characteristics of motor
TRANSCRIPT
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Lecturer Mr.PARTHASARATHY PDept. of EEE
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KIRAN GEORGE AKKARA MEB1314
KISHORE KUMAR P MEB1315
MAHADEVAN L MEB1316
MANIKANDAN MEB1317
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Mechanical Characteristics
Speed (N) Vs Torque (T)
Electromechanical Characteristics
Torque (T) Vs Armature Current (Ia )
Speed (N) Vs Armature Current (Ia)
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Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force tending to move it
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IL Ia
Ish IL = Ia + Ish
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Electrical characteristics
T ∞ φ Ia
T ∞ Ia Armature Current Ia To
rqu
e T
Ta Tsh
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Armature Current Ia
Sp
eed
N
Speed equation of DC motor is N = KV-IaRa
φ
N = K(V- Ia Ra )
•The speed is nearly constant
•Applications
Machine toolsLathesDriving ShaftingWood working machines
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T ∞ Ia
From above equation
N∞( v - T Ra )
Torque TS
pee
d N
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IL
Ia
Ise
IL = Ise = Ia
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T ∞ φ Ia
Φ ∞ Ia
T ∞ Ia2 (before saturation)
T ∞ Ia (after saturation)
Armature Current Ia
Torq
ue
T
T ∞ Ia2
T ∞ Ia
At light load the Ta increases as square of current
and hence the curve is parabola
After saturation the flux is constant , hence T ∞ Ia
therefore it becomes straight line
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Armature Current Ia
Speed equation of DC motor is N = KV-IaRa
φ
N = KV-IaRa
Ia
(φ = Ia)
N ∞Eb
Ia
oSpeed decreases when armature current is increased
oIf motor is started without some load the speed increases to adangerous value and hence destroys it
Sp
eed
N
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T ∞ Ia
N ∞
Torque T
Sp
eed
N
Eb
T
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The characteristics of a compound motor depends on whether the series and shunt field windings are assisting each other (cumulative) or opposing each other (differential)
Sp
eed
N
Armature Current Ia
Differential
Shunt
cumulative
series
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TORQUE - SLIP
CHARACTRISTICSThe curve drawn between torque and
slip is known as torque slip characteristics of induction motor.
Torque equation
T ∞ sE22 R2
R22+(sX2)
2
T ∞ sR2 Eb = constant
R22 + (sX2)
2
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The Torque – Slip characteristics
consists of three regions ,
1) Stable Operating Region.
2) Unstable Operating Region.
3) Normal Operating Region.
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In stable region, the slip value is very
small
T ∞ sR2
R22
T ∞ s (R2 = constant)
Therefore the graph is straight line
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When slip is further increased the region is unstable.
The slip value is high
T ∞ s (sX2)
2
T ∞ 1 (X2 is constant)S
Hence the curve is rectangular hyperbola
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The region AC is also known as operating region
From the curve we can understand the following terms
1. Starting torque
2. Maximum torque or pull out torque
3. Full load torque
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Starting torque
In torque-slip characteristics, the slip is 1 and speed is 0. At this time the torque is called starting torque.
• Maximum torque
The torque at slip s = sm is called maximum torque.
It is also known as breakdown torque or pull out torque
Full load torque
In the curve the point c is called full load torque of motor. Normally full load torque is less than the maximum torque.
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This curve is divided into regions according to slip value
Motoring Region (0 ≤ s ≤ 1)
Generating Region (s < 0)
Plugging Region (1.0 ≤ s ≤ 2.0)
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Motoring Region (0 ≤ s ≤ 1)
Induction motor rotates in the same direction as that of the field.
The speed decreases and torque increases till breakdown torque is reached.
In this region the air gap flux is nearly constant.
After breakdown torque Tmax, torque decreases and slip increases.
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Generating Region (s < 0)
The machine operates as a generator.
The rotor rotates at a speed greater than synchronous speed in the same direction as that of the rotating magnetic field.
Due to super synchronous speed the slip becomes negative, creating negative, or regeneration torque.
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Plugging Region (1 ≤ s ≤ 2)
The slip becomes greater then unity
This region occurs only when the stator field is reversed and also the rotating direction.
The change in direction results to come to rest and hence it is known as braking torque.
The torque is positive, but the speed is negative.
This method of braking is known as plugging.
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