MOTOR CONTROLSoft-Start Control of ElectricMotors

ISSUE 1 – JANUARY 2007

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26 MOTOR CONTROL www.semikron.com

Issue 1 2007 Power Electronics Europe

Soft-Start Control of ElectricMotorsFor a three-phase induction motor direct on-line start involves a very high motor starting torque and a veryhigh starting current. Semiconductors in soft-start devices must be extremely robust to resist considerablechip temperature changes and must demonstrate very good load cycle capability. The antiparallel thyristormodule SEMISTART for soft-starters provides half the internal thermal impedance of conventionalcomponents, thanks to double-sided thyristor chip cooling and thus, ensures high over-current capability forthe starting period. Norbert Schäfer and Ralf Herrmann, SEMIKRON, Nuremberg, Germany

The anti-parallel thyristor moduleSEMiSTART (Figure 1), designedspecifically for use in soft-start devices,provides half the internal thermalimpedance of conventional componentsin modular designs, thanks to double-sided thyristor chip cooling. This compactmodule also uses proven pressure contacttechnology.

In practice, three different types ofmotor starter control are used which aredescribed in the following.

Direct on-line startersFor a three-phase induction motor

(asynchronous motor), direct on-line startinvolves a very high motor starting torqueand a very high starting current. The highmotor starting torque can lead tomechanical damage; for instance, theconveyor belt driven by the three-phaseinduction motor may tear. The highstarting current can also result in voltagespikes in the grid. The larger the motor,the more serious the effects.

To combat such undesired effects, thevoltage applied to the induction motorduring the start-up phase is controlled.This means that the starting current and,consequently, the starting torque can belimited (see Figure 2).

Star-delta startersA simple solution is the star-delta starter

(also known as the wye-delta starter). Here, the motor stator windings areconnected in star (or wye) connection asthe motor accelerates up to its runningspeed; once the motor reaches near ratedspeed, the windings are connected indelta. The effect of starting in starconnection is that the voltage across eachstator winding during the build-up tonormal running speed is 1/�3 of thenormal. The changeover from star to deltaconnection is normally done using amechanical contactor. As, however, there

are only 2 switch connections (star anddelta), ‘controlling’ is not a particularlyappropriate term in this case. Moreover,this type of starter ‘control’ is not low-maintenance, as the mechanicalcontactors are prone to wear caused by

sparking and need to be replaced.

Soft-startersTo control the voltage applied to the

induction motor during the start-up phase,a soft-start device (soft starter), is needed.

Figure 1: The anti-parallelthyristor module SEMiSTARTis designed specifically for usein soft-start devices

Figure 2:Motor currentat direct on-line startand with softstart

In soft starters, thyristors are used forvoltage control (see Figure 3).

Two anti-parallel thyristors areconnected in series between the motorwindings and the grid. During the run-upto normal running speed (ramp-up), thevoltage across the motor windings is thencontrolled by way of phase control.Depending on when the thyristors arefired (trigger delay angle a), this meansthat the starting torque and the startingcurrent can be set to desired values. Afurther advantage of soft-start control isthat the starting time can also becontrolled.

The current flowing through thethyristors produces power dissipation inthe semiconductors. This powerdissipation heats up the semiconductors,which then have to be cooled. To preventfurther power dissipation in thesemiconductors after the ramp-up phase,the semiconductors are bypassed by amechanical switch (mechanicalcontactor). This bypass switch can berelatively small since it does not have toswitch large loads. A further plus is thatthe contacts of the bypass switch do not‘burn’ down. As the system has alreadyreached normal running speed, no largevoltage drop that has to be switched bythe contacts of the bypass switch occurs.The only voltage drop is that resultingfrom the mechanical design and thatacross the fired thyristors. This meansthat no large loads are being switched,which is why soft-starters are low-maintenance devices.

Semiconductor requirementsTo ensure that a soft-starter is both

compact and cost-efficient withoutcompromising reliability, thesemiconductors used in a soft-startermust meet a number of importantrequirements.

Even when a soft-starter is used thestarting current during the startingphase of a drive system is still severaltimes larger than the rated current (3-5

times higher). In large-scale systems,the peak starting current is oftenseveral thousand amperes. Thesemiconductors used therefore have tobe able to carry this high startingcurrent during the start phase. At thesame time, however, the soft-startermust be cost-optimised and as compactas possible. For this reason, thesemiconductors used (includingheatsink) must be as small as possible.

