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11 – AC Motors
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The intent of this presentation is to present enough information to provide the reader with a
fundamental knowledge of AC Motors used within Michelin and to better understand basic
system and equipment operations.
By enrolling in this self-study course, you have demonstrated a desire to improve yourself and
Michelin Manufacturing. However, this self-study course is only one part of the total Michelin
training program. Practical experience, IMS (AP) school, selected reading, and your desire to
succeed are also necessary to successfully round out a fully meaningful training program.
Although the words “he,” “him,” and “his” are used sparingly in this course to
enhance communication, they are not intended to be gender driven or to affront or discriminate
against anyone.
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11 – AC Motors
Three Phase AC Induction Motors
There are two main types of induction motors:
- Squirrel-Cage
- Wound Rotor
Both motors operate on induction and the principles are very similar. Let's first discuss the squirrelcage motor and its construction since it is the simplest form. We will then look at the differences and
applications of the wound rotor induction motor.
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11 – AC Motors
Squirrel-Cage Induction Motor
Construction
Stator - The stationary winding of the motor and it provides the rotating magnetic field. The stator
windings are wound around the laminated pole pieces, which are mounted to the outside housing of the
motor. It is also connected to the AC supply.
Terminal
Box
Stator
Frame
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11 – AC Motors
Squirrel-Cage Induction Motor
Rotor - The rotating member of the motor and it provides the torque or power to do the mechanical
work. The rotor is made of circular laminations with copper or aluminum bars imbedded around the
outside edge. It is connected to the shaft of the motor.
Fan
Roto
r
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11 – AC Motors
Squirrel-Cage Induction Motor
End Bells - Support the shaft of the motor and house the bearings.
End
Bell
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11 – AC Motors
Squirrel-Cage Induction Motor
End Bells - Support the shaft of the motor and house the bearings.
End
Bell
End
Bell
End
Bell
Terminal
Box
End
Bell
Fan
Stator
Roto
r
Frame
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11 – AC Motors
The Wound Rotor Induction Motor
This motor consists of three electrical parts:
1. A set of insulated windings mounted on the body of the motor. These windings are similar to those of
the squirrel-cage induction motor and are also called the stator windings.
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11 – AC Motors
The Wound Rotor Induction Motor
2. A set of insulated windings recessed into the laminated plates of the rotor. These windings are
always connected in the wye configuration with the three free ends brought out and each one
connected to each of three brass rings mounted on the motor shaft. These rings are the slip rings.
3. A means of making electrical connection
with the slip rings is needed so that current
may flow through the rotor windings when
they are in rotation.
These connections are by means of brushes,
which are held in position in contact with
the slip rings by the brush-holder.
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11 – AC Motors
The Wound Rotor Induction Motor
Construction
The wound rotor is constructed of windings instead of shorting bars, as in the squirrel-cage induction
motor. Like the squirrel-cage induction motor, it receives its power through induction.
Its windings are connected in a star configuration, and are accessible through the use of slip rings and
brush assembly.
This access to the rotor circuit is the major advantage of the wound rotor induction motor. It allows
changes in rotor impedance, through external resistance changes.
This allows the torque of the motor to be controlled. The wound rotor induction motor can be used
where the maximum torque is desired throughout the entire speed range.
As the speed changes, rotor circuit resistance is varied to maintain the resistance of the rotor equal to
the inductive reactance of the rotor (R = XL). When R and XL are equal, the phase angle of the
impedance of the rotor is 45°.
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11 – AC Motors
The Wound Rotor Induction Motor
The following basic block diagram illustrates the connection of external resistors to the rotor:
Three
Phase
Source
Resistor Bank
Stator
Brushes
Con
Rotor
Con
Slip Rings
The external resistance is usually of a value such that the resistance of the rotor circuit is equal to, or
greater than, the rotor reactance at standstill. The wound-rotor motor is usually started with all the
resistance in the rotor circuit. Starting torque with all the resistance is at maximum.
As the motor comes up to speed, the rotor resistance is shorted out by a contactor. With all external
rotor resistance cut out, the motor has good speed regulation, but somewhat less than that of the
standard squirrel-cage motor. This can also be done in stages with numerous contactors as is illustrated
on the following page.
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11 – AC Motors
The Wound Rotor Induction Motor
208 V / 3 Ph / 60 Hz
L1
L2
L3
FUSIBLE
DISCONNECT
1L1
1FU
2L1
1L2
2FU
2L2
1L3
3FU
2L3
1M
1OL
ADJUSTABLE RESISTORS
1T1
1T2
MTR
1T3
TO NEXT PAGE
2T3
2T2
3M
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2T1
3T3
3T2
3T1
2M
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11 – AC Motors
The Wound Rotor Induction Motor
Operating Characteristics
The insertion of resistance in the rotor circuit limits the starting surge of current and also permits high
starting torque and adjustable speed. If all the resistance of the resistor banks is connected into the
rotor circuit when the motor is running, the rotor current becomes less, and the motor slows down.
As the rotor speed decreases, more voltage is induced in the rotor windings due to the stator rotating
magnetic field speed being constant.
