imdrives-updated

8/7/2019 imdrives-updated

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Feb. 2011

AC DRIVES

I- INDUCTION MOTORS STARTING & SPEED CONTROL

1. Starting Codes of 3- Phase Induction Motors

Induction motors are self-starting i.e. they are started by simply plugging them into a

3-phase power source. However in some cases, the starting current may be high

enough to cause a dip in the system.

Dip voltage reduction due to the inrush current.

(Ex: starting a fridge causes the lights in the house to dim momentarily.)

Wound-rotor motors present no starting problem since the external resistance can be

increased so as to secure a smooth run- up operation.

Squirrel-cage motors are generally capable of starting at full-rated voltage but the

starting current may be high depending on the motor rated power and design.

(Starting current may be reduced by reducing V, but this may reduce the startingtorque which is proportional to 2

ThV ).

1.1- Code letters:

To estimate the rotor current at starting all squirrel cage motors now have a starting

code letter on their nameplates. The code letter sets limits on the amount of current

the motor can draw at starting conditions. These limits are expressed in terms of the

starting apparent power (KVA) as a function of the horsepower (hp) rating.

Table of NEMA code letters for starting induction motors

Nominal Code letter Locked rotor KVA/hp

A 0 _____ < 3.15

B 3.15 _____ < 3.55

C 3.55 _____ < 4

D 4 _____ < 4.5

E 4.5 _____ < 5

F 5 _____ < 5.6

G 5.6 _____ < 6.3

H 6.3 _____ < 7.1

J 7.1 _____ < 8

K 8 _____ 9L 9 _____ 10

M 10 _____ 11.2

N 11.2 _____ 12.5

P 12.5 _____ 14

R 14 _____ 16


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S 16 _____ 18

T 18 _____ 20

U 20 _____ < 22.4

V 22.4 and UP

To determine the starting current, the rated voltage, horsepower, and code letter are

read from the nameplate.

- The starting reactive power (KVA) : SST = hp x Code letter factor.

- The starting line current is expressed as:

t

ST

lV

SI

.3=

-----------------------------------------------------------------------------

Ex: A 10 - hp, 230 - V, 3- phase induction motor is marked with code letter G. What

is the max. starting current which may be expected at 230 V?

From the Table: Code G 3.66.5 =hp

KVA

For upper limit 3.6=hp

KVA

Then: SST = hp x 6.3 = 10 x 6.3 = 63 KVA.

IST = A1582303

63000=

Ex: What is the range of the starting current of a 15-hp, 208-V, 3-phase, code-F

induction motor?

Code F 5 5.6

The lower limit:AI

KVAS

ST

ST

2082083

75000

75515

==

==

The upper limit:

(range),)233208(

2332083

84000

846.515

=

=

=

==

AIcurrentstarting

AI

KVAS

ST

ST

ST

------------------------------------------------------

1.2- Induction Motor Starting Circuits (Across the line)

A typical full-voltage or across-the-line magnetic induction motor starter circuit is

shown in Fig.1. Having the disconnect switch shut, and when the start button is

pressed, the relay (or contactor) coil M is energized, causing the normally open

contacts M1, M2, and M3 to close. When these contacts do close, power is applied to

the induction motor, and the motor starts. Contact M4 also shuts, which shorts out the

starting switch, allowing the operator to release it without removing power from the

M relay. When the stop button is pressed, the main M relay is de-energized, and the

M1-3 contacts open, disconnecting the motor from the supply and causing it to stop.


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(The disconnect switch is used only for long term or protective shut down of the

system).

The above magnetic motor starter circuit has several built-in protective features

including:

a. Short-circuit protectionb. Overload protectionc. Under-voltage protection

Fig.1: Across- the- line starting

a. Short-circuit protection for the motor is provided by fuses F1, F2, and F3. If asudden short circuit develops within the motor and causes a current flow many times

larger than the rated current, these fuses will blow, disconnecting the motor from the

power supply and preventing it from burning up. However, these fuses must notburn

up during normal motor starting, so they are designed to require currents many times

greater than the full-load current for a short period before they open the circuit. This

means that short circuits through a high resistance and/or excessive motor loads will

not be cleared by the fuses.

b. Overload protectionfor the motor is provided by the devices labeled OL in Fig.1.

These overload protection devices consist of two parts, an overload heater element

and overload contacts. Under normal conditions, the overload contacts are shut.

However, when the temperature of the heater elements rises far enough, the OL

contacts open, de-energizing the M relay, which in turn opens the normally open M 1-3

contacts and removes power from the motor.

When an induction motor is overloaded, it will be eventually damaged by the

excessive heating caused by its high currents. However, this damage takes time, and

an induction motor will not normally be hurt by brief periods of high currents (such as

starting currents). The damage will occur only if the high current is sustained. The

overload heater elements also depend on heat for their operation, so they will not be

affected by brief periods of high current during starting, and yet they will operateduring long periods of high current, removing power from the motor before it can be

damaged.

c. Undervoltage protection is provided by the controller as well. Notice from the

figure that the control power for the M relay comes from directly across the lines to

the motor. If the voltage applied to the motor falls too much, the voltage applied to the


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M relay will also fall and the relay will de-energize. The M1-3 contacts then open,

removing power from the motor terminals.

