Sped

Cmtall imiter
PI
Voltuye
Sowe
Wm Tnvertue
Wm

TM

wm

.Wm

"No-1 Wo-d

P2 -
No- 2 y IH is bagteally
9

but a
Blak- 3,
N3-BArh

Beeaw

Rotr cweul,
But

elutlel
Meckaiel.

Makuatalpe)
X

to get deitruadr)

Blrek5 Mrulhipb
Rotr
Ret elekadeo

y.
 (A)

(afuae ).

kfeunu
Aaee invater (slak-1).
VsI Is be (ed)
Vaulable Ae

potade a
pul ad

and adle (8lek-s s Blek-4)

 be

e aolied.
192 Fundamentals of Electrical Drives
6.13.1 VSI Induction Motor Drives
Voltage sourve inverter allows a variable frequency supply to be obtained from
Fig. 6.37(a) shows a VSI employing transistors. Any other self-comnutated device a dc
instead of atransistor. Generally MOSFET is used in low voltage andlow power can be uge
(insulated gate bipolar transistor) and power transistors are used up to medium inverters,
IGB
GTO (gate turn off thyristor) and IGCT (insulated gate commutated thyristor) are power levels an
power levels. used for hig
VSI can be operated as a stepped'wave inverter or a pulse-width modulated
When operated as a stepped wave inverter, transistors are switched in the (PWM) invete.
sequence
numbers with a time difference of T/6 and each transistor is kept on for the duration T/n of Mtheir
T is the time period for one cycle. Resultant line voltage waveform is shown
in
Frequency of inverter operation is varied by varying T and the output voltage of theFig.inveteri
6.3170,
varied by varying dc input voltage. When supply is dc, variable dc input voltage is obtained h
Connecting a chopper between dc supply and inverter (Fig. 6.38(a)). When supply is ac, variah
dc input voltage is obtained by connecting a controlled rectifier between ac supply and inverter
(Fig. 6.38(b). A large electrolytic filter capacitor C is connected in dc link to make invete
operation independent of rectifier or chopper and to filter out harmonics in dc link voltage.

A B Ts
Va
T4

A BC

Induction
motor
(a) Transistor inverter-fed induction motor drive

VAB‘ VABT

V 27 20
5x/6 0
n/6
-VA

(b) Stepped wave inverter line voltage waveform (c) PWM inverter line voltage
w a v e f o r m

Fig. 6.37 VSI fed induction

motor drives:
Inverter output line and phase voltages are given by the
following Fourier serieS.
2 W3 1
VABt Va sin wr- sin 5or-sin 7 ot+ 111 sin llwt+13
1
Sin 13 or.
. . ](6.77
 Induction Motor Drives 193

Filter la
DC L
Chopper Six step IM
supply V inverter

de link

(a)
Filter

AC
Controlled Six step
supply C V IM
rectifier inverter

dc link

(b)
Filter Ia
+
DC PWM
IM
supply inverter

(c)

Filter

+
AC Diode PWM IM
supply bridge inverter

dc link

(d)
Fig. 6.38 VSI controlled IM drives

1
VaN = V, sin 01+sin Sor +sin 7ot (6.78)

The rms value of the fundamental phase voltage

V= -Va (6.79)

The torque for a given speed can be calculated by considering only fundamental component as
explained in Sec. 6.4. The main drawback of stepped wave inverter is the large harmonics of low
frequency in the output voltage. Consequently, an induction motor drive fed from astepped wave
inverter suffers from the following drawbacks:
(a) Because of low frequency harmonics, the motor losses are increased at all speeds causing
derating of the motor.
(b) Motor develops pulsating torques due to fifth, seventh, eleventh and thirteenth harmonics
which cause jerky motion of the rotor at low speeds as explained in Sec. 6.4.
 194
Fundanentals of Electrical Drives
(c) Harmonic content in motor current increases at low speeds. The
machine
loads at low speeds due to high (V/f) ratio. These two effects overheat the
saturates at light
speeds, thus limiting lowest speed to around 40% of base speed.
machine at low
Harmonics are reduced, low frequency harmonics are eliminated, associated losses
are reduced
and smooth motion is obtained at low
speeds also when inverter is operated as a pulse-width
modulated inverter. Fig. 6.37(c) shows output voltage waveform for sinusoidal
modulation. Since output voltage can now be controlled by pulse-width modulation, no pulse-width
isrequired for the variation of input de voltage, hence inverter can be directly arrangement
cOnnected
the supply is dc [Fig. 6.38(c)] and through a diode rectifier when supply is ac. when
The fundamental (Fig. 6.38(d)1.
component in the output phase voltage of aPWM inverter operating
sinusoidal PWM is given by with

V= m
Va
242 (6.80)
where m is the modulation index.
The harmonics in the motor current produce torque
For a given harnmonic content in motor teminal pulsation and derate the motor (Sec. 6,4)
the motor has higher leakage inductance, this voltage, the current harmonics are reduced when
when fed from VSI, induction motors with reduces derating and torque pulsations. Therefore.
large (compared to when fed from
leakage inductance are used. sinusoidal suppl)
6.13.2 Braking and
.Thae ae

Civ cuit Diayan

A
B TM
 A De
the cas

toode.

