdowns of oils causing bearing failure.
Primary causes of overheating are:
Overloading
Too frequent starts (NEMA recommends two cold starts or
one hot start per hour)
High ambient temperatures (NEMA typical design is 40 ºC)
Low or unbalanced voltages
High altitude operation
Inadequate ventilation i.e. damaged cooling fan,
contaminated motor
71. Why do Motors Fail?
Power Supply Anomalies
Ideal power is a perfect sine wave on each phase at the motor's rated voltage
& frequency-rarely achieved. The following problems appear.
Harmonics: Cause overheating and decreased efficiency.
Overvoltage: At moderate levels is usually not damaging, but can reduce
efficiency and power factor. (NEMA limit 110%)
Under-voltage: Increases current and causes overheating and reduced
efficiency in fully loaded motors. It is relatively harmless in under-loaded
motors. (NEMA limit 90% of rated).
Voltage unbalance: Causes overheating and reduced efficiency.
Unbalance greater than 1% requires motor de-rating and motors should
never be powered by a system with more than
5% unbalance.
72. Why do Motors Fail?
Power Supply Anomalies
Voltage spikes: Commonly caused by capacitor switching, lightning,
or cable stranding waves from a variable frequency drive (VFD).
These tend to cause turn-to-turn failures.
Frequencies under 60 HZ from VFDs: The application should be
reviewed to insure motor is suitable for the application without
installation of supplemental cooling.
Bearing damage from shaft currents: This usually originates from
VFDs. Consult the drive provider, motor manufacturer, or L&S
Electric for information on strategies such as an insulated bearing
sleeve, electro-conductive grease, or a shaft grounding system.
73. Why do Motors Fail?
Humidity
Humidity becomes a problem when the motor is
de-energized long enough to drop near the dew point
temperature.
Moisture weakens the dielectric strength of electrical varnish
and other insulating materials
Contributes to corrosion of bearings and other mechanical
components
Moisture from the air can mix with certain
particulate contaminants to create highly
electro-conductive solutions.
Insulation moisture can be significantly
reduced if the motor is kept warm.
74. Why do Motors Fail?
Humidity Control Strategies:
By heating or dehumidification, keep the environment of
unpowered motors below 80% relative humidity.
Specify new or rewound motors with heating elements for
the windings and use these when the motor is unpowered.
Periodically rotate the shaft of stored motors to keep
lubricant on the bearing surfaces.
75. Why do Motors Fail?
Abrasion
Corrosion
Overheating
Contamination
Contamination cannot be completely excluded by total enclosure or even
an explosion proof enclosure. Contamination destroys motors in three
ways:
Some airborne particulates are very abrasive. Motor coils flex
when in use and contamination with abrasive particles eat away
the wire enamel. Some substances, such as salt or coal dust are
electrically conductive. Heavy accumulation of contaminants
typically obstructs cooling passages.
76. Why do Motors Fail?
Improper Lubrication
Unfortunately, there are more ways to get it wrong than right. One
can over-lubricate as well as under-lubricate.
Grease itself introduces contaminants into bearings if careful
control is not practiced. Mixing greases with different bases
may cause grease constituents to separate and run out.
Different motors pose different requirements for the
introduction of lubricant and removal of old lubricant.
Each individual application dictates the amount, type, and
frequency of lubrication required.
This is a complete subject in itself. L&S Electric
provides additional information for discussion.
77. Why do Motors Fail?
Misaligned couplings
Over-tightened belt; or mis-alignment sheaves
Overly-compliant base or poor shimming of motor mounting feet
"Soft Foot," (i.e. motor feet) not in the same plane
Dynamic imbalance of load or internal imbalance of motor rotor
Failure to bypass resonant speed point in
VFD powered motors
Misapplication of bearings
Unusual Mechanical Loads
A variety of mechanical conditions can either overstress bearings,
leading to early failure, or distort the motor frame causing asymmetric
air gap, which in turn can cause vibration and bearing failure or winding
overheating. Conditions to avoid are:
78. Repair vs. Replacement
Difference in cost of repair vs. new purchase
Difference in efficiency of existing and proposed new motor
Availability of a new motor
Lifetime discounted cost of electric energy for each scenario
Possible mounting modifications
Cost in downtime and repairs from a possible
early failure in either scenario
Simple answer in principle. Rewind or otherwise repair a motor when
cheaper than buying a new motor. Implementing this is a little more
difficult because you need to consider the total cost of ownership.
