Unit 5-6-7 a

Introduction
 Three-phase induction motors are the most common and
frequently encountered machines in industry
- simple design, rugged, low-price, easy maintenance
- wide range of power ratings: fractional horsepower to
10 MW
- run essentially as constant speed from no-load to full
load
- Its speed depends on the frequency of the power
source
• not easy to have variable speed control
• requires a variable-frequency power-electronic
drive for optimal speed control

Construction
 An induction motor has two main parts
- a stationary stator
• consisting of a steel frame that supports a hollow,
cylindrical core
• core, constructed from stacked laminations (why?),
having a number of evenly spaced slots, providing the
space for the stator winding
Stator of IM

Construction
- a revolving rotor
• composed of punched laminations, stacked to create a
series of rotor slots, providing space for the rotor winding
• one of two types of rotor windings
• conventional 3-phase windings made of insulated wire
(wound-rotor) » similar to the winding on the stator
• aluminum bus bars shorted together at the ends by two
aluminum rings, forming a squirrel-cage shaped circuit
(squirrel-cage)
 Two basic design types depending on the rotor design
- squirrel-cage: conducting bars laid into slots and shorted at
both ends by shorting rings.
- wound-rotor: complete set of three-phase windings exactly
as the stator. Usually Y-connected, the ends of the three
rotor wires are connected to 3 slip rings on the rotor shaft.

Squirrel cage rotor
Wound rotor
Notice the
slip rings
Construction

Cutaway in a
typical wound-
rotor IM.
Notice the
brushes and the
slip rings
Brushes
Slip rings
Construction

Rotating Magnetic Field
 Balanced three phase windings, i.e.
mechanically displaced 120
degrees form each other, fed by
balanced three phase source
 A rotating magnetic field with
constant magnitude is produced,
rotating with a speed
Where fe is the supply frequency and
P is the no. of poles and nsync is called
the synchronous speed in rpm
(revolutions per minute)
120 e
sync
f
n rpm
P
=

Synchronous speed
P 50 Hz 60 Hz
2 3000 3600
4 1500 1800
6 1000 1200
8 750 900
10 600 720
12 500 600

( ) ( ) ( ) ( )net a b cB t B t B t B t= + +
sin( ) 0 sin( 120 ) 120 sin( 240) 240M M MB t B t B tω ω ω= ∠ °+ − ° ∠ °+ − ∠ °
ˆsin( )
3
ˆ ˆ[0.5 sin( 120 )] [ sin( 120 )]
2
3
ˆ ˆ[0.5 sin( 240 )] [ sin( 240 )]
2
M
M M
M M
B t
B t B t
B t B t
ω
ω ω
ω ω
=
− − ° − − °
− − ° + − °
x
x y
x y

1 3 1 3
ˆ( ) [ sin( ) sin( ) cos( ) sin( ) cos( )]
4 4 4 4
3 3 3 3
ˆ[ sin( ) cos( ) sin( ) cos( )]
4 4 4 4
net M M M M M
M M M M
B t B t B t B t B t B t
B t B t B t B t
ω ω ω ω ω
ω ω ω ω
= + + + −
+ − − + −
x
y
ˆ ˆ[1.5 sin( )] [1.5 cos( )]M MB t B tω ω= −x y

Principle of operation
 This rotating magnetic field cuts the rotor windings and
produces an induced voltage in the rotor windings
 Due to the fact that the rotor windings are short circuited,
for both squirrel cage and wound-rotor, and induced current
flows in the rotor windings
 The rotor current produces another magnetic field
 A torque is produced as a result of the interaction of those
two magnetic fields
Where τind is the induced torque and BR and BS are the magnetic
flux densities of the rotor and the stator respectively
ind R skB Bτ = ×

Induction motor speed
 At what speed will the IM run?
- Can the IM run at the synchronous speed, why?
- If rotor runs at the synchronous speed, which is the
same speed of the rotating magnetic field, then the
rotor will appear stationary to the rotating magnetic
field and the rotating magnetic field will not cut the
rotor. So, no induced current will flow in the rotor and
no rotor magnetic flux will be produced so no torque is
generated and the rotor speed will fall below the
synchronous speed
- When the speed falls, the rotating magnetic field will
cut the rotor windings and a torque is produced

Induction motor speed
 So, the IM will always run at a speed lower than
the synchronous speed
 The difference between the motor speed and the
synchronous speed is called the Slip
Where nslip= slip speed
nsync= speed of the magnetic field
nm = mechanical shaft speed of the motor
slip sync mn n n= −

The Slip
sync m
sync
n n
s
n
−
=
Where s is the slip
Notice that : if the rotor runs at synchronous speed
s = 0
if the rotor is stationary
s = 1
Slip may be expressed as a percentage by multiplying the
above eq. by 100, notice that the slip is a ratio and doesn’t
have units

