characteristics of motor

Lecturer Mr.PARTHASARATHY PDept. of EEE

KIRAN GEORGE AKKARA MEB1314

KISHORE KUMAR P MEB1315

MAHADEVAN L MEB1316

MANIKANDAN MEB1317

Mechanical Characteristics

Speed (N) Vs Torque (T)

Electromechanical Characteristics

Torque (T) Vs Armature Current (Ia )

Speed (N) Vs Armature Current (Ia)

Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force tending to move it

IL Ia

Ish IL = Ia + Ish

Electrical characteristics

T ∞ φ Ia

T ∞ Ia Armature Current Ia To

rqu

e T

Ta Tsh

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

T ∞ Ia

From above equation

N∞( v - T Ra )

Torque TS

pee

d N

IL

Ia

Ise

IL = Ise = Ia

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

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

T ∞ Ia

N ∞

Torque T

Sp

eed

N

Eb

T

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

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

The Torque – Slip characteristics

consists of three regions ,

1) Stable Operating Region.

2) Unstable Operating Region.

3) Normal Operating Region.

In stable region, the slip value is very

small

T ∞ sR2

R22

T ∞ s (R2 = constant)

Therefore the graph is straight line

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

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

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.

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)

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.

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.

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.