1.3. Methods of StartingIf the starting current is very high, then the applied voltage has to be reduced since the

current is directly related to the voltage (Ohms law). The starting torque is

proportional to the square value of the applied voltage since according to Thevenin

parameters:

TV R

R R X X ST

Th

S Th Th

=+ + +

3 22

2

2

2

2

.

[( ) ( ) ]

V

05. V

05. Tst

T

Tst

To reduce the applied voltage, the following methods can be used:

a) Series-Resistance starting:

Three resistors are inserted in series with the stator wdg during starting and are

gradually reduced as the motor speeds up (smooth operation).

Vl

Vm

Vl

b) Autotransformer starting:

This method leads to even more current reduction depending on the turns ratio

and on whether the motor is Yor

- connected. (not as smooth due to thecontacts and more expensive).


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1 1

2 2

3

Motor Terminals

1

3

( )Yor

Im

c

VR

Starting sequence:

close 1 & 3 reduced voltage.contact 3 is closed to develop a Y- connection.

- As the motor accelerates, contacts 1 & 3 are opened and 2 is closed fullline voltage is now connected.

- To stop the motor, open all contacts.

c)Y- starting:

A -connected I.M. can be started with its terminals switched to

Y-connection. This leads to a voltage (and current) reduction by3

1(0.58) of

its value. Consequently the starting torque will be reduced by31 .


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Y S tart ing Operation

(1) (2)

d- Other methods:Split wdg starting: using 2 identical wdgs in the stator with dual voltage rating.

For starting: 2 wdgs are connected in series.

For running: 2 wdgs are connected in parallel.

1.4. Effect of Rotor Resistance on I.M. Starting

Wound-rotor designs have the rotor winding terminals available outside the machine

through brushes and slip rings.

At starting, an external variable resistance (rheostat) is usually connected across the

rotor winding terminals thus providing high starting torque (to encounter inertia) andlow starting current (to avoid voltage dips). As the motor accelerates, the external

resistor is gradually reduced down to zero (at rated speed) thus allowing high-

efficiency operations at low slip values.

Based on the torque expression:

( )22

2

2

223

2

1

XXs

RR

s

RV

T

ThTh

Th

s++

+

=

and at constant V and f, T is affected by changes in the rotor through the expression

s

R2 only. Therefore, if both R2 and s are doubled, the following observations are

made:

(i)s

R2 is unchanged and therefore all stator parameters such as current, power factor,

and air gap power are unchanged. The full load torque is also unchanged.


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(ii) Doubling s means the rotor speed is reduced thus increasing the relative motion

between rotor and stator. This increases (almost doubles) the induced voltage (E).

(iii) The rotor frequency (f2 = s.f) is doubled leading to doubling the value of the

reactance ( 22 ..2 LfX = ) and the impedance (Z2).

(iv) The rotor current

2

2

Z

EI = would remain unchanged.

(v) Reducing the rotor speed means less mechanical power is developed (P=WT), and

more copper loss being conceded ( 22

2 RI ) due to the increase in R2 (R2 is doubled

with s).

To examine these effects numerically, use will be made of the following illustrative

example:

Ex: A 500-hp wound-rotor induction motor, with its slip rings being short-circuited,

has the following characteristics:

Full-load slip: s = 1.5%

Full load rotor copper loss: PRCL = 5.69 KW

Slip at max. torque: maxTs = 6 %Rotor current at maximum torque I2Tmax = 2.82 I2fl

Torque at 20% slip = 1.2 Tfl

Rotor current at 20% slip = 3.95 I2fl

- If the rotor resistance is increased 5x, determine:

a- The slip (s) at which the motor will develop the same full-load torque.

To keep the torque unchanged,s

R2 must be kept constant s=5xs = 0.075.

b- The total I2R loss in the rotor

Rotor current is unchanged whereas R2 = 5.R2 PRCL = 5 x 5.69 = 28.45 KW

c- The new slip at max. torqueSince

maxTs is directly related to the increase in R2, then:

maxTs = 5.5Tmax = 0.3

d- The starting torque

The starting torque corresponds to a slip S = 1, then the new Tst will be the same as

the torque defined at a slip of 20% Tst = 1.2 Tfle- Rotor current at starting

Similarly, rotor current at starting is the same as that of 20% slip

I Ifl2 2

395' .=

Example:

A 220-V, 3-phase, 6-pole, 50-Hz wound rotor I.M. develops an internaltorque of 180% with a line current of 190% at a slip of 5% when operated from rated

voltage and frequency with the terminals of the Y-connected rotor winding being

short-circuited The resistance measured between any 2 terminals of the rotor wdg is

0.18 and is assumed constant. What resistance should be inserted into the rotor wdgso that the starting current will be limited to 190% of the rated value?


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Solution:

TS t

'

TSt

T

s

(%)s

(%)s

R1 X1 X2I

1

I2

R

s

2X

I

V E1

per-phase rotor resistance is: == 09.02

18.02R

Since the required Ist is the same as that at rated operation (both are 190% of rated

full-load value)

The air gap voltage and I are also the same for both conditions.