Induction Motors and Transformers
 Both IM and transformer works on the principle of
induced voltage
- Transformer: voltage applied to the primary windings
produce an induced voltage in the secondary windings
- Induction motor: voltage applied to the stator windings
produce an induced voltage in the rotor windings
- The difference is that, in the case of the induction
motor, the secondary windings can move
- Due to the rotation of the rotor (the secondary winding
of the IM), the induced voltage in it does not have the
same frequency of the stator (the primary) voltage

Frequency
 The frequency of the voltage induced in the rotor is
given by
Where fr = the rotor frequency (Hz)
P = number of stator poles
n = slip speed (rpm)
120
r
P n
f
×
=
( )
120
120
s m
r
s
e
P n n
f
P sn
sf
× −
=
×
= =

Frequency
 What would be the frequency of the rotor’s induced
voltage at any speed nm?
 When the rotor is blocked (s=1) , the frequency of
the induced voltage is equal to the supply frequency
 On the other hand, if the rotor runs at synchronous
speed (s = 0), the frequency will be zero
r ef s f=

Torque
 While the input to the induction motor is electrical
power, its output is mechanical power and for that we
should know some terms and quantities related to
mechanical power
 Any mechanical load applied to the motor shaft will
introduce a Torque on the motor shaft. This torque is
related to the motor output power and the rotor speed
and.out
load
m
P
N mτ
ω
= 2
/
60
m
m
n
rad s
π
ω =

Horse power
 Another unit used to measure mechanical power is
the horse power
 It is used to refer to the mechanical output power
of the motor
 Since we, as an electrical engineers, deal with
watts as a unit to measure electrical power, there is
a relation between horse power and watts
746hp watts=

Equivalent Circuit
 The induction motor is similar to the transformer with
the exception that its secondary windings are free to
rotate
As we noticed in the transformer, it is easier if we can combine
these two circuits in one circuit but there are some difficulties

Equivalent Circuit
 When the rotor is locked (or blocked), i.e. s =1, the
largest voltage and rotor frequency are induced in
the rotor, Why?
 On the other side, if the rotor rotates at synchronous
speed, i.e. s = 0, the induced voltage and frequency
in the rotor will be equal to zero, Why?
Where ER0 is the largest value of the rotor’s induced voltage
obtained at s = 1(loacked rotor)
0R RE sE=

Equivalent Circuit
 The same is true for the frequency, i.e.
 It is known that
 So, as the frequency of the induced voltage in the
rotor changes, the reactance of the rotor circuit also
changes
Where Xr0 is the rotor reactance
at the supply frequency
(at blocked rotor)
r ef s f=
2X L f Lω π= =
0
2
2
r r r r r
e r
r
X L f L
sf L
sX
ω π
π
= =
=
=

Equivalent Circuit
 Then, we can draw the rotor equivalent circuit as
follows
Where ER is the induced voltage in the rotor and RR is the
rotor resistance

Equivalent Circuit
 Now we can calculate the rotor current as
 Dividing both the numerator and denominator by s
so nothing changes we get
Where ER0 is the induced voltage and XR0 is the rotor
reactance at blocked rotor condition (s = 1)
0
0
( )
( )
R
R
R R
R
R R
E
I
R jX
sE
R jsX
=
+
=
+
0
0( )
R
R
R
R
E
I
R
jX
s
=
+

Equivalent Circuit
 Now we can have the rotor equivalent circuit

Equivalent Circuit
 Now as we managed to solve the induced voltage
and different frequency problems, we can combine
the stator and rotor circuits in one equivalent
circuit
Where
2
2 0
2
2
2
1 0
eff R
eff R
R
eff
eff R
S
eff
R
X a X
R a R
I
I
a
E a E
N
a
N
=
=
=
=
=

Power losses in Induction machines
 Copper losses
- Copper loss in the stator (PSCL) = I1
2
R1
- Copper loss in the rotor (PRCL) = I2
2
R2
 Core loss (Pcore)
 Mechanical power loss due to friction and windage
 How this power flow in the motor?

Power relations
3 cos 3 cosin L L ph phP V I V Iθ θ= =
2
1 13SCLP I R=
( )AG in SCL coreP P P P= − +
2
2 23RCLP I R=
conv AG RCLP P P= −
( )out conv f w strayP P P P+= − + conv
ind
m
P
τ
ω
=

Equivalent Circuit
 We can rearrange the equivalent circuit as follows
Actual rotor
resistance
Resistance
equivalent to
mechanical load

Power relations
3 cos 3 cosin L L ph phP V I V Iθ θ= =
2
1 13SCLP I R=
( )AG in SCL coreP P P P= − +
2
2 23RCLP I R=
conv AG RCLP P P= −
( )out conv f w strayP P P P+= − +
conv RCLP P= + 2 2
23
R
I
s
=
2 2
2
(1 )
3
R s
I
s
−
=
RCLP
s
=
(1 )RCLP s
s
−
=
(1 )conv AGP s P= −
conv
ind
m
P
τ
ω
=
(1 )
(1 )
AG
s
s P
s ω
−
=
−

