topic 3: induction motor modeling - steady state spring 2004 ece 8830 - electric drives

Topic 3: Induction Motor Modeling -

Steady State

Spring 2004

ECE 8830 - Electric Drives

Introduction

Induction machines are the most widely used of all electric motors. They offer the following attractive features:

Generally easy to build and cheaper than corresponding dc or synchronous motors

Rugged and require little maintenance Offer reasonable asynchronous performance A manageable torque-speed curve Stable operation under load Generally satisfactory efficiency Range in size from few Watts to several MW

Introduction (cont’d)

Some disadvantages of induction motors are:

Speeds not as easily controlled as dc motors Draw large starting currents, typically 6-8 x

their full load values Operate with a poor lagging power factor

when lightly loaded

Introduction (cont’d)

New designs for high performance induction machines, such as in high speed motors for gas compressors, will be required to have new characteristics from existing machines, it is important to have a good fundamental understanding of these types of machines.

Goal: To develop a “simple” model for the induction machine that is useful for control and simulation.

Structure of an Induction Machine

Two types of induction machine:Wound rotor or squirrel cage rotor

Rotating Magnetic Field and Slip

We previously showed that a balanced set of three-phase currents flowing in a set of symmetrically placed, three-phase stator windings produces a rotating mmf given by:

[eq. (6.1) Ong, eq. (2.9) Bose]

where ae is the electrical angle measured from the a-

phase axis and e is the angular speed of the stator mmf in electrical radians/second.

3 4( , ) cos( )

2e ea m a e

NF t I t

P

Rotating Magnetic Field and Slip (cont’d)

Rotating Magnetic Field and Slip (cont’d)

In mechanical radians/sec. the synchronous speed is related to the electrical speed by:

If the rotor is rotating at an angular speed rm the slip speed is simply equal to sm - rm. The “slip”,s, is the normalized slip speed and is given by:

2sm eP

sm rm e r

sm e

s

Torque Production

The torque produced by an induction motor may be derived and expressed by the following equation: (see ref. [1] in Bose)

where P= # of poles l = axial length of motor r = radius of motor Bp= peak air-gap flux density

Fp= peak value of rotor mmf

and

sin2e p p

PT lrB F

2 r

Per-Phase Equivalent Circuit Model

A per-phase transformer-like equivalent circuit is shown below:

Per-Phase Equivalent Circuit Model (cont’d)

Synchronously rotating air gap flux wave generates a counter emf Vm. This in turn is converted to a slip voltage in the rotor phase, Vr’ = nsVm, where n=rotor:stator turns ratio, and s=normalized slip.

Stator terminal voltage, Vs = Vm + VRs +VLls

where VRs=voltage drop across stator resistance (Rs) and VLls=voltage drop across stator leakage inductance (Lls).


Excitation current, I0 = Ic + Im

where Ic is core loss current (=Vm/Rm)

and Im is magnetizing current (=Vm/ )

Rotor-induced voltage, Vr’ = VRr’ + VLl’

where VRr’ = voltage drop across rotor resistance

and VLl’ = voltage drop across rotor leakage

inductance

The induced voltage in the rotor leads to a rotor current Ir’ at slip frequency sl.

e mL


The stator current, IS = I0 + Ir

where Ir is the rotor-reflected current induced in the stator.

I0


2'

' 'm m

r rrr sl lr

e lr

n sV VI nI

RR j Lj L

s

'

2r

r

RR

n

'

2lr

lr

LL

n


Torque expression can be written as:

where = peak value of air gap flux linkage/pole and = peak value of rotor current

3 ˆˆ sin2 2e m r

PT I

ˆm

r̂I

Per-Phase Equivalent Circuit Model: Power Expressions

Input Power: where cos is input PF

Stator copper loss:

Rotor copper loss:

Core loss:

Power across air gap:

Output power:

Shaft Power: where PFw is friction and windage power loss

3 cosin s sP V I

23ls s sP I R23lr r rP I R

23 /lc m mP V R

23 /g r rP I R s

23 (1 / )o g lr r rP P P I R s s

sh o FwP P P

Per-Phase Equivalent Circuit Model: Torque Expression

The torque can be expressed as:

where is the rotor

mechanical speed (radians/sec.)

2 23 13

2o r

e r r rm m e

P Rs PT I R I

s s

2 2(1 )m r es

P P

Per-Phase Equivalent Circuit Model: Torque Expression (cont’d)

Using a little algebra (see Bose) it can be shown that the torque may be further expressed as:

where .

This torque expression is similar to that for a dc motor, where Im = magnetizing

component of stator current and Ia =

armature component of stator current.

