Transcript Design Of A Brushless Doubly-fed Induction Motor For

DESIGN OF A BRUSHLESS DOUBLY-FED
INDUCTION MOTOR FOR ADJUSTABLE SPEED
DRIVE APPLICATIONS
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By
SINDHUJA SAYEEBABA
Edited By
Sarath S Nair
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m
OVERVIEW
Introduction
 Adjustable Speed Drives (ASD) and its need
 Evolution of the BDFM
 Principles of Operation
 Design of a Brushless Doubly-fed Induction Motor
 Comparison of BDFIM and Induction motor
 Potential Applications
 Merits and demerits
 Summary

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INTRODUCTION
 BDFIM is an AC machine which can operate as both a
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motor and a generator
 The stator has two sets of winding one is power winding
which is connected to the grid and the control winding
which is supplied from a converter
 A specially designed rotor assembly tries to focus most
of the mutual magnetic field to follow an indirect path
across the air-gap.Hence brushes are absent.
 Power is supplied to the rotor by means of
electromagnetic induction. Winding sets are excited
independently and actively participate in the electromechanical energy conversion process, which is a
criterion of doubly-fed electric machines
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Schematic of BDFM
The shaft speed is adjusted by varying the
frequency of the control winding. As a doubly-fed
electric machine, the rating of the frequency
converter need only be fraction of the machine
rating.
 A variety of rotor structures can be fitted into
the stator namely BDFIM and BDFRM.
 In BDFIM, the rotor is made of laminated steel
and a nested cage of aluminum or copper rods
and the operation is pertained to the squirrelcage induction motor
 In the BDFRM, the rotor is made of salient pole
of laminated steel, similar to the operation of
synchronous reluctance motor

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BRUSHLESS DOUBLY FED MOTOR
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Adjustable Speed Drives (ASD)

The equipment to control the speed of machinery. Also

Primary function of adjustable speed drive is to control
the speed, torque, acceleration, deceleration and
direction of rotation of a machine.

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known as Variable speed Drive(VSD)
The speed control of either an induction motor or a
synchronous motor is done by adjusting the frequency of
the power supplied to the motor.

Speeds must change to adapt to various tasks for
improved product quality, production speed or

There are two electric adjustable speed drives
namely DC drives and AC drives.

Electrical drive is preferred due to its low initial
cost, high efficiency, low maintenance requirements,
excellent performance, saves energy and versatility.
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safety.
Need For The Electrical Drive

In IM at start high inrush current cause high power loss
performance of the utility.

When the motor is operated at a minimum load
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which leads to insulation failure and affect the
magnetizing current is drawn and the PF is very low due
to the inductive load.

When the motor operates at a PF less than unity, the
current drawn by the motor is not sinusoidal in nature.
Power factor is the ratio of real power to reactive power.
PF=P/ (Irms*Vrms)
 Poor power factor is due to the phase displacement or due
to the high harmonic current.
 The PF is very important and the customer is forced to
maintain the full-load condition for the entire operating
time or else pay penalties for the light load condition.
 In applications like cranes or hoists, stop and reverse of
the motor is needed often
 So for all the above problems there is a need for an
electric drive which runs at various speed ranges.

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Evolution Of The BDFM
The adoption of AC brought the induction
machine into industrial service. However
industry required variable speed operation
 Fundamental operating speed for an induction
machine is fixed by the mains frequency
 The common method of controlling the speed of
induction machines was the introduction of series
resistance to the rotor
 Later on connecting two induction machines
together allowed to achieve three different speeds
of efficient operation. Such an arrangement was
known as `cascaded induction machines'

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cascaded
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connection consists of
two wound-field induction
machines coupled mechanically to
the same shaft and load
The slip power of the first
machine is therefore used to excite
the second machine .
The stator windings of the second
machine can be short-circuited or
terminated in external resistances
Thus the doubly-fed motor control
can be realized without the use of
brush gears.
Three different efficient speeds of
operation could be achieved.
DOUBLY FED WOUND FIELD IM DRIVE
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
Two multiphase winding sets with similar polepairs are placed on the rotor and stator
A multiphase slip ring assembly is used to
transfer power to the rotating winding set and to
allow independent control of the rotor winding
set.

Rotor winding set actively participates in the
energy conversion process with the stator
winding set.
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
PRINCIPLES OF OPERATION

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The operating principles of the BDFIM are
illustrated with the help of cascade connection of
induction motor consisting of two wound field
induction machines having different pole
numbers of 2p and 2q which are coupled
mechanically to the same shaft and load.
A set of symmetrical sinusoidal currents of
frequency f1 are flowing in the primary windings
of the first machine, a counter-clock wise rotating
field of w1=2пf1/p is set up in the airgap
 The second machine is of 2q poles, the rotor
currents will set up a rotating field of w2r=2пf2/q
which will rotate in clock-wise with respect to the
rotor.
 fl is fixed and f2 is varied properly, the rotor
speed can be controlled to be
wr= 2п (f1+f2)/(p+q)
Hence, the combination acts like a 2(p+q) pole
machine.

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
The electromagnetic torque found in the d-q
reference frame is
Te=pMp (iqs1idr1-ids1iqr1) +qMq (iqs2idr2-idr2iqr2)
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
The first machine, usually connected to the 60 Hz
utility, acts like a high-slip induction machine
while the second machine can be in synchronous
machine action or induction machine action

Comparison of BDFIM &IM
2/6 P BDFIM
4 P IM
245
215
SLOT/PHASE/POLE
6/2
3
HORSEPOWER
10
10
EFFICIENCY (%)
91
92.4
POWER FACTOR
0.89
0.83
National Electrical Manufacturers Association
(NEMA)
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NEMA FRAME SIZE
BDFIM
 Ratings:
Motor: 10 HP Drive: 5 HP
 Prices:
 Motor: $811
Drive: $1450
Total: $ 2261
 SQUIRREL CAGE IM DRIVE
 Ratings:
Motor: 10 HP Drive: 10 HP
 Prices:
Motor: $614 Drive: $2375
 Total: $2989 (32% higher than BDFlM)

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POTENTIAL APPLICATIONS
Turbo-machinery
 hydro power generation
 commercial and industrial heating
 ventilation and air conditioning
 Wind power generation
 If the BDFM is maintained at its synchronous
speed, the secondary windings need to supply DC
excitation to the motor and a fully rated variable
frequency power converter can be used to supply
the primary windings. Hence a brushless
synchronous machine can be realized

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MERITS
The merits of this emerging technology can be
DEMERITS
 poor electromagnetic design
 Avoids slip ring assembly
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summarized as
 Brushless
 High power factor operation when operating as
VSD
 Minimum system cost
 Energy saving

SUMMARY
In this paper the basics of BDFIM, evolution
of BDFM, design of the brushless doubly-fed induction
cost, potential applications, merits and demerits of the
BDFIM are considered
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motor, comparison of BDFM with IM based on design and
REFERENCES
[1] Yuefeng LIAO,’ Design of a Brushless Doubly-Fed Induction Motor
for Adjustable Speed Drive Applications’, GE-Corporate Research
and Development Center, NY 12301, USA.
[3] www.microchiptechnologies.com
[4] www.google.com
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[2] Paul C. Roberts,’ A Study of Brushless Doubly-Fed (Induction)
Machines’, University of Cambridge, September 2004.
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