Power relations
AGP
RCLP
convP
1
s
1-s
: :
1 : : 1-
AG RCL convP P P
s s

Torque, power and Thevenin’s Theorem
 Thevenin’s theorem can be used to transform the
network to the left of points ‘a’ and ‘b’ into an
equivalent voltage source VTH in series with
equivalent impedance RTH+jXTH

1 1( )
M
TH
M
jX
V V
R j X X
φ=
+ +
1 1( )//TH TH MR jX R jX jX+ = +
2 2
1 1
| | | |
( )
M
TH
M
X
V V
R X X
φ=
+ +

 Since XM>>X1 and XM>>R1
 Because XM>>X1 and XM+X1>>R1
1
M
TH
M
X
V V
X X
φ≈
+
2
1
1
1
M
TH
M
TH
X
R R
X X
X X
 
≈  ÷
+ 
≈

Then the power converted to mechanical (Pconv)
2 2
22
2( )
TH TH
T
TH TH
V V
I
Z R
R X X
s
= =
 
+ + + ÷
 
2 2
2
(1 )
3conv
R s
P I
s
−
=
And the internal mechanical torque (Tconv)
conv
ind
m
P
τ
ω
=
(1 )
conv
s
P
s ω
=
−
2 2
23
AG
s s
R
I
Ps
ω ω
= =

2
2
2
22
2
3
( )
TH
ind
s
TH TH
V R
sR
R X X
s
τ
ω
 
 ÷
 ÷  
=  ÷ ÷
   ÷+ + + ÷ ÷  
2 2
2
22
2
3
1
( )
TH
ind
s
TH TH
R
V
s
R
R X X
s
τ
ω
 
 ÷
 =
 
+ + + ÷
 

Torque-speed characteristics
Typical torque-speed characteristics of induction motor

Comments
1. The induced torque is zero at synchronous speed.
Discussed earlier.
2. The curve is nearly linear between no-load and full
load. In this range, the rotor resistance is much
greater than the reactance, so the rotor current,
torque increase linearly with the slip.
3. There is a maximum possible torque that can’t be
exceeded. This torque is called pullout torque and
is 2 to 3 times the rated full-load torque.

Comments
4. The starting torque of the motor is slightly higher
than its full-load torque, so the motor will start
carrying any load it can supply at full load.
5. The torque of the motor for a given slip varies as
the square of the applied voltage.
6. If the rotor is driven faster than synchronous speed
it will run as a generator, converting mechanical
power to electric power.

Maximum torque
 Maximum torque occurs when the power
transferred to R2/s is maximum.
 This condition occurs when R2/s equals the
magnitude of the impedance RTH + j (XTH + X2)
max
2 22
2( )TH TH
T
R
R X X
s
= + +
max
2
2 2
2( )
T
TH TH
R
s
R X X
=
+ +

Maximum torque
 The corresponding maximum torque of an induction
motor equals
The slip at maximum torque is directly proportional to
the rotor resistance R2
The maximum torque is independent of R2
2
max 2 2
2
31
2 ( )
TH
s TH TH TH
V
R R X X
τ
ω
 
 ÷=
 ÷+ + + 

Maximum torque
 Rotor resistance can be increased by inserting
external resistance in the rotor of a wound-rotor
induction motor.
The
value of the maximum torque remains unaffected
but
the speed at which it occurs can be controlled.

Maximum torque
Effect of rotor resistance on torque-speed characteristic

Determination of motor parameters
 Due to the similarity between the induction motor
equivalent circuit and the transformer equivalent
circuit, same tests are used to determine the values
of the motor parameters.
- DC test: determine the stator resistance R1
- No-load test: determine the rotational losses and
magnetization current (similar to no-load test in
Transformers).
- Locked-rotor test: determine the rotor and stator
impedances (similar to short-circuit test in
Transformers).

DC test
- The purpose of the DC test is to determine R1. A variable
DC voltage source is connected between two stator
terminals.
- The DC source is adjusted to provide approximately
rated stator current, and the resistance between the two
stator leads is determined from the voltmeter and
ammeter readings.

DC test
- then
- If the stator is Y-connected, the per phase stator
resistance is
- If the stator is delta-connected, the per phase stator
resistance is
DC
DC
DC
V
R
I
=
1
2
DCR
R =
1
3
2
DCR R=

No-load test
1. The motor is allowed to spin freely
2. The only load on the motor is the friction and windage
losses, so all Pconv is consumed by mechanical losses
3. The slip is very small

No-load test
4. At this small slip
The equivalent circuit reduces to…
2 2
2 2
(1 ) R (1 )
&
R s s
R X
s s
− −
? ?

No-load test
5. Combining Rc & RF+W we get……

No-load test
6. At the no-load conditions, the input power measured by
meters must equal the losses in the motor.
7. The PRCL is negligible because I2 is extremely small
because R2(1-s)/s is very large.
8. The input power equals
Where
&
2
1 13
in SCL core F W
rot
P P P P
I R P
= + +
= +
&rot core F WP P P= +

No-load test
9. The equivalent input impedance is thus approximately
If X1 can be found, in some other fashion, the magnetizing
impedance XM will be known
1
1,
eq M
nl
V
Z X X
I
φ
= ≈ +

Blocked-rotor test
 In this test, the rotor is locked or blocked so that it
cannot move, a voltage is applied to the motor, and
the resulting voltage, current and power are
measured.