As a result, more current flows in the rotor windings, which creates the necessary torque at the reduced
speed.
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11 – AC Motors
The Wound Rotor Induction Motor
Speed Regulation
To increase the speed, for full speed operation, the rotor winding resistance is shorted out through the
contactor.
Although the insertion of resistance in the rotor circuit improves the starting torque at low speeds, it has
the opposite effect at normal speeds. Thus the speed regulation of the motor is poorer with resistance in
the rotor circuit. The resistance is shorted out as the motor comes up to speed in order to maintain
running torque and better speed regulation.
Although inserting the external resistance into the rotor circuit will vary the speed of the motor by
changing the torque, this method is only seen in older installations.
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11 – AC Motors
The Wound Rotor Induction Motor
Applications
Because the wound rotor induction motor is similar to the squirrel-cage induction motor, it is used in
similar applications. However, the wound rotor induction motor is seen more often in applications where
some speed or torque control is needed.
As discussed earlier, we can manipulate the torque of the wound rotor induction motor by changing the
value of the externally connected resistances. Because torque has a direct affect on speed, we will also
be varying the speed of the motor.
Wound rotor motors can be used as variable-speed motors. Their initial cost is much greater, so they
are usually used only where frequent large starting currents exist and for loads that require slow
acceleration with controlled torque.
Wound rotor motors are most frequently used in applications where high values of starting torque and
low starting currents are required; also, where the inertia of the driven machine is extremely high.
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11 – AC Motors
Theory of Operation
Rotating Magnetic Field
The speed of the rotating magnetic field is called the synchronous speed of the motor. The following
formula can be used to determine the speed of the rotating magnetic field created by the stator:
Ns =
Where
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Ns
f
P
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60  f
P
is the speed of the stator rotating magnetic field in RPMs
is the frequency of the applied voltage in hertz (Hz)
is the number of pair of poles per phase in the stator winding
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11 – AC Motors
Theory of Operation
When motor manufacturers describe a 2-pole motor, they are talking about the numbers of poles that are
created in the rotor due to the rotating magnetic field.
For a 2-pole motor which has only 1 pair of poles per phase:
Ns = 3600 rpm (assuming 60 Hz)
For a 4-pole motor which has 2 pair of poles per phase:
Ns = 1800 rpm (assuming 60 Hz)
For a 6-pole motor which has 3 pair of poles per phase:
Ns = 1200 rpm (assuming 60 Hz)
For a 8-pole motor which has 4 pair of poles per phase:
Ns = 900 rpm (assuming 60 Hz)
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11 – AC Motors
Theory of Operation
As we saw in the generation of a three phase AC voltage, phase A is +VMax, phase B is -1/2 VMax, and
phase C is -1/2 VMax.
Phase A
Phase B
Phase C
+ Voltage
Time
- Voltage
A
Also, notice that phase B will be +VMax next, then phase C. If we reversed the phasing for phase B and
C, phase C would be next, then B. This is important in understanding how to reverse the rotation of a
three phase motor. All you have to do is reverse any two phases. The illustration on the next page
simulates the rotating magnetic field produced by the stator and its effect on the rotor. Notice where the
voltages are assigned and, also, how the currents are flowing.
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11 – AC Motors
Theory of Operation
The results of this analysis are shown for voltage points 1 through 7 in the figure below. At point 1, the
magnetic field in coils 1-1A is maximum with polarities as shown. At the same time, negative voltages
are being felt in the 2-2A and 3-3A windings.
These create weaker magnetic fields, which tend to
aid the 1-1A field. At point 2, maximum negative voltage
is being felt in the 3-3A windings. This creates a strong
magnetic field which, in turn, is aided by the weaker fields
in 1-1A and 2-2A.
As each point on the voltage graph is analyzed, it can
be seen that the resultant magnetic field is rotating in
a clockwise direction. When the three-phase voltage
completes one full cycle (point 7), the magnetic field
has rotated through 360º .
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11 – AC Motors
Theory of Operation
In AC theory we learned the left-hand rule for conductors, the left-hand rule for coils, and the left-hand rule for
generators. We now can use those rules to show how the rotating magnetic field is created in the stator. The
following diagram can be used to illustrate the creation of the stator rotating magnetic field and explain why the rotor
+V
rotates.
Max
F
A
Relative motion of the conductor
Direction of the magnetic flux lines N to S
Induced current into the conductor
F
B
I
B
IMax
I
S
Notice the voltage at point A versus points B and C. These voltages
S
and currents are happening at a given instant in time (A on the three
phase sine wave). Also notice, that one conducting bar of the rotor is
darker than the others.
If we evaluate what happens when the rotating magnetic field is
N
rotating clockwise (from A to B, then C), this rotation of the stator
magnetic field causes the relative motion of the darkened conductor
-1/2 V
1/2 I
to be to the opposite direction. If the induced current is coming out of
the board, as illustrated, then the flux lines cause the rotor to also rotate clockwise.
S
N
1/2 IMax
B
-1/2 VMax
N
Max
C
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11 – AC Motors
Theory of Operation
Induction Motor Slip
An induction motor cannot run at synchronous speed since the rotor would be standing still with respect
to the rotating field and no current would be induced in the rotor.