Thus, for reasons of cost, thyristorcomponents whose rated current is farlower than the large system startingcurrent are used in practice. This is whythe thyristor chips heat up substantiallyduring the brief start phase, e.g. from TStart

= 40°C to TRamp-up = 130°C, resulting in achip temperature difference of 90K. If asystem is switched on 3 times per hour,8 hours a day on 365 days a year, thetotal number of load changes after 10years is 87,600. These thyristors must beable to carry the overload current thatoccurs during the start phase fordecades.

Up till now, manufacturers of soft-starters have had difficulties in findingthe optimum semiconductors for theirdevices on the market. This is wherethe anti-parallel thyristor moduleSEMiSTART steps in, as this module wasdeveloped specifically for use in soft-start devices.

Mounting and connecting technology There are a number of different ways

of assembling and connecting a silicon

chip. In many modules, the silicon chip issoldered on both sides (anode andcathode side) with single-sided modulecooling (see Figure 4).

The heat that builds up in the moduleis dissipated to the heatsink via thebaseplate (single-sided cooling). Aparticular problem here is the differentthermal expansion coefficients of theindividual components used in a thyristormodule. In modules with solderedconnections, the thyristor chip, solderand copper (main terminals) havedifferent expansion coefficients.

Over time, these different coefficientslead to fatigue in the solder thatconnects the chip and the copperterminal due to load cycle operation. Asa result, delamination of the solder layeroccurs, i.e. fine hairline cracks appear inthe solder layer. The solder fatiguecracking then results in an increase inthermal impedance which, in turn, leadsto an increase in chip temperature and,ultimately chip failure. In fact, it is notunusual for chip failure to occur insoldered modules.

In modules based on pressure contacttechnology, by way of contrast, the chipis connected between the mainterminals by contact pressure. In thesemodules, the chip is not solderedbetween the main terminals. Instead,very high contact pressure (several kN)is applied to ‘retain’ the chip betweenthe main terminals. In practice it hasbeen shown that, especially inapplications with large power loads

28 MOTOR CONTROL www.semikron.com

Issue 1 2007 Power Electronics Europe

Figure 3: Principle schematic of a soft starter

Figure 4: Principlebehind the soldercontact module

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(rated currents >200A), the load cyclecapability of the components connectedusing pressure contact technology is farsuperior owing to the non-use ofsoldered connections.

This is why Semikron recommendsusing pressure-contacted components insoft-start devices with larger ratedcurrents. And it is this very pressurecontact technology that is used inSEMiSTART (see Figure 5).

In SEMiSTART modules the two thyristorchips are ‘pressed’ between twoheatsinks.

This type of mounting and connectiondoes not contain solder layers, which is

why the SEMiSTART modules boast verygood load cycle capability and,consequently, a long service life.

The heatsinks are optimallydimensioned for the chip dimensions andfor use in soft-start devices. The result isvery compact modules. The total thermalresistance between the thyristor chips andheatsink is far lower than that of otherconventional components. As the chipsare pressed directly between twoheatsinks and are cooled on both sides,the thermal resistances are very low.Another advantage is that very littlemounting effort is necessary forSEMiSTART modules: no special clamps

are needed as is the case whenassembling capsule thyristors. Plus, nothermal paste is needed as in the case ofmodule assembly.

SEMiSTART modules can, of course,also be used in other applications, e.g.protective circuits. SEMiSTART modulescome in three different sizes and a totalof 5 different current classes. Thecurrent range is 500A – 3000A for amaximum current flow time (ramp-uptime) of 20 seconds. The thyristorshave a maximum off-state voltage of1800V.

ConclusionThe market for soft-starters will

continue to grow over the coming yearsas the advantages that thesecomponents boast over conventionalsolutions become more apparent. Theantiparallel thyristor module SEMISTARTfor soft-starters provides half theinternal thermal impedance ofconventional components thanks todouble-sided thyristor chip cooling andits compact design. Extremely highovercurrents are therefore possible for ashort period. What’s more, thanks to theuse of pressure contact technology,these modules offer a high degree ofreliability.

www.semikron.com MOTOR CONTROL 29

Figure 5: Pressurecontact technologyused in theSEMiSTART module