Blocked-rotor test
 The AC voltage applied to the stator is adjusted so
that the current flow is approximately full-load
value.
 The locked-rotor power factor can be found as
 The magnitude of the total impedance
cos
3
in
l l
P
PF
V I
θ= =
LR
V
Z
I
φ
=

Blocked-rotor test
Where X’1 and X’2 are the stator and rotor reactances at
the test frequency respectively
'
cos sin
LR LR LR
LR LR
Z R jX
Z j Zθ θ
= +
= +
1 2
' ' '
1 2
LR
LR
R R R
X X X
= +
= +
2 1LRR R R= −
'
1 2
rated
LR LR
test
f
X X X X
f
= = +

Blocked-rotor test
X1 and X2 as function of XLR
Rotor Design X1 X2
Wound rotor 0.5 XLR 0.5 XLR
Design A 0.5 XLR 0.5 XLR
Design B 0.4 XLR 0.6 XLR
Design C 0.3 XLR 0.7 XLR
Design D 0.5 XLR 0.5 XLR

Induction Motor
The induction motors are widely used as industrial
drives due to its simplicity, reliability and low cost.
 Induction motors works with better efficiency,
appreciable over-load capacity and maintenance
required is minimum.
With the application of thysristor control, induction
motor can be used for variable speed drive.
Three - phase induction motors are available with
various ratings from fractional hp to several thousands of
hp (say 10,OOOhp).

Induction Motor
Small motors below 1 hp are usually single phase
induction motors.
Induction motors are available with different voltage
ratings i.e. 440V, 3.3 KV, 6.6 KV, 11 KV.
Induction motors are not economical above 11 KV.
 Because it is difficult to design slot insulation.
 The rated voltage generally depends on KW rating.

Specifications of 3-phase induction motors
•Output Rating:
The preferred output rating for induction motor up
to and including 110 kW are
0.06,0.09,11,15,22,90,100 kW.
•Type of mounting:
The mounting is to be specified like foot mounting
and bed mounting etc.

Specifications of 3-phase induction motors
• Rated voltage and Rated frequency with variations
Motors shall be able to deliver rated output with
a) A terminal voltage differing from its rated value by not
more than +/-6% or ,
b)The frequency differing from its rated value by not
more than +/-3% or,
c) Combination of a and b

Specifications of 3-phase Induction Motors
• The voltages preferred for three phase, 50Hz
machines are 415V,3.3kV,6.6kV and 11kV
Coordination of voltages and outputs:
Rated voltage(kV) Minimum Rated Output (kW)
2<kV<=3.3
3.3<kV<=6.6
6.6<kV<=11
100
200
1000

•The frequency shall be standard frequency of 50Hz.
•Class of Insulation: The class of insulation used for the
winding is to be given (class A, E, B, F, H).
•Ambient temperature
•Type of construction and bearing arrangements
• Type of enclosure and cooling system
•Method Of starting and drive details
•Performance requirements with respect to-efficiency
and related parameters.

Speed : measured in Rpm
Number of Phases:
single phase induction motors
Three phase induction motors
Rated Current : In amperes
Efficiency: (input-loss)/input
Type of duty: Continuous duty
Short term duty
Intermittent duty (etc)

Class of insulation :
Class Temperature
A 1050
C
E 1200
C
B 1300
C
F 1550
C
H 1800
C
Type of construction of rotor: wound rotor & cage rotor
Type of cooling system: radial cooling & axial cooling
Methods of starting: Direct online starting , auto transformer
starting & star-delta starting
Type of mounting :mounting is to be specified like Foot
mounting, Bed mounting

Procurement of induction motor
Information to be given with enquiry and order as per:
IS 3251978.
When enquiring for and placing an order for induction
motor the following particulars should be supplied.
1) Site and operating conditions
2) Reference to this standard i.e. IS code number
3) Type of enclosure
4) Type of duty
5) Method of cooling
6) Type of construction
7) Frequency in Hz
8) No. of phases
9) Mechanical output in KW

10) Rated voltage and permitted variation
11) Class of insulation
12) Speed in revolutions per minute, approximate, at
the rated output
13) Direction of rotation, looking from the driving end.
14) Unit or bidirectional of rotation required.
15) The maximum temperature of air and water used for
cooling.
16) Maximum permissible temperature rise
17) The height at which the motor is intended to work
18) Variation of voltage, current, frequency and speed
19) Particulars of tests required and where the tests are
to be carried out