The rotor speed must be slightly less than synchronous speed in order that current be induced in the
rotor to permit rotor rotation. The difference between rotor speed and synchronous speed is called slip:
Where
NS = synchronous speed, in rpm
Nr = rotor speed, in rpm
Slip can also be expressed as a percent of synchronous speed:
Where
NS = synchronous speed, in rpm
Nr = rotor speed, in rpm
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11 – AC Motors
Theory of Operation
Induction Motor Slip
To calculate the actual shaft speed of an induction motor this relationship will be used:
Where:
Nr = is the actual shaft speed in RPMs
f = is the frequency of the applied voltage in hertz (Hz)
P = is the number of pair of poles per phase in the stator winding
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11 – AC Motors
Theory of Operation
Induction Motor Efficiency
The three-phase, squirrel-cage induction motor operates at a relatively constant speed from no-load to
full-load. Because of the extremely low impedance of the rotor, only a slight decrease is speed is
necessary to cause a large increase in rotor current to develop the necessary torque to turn the
increased load.
The percent slip at no-load is less than 1% while at full-load it is usually between 3-5%.
This small change in percent slip from no-load to full-load likewise indicates why a squirrel-cage
induction motor is considered a fairly constant speed motor. As the slip increases in a straight line
characteristic, the rotor current will likewise increase in practically a direct proportion and cause the
torque to increase as a straight line characteristic.
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11 – AC Motors
Theory of Operation
Induction Motor Efficiency
The losses in an induction motor consist of the stray power losses and the copper losses. The stray
power losses include mechanical friction losses, windage losses, and iron losses.
These remain relatively constant at all load points and are often called fixed losses. The second group
of losses, called copper losses, is the I2R losses in the windings of the motor. As the current increases
in the motor windings with an increase in load, the I2R losses increase. At light loads the percent
efficiency () is low because the fixed losses become a smaller part of the input and the efficiency
increases to its maximum value.
However, when the rated capacity of the motor is exceeded, the copper losses become excessive and
the efficiency decreases.
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11 – AC Motors
Theory of Operation
Induction Motor Efficiency
The efficiency of an AC induction motor can be determined by:
Where
Example:
Given:
Find:
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 = Greek letter eta, percent efficiency
Pout = the output power produced by the shaft in watts
Pin = the input power required in watts
Pin = 8500 watts
Pout = 10 hp
=
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11 – AC Motors
Nameplate Data
NEMA Nameplate Data
What Do All Those Things on an AC Motor Nameplate Mean?
Introduction:
What does all that extra information on the nameplate mean? To define the basic performance and
mounting parameters of a motor, the National Electrical Manufacturers Association (NEMA) defines
some basic design and dimensional parameters in NEMA Standard MG 1. These parameters are then
coded onto the motor nameplate to give you a basic definition of what you have received. Manufacturers
often include additional information to further define some key motor features.
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11 – AC Motors
Nameplate Data
NEMA Nameplate Data
This standard section, "Nameplate Marking for Medium Single-Phase and Poly-phase Induction Motors,"
of the NEMA standard requires that "The following minimum amount of information shall be given on all
nameplates of single-phase and poly-phase induction motors.“
* Manufacturer's type and frame designation
* Horsepower output.
* Time rating.
* Maximum ambient temperature for which
motor is designed.
* Insulation system designation.
* RPM at rated load.
* Frequency.
* Number of phases.
* Rated load current.
* Voltage.
* Code letter for locked rotor kVA.
* Design letter for medium motors.
* NEMA nominal efficiency
* Service factor if other than 1.0.
* For motors equipped with thermal protectors,
the words "thermally protected".
.
* For motors rated above 1 HP equipped with over-temperature devices or systems, the words “OVER
TEMPERATURE PROTECTED _____". A type number inserted in the blank would identify the
protection type.
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11 – AC Motors
Nameplate Data
NEMA Nameplate Data
The information on a motor nameplate can be arranged in categories.
By definition, an induction motor converts electrical energy to useful mechanical energy. With rated
electrical input the motor will deliver rated output shaft power.
There are established standard indicators of how effective the motor does its job, as well as data on the
nameplate concerning safety and reliability.
The following information provides a brief definition and some application considerations regarding motor
data on the nameplate.
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11 – AC Motors
Nameplate Data
Electrical Input
Voltage
The voltage at which the motor is designed to operate is an important parameter. One common
misapplication is of motors (rated) at one voltage but applied on a different voltage using the + 10%
voltage tolerance for "successful" operation.
Nameplate-defined parameters for the motor such as power factor, efficiency, torque, and current are at
rated voltage and frequency. Application at other than nameplate voltage will likely produce different
performance. It is common for manufacturers to nameplate a wide variety of voltages on one motor
nameplate. A common example is a motor wound for 230 and 460 V (230/460 V) but operable on 208 V.
This 208-230/460V motor will have degraded performance at 208 V.
Another common misconception is to request a motor rated at network voltage; for example, at 480 V.