A- 3-ph Thduetn' m t a e h an a

inpit (,A)
.

's tuneo
 126 Z40

Sck's ll omduut i pait.

tndueti
an
as,

12s 240
 Cos 36
2 A

4 Lix
2

con alo clag

7
Im

L Cwreu
A

). Tnput tanat shloo

7 =

3
T:

very sell)
 *.()
beeme
.

the Cnnut
to mest of
teae tanee

JM.

Cae
 Consfat air j9

Xs

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be comfaut for
curreut

mgntiny
Cnotant air y P .
+fen fan.
'y

,2

/Ro

2
 conbial
Conlad

dalily fnnll,

(
Rane
ofsny
in numatal
 2
2

Sf
24 (nt')
1
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macntaina

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 2r(et y)

Bbe cluy?
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cktodats, pad
tontalel Rotr
G

chraterls his

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Here we
be cau ee,
lemtant but

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Rot

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A (20 360°

eutreuttf.

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Sensor Cent
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f
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en se
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37
Cmwl
fde nk cwnent
 Slip Power Recovery
Slip Power Recovery Scheme used in Induction Motor - Figure 6.53 shows an
equivalent circuit of awound-rotor induction motor with voltage V. injected into
its rotor, assuming stator-to-rotor turns ratio unity. When rotor copper loss is
neglected
Pm=P- P, (6.92)
where P. is the power absorbed by the source V. The magnitude and sign of
P, can be controlled by controlling the magnitude and phase of V.. When P. is
zero, motor runs on its natural speed torque characteristic. A positive P, will
reduce Pm, and therefore, motor will run at a lower speed for the same torque.
When P, is made equal to P, then Pm and consequently speed will be zero.
Thus, variation of P, from 0 to P, willallow speed control from synchronous to
zero speed. Polarity of V,for this operation is shown in Fig. 6.53 by a continuous
line.
X R,

Fig. 6.53 Induction motor operalion with an injccted voltage in its rotor
than
When P, is negative, i.e. V,acts as a source of power, Pm will be larger
Polarity of
P, and motor will run at a speed higher than synchronous speed.
dotted line in Fig.
V, for speed control above synchronous speed is shown by a
6.53.
Power Recovery
When rotor copper loss is neglected, P, is equal to Slipsynchronous speed
Scheme used in Induction Motor, sP. Speed control below
same approach was adopted in
is obtaind by controlling the slip-power. thewasting power in externalresistors,
rotor resi_tance control. However, instead of methods of speed control are
it is usefully employed here. Therefore, these in Induction Motor recovery
classified as Slip Power Recovery Scheme used
schemes. Two such schemes, Static Sherbius
and Static Kramer Drives, are
described here.
Static Scherbius Drive (Fig. 6.54(a)):
motor below synchronous speed.
It provides the speed controlof awound rotor by adiode bridge. The controlled
dc
A portion of rotor ac power is converted intoback to ac and feeds it back to the ac
rectifier working as an inverter converts it
controling inverter counter
source. Power fed back (i.e. P.) can be controlled by
inverter firing angle. The dc
emf Vo, which in turn is controller by controling the
la.
link inductor is provided to reduce ripple in dc link current
Motor is fed back to the
Since SIip Power Recovery Scheme used in Induction
resistors, drive has
source, unlike rotor resistance control where it is wasted in
 ahigh efficiency. The drive has higher efficiency than stator voltage control by
ac voltage controllers because of the same reasons.