20) Any information regarding the assosiated machine.
21) Rotor: squirrel cage or slip ring
22) System of earthing if any, to be adopted
23) Details of shaft extension required
24) Methods of starting employed
25) Breakaway torque
26) Nature of load
27) For HV motors, fault capacity, duration of fault
along
with the details of protective devices
28) Methods of drive
29) Any specific requirements

Rating plate of induction motor
•Rating plate giving the following details should be
supplied with each motor
• Reference to the standard i.e. Ref. IS: 325
• Induction motor
• Name of the manufacturer
• Manufacturer's number and frame reference
•Type of duty
• Class of insulation
• Frequency in Hz
• Number of phases
• Speed in rpm

•Rating plate giving the following details should be
supplied with each motor
• Reference to the standard i.e. Ref. IS: 325
• Rated output in KW
• Rated voltage and winding connections
• Current in amperes at rated output
• Rotor (secondary) voltage and winding connections
• Rotor (secondary) current in amperes at rated output
• Ambient temperature when above 40°C.

Types of Enclosures
The different types of enclosures are as follows
i) Open ventilated, motor
ii) Ventilated motor
iii) Drip proof motor
iv) Water protected motor
v)Totally enclosed motor
vi) Totally enclosed fan cooled motor
vii) Environment proof motor
viii) Weather proof motor
ix) Hose proof motor

Types of Enclosure
•The method of cooling is closely related to the
construction and the type of enclosure of the
machine.
•Open - pedestal: In this the stator and rotor ends
are open to the outside ambient air, the rotor being
supported on pedestal bearings mounted on the
bed plate.

•Open end bracket: In this the
bearings forms part of the end
shields which are fixed to the stator
housing. The air is in comparatively
free contact with the stator and
rotor through the openings. This is
common for small and medium size
motors and generators.
Types of Enclosures

•Protected or end-cover type with guarded openings:
The protector may be screen or fine-mesh over.
Types of Enclosures

•Drip, splash or hose proof: This is a protected
machine with the openings in the end shield for
cooling. The end shields are designed to prevent entry
of falling water or dirt or jets of liquid.
Types of Enclosures

•Pipe or duct cooled: With end covers closed except
for flanged openings for connection to cooling pipes.
Types of Enclosures

•Totally enclosed: The air will not be in contact with the
ambient air. The machine is totally air tight. Total
enclosure may be associated with an internal rotor fan,
an external fan, cooling or closed air circuit cooling in
which the air is circulated to a cooler and returned to the
machine.
Types of Enclosures

•Water cooled
Types of Enclosures

•Flame proof or explosion
proof: This motor is used in
hazardous location such as
mines, chemical industries
etc.
•Note: The ratings of machines are dependent upon
their respective cooling systems. For complex
cooling systems, the machines may have to be de-
rated.
Types of Enclosures

Duty of Rotating Machines
•The variations of load with time is termed as duty of
motor.
•The duty requirement may be declared numerically or
with the aid of time sequence graphs.
•The duty is very important in case of electrical motors
as they have a time rate of temperature rise.

Classes of Duty
Following are the classes of duty:
1. S1 – Continuous duty
2. S2 – Short time duty
3. S3 – Intermittent periodic duty
4. S4 – Intermittent periodic duty with starting
5. S5 – Intermittent periodic duty with starting and electric
braking
6. S6 – Continuous duty with intermittent periodic loading
7. S7 – Continuous duty with starting and electric braking
8. S8 – Continuous duty with periodic speed changes

Continuous duty (S1)
 Motor is running long enough and temperature
reaches steady value.
 Used in paper mill drives, conveyors, compressors.

Short time duty(S2)
 In these motors, the time of operation is very low
and the heating time is much lower than the cooling
time.
 These motors are used in crane drives, drives for
house hold appliances, valve drives etc.

Intermittent periodic duty (S3)
 Here the motor operates for some time and then
there is rest period.
 This is seen at press and drilling machine drives.

Intermittent periodic duty with starting (S4)
 In this type of duty, there is a period of starting,
which cannot be ignored and there is a heat loss at
that time.
 This motor duty class is widely used in metal
cutting and drilling tool drives, mine hoist etc.

Intermittent periodic duty with starting and
braking
 In this type of drives, heat
loss during starting and
braking cannot be ignored.
 The corresponding periods
are starting period,
operating period, braking
period and resting period.
•These techniques are used in billet mill drive,
manipulator drive, mine hoist etc.

Continuous duty with intermittent periodic
loading
 In this type of motor duty, everything is same as
the periodic duty but here a no load running
period is occurred instead of the rest period.
 Pressing, cutting are the examples of this system.

Continuous duty with starting and braking
 Consists of periodic cycles each having a period of
starting, a period of running at a constant load and
a period of braking.
 There is no period of rest.
 The main drive of Blooming mill is an example.

Continuous duty with periodic speed changes
 Consists of periodic duty cycle each having a period
of running at one load and speed, and another
period of running at different load and speed.
 There is no period of rest.
 Several paper mill drives are examples.