The NEMA standard is 460 V. The voltage rating assumes that there is voltage drop from the network to
the motor terminals. Thus, the 460V motor is appropriate on a 480V network.
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Nameplate Data
Frequency
Input frequency is usually 50 or 60 Hz. When more than one frequency is rated, other parameters that
will differ at different input frequencies must be defined on the nameplate.
The increasing use of adjustable frequency drives (AFDs) is also making it necessary to nameplate a
frequency range, especially for hazardous-duty-listed applications.
Phase
This represents the number of ac power lines supplying the motor. Single and three-phase are the
norms.
Current
Rated load current in amps is at nameplate horsepower (HP) with nameplate voltage and frequency.
When using current measurement to determine motor load, it is important that correction be made for the
operating power factor. Unbalanced phases, under-voltage conditions, or both, cause current to deviate
from nameplate AMPS.
Review both motor and drive for a matched system regarding current on AFD applications.
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Nameplate Data
Code
A letter code defines the locked rotor kVA on a per-hp basis. Codes are defined by a series of letters
from A to V. Generally, the farther the code letter from A, the higher the inrush current per hp. A
replacement motor with a "higher" code may require different upstream electrical equipment, such as
motor starters.
Type
The NEMA standard requires manufacturer's type, but there is no industry standard regarding what this
is. Some manufacturers use "Type" to define the motor as single or polyphase, single or multispeed, or
even by type of construction.
Type is of little use in defining a motor for replacement purposes unless you also note the specific motor
manufacturer.
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Nameplate Data
Power factor
Also given on the nameplate as "P.F." or PF," power factor is the ratio of the real power (W) to the
apparent power (VA) expressed as a percentage. It is numerically equal to the cosine of the phase
angle. For an induction motor, power factor also varies with load. The nameplate provides the power
factor for the motor at full load.
Real power is the power that does work; apparent power has a reactive component. This reactive
component is undesirable - the utility company must supply it, but it does no work. A power factor close
to unity (100%) is most desirable.
Because there are tradeoffs when designing an induction motor for improved efficiency or other
performance parameters, power factor sometimes suffers. It can be improved by adding capacitors.
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Mechanical Output
Horsepower
Shaft horsepower is a measure of the motor's mechanical output rating, its ability to deliver the torque
required for the load at rated speed. It is usually given as "HP" on the nameplate. In general: HP =
(Torque) x (speed)/5,250 where:
Torque is in lbs.-ft. Speed is in rpm.
Full-load speed
The speed at which rated full-load torque is delivered at rated power output is full-load speed. It is
generally given as "RPM" on the nameplate. This speed is sometimes called "slip" speed or actual rotor
speed rather than synchronous speed.
An induction motor's speed is always less than synchronous speed and it drops off as load increases.
For example, for 1800 rpm synchronous speed, an induction motor might have a full-load speed of 1748
rpm.
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Design
The NEMA standard defines "design," which defines the torque and current characteristics of the motor.
Letters are assigned the defined categories. Most motors are Design B, although the standard also
defines Designs A, C, and D. Common headings on nameplates include "Des," "NEMA Design," and
"Design." Some motors may not conform to any torque-current characteristics.
The motor manufacturer may assign them a letter that is not a defined industry standard. It is important
to check the design letter when replacing a motor in an existing application. Another note on Design B:
Design B constrains the motor designer to limit inrush current to established standards. This insures that
the user's motor-starting devices are suitable. A Design A motor has torque characteristics similar to
those of the Design B motor, but there is no limit on starting inrush current.
This may cause starter sizing problems. You should be aware of this and work with the motor
manufacturer to insure successful operation of your motor systems.
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Performance
NEMA Nominal Efficiency
Efficiency is defined as output power divided by input power expressed as a percentage:
(Output Power / Input Power) x 100
NEMA nominal efficiency on a nameplate represents an average efficiency of a large population of like
motors. The actual efficiency of the motor is guaranteed by the manufacturer to be within a tolerance
band of this nominal efficiency. The band varies depending on the manufacturer.
However, NEMA has established the maximum variation allowed. The maximum allowed by NEMA
standards represents an additional 20% of motor losses from all sources, such as friction and windage
losses, iron losses, and stray load losses.
Therefore, you should pay attention to guaranteed minimum efficiencies when evaluating motor
performance.
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11 – AC Motors
Service factor
The service factor (S.F.) is required on a nameplate only if it is higher than 1.0. Industry standard service
factor includes 1.15 for open-type motors and 1.0 for totally-enclosed-type motors.
However, service factors of 1.25, 1.4, and higher exist. It is not considered good design practice to use
the rating afforded by S.F. continuously; operating characteristics such as efficiency, power factor, and
temperature rise will be affected adversely.
Duty
This block on the nameplate defines the length of time during which the motor can carry its nameplate
rating safely. Most often, this is continuous ("CONT").
Some applications have only intermittent use and do not need motor full load continuously. Examples are
crane, hoist, and valve actuator applications. The duty on such motors is usually expressed in minutes.
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11 – AC Motors
Safety
Special markings
Many motor nameplates have special markings to reflect third-party certification or recognition.