At given condition:

+= 2

221 jX

S

RIE (rated)

At starting: ( )2221 ''' jXRIE +=

Since

===

===

==

71.109.08.1'RR

resistancerotortotalnewrequiredtheis8.105.0

09.0'

''

22ext

22

1122

R

S

RR

EEII


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II. Solid-state Induction Motor Drives

The continuous developments in the power electronics and semiconductor switching

devices, accompanied by a consistent drop in their prices, have lead to a world- wide

spread of induction motor drives. These drives can provide optimum speed control so

as to improve efficiency of the energy conversion process in which the electric drive

is used.

Various devices used for this purpose are built using the pulse width modulation

(PWM) technique, a process of modifying the width of pulses in a pulse train (wave)

according to a certain control signal.

Its principle of operation can be expressed as shown in Fig.2:

Induction

MotorFilter

Drive

(transistors)Comparator

Fig.2: Typical induction motor drive

In reference to Fig.3, two signals are fed to the comparator, a reference signal and a

carrier signal.

Reference

Carrier

comparator

signal to the

transistor to

make it conduct

or open

+-

Fig.3: PWM layout and connections.

The amplitude of the carrier wave can be adjusted by means of a potentiometer. In the

comparator, the 2 waves are compared and the transistor (drive) is turned on when the

carrier is greater than the reference, as shown in Fig.4.


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Carrier

Reference

Fig.4: PWM operation.The width of the pulse can be varied by changing the shape of the carrier using the

potentiometer.


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PWM principle is to switch the input voltage ON and OFF many times during each

half cycle and to vary the frequency and the duration of the ON pulses in relation to

the OFF. The aim is to stimulate a sinusoidal shape for the voltage supplied to the

motor. PWM are used to control the:

a) Frequency at a constant voltage, fig.5.


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Voltage at a constant frequency , fig.6.


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b) Voltage and frequency, both variable (most desirable), fig.7.


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2.1. Voltage and Frequency Patterns:

a. Variable Voltage Control

The torque developed by an induction motor is proportional to the square of

terminal voltage. Speed control is therefore achieved by varying the terminal voltage

until the torque required by the load is developed at the desired speed. Since one

cannot allow the terminal voltage to be more than the rated value, this method allows

speed control only below the rated speed. If the stator copper loss and the friction,

windage, and core losses are ignored, the converted power is:

( )sPP gconv = 1This power decreases but the rotor copper loss increases with the increase in slip.

Consequently, the motor efficiency is very poor at low speeds.

b. Variable frequency controlThe synchronous speed is directly proportional to the supply frequency.

(p

fns

120= ). Hence, the synchronous speed and the motor speed can be controlled

below and above the normal full-load speed by changing the supply frequency.

The voltage induced in the stator E is proportional to the product of the supply

frequency and the air-gap flux such that:

fKE ..= f

EK=

Induction motors are designed to operate at the knee point of the magnetization

characteristic to make full use of magnetic material and simultaneously to avoid

saturation. Therefore, the increase in flux will saturate the motor. This will increase

the magnetizing current, distort the line current and voltage, increase the core loss

and the stator copper loss, and produce acoustic noise.

While an increase in flux beyond the rated value is undesirable from the consideration

of saturation effects, a decrease in flux is also avoided to retain the torque capability

of the motor. Therefore, frequency control below the rated frequency is generally

accompanied by reducing the machine phase voltage V along with the frequency f in

such a manner that the flux is maintained constant, (constant V/f control). Above the

rated frequency, the motor is operated at a constant voltage because of the limitation

imposed by the stator insulation or by supply voltage limitations.

Control Patterns

Since loads connected to an induction motor can vary in size significantly, then

patterns ofV& f (V/f) would vary accordingly. In what follows, are the three main

patterns:

a- Standard Pattern:

at small frequencies, (R>>X) inside the motor windings, therefore, it is necessaryto have V=const i.e. (V/f)so as to ensure having sufficient Tstat the lowestspeeds.


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V=const forf>frated with no effect on saturation.

0 frated(50 or 60

Hz)

[Hz]

f

V

Vrated

Fig.8.a: Standard operation

b- High Tst loads:

Vchanges linearly with f

Shallower curve forf


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0frated

[Hz]

f

V

Vrated

Fig.8.c: Low starting torque.

2.2. Operating Modes

Electric drives can operate in different modes depending on the direction of rotation

and the power flow. In reference to figure 8-d, the following operating modes are

observed: First quadrant: Motoring operation only in one direction (forward), Second quadrant: Regenerative braking, the torque is reversed (braking) while the

motor is still rotating in the same (forward) direction.

Third quadrant: Reverse motoring, both speed and torque are reversed, but thepower is still in the same (forward) direction.

Fourth quadrant: Plugging, torque and speed have opposite directions (normallybecause the speed direction is reversed), leading to a braking condition.

2.2. DC Link in Adjustable Speed Drives:

Adjustable speed drives of ac motor requiring variable V, variable ffrom a fixed V-

fixed f source can be designed using an intermediate dc link. In such systems, ac

power is converted into dc and back to ac of different voltage and frequency.