Three phase Induction Motor with type of
Protection ‘n’
•It is essentially standard industrial equipment with
additional attention paid to certain features which is
suitable for operating in any gas mixed with air having
an ignition temperature higher than that of the
temperature class marked on it.
•Flame proof enclosure, intrinsic safety or type of
protection 'e' motors are also operated in hazardous
Locations.

Three phase Induction Motor with type of
Protection ‘n’
•Saving in cost may be achieved by using equipment
with type of protection 'n'.
•IS 9628-1980 is referred to provide a common basis for
the construction and testing of motors with type of
protection 'n'.

Three phase Induction Motor with Type of
Protection ‘n’
•A flame proof equipment installed in hazardous
locations like mining industry ensures that ignition
occurring within the enclosure of the motor will not
transmit the flame to the atmosphere surrounding the
motor.
•Flame proof motors are expensive and difficult to
manufacture for higher rating.

Protection ‘n’
•The advent of technology and improvement in
installation lead to the development of type of
protection 'e'.
•With type of protection 'e' the temperature rise of the
apparatus 100
less than the normally permitted
temperature rise for the class of insulation.

Protection ‘n’
•For motor operating normally so as to prevent auto
ignition of the surrounding gases/vapours which may be
released under abnormal conditions resulted in the
develop of type of protection ‘n’ motors.

Protection ‘n’
•The type 'n' motor is an improved version of a normal
induction motor with the following additional
requirements .
1.Non-sparking (vibration proof) terminals
2.Adequate creep age or clearances
3.Adequate tightness of rotor bars in the rotor and
brazed or welded end rings to prevent sparking while
starting.
4.Adequate clearance between stator and rotor
5.Adequate clearance between shaft and bearing

Protection ‘n’
6. Suitable axial and radial clearance between the fan
and finned portions of the motor .
7. Provision against accidental reduction in clearance
8. between cable Lugs at the cable terminations
9. Terminal box to withstand through faults, without
shattering and
10. Restriction of surface temperature rise to2000
C for
temperature class T3 (Ref IS:8239-1976) during
normal operation as well as starting can be very
frequent.

Protection ‘n’
Hazardous locations and explosion or flame proof
machine:
•Special enclosures are used for machines operating in
hazardous Locations
•i.e In presence of highly inflammable gas vapours,
combustible dust, highly inflammable liquids such
petrol, naphtha, benzene etc., will explode in, presence
of electric spark.

Protection ‘n’
•In case of explosion, it is confined within the machine
and will not be spread to atmosphere.
•Hence the enclosure should be strong to withstand
high pressure built up.
•Motors used in such locations are also called as non
sparking machines.

Installation and Foundation of induction motor
Various stages in the installation of induction motor
Acceptance and proper storage at site.
Foundation and civil work.
Drawing of supply and control cables.
Preparing motor for installation.
Preparing driven machine and shaft alignment ready.
Checking the insulation, starter, supply and control cables.
Drying out.
Checks and tests on the machine and related accessories.
Trial run on load under observations.
Setting of protective relays.
Final commissioning and handling over to operating staff.

Acceptance & Proper Storage at site:
Acceptance
Check carefully for any damage that may have occurred
in transit. If any damage or shortage is discovered, do not
accept until an appropriate notation on the freight bill is
made. Any damage discovered after receipt of
equipment should be immediately reported to the
carrier.
Storage
•Keep motors clean.
•Keep motors dry.
•Keep Bearings Lubricated

Foundation and Civil Work:
•Location
In selecting a location for the motor, consideration should
be given to environment and ventilation. A motor with the
proper enclosure for the expected operating condition
should be selected.

•Floor Mounting
Motors should be provided with a firm, rigid foundation,
with the plane of four mounting pads flat within 0.25 mm
(0.010 in.) for 56 to 210 frame; 0.38 mm (0.015 in.) from
250 through 500 frame.

V-Belt Drive:
Select proper type and number of belts and sheaves.
 Excessive belt load will damage bearings.
Align sheaves carefully to avoid axial thrust on motor
bearing.
Adjust belt tension to belt manufacturers
recommendations. Excessive tension will decrease bearing
life.

Preparing motor for installation:
•Only qualified personnel who are familiar with the
appropriate national code, local codes and sound
practices should install or repair electrical motors and
their accessories.
•Installation should conform to the appropriate national
code as well as local codes and sound practices.
•Failure to follow these instructions could result in serious
personal injury, death and/or property damage.

Preparing driven machine and shaft alignment ready
Shaft Rotation: It is recommended that the motor shaft
be rotated 5 to 10 rotations every three months to
distribute the grease in the bearings.
This will reduce the chance for corrosion to form on the
bearing rolling elements and raceways.

Checking insulation, starter, supply & control cables:
Electrical live circuit hazard: Do not touch electrically live parts.
Disconnect, lockout and tag input power supply before installing or
servicing motor (includes accessory devices).
AC power supply limits : Motors are designed to operate within
the following limits at the motor terminals. AC power is within +/-
10 % of rated voltage with rated frequency applied. (Verify with
nameplate ratings)
Terminal box-conduit opening: For ease of connections, motors
are typically provided with large terminal boxes.
Lead connections: Electrical connections to be made per nameplate
connection diagram or separate connection plate.
Insulation: Use a “Megger” periodically to ensure that the integrity
of the winding insulation has been maintained. Record the Megger
readings.