Some common markings are:
* CSA Indicates that the manufacturing system and the motor components meet the standards of, the
Canadian Standards Association.
• UL indicates that the manufacturing system and the motor components meet the standards of,
Underwriters Laboratories.
Reliability
Insulation class
Often abbreviated "INSUL CLASS" on nameplates, it is an industry standard classification of the thermal
tolerance of the motor winding. Insulation class is a letter designation such as "A," "B," or "F," depending
on the winding's ability to survive a given operating temperature for a given life. Insulation classes of a
letter deeper into the alphabet perform better. For example, class F insulation has a longer nominal life at
a given operating temperature than class A, or for a given life it can survive higher temperatures.
Operating temperature is a result of ambient conditions plus the energy lost in the form of heat (causing
the temperature rise) as the motor converts electrical to mechanical energy.
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Maximum ambient temperature
The nameplate lists the maximum ambient temperature at which the motor can operate and still be within
the tolerance of the insulation class at the maximum temperature rise. It is often called "AMB" on the
nameplate and is usually given in degrees C.
Altitude
This indicates the maximum height above sea level at which the motor will remain within its design
temperature rise, meeting all other nameplate data. If the motor operates below this altitude, it will run
cooler. At higher altitudes, the motor would tend to run hotter because the thinner air cannot remove the
heat so effectively, and the motor may have to be derated. Not every nameplate has an altitude rating.
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11 – AC Motors
Construction
Enclosure
This designation, often shown as "ENCL" on a nameplate, classifies the motor as to its degree of
protection from its environment, and its method of cooling. NEMA describes many variations. The most
common are Open Drip-Proof (ODP) and Totally Enclosed Fan Cooled (TEFC).
• ODP-An open drip-proof motor allows a free exchange of air from outside the motor to circulate
around the winding while being unaffected by drops of liquid or particles that strike or enter the
enclosure at any angle from 0 to 15 deg. downward from the vertical.
• TEFC-A totally enclosed fan cooled motor prevents free exchange of air between inside and outside
the motor enclosure. It has a fan blowing air over the outside of the enclosure to aid in cooling. A
TEFC motor is not considered air or water-tight; it allows outside air containing moisture and other
contaminants to enter, but usually not enough to interfere with normal operation. If contamination is a
problem in a given application, most manufacturers can provide additional protection such as mill &
chemical duty features, special insulations and internal coating, or space heaters for motors subject to
extended shutdown periods and wide temperature swings that could make the motor "breathe"
contaminants.
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11 – AC Motors
Construction
Frame
This nameplate block can offer a lot of information if the motor is nearly standard. The frame size sets
important mounting dimensions such as foot hole mounting pattern, shaft diameter, and shaft height.
NEMA standards do not set some dimensions that can turn out to be important if the motor must fit into a
confined space.
These include maximums of overall height and length, and maximum conduit-box extensions. The data
in the "Frame" block can be hard to interpret when special shafts or mounting configurations are used.
Some examples of frame designation:
• 445T-This motor is a modern standard T-Frame motor. Critical mounting dimensions for all
manufacturers are as defined in the NEMA Standard.
• 445TC This T-Frame motor has a standard NEMA-defined C-face.
• 445TD This T-Frame motor has a standard NEMA-defined D-flange.
• 445U The dimensions of a U -Frame motor are defined by NEMA standards prior to 1965. The UFrame is the predecessor to the present T -Frame motor, and typically it has the equivalent
horsepower capability of a T Frame motor that is two frame sizes smaller.
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11 – AC Motors
Construction
Frame
For example, the T -Frame equivalent of a 445U Frame motor for 100 hp at 1,800 rpm is a 405T motor
for the same power and speed.
The first two digits of the frame size divided by 4 defines the height of the shaft centerline from the
bottom of the feet. Thus, the shaft height of a 445T motor is 44 / 4 = 11 in.
The third digit in the frame size determines the distance between the foot holes nearest the shaft and the
opposite drive-end foot holes. Many manufacturers drill multiple foot holes in motor bases to allow
mounting in short or longer frame positions.
For example, a 445T motor base may have mounting holes for 444T and 445T motors. If special
dimension designations appear, be sure to contact the motor manufacturer for dimensional information
for a replacement.
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11 – AC Motors
Construction
Bearings
Though NEMA does not require it, many manufacturers supply nameplate data on bearings, because
they are the only true maintenance components in an AC motor.
Such information is usually given for both the drive-end bearing and the bearing opposite the drive end.
Nameplate designations vary from one manufacturer to another.
For rolling-element bearings, the most common is the "AFBMA Number." That is the number that
identifies the bearing by standards of the Anti-Friction Bearing Manufacturers Association.
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Construction
Other data
A typical nameplate also includes the motor's brand name, "Serial No." or other identifying number
unique to that motor. This allows the manufacturer to trace the motor back through manufacturing.
The nameplate also includes the manufacturer's name, and its principal city and state and "Made in
U.S.A." if U.S.-made. The nameplate is a treasury of important information about a motor. If you specify,
buy, maintain, or replace motors, you should know how to read them.