Evolution of Dc link and drives could be briefed as shown in Fig.9:

Fig.8-d: Torque- speedquadrants


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FixedV&fsource

Variable

Transf.Diode rectifier Inverter

IM

(ac)

Variable

Vac

Variable

V dc

Variable

V&fa-

Transformer output is controlled according to the desired frequency level of

the output (V/f= const)

FixedV&fsource

VariableTransf.

Diode rectifier Inverter IM(ac)

Variablef

fixed

V& f

dc link

VariableV&fb-

FixedV&fsource

Phase

Controlled

Thyristor

Bridge

Rectifier

InverterIM

(ac)

Variable V

dcc-

Variable

V&f

FixedV&f

source

InverterDiode rectifier Chopper IM

(ac)

Variable

Vdc

fixed V

dc

Variable

V&fd-

Instead of thyristor bridge of (c), a combination of diode rectifier and

chopper would provide variable dc voltage

FixedV&fsource

Diode rectifier PWM Inverter IM

(ac)

fixed V

dce-

Waveforms to the motor are closer to sine waves (more attractive)

Fig.9: Developments in ac drives


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2.3. Solid State Control Methods

Solid- state control methods used in association with induction motors are rotor slip

energy recovery, ac power motors, and motors with inverters and cycloconverters.

2.3.1- Rotor Slip Energy Recovery:In the steady state analysis of induction motors, it was mentioned that a fraction s of

the air gap power is wasted in the short-circuited rotor circuit. The lower the rotor

speed (highers) the higher is the loss.

In Fig.10, the energy at rotor frequency has to be converted into energy at the supply

frequency before having it injected back into the supply.

Diode Rectifier

acdc

Inverter

dcac

wound rotor

induction motor

II

r

Vr

Smooting

choke

Fig.10: Schematic of rotor slip energy recovery.

The diode rectifier receives the power out of the wound rotor winding through the

brushes, Fig.8. The smoothed output (Vr) is applied to the DC terminals of an inverter

while the AC output from this inverter has the same frequency of the supply. The 3-

phase power from the AC terminals of the inverter is then fed to the induction motor

supply. This method is applicable but usually very expensive and could be a part oflong-term investment. A further advantage of this system is that it allows closed-loop

speed control of the induction motor. That way the slip energy recovery process can

be classified as one method for speed control besides the other four. A schematic of

this system is shown in Fig.11.

TransformerInverterDiode

Rectifier

Induction

Motor

GatingSignals

Speed

ControllerK

T + -+

VRReference

Signal

I

Power

Ia

Ir

VDR

IT-I

R

IT

IRVR-VT

VT

I0

Tachogenerator

Current Speed

Signal

++

( )

I0

Fig.11: Speed control using rotor slip energy recovery.


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The current feedback signal (IT) is obtained from the current transformer connected to

one of the AC lines of the inverter. At normal operation, the speed signal is obtained

from the motor through a tachogenerator coupled to and driven by the motor. The

control process can be briefed as follows:

-

If an increase of speed is requested then the signal reference voltage (VR) is

increased.- VR is compared to the existing tachogenerator voltage (VT) and the difference (VR -

VT>0) is fed to the speed controller.

- The speed controller generates a current reference signal (IR) greater than theexisting value (IT).

- (IR-IT) is fed to the gating circuits which call for a lower inverter input voltage(VDR) for the given output current (I0).

- As (VDR) drops, the rotor current increases and this increase is reflected in anincrease in the stator current to compromise the increase in power demand.

- As (Ia) increases, the developed torque will increase and the motor will accelerate.


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2.3.2- Squirrel-Cage Motor with AC Power Controller:

Introduction of impedance into the lines that supply the stator of an induction motor

reduces the terminal voltage of the motor, particularly when it is loaded. The

introduction of controlled semiconductor devices into the supply lines by a power

controller also provides control of the effective terminal pd in somewhat the same

way as variable impedances.

Apart from providing speed control, power controllers may be used for starting large

squirrel-cage motors for by this means the current drawn from the line can be held to

a reasonably low value.

Control of the ac input power to the induction motor can be achieved by variation of

the point in the cycle at which the thyristors are triggered (turned on), without the

need for changing the supply voltage. This delay in the firing angle leads to an

effective value of the voltage (or current) being lower than the rated one. As a result

the motor speed will drop.

Q1

Q4

Q3

Q6

Q5

Q2

Induction Motor

I'a

I'b

I'c

Ia

Ib

Ic

R L

R L

R L

N n

Fig.12: Ac power controller on induction motors

Looking at one phase: Q5 and Q2 of Phase C of Fig.12:If the triggering delay angle of Q5 & Q2 is =60 for example, then 2 conditions mayarise, in reference to Fig.12:

i. If the load is purely resistive then the load voltage and current waveformswould be as shown, i.e., with instantaneous rise when = t.