Drying Out:
If the resistance is lower than one megaohm the windings
should be dried in one of the following two ways:
• Bake in oven at temperatures until insulation resistance
becomes constant.
• With rotor locked, apply low voltage and gradually
increase the current through windings until temperature
measured with a thermometer reaches 194°F. Do not
exceed this temperature.

Checks & Tests on the machine & related accessories:
• Trial run on load under observation
• Settings of protective relays
• Final commissioning and handing over to the
supporting staff.

Foundation
•Usually concrete foundation is preferred.
Requirements :
Space required.
Overhead travelling cranes.
Size of the motor.
Foundation should be strong.
Functions:
To transmit the static and dynamic load of the running
motor to the ground.

Foundation
Essential features needed for installation
Foundation bolts.

Foundation
•Concrete bed
•Foundation made of cement concrete.
•Bed plate

Foundation
Shimming work during installation:
•Shims are thin strips of steel sheet or size 0.2 mm to 2 mm.
•These are used to insert under the foot of a motor to raise or
align the shaft with the driven equipment.
•After alignment the exact height of driven and driving axes is
achieved by removing or adding the shims.
•Based on the size of the motor appropriate mounting is
chosen.
•For small and medium machines anti vibrating mounts called
as vibramounts are used to absorb the vibrations developed in
the machine effectively and to transmit the same to the
ground.

Foundation
Shaft alignment:
•The misalignment will affect the machine operation.
• The radial and axial clearances between the couplings
of two shafts are measured after alignment.
•Rotor is turned through approximately 0°, 900
, 1800
,
2700
and 3600
should not differ by the following values.
• 0.03 mm for 300 mm diameter coupling.
• 0.5 mm for 500 mm diameter coupling.

Foundation
Three steps to align the flexile coupling
•Axial positioning of the shafts
•Paralleling the shaft axis
•Centering the shaft axis
•The shafts driving and driven machines aligned on bed-plates in
their final positions by using shims under the feet of the machines.
• A feeler gauge is used to know the difference by turning till rotor
through 90°, 1800
, 2700
and 3600
.

Drying of winding
•The insulation of rotating machines will absorbs
moisture from the atmosphere
•The moisture reduces the insulation resistance
•Drying out of induction motor by applying the heat to
the windings.
•In the first phase the insulation resistance starts
decreasing due to distribution of moisture in entire
insulation.
•In the second phase is a steady temperature phase over
certain time and insulation resistance remains almost
constant.
•In the third phase the insulation resistance increases
there by indicating the moisture is removed.

Drying of winding
•Polarization Index to be found by using resistance
at 10 minutes to resistance at 1 minute.
•PI gives the quantitative information regarding
the presence of moisture, dust and dirt.
• For class A insulation PI is 1.5 or more, for class B
insulation 2.0 or more. PI value less than 1
indicates the immediate need of reconditioning.

Drying of winding
Log sheet of drying out of an induction motor:
1)Technical particulars of machine
•Rated voltage, Rated frequency
•Rated kW, Full load current
•Connection diagram, number of windings
2) Technical particulars about connections for drying out
3) Check prior to starting the drying
4) Ambient temperature
5) Initial values of insulation resistance
6) Time of start, date, hours
7) Operators name
8) Additional remarks

Drying of winding
•Permissible hot-spot temperature of windings
during drying out are
Method Temperature in degree C
Thermometer method 650
C
Resistance method 800
C
Thermocouple method 700
C
Temperature of the outlet air by
thermometer method
800
C

Drying of Induction Motor by drying chamber &
resistance method
•The machine to be dried is housed in a drying chamber.
•The volume of drying chamber should be nearly four
times the volume of the induction motor.
•The heated air by using resistor heaters is circulated by
means of fans and air circulation system.
•The air temperature is measured using thermometers.
•The moisture is expelled from the machine is let out of
the drying chamber through air outlet.

resistance method
•The ratings of heaters used for drying is given by the
equation,
P = 0.025 (T2 – T1) kW/Vol
where,
P = kW rating of heater
T1 = Ambient temperature
T2=Temperature of hot air
Vol = Volume-of air inlet m2
/min

resistance method
•The temperature is
gradually raised not
faster than l00
C per
hour.
•It is required to
preferably maintain
steady temperature
throughout the
heating.

Drying out by Radiating Lamps
•This method is used for medium and small motors.
•The infrared lamps are located in chamber facing the
motor winding with rotor removed.
•This method is applicable for dismantled motor for
drying the stator and rotor winding separately.

Drying out by Circulating Short Circuit Current
•This is convenient method for drying out slip ring induction motor
•By short circuiting the rotor, large current passes through the
windings, due to this current heat will be produced in the winding
•The current through the stator winding not to exceed 50% of the
rated current.