Note: This material is not intended to provide operational instructions. Appropriate manufacturer
instruction manuals and precautions should be studied prior to installation, operation, or maintenance of
equipment.
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11 – AC Motors
Construction
Other data
A typical nameplate also includes the motor's brand name, "Serial No." or other identifying number
unique to that motor. This allows the manufacturer to trace the motor back through manufacturing.
The nameplate also includes the manufacturer's name, and its principal city and state and "Made in
U.S.A." if U.S.-made. The nameplate is a treasury of important information about a motor. If you specify,
buy, maintain, or replace motors, you should know how to read them.
Note: This material is not intended to provide operational instructions. Appropriate manufacturer
instruction manuals and precautions should be studied prior to installation, operation, or maintenance of
equipment.
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11 – AC Motors
IEC Nameplate Data
1.
2.
3.
4.
5.
6.
7.
8.
Motor wiring = 3 phase.
Design frequency = 50 Hz.
Ratings according to IEC frame sizes = IEC 34-1
Production number.
Power of motor = 15 kW. (Power on the shaft not electric power)
Nominal speed = 1450 r/min.
Insulation Class = Cl. F.
Power factor = Cos F 0.90. (at 1450 r/min)
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IEC Nameplate Data
9. Connection terminal Star (Y). =
10. Input voltage Y connection = 380 V.
11. Total current Y connection = 29 A.
12. Connection terminal = Delta (Δ).
13. Input voltage Δ connection = 220 V.
14. Total current Δ connection = 50 A.
If the supply line voltage is 380 VAC, then
motor must be Star (Y) connection.
If the supply line voltage is 220 VAC, then
motor must be Delta (Δ) connection
15. Type of motor = Cat. No. …..
16. IP ratings = IP 54.
17. Weight of motor = ….. Kg.
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IEC Nameplate Data
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11 – AC Motors
Wiring Configurations
U.S. Dual Voltage Motor Connections
It is common for U.S. electric motor manufacturers to build motors for connection to many different
voltage sources. Manufacturing motors for dual voltages, such as 480/240 volt, enables the same motor
to be used in applications where the line voltages are different.
These motors have the same characteristics on either voltage. The speed and horsepower remain the
same if the motor is operated on either the higher or lower voltage.
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11 – AC Motors
Wiring Configurations
U.S. Dual Voltage Motor Connections
T1
T4
Dual-voltage connections are either wye-connected or delta-connected.
Each phase is divided into two sections with the same number of coils in
each section and nine external connections are brought out from these
sections when the motor is wound.
T7
T8
T9
T5
T6
WYE
T3
T2
Each external lead or connection is stamped on the wire or at the
terminal block with a number from one to nine.
T1
T4
T9
DELTA
T6
T3
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T8
T5
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T7
T2
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11 – AC Motors
Wiring Configurations
Wye-Connected Motors
Below illustrates the connections and terminal markings for a wye-connected, dual-voltage induction
motor. In the series-wye connection illustrated on the left, the ascending numbers go in succession in a
clockwise direction to the three points of the wye. Then starting at the end of the T1 winding, go around
clockwise again, completing the three outside groups. The third start is on the inside wye just below T4,
and go around clockwise again. This makes the nine external terminals of the six windings. The other
three terminals of the internal wye are connected inside the motor for a nine lead motor.
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11 – AC Motors
Wiring Configurations
Wye-Connected Motors
Note on the diagram on the left that this connection has two windings for each phase connected in
series. This is known as a series-wye connection and is always the higher voltage connection in a
dual-voltage, wye-connected motor.
The original
internal wye
and an
external wye
formed by
connecting
leads T4,
T5, and T6
together as
illustrated.
The drawing on the right illustrates how the same nine external leads are connected for the lower voltage
operation. The two windings of each phase are now connected in parallel and have the same voltage
drop across each winding. This is known as a parallel-wye connection, and is always the lower voltage
connection for any dual-voltage, wye-connected motor. In this connection there are two wye center
points.
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11 – AC Motors
Wiring Configurations
Delta-Connected Motors
The diagrams below illustrate the connections and terminal markings for a delta-connected, dual-voltage
induction motor. The diagram on the left shows the method of numbering and connecting the nine
external leads for high voltage operation. The method of numbering for the delta is much the same as
the wye. Starting at the top corner of the delta, number the three corners T1, T2, T3. Then start at the
end of the T1 winding and go clockwise to number for T4, T2 toT5, and T3 to T6. For the next terminal,
start at the T4 winding and go clockwise to the next terminal for T7, then go clockwise of T5 and label
T8, and finally label T9.
Ph A
Ph A
T1
T1
T9
T4
T6
T3
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T5
T4
T6
T7
T8
Ph C
T9
T2
Ph B
T3
T7
T8
T5
Ph C
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T2
Ph B
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11 – AC Motors
Wiring Configurations
Delta-Connected Motors
Note the diagram on the left, the two sections of each phase are connected in series for the high
voltage connection.