3-phase ac supply a= 90
90° < a < a, < l80º
Induction
Transforner
motor

Vai
Vazt
+}

(a) The drive circuit (b)Speed-torque curves

Fig, 6.54 Static Scherbius drive
Drive input power is the difference between motor input power and the power
fed back. Reactive input power is the sum of motor and inverter reactive powers.
Therefore, drive has a poor power factor throughout the range of its operation.
From Fig. 6.54(a), neglecting stator and rotor drops

Va = (6.93)

COS (6.94)
where a is the inverter firing angle and, n and m are, respectively, the stator to
the
rotor turns ratio of motor and source side to converter side turns ratio of
transformer. Neglecting drop across inductor

Va + V42 =0
Substituting from Egs. (6.93) and (6.94) yields

S (6.95)

where a n/m.
commutation of inverter
Maximum value of a is restricted to 165° for safe
from 90 to
thyristors, Slip can be controlled from 0to 0.966a when a is changed
can be obtained.
T05".By appropriate choice of a, required speed range
Iransformer is used to match the voltages Va and Vi. At
the lowest speed
required from the drive, Va will have the maximum value Vatm given by
 Vatn = Vsmaxn
where smak, iS the value of slip at the lowest speed. If a is restricted to 165°, m is
chosen such that the inverter voltage has a value Vitm When a is 165° i.e.
Y cos 165° +

)1 =
ncos l65° .0.966
Sux
Such a choice of m ensures inverter operation at the highest firing angle at the
lowest motor speed, giving highest power factor (Eqn. (5.109)) and lowest
reactive power at the lowest speed. This improves the drive power factor and
reduces reactive pOwer at allspeeds in the speed range of the drive.
sR sX 2(sR{ + R,) +Ra

Va
sV/n

(a) Equivalent circuit of the (b) Equivalcnt circuit

motor referred to the rotor of the drive

Fig. 6.55 Motor and drive equivalent circuits

to the rotor, neglecting
Figure 6.55(a) shows equivalent circuit of motor referredwhen referred to dc link,
magnetizing branch. Derivation of Eq. (6.90) shows thatapproximate dc equivalent
resistance (sR, + R.) will be 2(sR', + R,). This gives
are given in Eqs. (6.93) and
circuit of the drive (Fig. 6.55(b), where Vaa and VeeEquivalent circuit ignores the
(6.94). R is the resistance of dc link inductor.
commutation overlap in the diode bridge. Now
cos
+
Va + Va2 (6.96)
2 (sR; + R) + Ro
la =2(sR +R,) +Ry
If rotor copper loss is neglected
(6.97)
sP, =|Vala
|Valla
(6.98)
T=
PVela
curves is shown in Fig. 6.54(6).
The nature of speed torque
control
and pumpdrives which require speed
The drive has applications in fan denotedby smax, then power ratings of
in a narrow range only. If maximum slip is times the motor power
inverter and transformer can be just smax synchronous
bridge,
diode
For example, when speed is to be reduced below
rating (Eg. 6.97).
speed by only 20%, power ratings of diode bridge, inverter and transformer will
be just 20% of motor power rating. Consequently, drive has a low cost.
Drive is started by resistance control with Si closed and S;
When speed reaches within control range of the open (Fig. 6.54).
drive, S, is closed to connect
diode bridge and inverter is activated. Now S, is
resistances. opened to remove the
In fan and pump drives braking is not
required, because the fluid pressure
provides adequate braking torque. To maintain constant fluid flow with variations
in pressure head and the nature of pumped
closed loop speed control. Aclose loop speed fluid, the drive is operated with a
Control is shown in Fig. 6.56. It operates in the control
same
scheme with inner current
3.5. way as the scheme of Fig.

This drive is widely used in medium and high power

and pump drives, because of high efficiency and (up to around 10 MW) fan
This drive provides a constant torque control low cost.
(Eqn. (6.98). Constant power
control is obtained by static Kramer drive described
below.

ac supply

Transformer

Sped
sensor

Piring
circuit
Current
limiter

Speed Current
controlier Controllcr

Fig. 6.56 Closed-loop control of static Scherbius drive

Static Kramer Drive:
The dc
Rotor slip power is converted into dc by a diode bridge (Fig. 6.57(a). Torque
power is now fed to dc motor mechanically coupled to induction motor. Speed
supplied to load is sum of torque produced by induction and dc motors.
control is obtained by controlling field current of dc motor.

supply

+ Vo2

Vai

(a) The drive circuit

Vat
Vae
B Vai
C

(b) Field control with diode (c) Firing angle control of thyristor bridge
with constant motor field
bridge
Fig. 6.57 Static Kramer drive

shows variations of Va and Vaz With speed for twO values of dc

Figure 6.57(b) at
field current. The steady state operation is obtained when V = V, i.e.
motor control is possible from synchronous
Aand B for field currents ln and Iz. Speedspeed. When larger speed range is
speed to around half of synchronous thyristor bridge. Now relationship
required, diode bridge is replaced by a controlling firing angle of thyristor
between Va and speed can be altered by
controlled up to standstil.
rectifier (see Fig. 6.57(c)). Speed can now be
 Main
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Back anf
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dide b, nent Aall freaked,

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tene we ane tnin 4 tranis tr oitch, T, a, ,,
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