Drying by windage losses
•This method is applicable to high frequency motors
having high speed.
•The inlet and outlet air ports are blocked.
•The windings gets dried by windage losses dissipated in
the form of heat.

Testing of induction motor
Type test : These test are conducted on motor of new
type to confirm the design.
Routine test: These tests are conducted on each motor to
confirm proper manufacture and to ensure smooth
performance at site.
Commissioning tests: These tests are conducted at site,
after installation, before final commissioning to ensure
desired performance under practical conditions.
Special tests: These tests are conducted for special
investigation as per the contract made between the
manufacturer and purchaser, preferably in presence of
representative of purchaser at the floor before dispatch .

Development test: These tests are conducted to analyze
for design parameters and stresses will be helpful in
development/improvement of the earlier machine or new
machine.
Reliability tests: These tests are conducted to ascertain
reliability of the motor under operating conditions.
Periodic maintenance checks and tests: These tests are
included in the preventive maintenance schedule which
depends on the service conditions.

Before commissioning test following tests are to be carried out to get
trouble free performance.
1.Measurement of dc resistance
2.Open circuit voltage test
3.No-load test
4.Starting test
5.Short circuit test
6.Load test
7.Temperature rise test
8.Measurement of slip
9.Insulation test
10.Tests on cooling system
11.Tests on lubricating system
12.Special tests viz. vibration tests. oscillographic tests to record starting
currents, switching voltages etc.

Commissioning test:
•These are conduct on at site, after installation.
•The following test are carried out to get trouble free performance
1.Measurement of resistance of winding
2. Measurement of insulation resistance of winding insulation
3.Power frequency high voltage test
4.Viberation test
5.On load test
6.Testing on cooling system
7.Testing on setting and protection

Mechanical alignment:
•When the rotor is supplied without shaft is assembled, is
to be fitted on to the shaft before installation.
•While fitting the rotor on to the shaft the difference
between the rotor and shaft temperature are taken into
account
•The rotor hub bore and the shaft diameter are to be
matched properly.

Mechanical alignment:
•Place the rotor in position such that the air gap between
the rotor and rotor stocks in approximately uniform.
•Once the stator and the rotor are mounted in position,
check for clearances between shaft journal necks and the
butt end of the bearing shells.

Bearings The selection of bearings depends upon the fallowing
factors:
•Speed
•Temperature limit
•Load capacity
•Space and weight limitation
•Noise and vibrations
•End thrust
•Corrosion resistance
•Cost

Bearings :
Sleeve (journal) bearings and ball bearings are used for ,BHP motors.
Ball bearings, cylindrical roller bearings, needle bearings are used for
medium and large motors.
A proper lubrication is to be provided for bearings, using lubricating
oil or grease.
• The amount of bearing wear can be determined by measuring the
air gap between the rotor and stator.
•The schedule of maintenance of bearings depends upon the type of
duty and rating of the motor.
•The bearings should be dismantled carefully without causing
damage.
•The prescribed method is to be strictly adhered while carrying out
the assembly of bearings on the shaft.

Mechanical Test
1. Vibrations
2. Balancing
3. Gap length

Mechanical Test
Reasons for vibration:
1.Misalignment between motor and driven equipment
2.Loose foundation bolts
3.Badly worn bearing
4.Mechanically unbalanced rotor
5.Bent or cracked shaft
6.Highly pulsating load
7.Magnetic effects of high frequency

Mechanical Test
1.Vibration/displacement indicator
2.Vibrometer
Prevention of vibration:
1.Checking of bolts , coupling , foundation and bearing.
2.Checking the balance if the vibration is present even
when the driven machine is decoupled and motor is run
alone.
3.If the vibration is present when the driven machine is
coupled then vibration may be in the driven equipment
or shaft misalignment.

Mechanical Test
Balancing
•Balancing is required for smooth and vibration
free running of motor
•While balancing motor , rotor including slip rings
and couplings are considered
•Balancing is obtained by adding or shifting
weights fixed on the rotor for counter balancing
•It is also done by etching or drilling

Mechanical Test
Methods of balancing :
1.Static balancing for low speed machines
2.Dynamic balancing for high speed machines
Static balancing

Mechanical Test
Static balancing : Rotor to balanced is fixed
between two knife edges which are in horizontal
plane and balanced rotor will remain standing in
any positions when turned about axis any direction
position and will not oscillate .
But when rotor is unbalanced rotor will not
remain in position and will try to come down.
Dynamic balancing: Special balancing machines are
used and rotor of the machine to be balanced is
mounted on the axis of balancing machine and
driven at high speed unbalanced motors will
vibrate at high speeds

Air gap:
•The air gap b/w the stator & the rotor are checked & adjusted
after the shaft is fully aligned.
• Set the air gap with the help of wedge type gauges on both sides
of the rotor.
• Permissible values of difference b/w max & min air gap for an
induction motor is 10%

Unit 5-6-7 a

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