tm008 en rev b 2007 starting methods for ac motors lowres
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8/7/2019 tm008 en rev b 2007 starting methods for ac motors lowres
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Te
chni cal not e
s
M
UN
Ist
UN
= Rated net voltage
Ist
= Start current at full voltage
M
UN
Ist
UN
= Rated net voltage
Ist
= Start current at full voltage
M
UN
IstR
UN
= Rated net voltage
Ist
= Start current at full voltage
IstR
= Start current at red. voltag
XM
= Motor reactance
XR
= Reactor reactance
XM
XR
I stR
= ( )U N
X M
+ X R
T stR
= ( )I stR
I st
T st
2
Starting methods for AC motors
Reducing electrical and mechanicalstress at start-upThe starting current o an AC motor can vary
rom 3 to 7 times the nominal current.This isbecause a large amount o energy is requiredto magnetise the motor enough to overcomethe inertia the system has at standstill. The highcurrent drawn rom the network can causeproblems such as voltage drop, high transientsand, in some cases, uncontrolled shutdown.High starting current also causes greatmechanical stress on the motor’s rotor bars andwindings, and can aect the driven equipmentand the oundations. Several starting methodsexist, all aiming to reduce these stresses.
The load, the motor and the supply networkdetermine the most appropriate startingmethod. When selecting and dimensioningthe starting equipment and any protectivedevices, the ollowing actors must be takeninto account:
• The voltage drop in the supply networkwhen starting the motor
• The required load torque during start• The required starting time
Direct-on-line (DOL) start
Direct on line starting is suitable or stablesupplies and mechanically sti and well-dimensioned shat systems. It is the simplest,cheapest and most common starting method.Starting equipment or small motors that donot start and stop requently is simple, otenconsisting o a hand operated motor protectioncircuit breaker. Larger motors and motors thatstart and stop requently, or have some kind o control system, normally use a direct-on-linestarter which can consist o a contactor plusoverload protection, such as a thermal relay.
Star-Delta (Y/D) startingMost low voltage motors can be connected torun at either 400 V with delta connection or
at 690 V with star connection. This fexibility can also be used to start the motor with alower voltage. Star/delta connection gives alow starting current o only about one-third o that during direct-on-line starting, although thisalso reduces the starting torque to about 25%.The motor is started with Y-connection andaccelerated as ar as possible, then switched toD-connection. This method can only be usedwith induction motors delta connected or thesupply voltage.
TM008 EN Rev B 2007
ABB has the widest
range of motors of any
manufacturer, from 0.055 kW
to 70 MW. A number of
starting methods are used
within this vast range; your
ABB representative can
advise on the one most
suitable for a particular
application.
Reactor startBy connecting a coil with an iron core (areactor) in series with the motor during start,the starting current is limited in proportionwith the voltage. However, this also meansa substantial (quadratic) reduction in theavailable starting torque. The advantage o this
method is its low cost in comparison with othermethods.
Auto transformer startThe eect o auto transormer start is similar tothat o reactor start. Using a transormer to limitthe voltage reduces the starting current and thetorque, but less so than the reactor start. Themethod is more expensive than reactor start.
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T e
ch ni cal not e
s
M
UN
IstR
UN
= Rated net voltage
Ist
= Start current at full voltage
IstR
= Start current at red. voltage
UM
= Motor voltageU
M
M
UN
IstR
UN
= Rated net voltage
IstR = Start current w/capacitor
XN
= Net reactance
XC
= Capacitor bank reactance
XM
XC
XM
= Motor short circuit
reactance
XN
M
UN
IstR
UN
= Rated net voltage
IstR
= Start current at red.voltage
UM
UM
= Motor voltage
M
UN
IstR
UN
= Rated net voltage
IstR
= Start current at red.
voltage
UM
UM
= Motor voltage
UN
= Rated net voltage
IstR
= Start current at red. voltage
UM
= Motor voltage
UN
IstR
UM
M
T stR
= ( )I stR
I st
T st I
stR = ( )U
M
U N
I st
2
T stR
=X
C X
M
X C +
X
M
X N +
X C X
M
X C +
X
M T
st I stR
=
( )X
C X
M
U N
X C +
X
M
X N +3
08 EN Rev B 2007
© Copyright 2007 ABB. All rights reserved. Specifcations subject to change without notice.
Capacitor startBy storing the power required or magnetisationin capacitor banks, it is possible to start with ullstarting torque without disturbing the network.To avoid over-compensation, the capacitorbank must be uncoupled ater start-up. Thedisadvantages o this method are the high cost,and the large space requirement o the capacitorbanks.
Soft startersSot starters are based on semiconductors,which, via a power circuit and a control circuit,initially reduces the motor voltage, resulting inlower motor torque. During the starting process,the sot starter progressively increases themotor voltage so that the motor becomes strongenough to accelerate the load to rated speedwithout causing torque or current peaks. Sotstarters can also be used to control the stoppingo a process.
Sot starters are less costly than requency
converters but like requency converters, they may inject harmonic currents into the grid,disrupting other processes.
Rheostat startingRheostat starting can only be used with slip ringmotors. On these motors, the resistance o the
rotor circuits can be increased with an externalresistor. This method is usually chosen whenthe supply net is weak and the required startingtorque and moment o inertia are very high. By switching in the additional resistances in steps,normally 4 to 7 steps, the desired accelerationtorque can be obtained. The normal DOLstarting equipment also required.
Frequency converter start
Although a requency converter is designedor continuous eeding o motors, it can alsobe used or start-up only. The requency converter enables low starting current becausethe motor can produce rated torque at ratedcurrent rom zero to ull speed. As the price o requency converters continues to drop, they are increasingly being used in applicationswhere sot starters would previously have beenused. However in most cases they are still moreexpensive than sot starters, and like these, they inject harmonic currents into the network.