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i

t

Vmsint

Q5

Q2 R

Fig 12: Output Waveform of an AC Power Controller.

ii- If the load consists of an inductance and a resistance, which is how an induction

motor is represented, then there would be no instantaneous rise. Instead, the

current (or voltage) would increase almost exponentially from zero and also the

inductance would extend the current beyond point t=, i.e., almost puresinusoidal waveform is still feeding the motor but the input power (area under the

curve) is less. Optimum operation occurs when the current of one half-cycle

would be prolonged until the beginning of the other half-cycle exactly, so that the

circuit would behave as if no thyristors are present. This optimum operation

occurs when:

=

R

L

1

tan

Where is the angle beyond which the waveform would become discontinuous.

Therefore for a continuous waveform, the triggering angle would vary from zero up to

.

For values beyond this limit, the waveform would still have a fundamental component

of continuos wave but the higher harmonics would be more significant.

This method does not affect the frequency and therefore the v/f ratio is reduced which

means that the motor will not be operated with optimum saturation level.

Furthermore, the motor would be operating at much higher slip leading to more losses

inside the motor since the synchronous speed of the motor is not changed. The

resulting torque - speed characteristics are similar to those of voltage control method,Fig.13.


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Speed (rpm)

Torque

n

Fig. 13: Torque - speed characteristics

AC power controllers, normally used for class D motors, result in a drive system of

poor power factor. This could be avoided by having a high rotor resistance, which

leads to:

- an increase in the slip value at which the torque is max. (shift of curveleftwards)

- starting current reduction and power factor improvement.Schematic of a simple closed loop control system bases on an AC power controller is

shown in Fig.14.

Three phase ac

power controller

Controller Logic

Tachometer

Induction Motor

VT

Mechanically

CoupledGating (trigerring)

signal

Speed Reference signal

(gives the desired speed value)

Fig.14: Closed- loop ac power controller.


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2.3.3. Squirrel-Cage - Motor with Inverter:

In reference to Fig.15, the inverter drive set has the following characteristics:

a. The inductance in the DC link provides smoothing whereas the capacitancemaintains a fixed value for the DC link voltage.

b. The output voltage from the inverter is usually a square wave with higherharmonics and the armature current may, as a result, have peak pulses. These

peaks cause additional losses and heating. This could be avoided by motor de-

rating.

c. The stator current peaks are inversely related to the motor leakage reluctance, andhence motors with high leakage reactances are usually used in association with

inverter drives.

d. These inverters have problems at low frequencies because the DC link voltagewould not be sufficient to commutate thyristors. Minimum operable frequencies

are 5 Hz and higher.

e. General applications are for low and medium- power industrial drives in ratingsup to 200 KVA.

The following schematic, Fig.15, shows a combination of semiconductor converters

required for such variable-frequency, variable-voltage control (open loop).

controlled rectifier

(ac dc)Inverter (dc ac)

Induction

Motor

Filter

Vo

Frequency ControlVoltage Control

Fig.15: Open- loop inverter drive.

The voltage control of the rectifier ensures that the voltage from the rectifier (V 0) will

vary with the frequency according to the above characteristics. The drive does not try

to make an instantaneous jump form one speed to another due to the motor inertia;

instead special circuits are added to ensure a smooth acceleration or deceleration. The

drive also contains devices to protect the motor against short circuit, overheating, and

under voltage as discussed earlier.

The output line voltage from a typical three-phase inverter can be resolved using

Fourier analysis into a fundamental component and a set of higher odd harmonics (3rd

harmonic will have a sum of =0 in 3 phases)

])6

7sin(7

1)

65sin(

5

1)

6[sin(

6cos

40 L+++++=

wtwtwtVVab


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The fundamental of which has an amplitude value of:

000max 1.132

2

34VVVVab ===

The rms line value is:

0

0max 6

232

2VVVV abl =

== 0

0 78.06 VVVl ==

The corresponding phase value fed to the motor is:

045.03

VV

V l == .

2.3.4. Cyclo-Converter:

A cycloconverter is a system of switches that convert directly ac power from a

constant - voltage, constant-frequency supply to a variable - voltage, variable -

frequency output without the need for a dc link. Analysis will be conducted for a

square wave ac voltage but it could be extended to cover sinusoidal waveforms. To

clarify the cycloconverter operation, consider the following circuit:

(a) Thyristors T1 and T2 are on

for a number of half cycles then

T3 and T4 will be on after a

delay of 2 half cycles.

Group 1

Group 2

( ) m =

(c) Notched Output Voltage

T1

T2

T3

T4

Group 1

Group 2

i1

Load

T1 Conducting

T2

Conducting

v

v1

v2


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Limitations:

To avoid overlap and higher harmonics in the output, cycloconverters are used

to generate only 13 of the supply frequency.

III. PWM Voltage Source Inverter for Induction Motors

The difficulty with most 6- step inverters is that their performance at low speeds

is not reliable. The stepping nature of the rotating stator field causes the torque to be

applied in pulses rather than smoothly. PWM is the most widely used method of

improving the low speed performance of DC link inverter systems. The principle is

to use high speed switching to enable the motor current waveforms at low speed to

be more sinusoidal and hence lead to a smoothly rotating magnetic field in the motor.