32e m m a

PT L I I

sina rI I

Simplified Per-Phase Equivalent Circuit

A simplified circuit dropping Rm and shifting Lm to the input is applicable to integral horsepower machines.

Performance of this equivalent circuit is typically

within 5%.

Simplified Per-Phase Equivalent Circuit Model (cont’d)

The current Ir in this circuit is given by:

The torque of the motor using this circuit is given by:

2 2 2( / ) ( )s

r

s r e ls lr

VI

R R s L L

2

2 2 23

2 ( / ) ( )sr

ee s r e ls lr

VRPT

s R R s L L

Example of Calculating Efficiency of an Induction Motor

Example 5.1 Krishnan

Flowchart for Computing Steady State Performance of Induction Motor

Ref: R. Krishnan, “Electric Motor Drives”

Torque-Speed Curve of Induction Motor

The torque-speed curve as a function of slip can be calculated from the equation two slides back.

Torque-Speed Curve of Induction Motor (cont’d)Three regions in torque-speed curve:1) Plugging (braking) region (1<s<2) Rotor rotates opposite to direction of air gap

flux. Can happen, for example, if stator supply phase sequence reversed while rotor is moving.

2) Motoring region (0<s<1) Te=0 at s=0. As s increases (speed decreases),

Te increases until max. torque (breakdown torque) is reached. Beyond this point, Te decreases with increasing s.

Torque-Speed Curve of Induction Motor (cont’d)

3) Regenerating Region (s<0) Here the induction machine acts as a

generator. Rotor moves faster than air gap flux resulting in negative slip.

Torque-Speed Curve of Induction Motor (cont’d)


Performance Characteristics of Induction Motor


Starting Torque of Induction Motor

The starting torque of an induction motor is given by substituting for s=1 and is given by:

2

2 2 23

2 ( ) ( )sr

ee s r e ls lr

VRPT

R R L L

Starting Torque of Induction Motor (cont’d)

This torque can be enhanced for line start motors (ones started directly with full line voltage) by increasing the rotor resistance. This can be achieved by connecting external resistors in the case of slip ring rotors. However, with squirrel cage rotors where the rotor is shorted, deep bar or double-cage rotors can be used to increase starting torque.

Characterizing Induction Motors

One way to characterize an induction motor is with the No-load/blocked rotor tests which yield the per-phase equivalent circuit model shown earlier (see figure below).

iarias

vas =M

Characterizing Induction Motors (cont’d)

We can characterize an induction motor with the variables Rs, Lls, M, Llr, Rr determined through lab tests using balanced 3 excitation. This circuit described the impedance perceived per phase on a line-neutral connected machine. Everything in the dashed box is a rotor quantity that has been “referred” to the stator by the ideal transformer in the machine model. From now on, assume that Llr, Rr and iar are referred to the stator.


No-Load Test (s=0) Equivalent circuit:



No-Load Test (s=0) yields: In sinusoidal steady state, ignoring

resistances:

But -ias = (ibs+ics)

From transformer model:

as as s bs ab cs abv i X i X i X

3[ ]

2as as s ab as ls srv i X X i L L

Las

as as lsv i L M =>3

2 srM L


Blocked rotor test (s=1) yields estimates of Lls and Llr. Equivalent circuit at standstill is shown below:



Ohmmeter/Power loss tests give Rs

and Rr.

So, with Llr, Rr and all ir’s understood as

referred rotor quantities, the “stator-side” tests identify all the model parameters for the induction motor.

Example of Determining Induction Motor Model Parameters

Example 5.2 Krishnan

NEMA Classification of Induction Motors

The National Electrical Manufacturers Association (NEMA) has classified induction motors based on their torque-slip characteristics. (see text for details)

Circuit Model of a Three-Phase Induction Machine (State-Space Approach)

Voltage Equations

Stator Voltage Equations:

asas as s

dv i r

dt

bsbs bs s

dv i r

dt

cscs cs s

dv i r

dt

Voltage Equations (cont’d)

Rotor Voltage Equations:

arar ar r

dv i r

dt

brbr br r

dv i r

dt

crcr cr r

dv i r

dt

Flux Linkage Equations

Model of Induction Motor

To build up our simulation equation, we could just differentiate each expression for , e.g.

But since Lsr depends on position,

which will generally be a function of time, the trig. terms will lead to a mess!

Park’s transform to the rescue!

asas

d dv

dt dt

[first row of matrix]

topic 3: induction motor modeling - steady state spring 2004 ece 8830 - electric drives

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