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Induction Machines
Principle of Operation Equivalent Circuit Power and Torque Torque-speed characteristic Loading & Stability Induction Machine Modes of Operation Starting of Induction motors Braking of Induction Motors Testing Practical Motors
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Induction Machines: Motor Braking
In many systems, motors are stopped simply by natural deceleration. The time this takes depends solely on the inertia and resistive torque of the machine the motor drives. However, the time often needs to be cut down and electrical braking is a simple and efficient solution.1. Plugging2. Dynamic Braking3. Regenerative Braking
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Induction Machines: Motor Braking
1. Plugging3
Induction Machines: Motor Braking
1. Pluggingm L
dT T Jdt
m LdT T Jdt
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Induction Machines: Motor Braking
2. Dynamic Brakinga. DC Dynamic brakingTo obtain this type of braking the stator of a running induction motor is connected to a dc supply.
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Induction Machines: Motor Braking
2. Dynamic Brakingb. Zero Sequence Dynamic braking All the three stator phases are connected in series and single phase ac or dc is connected across them (as shown in the figure). This type of connection is called zero-sequence connection, because current in all the stator windings are co-phasal.
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Induction Machines: Motor Braking
2. Dynamic Brakingc. AC Dynamic braking (wound rotor)This type of braking is obtained when the motor is made to run on a single phase supply by disconnecting any one of the three phase from the source, and it is either left open or it is connected with another phase.
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Induction Machines: Motor Braking
3. Regenerative BrakingIt take place only if the speed of the motor is greater than synchronous speed and in the same direction as the rotating field. This happens by switching the operating frequency to a lower frequency, so the torque and slip is negative and the machine acts as a generator supplying power to the supply network.
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Induction Machines: Motor Braking
3. Regenerative Braking
m LdT T Jdt
m LdT T Jdt
snn
dT
sn n
V/f
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Induction Machines: Testing
Squirrel Cage Machine1. DC Test2. Locked rotor test3. No-load testWound Rotor Machine1. DC Test2. Locked rotor test3. Open Circuit test4. No-load test
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Induction Machines: Testing
1. DC TestDC voltage is adjusted such that stator current = full load current
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1
DCeq
DC
VRI
1DC FLI I
1 12eq DCR R1 1
12 eqR R
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Induction Machines: Testing
2. Locked (Blocked) Rotor Test Motor is forced to standstill
0LRn 1s 2 2, ,c mR X R X
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Induction Machines: Testing
2. Locked (Blocked) Rotor Test Voltage is adjusted such that stator current ≤ full load current
1LRV
1LRI1LRW
2LRW1 2 1 2LR LR cu cuW W P P
'1 2LRR R R
21 2 3LR LR LR LRW W I R
'2 1LRR R R
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Induction Machines: Testing
2. Locked (Blocked) Rotor Test
1
1
LRLR
LR
VZ
I
2 2LR LR LRZ R X
2 2LR LR LRX Z R
'1 2LRX X X
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Induction Machines: Testing
3. No-Load Test Motor running atno mech. Load
INL = 30-40% IFL
NL sn n 0s
'2 0I
NL mech lossesT T
2cu mechP P
&mech ironR
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Induction Machines: Testing
3. No-Load Test
1NLV
1NLI 1NLW
2NLW
1 2 1NL NL iron mech cuW W P P P
21 1 13cu NLP I R
mech ironP P
&mech ironP &mech ironR
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Induction Machines: Testing
3. No-Load Test
1
1
NLNL
NL
VZ
I
2 2NL NL NLZ R X
2 21 1NL NLX Z R X X
1NLX X X
1NLR R
&mech ironX R
&mech ironR
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Induction Machines: Testing
1. DC TestDC voltage is adjusted such that stator current = full load current
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1
DCeq
DC
VRI
1 FLI I
22
2
DCeq
DC
VRI
1 12eq DCR R
2 223eq DCR R
1 112 eqR R
2 232 eqR R
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Induction Machines: Testing
2. Locked (Blocked) Rotor Test Voltage is adjusted such that stator current ≤ full load current
1LRV
1LRI1LRW
2LRW'
1 2LRX X X
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Induction Machines: Testing
3. Open Circuit Test (for wound rotor)
1oV 2oV
1oI1oW
2oW1 2 1o o iron cuW W P P
13 cosiron o o oP E I
1 coso
co o
ERI
1 sin
o
o o
EXI
1
2
oeff
o
Va
V
21 1 13cu oP I R 1 1 1o oE V I Z
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Induction Machines: Testing
4. No-Load Test
1NLV
1NLI 1NLW
2NLW
1 2 1NL NL iron mech cuW W P P P
21 1 13cu NLP I R
mechP
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Induction Machines: Practical Motors
Space Harmonics
( )60s
s hn fnh hp
1 3 5cos cos3 cos5 ...gB B B B
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Induction Machines: Practical Motors
Space Harmonics
sn
T ve
5sn
T zero T ve
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Induction Machines: Practical Motors
Space HarmonicsCrawling
Crawling: Motor runs at almost 1/7 of its synchronous speed
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Induction Machines: Practical Motors
Time Harmonics
1,3,5,..
cos( )a mh eh
i I h t
1,3,5,..
cos( ( 120))b mh eh
i I h t
1,3,5,..
cos( ( 120))c mh eh
i I h t
1,3,5,..