Also, the inverter is controlled so that the output voltage is variable and of pulsed

wave, instead of the square wave, which tends to be more sinusoidal. This leads to

good performance at low speeds as well as high and the ability to control the motor

accurately even around zero value.

PWM drives cost has been reducing steadily with the drop in prices of large scale

integrated circuits and microprocessors, so that this system is nowadays oftenemployed for general purpose drives at various speeds. PWM use has increased due

to the availability of faster switching devices like transistors and gate turn- off

thyristors (GTOs).

3.1. PWM Drive System

The elements of PWM drive systems are generally similar to those of the 6- step

system with the exception that the mains convertor can be a diode rectifier only, and no

control is required from the input side of the DC link. PWM systems are, in general,

voltage source DC link systems as shown in Fig.18. A constant DC link voltage is used

and all the control is done via the motor inverter operating in PWM mode. The circuit

also includes a DC link reactor as a means of reducing the level of high frequency

currents getting into the input circuit and to force these currents to flow in the DC link

capacitor. The reactor is not needed to smooth the DC link voltage because the diode

rectifier already produces a good and steady DC level and some manufacturers

dispense with this reactor for economy reasons.

The DC link capacitor is used to provide a path for the currents which flow through

the feedback diodes in the inverter. When the inverter operates at high frequency, large

AC ripple currents flow in this capacitor and it has to be correctly selected for these

conditions.


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Fig.18: PWM drive system

Control over the drive, in all respects, is now carried out via the inverter alone and

most PWM pattern generating systems include inputs to enable independent setting of

voltage, frequency and phase sequence so that the correct conditions for the motor can

be produced.

If the frequency to the motor is reduced suddenly the motor can regenerate the load

energy into the inverter and the DC link rises in voltage due to the energy being fed

into the capacitor via the feedback diodes. To guard against this possible increase in

DC voltage which could quickly damage the semiconductors it is usual to include a

DC voltage measurement that will cause increase in inverter frequency if a high DC

voltage is detected. This prevents the motor slowing down too quickly. If fast

slow-down is required then some means of absorbing the regenerated energy on theDC link is required.

PWM inverter systems in general, provide superior performance to the six- step

alternatives since:

1) The range of speed control is much wider and operation at and around zero

speed is quite satisfactory.

2) Low frequency torque pulsation does not occur in the output and hence

there is less chance of exciting mechanical load resonance.

3) The current waveforms in the motor are always very near to sinusoidal

leading to more economic and quieter performance.

4) The diode input rectifier means that the input power factor is always high

irrespective of the speed and load.

5) In multi-drive systems it is possible to connect a number of inverters to the

same DC link to allow transfer of regenerated power from some drives to help

feed other motoring drives.

The major disadvantage, however, is the increased complexity and the increased

difficulty in protecting these systems, and in the losses due to the frequent switching.

(max. freq. is around 150Hz).


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3.2. Performance and Application

a. In general PWM system can provide very high quality performance over a very

wide speed range. With the larger number of voltage pulses per half cycle, even at

low speed, the current waveforms can be very near to a sine wave and very smooth

performance near zero speed is obtainable. These improved performance capabilitiescan be achieved by employing very high quality and highly specified semiconductor

switches. There is also the very complex and variable voltage waveforms produced by

the inverter that may make the system difficult to understand.

b. Because of the high specification of the inverter switches the operating voltage of

these systems has up to now been limited to the range up to 500 volts AC, but with

the increasing use of gate turn- off thyristors (GTOs) for PWM systems operating

voltage capabilities are increasing.

c. Being a voltage source system for induction motors, this drive is not affected by the

precise parameters of the motor connected to it. Therefore, it is possible to supply a

number of motors from the same drive as long as they are all required to operate at the

same frequency. In such cases load sharing is not seen to be a problem due to the

inherent slip of the induction motor and the ability of the inverter to provide the

currents which the individual motors may demand.

d. Most PWM pattern generators allow for the reversal of the output voltage

waveforms so that electronic reversal of the motor can be used if needed. This is

achieved simply by reversing the direction of modulation of the inverter switching at

the most satisfactory point in the cycle.

e. The inverters used in this system are usually fully capable of accepting power from

the motor and feeding it back into the DC link but this facility may not be used and it

may even be prevented to avoid over-voltages on the DC link. If no special

arrangements are made to absorb or feedback regenerated power then the energy will

be dumped into the DC link capacitor causing its voltage to rise quickly. When

regular motor braking is required with a PWM drive system, then either switchedresistor will be included to dissipate the energy or an additional feedback thyristor

converter will be included to pass the power back to the AC mains.

f. The system efficiency is relatively good as far as motor losses are concerned, the

motor current is much nearer to sinusoidal than most of the other DC link systems and

hence the conductor losses are very near to those occurring under sinusoidal

conditions. The voltage waveform applied to the motor does contain a substantial

harmonic content and this does increase the iron and stray losses in the motor by an

amount that will depend on the frequency of inverter switching.

g. One of the important features of PWM drive systems is the direct result of having a

supply side diode rectifier to give a constant DC link voltage. The result is that the

power factor of the input current to the drive is always high and it does not vary with

the speed of the drive. Drives of this type will have an input power factor of around

0.95 per unit.