Ph A
Ph A
T1
T1
T9
T4
T6
T3
T5
T4
T6
T7
T8
Ph C
T9
T2
Ph B
T3
T7
T8
T5
Ph C
T2
Ph B
The drawing on the right illustrates the nine external lead connections for lower voltage operation. The
two sections of each phase are connected in parallel. This is a parallel-delta connection and always the
lower voltage connection in a dual-voltage, delta-connected motor.
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11 – AC Motors
Wiring Configurations
European Motors
These motors have six leads, three coils, and have the capability of being connected either wye or delta.
The wye connection is used for high voltage, and the delta connection is used for low voltage. Another
point to consider is that these motors were designed for 380V or 220V. Normally in the U.S., 480 volts
will be used.
One additional factor is the European motor is designed to operate on 50 Hertz. In the U.S., we have 60
Hertz.
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Wiring Configurations
European Motors
Below is a European six lead motor connected in a wye configuration for high voltage (380volts).
Ph A
U (U1)
X (U2)
Z (W2)
Y (V2)
W (W1)
V (V1)
Ph C
Ph B
U (U1)
Z (W2)
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V (V1)
X (U2)
W (W1)
Y (V2)
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11 – AC Motors
Wiring Configurations
European Motors
Below is a European six lead motor connected in a Delta configuration for low voltage (220volts).
Ph A
Z (W2)
U (U1)
X (U2)
W (W1)
Ph C
Ph B
Y (V2)
U (U1)
Z (W2)
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V (V1)
V (V1)
X (U2)
W (W1)
Y (V2)
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11 – AC Motors
Wiring
Exercises
Use the diagram below to practice the connections for a low voltage wye-connected, 9-lead
American motor.
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11 – AC Motors
Wiring
Exercises
Use the diagram below to practice the connections for a high voltage wye-connected, 9-lead
American motor.
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Wiring
Exercises
Use the diagram below to practice the connections for a high voltage delta-connected, 9-lead
American motor.
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11 – AC Motors
Wiring
Exercises
Use the diagram below to practice the connections for a low voltage delta-connected, 9-lead
American motor.
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11 – AC Motors
Wiring
Exercises
Use the diagram below to practice the connections for a wye-connected, 6-lead European motor.
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Wiring
Exercises
Use the diagram below to practice the connections for a delta-connected, 6-lead European
motor.
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11 – AC Motors
Troubleshooting AC Motors
Care of Windings and Insulation
Except for expensive, high horsepower motors, routine inspections generally do not involve opening the
motor to inspect the windings. Therefore, long motor life requires selection of the proper enclosure to
protect the windings from excessive dirt, abrasives, moisture, oil and chemicals.
When the need is indicated by severe operating conditions or a history of winding failures, routine testing
can identify deteriorating insulation. Such motors can be removed from service and repaired before
unexpected failures stop production.
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11 – AC Motors
Troubleshooting AC Motors
Whenever a motor is opened for repair, service the windings as follows:
1. Accumulated dirt prevents proper cooling and may absorb moisture and other contaminants that
damage the insulation. Vacuum the dirt from the windings and internal air passages. Do not use high
pressure air because this can damage windings by driving the dirt into the insulation.
2. Moisture reduces the dielectric strength of insulation which results in shorts. If the inside of the motor
is damp, dry the motor using manufacturer recommendations.
3. Wipe any oil and grease from inside the motor. Use care with solvents that can attack the insulation.
4. If the insulation appears brittle, overheated or cracked, the motor should be replaced as soon as
possible.
5. Check the lead-to-coil connections for signs of overheating or corrosion. These connections are often
exposed on large motors but taped on small motors. Repair as needed.
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11 – AC Motors
Troubleshooting AC Motors
Testing AC Motor Windings Insulation
Routine field testing of windings can identify deteriorating insulation permitting scheduled repair or
replacement of the motor before its failure disrupts operations. Such testing is good practice especially
for applications with severe operating conditions or a history of winding failures and for expensive, high
horsepower motors and locations where failures can cause health and safety problems or high economic
loss.
The easiest field test that prevents the most failures is the ground-insulation test. It applies a voltage,
usually twice the nominal value (500 or 1000 volts), to the motor and measures the resistance of the
insulation.
NEMA standards require a minimum resistance to ground at 40 degrees C ambient of 1 megohm per
one thousand volts of rating plus 1 megohm. Medium size motors in good condition will generally have
megohmmeter readings in excess of 50 megohms. Low readings may indicate a seriously reduced
insulation condition caused by contamination from moisture, oil or conductive dirt or deterioration from
age or excessive heat.
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11 – AC Motors
Troubleshooting AC Motors
Testing AC Motor Windings Insulation
One megohm reading for a motor means little. A curve recording resistance, with the motor cold and
hot, and date indicates the rate of deterioration. This curve provides the information needed to decide if
the motor can be safely left in service until the next scheduled inspection time.
Note: Insure that the motor leads are completely disconnected from the rest of the circuit before using
the megohmmeter to test the insulation. The higher voltage used could damage other equipment in the
circuit.
Testing AC Motor Windings DC resistance
When a motor overload trips and/or blows fuses, certain procedures and tests should be carried out:
• Lockout and tagout main power supply if applicable.