4 cos( ) cos2a mh e
h
NM I h t pp
1,3,5,...
4 cos( ( 120))cos( 120)2b mh e
h
NM I h t pp
1,3,5,..
4 cos( ( 120))cos( 120)2c mh e
h
NM I h t pp
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Induction Machines: Practical Motors
Time Harmonics
3 4( , ) cos( )2 2th mh e
NM t I h t pp
60sh
hfnp
sn 5 sn
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Induction Machines: Practical Motors
Time Harmonics
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Induction Machines: Practical Motors
CoggingThe rotor of particularly squirrel cage induction motor sometimes refuse to start at all particularly a when the voltage is low. This happens of stator teeth is equal to the number of rotor teeth ,and therefore due to the magnetic locking or cogging. It is found that the reluctance of magnetic paths is minimum when the stator and rotor teeth comes in front of each other, it is in such position of minimum reluctance that the stator tends to remain fixed thus causes serious trouble during starting. This can be easily overcome by: 1. Making number of rotor slots more than the number of stator
slots.2. Giving slightly skew to the rotor slots (skewed) i.e. to arrange
the stack of rotor laminations so that the rotor slots are "skewed" or angled with respect to the axis of rotation.
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Induction Machines: Practical Motors
CoggingSkewing
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Induction Machines: Practical Motors
Rated Voltage (Vr )Rated Frequency (fr)Rated Power (kW or HP)Full Load Speed (rpm)Full load Current (IFL)Full load power factor
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Induction Machines: Practical Motors
Motor NEMA Classes
NEMA MG 1-2011 Motors and Generators
NEMA Locked-Rotor Code Letters kVA/hp
A 0–3.15 G 5.6–6.3
B 3.15–3.55 H 6.3–7.1
C 3.55–4.0 I 7.1–8.0
D 4.0–4.5 J 8.0–9.0
E 4.5–5.0 K 9.0–10.0
F 5.0–5.6 L 10.0–11.2
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Induction Machines: Practical Motors
Induction Machines: Practical Motors
Motor Service Factor
Service factor is defined as the permissible amount of overload a motor will handle within defined temperature limits. When voltage and frequency are maintained at nameplate rated values, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.The standard service factor is 1.15. The service factor is required to appear on the nameplate only if it is higher than 1.0.
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Induction Machines: Practical Motors
Motor Service Factor
For standard motors (60 Hz)
NEMA MG 1-2011 Motors and Generators
34
Induction Machines: Practical Motors
Motor Duty CyclesNormally, continuous duty three-phase induction motors are designed for the rated power. Most motors however are operated with a duty type which is not continuous.• S1: Continuous duty• S2: Temporary duty• S3: Intermittent periodic duty-type without starting• S4: Intermittent periodic duty with starting• S5: Intermittent periodic duty with starting and electrical braking• S6: Continuous-operation duty type• S7: Continuous-operation duty with starting and electrical braking• S8: Continuous-operation periodic duty with related load/speed changes• S9: Duty with non-periodic load and speed variations
IEC 34-7 Standard
35
Induction Machines: Practical Motors
Motor Insulation Classes
Insulation systems are rated by standard NEMA classifications according to maximum allowable operating temperatures.
NEMA MG 1-2011 Motors and Generators
36
Induction Machines: Practical Motors
Motor Environmental ProtectionIP protection: Protection of persons against getting in contact with (or approaching) live parts and against contact with moving parts inside the enclosure. Also protection of the machine against ingress of solid foreign objects. Protection of machines against the harmful effects due to the ingress of water
IEC 60034-5 Standard
37
Induction Machines: Practical Motors
Motor Environmental ProtectionIK protection: Classification of degrees of protection provided by enclosure for motors against external mechanical impacts.
IEC 60034-6 Standard
38
Induction Machines: Practical Motors
Motor Cooling (IC)
IEC 60034-6 Standard
39
Induction Machines: Practical Motors
Motor Mounting Arrangement (IM)
IEC 60034-7 Standard
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