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4. Induction Motors Braking

Braking methods of induction motors can be divided into the following categories:

1. Regenerative braking.

2. Plugging or reverse voltage braking.

3. Dynamic or rheostatic braking.

4.1. Regenerative Braking

[ ]T

V R

R R X X st

s

Th

Th Th

=+ + +

1 32

2

2

2

2

2.

( ) ( )

2

2

2

2

max

)(

3.

2

1

XXRR

VT

ThThTh

Th

s +++=

and:2

2

2

2

)(max

XXR

RS

ThTh

T

++=

These equations provide expressions for starting torque Tst, STmax and Tmax, and

expressions are valid for the full speeds range, i.e. below synchronous speed (s > 0),

speeds above synchronous speed (s < 0), and also for negative speeds (s > 1). Figure

19 shows the speed-torque curves for all the three ranges of speed.

The operations form > s (or s 1) produce negative power and

therefore correspond to the braking operation.

With a positive sequence voltage across the motor terminals, the operation above

synchronous speed gives the regenerative braking operation (portion BAE). In

regenerative braking, the motor works as an induction generator, converting

mechanical energy supplied by the load to electrical energy, which is fed to the

source. Thus the generated energy is usefully employed. It should be understood that

if the source cannot accept energy then the regenerative braking cannot be used. The

operation of the motor in regenerative braking can be explained as follows.

Operational Concepts

When the motor runs at a speed greater than the synchronous speed, the relative speed

between the rotating stator field and the rotor is negative. The rotor induced-voltage

and currents have directions opposite to those under the motoring operation. The

resulting stator currents will also be in the opposite direction. Thus, the power flows

will be from the motor to the source and the motor works as an induction generator.The magnetizing current required to produce flux is obtained from the source. It may

be noted that the machine cannot regenerate unless it is connected to a source.

For regenerative braking to take place, the motors speed should be greater than the

synchronous speed. When the motor is fed by a fixed frequency source, regenerative

braking is possible only for speeds greater than the synchronous speed. When the

motor is fed by a variable frequency source, the source frequency can be adjusted to


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give a synchronous speed less than the motor speed for any motor speed, i.e. by

reducing the motor frequency, we get a synchronous speed less the that of the rotor,

and hence regenerative braking can be maintained until zero frequency or zero speed.

Fig.19: Torque- speed characteristic

When regenerative braking is employed for holding the speed against an active load,

care should be taken to restrict the operation in the region between the synchronous

speed and the speed for which the braking torque is the maximum. That is, on the

portion AB in Fig.19 for which 0 > s >-sTmax. For slips more negative than sTmax,(portion AE), the braking torque reduces drastically, leading to runaway speeds

because the faster the motor runs, the lesser will be the braking torque. This

restriction on the slip range must also be observed when braking against an active

load by varying the supply frequency.

When holding an active load by regenerative braking, a short duration dip in the

supply voltage or a momentary increase in the load torque may shift the operation to

the unstable region. In such a situation mechanical brakes may be used to assist the

regenerative braking to prevent runaway speeds. Alternatively, capacitors may be

connected in series with the motor to increase the braking torque. If one is using a

wound-rotor motor, the rotor resistance may also be increased to increase the range of

stable operation.

The developed braking torque can be calculated by using the negative sign for the

slip. The shaft torque is obtained by adding friction windage and core loss torque to

the developed torque.

It may be noted that for the same absolute value of slip, the braking torque is higher

than the motoring torque. Since the braking speeds are also higher, the regeneratedpower is much higher than the motoring power.


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4.2. Plugging

An induction motor operates in the plugging mode for slips s>1. This takesplace when the motor is moving backwards relative to the rotating stator

magnetic field. (segment CD, Fig. 19).

Since the motor is running in the reverse direction to that of the stator mmf,the developed torque provides the braking operation, or plugging, to stop the

motor, and to accelerate it in the opposite direction.

The electrical power generated by the conversion of mechanical powersupplied by the load and inertia, and also the power supplied by the source, are

dissipated in the motor circuits resistances. Thus, this is a highly inefficient

method of braking.

The motor can be braked by changing the phase sequence of the motorterminal voltages by simply interchanging the connections of any two motor

phases. This will reverse the direction of rotation of the stator field. The motor torque is not zero at zero speed. To stop the motor, it should be

disconnected from the supply at or near zero speed.

An additional device will be required for detecting zero speed anddisconnecting the motor from the supply. Therefore, plugging is not suitable

for stopping, but rather for speed control, and for reversing the direction of

rotation.

Because of high values of slip (nearly 2 at point D), the equivalent rotorresistance R2/s has a low value. In the case of a wound-rotor motor, external

resistors are connected in the rotor to reduce the current and increase the

braking torque. The value of the external resistor can be chosen to provide the

maximum torque for s=2. As s falls, the resistance can be varied to brake and

reverse the motor at the maximum torque.