• Test leads at the motor terminals box with an ohmmeter for continuity and ohmic value of windings
between phases A to B, phases A to C and phases B to C. Each resistance should be within 1 or 5
ohms of each other.
• The next pages will illustrate what the readings could be in certain circumstances.
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11 – AC Motors
Troubleshooting AC Motors
What do these readings indicate?
The winding value is approx.
100 ohms per winding.
Ph A
U (U1)
X (U2)
Z (W2)
W (W1)
A-B = 200 ohms
A-C = 200 ohms
B-C = 200 ohms
A-Grd. Open Loop
B-Grd. Open Loop
C-Grd. Open Loop
Y (V2)
V (V1)
Ph C
Ph B
A normal Wye connected motor!
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Troubleshooting AC Motors
What do these readings indicate?
The winding value is approx.
100 ohms per winding.
Ph A
A-B = 200 ohms
A-C = Open Loop
B-C = Open Loop
U (U1)
A-Grd. Open Loop
B-Grd. Open Loop
C-Grd. Open Loop
X (U2)
Z (W2)
W (W1)
Y (V2)
V (V1)
Ph C
Ph B
An Open Winding on a Wye connected Motor
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Troubleshooting AC Motors
Ph A
U (U1)
X (U2)
Z (W2)
Y (V2)
W (W1)
V (V1)
Ph C
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Ph B
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11 – AC Motors
Troubleshooting AC Motors
What do these readings indicate?
Ph A
The winding value is approx.
100 ohms per winding.
U (U1)
A-B = 100 ohms
A-C = 100 ohms
B-C = 200 ohms
A-Grd. Open Loop
B-Grd. Open Loop
C-Grd. Open Loop
X (U2)
Z (W2)
W (W1)
Y (V2)
V (V1)
Ph C
Ph B
A Shorted Winding on a Wye connected Motor
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
Ph A
U (U1)
X (U2)
Z (W2)
Y (V2)
W (W1)
V (V1)
Ph C
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Ph B
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
What do these readings indicate?
Ph A
The winding value is approx.
100 ohms per winding.
U (U1)
X (U2)
Z (W2)
Y (V2)
W (W1)
A-B = 200 ohms
A-C = 200 ohms
B-C = 200 ohms
A- Grd. 0 ohms
B-Grd. 200 ohms
C-Grd. 200 ohms
V (V1)
Ph C
Ph B
A Grounded Winding on a Wye connected Motor
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
Ph A
U (U1)
X (U2)
Z (W2)
Y (V2)
W (W1)
V (V1)
Ph C
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Ph B
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
What do these readings indicate?
The winding value is approx.
120 ohms per winding.
Ph A
Z (W2)
A-B = 80 ohms
A-C = 80 ohms
B-C = 80 ohms
U (U1)
A-Grd. Open Loop
B-Grd. Open Loop
C-Grd. Open Loop
X (U2)
W (W1)
Ph C
Ph B
Y (V2)
V (V1)
A Normal Delta connected Motor
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
What do these readings indicate?
Ph A
The winding value is approx.
120 ohms per winding.
Z (W2)
U (U1)
A-B = 120 ohms
A-C = 120 ohms
B-C = 240 ohms
X (U2)
W (W1)
A-Grd. Open Loop
B-Grd. Open Loop
C-Grd. Open Loop
Ph C
Ph B
Y (V2)
V (V1)
An Open Winding on a Delta connected Motor
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
Ph A
Z (W2)
U (U1)
X (U2)
W (W1)
Ph C
Ph B
Y (V2)
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
V (V1)
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
What do these readings indicate?
Ph A
The winding value is approx.
120 ohms per winding.
Z (W2)
U (U1)
A-B = 60 ohms
A-C =0 ohms
B-C = 60 ohms
X (U2)
W (W1)
A-Grd. Open Loop
B-Grd. Open Loop
C-Grd. Open Loop
Ph C
Ph B
Y (V2)
V (V1)
A Shorted Winding on a Delta connected Motor
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
Ph A
Z (W2)
U (U1)
X (U2)
W (W1)
Ph C
Ph B
Y (V2)
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
V (V1)
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
What do these readings indicate?
Ph A
The winding value is approx.
120 ohms per winding.
Z (W2)
U (U1)
A-B = 80 ohms
A-C = 80 ohms
B-C = 80 ohms
X (U2)
W (W1)
A-Grd. 0 ohms
B-Grd. 80 ohms
C-Grd. 80 ohms
Ph C
Ph B
Y (V2)
V (V1)
A Grounded Winding on a Delta connected Motor
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
Classification : D3
Classification : D3
Conservation :
Page : ‹#›
11 – AC Motors
Troubleshooting AC Motors
Ph A
Z (W2)
U (U1)
X (U2)
W (W1)
Ph C
Ph B
Y (V2)
11 AC Motors
Presentation : IMS – Tech Managers Conference
Author : IMS Stafff
Author : IMS Staff
Creation date : 09 Nov 2012
Creation date : 08 March 2012
V (V1)
Classification : D3
Classification : D3
Conservation :
Page : ‹#›