Ideally you have to consider:
79. Maintaining Reliability
& Efficiency
To help assure a quality repair, you should:
Evaluate prospective motor
repair service providers
Don't pressure the provider
for unrealistic turnaround time
Clearly communicate your
requirements to the provider
80. Evaluate Repair Providers
Look for indicators of a quality control program, such as evidence of
participation in an ISO 9000 program, membership in EASA, &
participation in EASA–Q program.
Inquire about staff morale, training, turnover, etc.
Determine whether the service center has sufficient facilities &
materials to handle the size & type of motors you send them.
Make an point to spend time evaluating each potential
provider's service center.
81. Note what test equipment the service center owns and routinely
uses to verify successful repair. Examples:
Core loss tester
Surge comparison tester
Voltage regulated power supply for
running at rated voltage
Vibration testing equipment
Ask to see record-keeping system that the service center maintains
for repaired motors
Inquire about method of insulation removal, burnoff, mechanical
pulling, etc.
For burn off, ask about methods for preventing flames or hotspots & ensuring
uniform temperature when roasting multiple motors
Take note of the overall cleanliness of the service center
Evaluate Repair Providers
82. Motor Protection
Short-circuit / Instantaneous over current
Thermal overload
Phase current imbalance
Phase current loss
Over-current(instantaneous and temporized)
Ground fault / Instantaneous earth fault
Long start (stall) / Incomplete sequence
Jam (locked rotor)
Under-current
Phase current reversal
Motor temperature (by sensors)
Rapid cycle lock-out / Locking out
Load shedding
Notching or jogging / Number of starts
Phase voltage imbalance
Phase voltage loss
Phase voltage reversal
Under-voltage
Over-voltage
83. Difference between standard motor
and energy efficient motor
More copper in the windings.
Reduced fan loses.
Energy efficient motors operate with efficiencies that are
typically 2-6% higher than standard motors.
84. Need:
When there is a new installation or
modification to your plant.
Old motors are damaged and need rewinding.
Existing motors are underloaded or
overloaded.
Protecting other devices.
86. Losses :
Losses are primarily of two types i.e. core and copper
losses.
Copper loss
Core loss
Friction and windage
Loss
Stray load loss
87.
88. Cost of energy efficient motors:
Usually it is of normal cost and slightly more
than the normal motors. It is about 15% to 30%
more than the normal motors.
In Future, the initial cost may be available at the
same cost as a standard motor when the
population of EE Motors increases
The induction motor, rotor is not connected to an external source of voltage.
The squirrel cage rotor of the induction motor set in the rotating field of the stator should accelerate until it is running steadily at a speed which is slightly less than the synchronous speed at which the magnetic field rotates.
When the stator winding is energised with the rotor stopped, the slip is 100% and maximum emf is induced in the rotor. A heavy current is thus established into the rotor and this produces a flux which opposes and weakens the stator flux.
Since rotor is stationary or until at low speed the emf induced into the stator due to rotor is very low and therefore the stator draws a large amount of current during starting which may damage the stator winding.
To reduce this heavy starting current, the voltage applied to the stator winding is reduced by the use of star-delta starting switch. For normal running, the motor is designed to operate with the stator phases mesh or delta connected to the supply via the switch so that the phase voltage is equal to the line voltage.
For starting, the stator winding are connected up in star via the switch to the supply so that the phase voltage is 1/√3 of the normal voltage. This reduced voltage limits the starting current.
The frequency of the current induced in the rotor is the frequency With which the stator field rotates relative to each conductor. When the rotor is at rest, this equals the supply frequency. When the rotor is running lightly loaded the slip is small, and the frequency of induced rotor current may be only a few cycles per second. Now the resistance of the squirrel cage rotor is small and inductance high. Its reactance will therefore be large at the frequency of supply when the rotor is stationary and much less when it is running. Thus on starting rotor current and the rotor emf are nearly 90 degrees out of phase. The flux produced by this lagging rotor current is such that there is little interaction between it and the stator flux, and the starting torque is poor. As the rotor current come into phase with the rotor emf with increased rotor speed (decreased slip and inductive reactance) the rotor and the stator flux comes more into phase and the torque increases
This motor configuration works well at low horsepower. Though, usually applied to smaller motors