4.3. DC Dynamic Braking

In dc dynamic braking, the motor is disconnected from the ac supply and

connected to a dc supply. The ways in which the motor can be connected to a dc

supply are shown in Fig. 20. Connections c and f provide uniform loading for all

the three phases but complicate the switching operation within the motor 3- phase

winding. Connections a, b, d, and e are generally used because of the simpler

switching operations. The flow of direct current through the stator windings sets up a

stationary magnetic field. The relative speed between the stationary stator field andthe moving rotator is now negative. Consequently, 3-phase voltages of reverse

polarity and phase sequence (compared to the motoring in the same direction) are

induced in the rotor. The resultant three-phase rotor currents produce a rotating field,

moving at the rotor speed in the direction opposite to that of rotor, thus giving a

stationary rotor field. Since both stator and rotor fields are stationary and rotor current


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flows in the opposite direction, a steady breaking torque is produced at all speeds. It,

however, becomes a zero standstill due to zero rotor currents.

Figure 20: Stator connections for dc dynamic braking

Since the dc current flowing through the stator depends on its resistance which is low,

a low voltage dc supply is required. This is obtained from the ac supply by a step

down transformer and a diode bridge. When controlled braking (braking with variabletorque) is required, a thyristor bridge is used instead of the diode bridge. When quick

braking is required, to produce large braking torque, the stator current can be set as

high as ten times the rated value. But then either the supply must be removed or the

current must be reduced below the rated value soon after the motor stops, otherwise

the motor will be overheated.


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5. Variable- frequency Synchronous Motor Drives

Speed control of synchronous motors could be conducted using the devices

described in association with induction motors such as ac power controllers, inverters,

and cycloconverters. Due to the unique relation between stator frequency and the

rotor mechanical speed, synchronous motor drives have the following features:

i- Since they can operate at a synchronized speed, they are used for applications

where very precise motor operation and accurate speed control are required.

ii- Due to the field excitation adjustment capability, synchronous motors can

operate with optimum power factor conditions.

iii- On the other hand, synchronous motor are subject to loss of synchronism if

significant load changes occur, and if the load angle exceeds the 90o

angle set as its

stability limit.


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6. Dynamics of Induction Motors Starting

The proper choice of an induction motor requires a knowledge of its startingduty and the voltage dip it may cause to the system especially in the case of

large induction motors operating in industrial complexes. Insufficient torque at starting may expand the run-up period and cause motor

heating due to the long lasting inrush current.

In general, the torque developed by the motor is used to:1. Encounter the coupled load torque, including friction and windage (Tl)

2. Accelerate the whole system (motor and load) i.e.

dt

dJTT ml

+=

sec)/(radspeedmachine

timet

inertiaofmomentJtorqueloadT

developedtorqueTWhere

m

l

Therefore, the difference l= is the torque available to accelerate the rotatingmass i.e.

TdJdt

dt

dJTTT

m

ml

=

==

The time required to reach the operating speed m is :

=m

md

TJt

0

1

(t is the run up time).

This equation could be solved numerically or graphically in association withother equations that describe the motor performance. The magnitude of thesupply current remains large until the speed increases up to the rated value.

The speed - time curve could be typically illustrated as:


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Fig.21: Speed variation with time during acceleration.

Run- up Period Calculations

If the motor is not connected to a load (i.e. T1=0), then:

dt

dJT m

.=

The high IST will normally last until the motor reaches a speed that corresponds to

sTmax, then it starts to drop down to rated value.

Since the slip drops as speed increases, T can be expressed in terms of the slip such

that:

dt

dsJT s ..=

The ve sign represents the drop in s as the motor accelerates.

From which, the time needed to reach sTmax is:

=max

1..

St

sT

dsJt

If the stator resistance is neglected (R1=0), then according to S.S. analysis:

2

2

22

22

2

2

22

22

2

max

2

2

2max

12

2

2

2max

)()(

..3

.1

)()(

..3

.1

)00(,)(

XXs

Rs

RV

XXs

RR

s

RV

T

XXs

R

XX

Rs

RRXXR

Rs

TH

TH

STHTH

TH

s

TH

TTH

T

TH

THTH

T

++=

+++=

+=+

=

==++

=


35/35

)2

(

1.2

12,

..22

)11

.(.

2

.3

..2*

)()(

..3*

1

.3

2

1

)(

.3.

2

1

,

max

max

max2

max

max

max

22

max

max

max

maxmax

max22max

max

2

max

2

2

max

222

22

max

max

2

2

2

2

2max

T

T

MAX

T

T

T

TMAX

ST

T

T

T

T

TT

Th

Ts

T

Th

s

T

Th

sThThTh

Th

s

s

s

s

sTT

ss

ss

TTAlso

ss

ss

s

s

s

sT

T

ssss

T

T

V

s

R

s

R

s

Rs

RV

T

T

s

R

V

XXRR

VT

Also

+

==>

+=+=

+=

+

==>

+

=

+

==>

=+++

=

Hence, the time needed to reach sTmax is:

)]ln(..2

1[

.2

.

].[

.2

...

maxmax

max

max2

max

1 max

max

max1

maxmax

TT

T

Ts

S

T

TsS

s

sss

s

T

Jt

ds

s

s

s

s

T

J

T

dsJt

TT

=

+

==

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