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Danfoss VLT® Soft Starter - Applications like pumps, conveyers, centrifuges and bandsaws mustbe started slowly, and sometimes stopped slowly, to prevent mechanical shocks such as water

hammer, and strains on bands, couplings and shafts

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applications

Soft Starter In Industrial Applications

Introduction to Soft Starter

An induction motor connected directly to the power supply draws a constant magnetizing current at fullspeed, no matter what the load is. At anything less than full load, power factor of the induction motor isless than its rated power factor because the torque producing current decreases as the load

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Inductive impedance in series with the motor windings

decreases, but the magnetizing current does not change. The lighter the load, worse the power factor.

Motors selected from a standard range are almost always chosen with a rated power in excess of maximum load demand, with the result that in any installation the motors seldom operate at their fullrated load. Consequently, they can never achieve their rated power factor even at a maximum loaddemand. More over, if the load is variable.

The wastage of energy is worse Electrical energy supply tariffs for industrial users almost invariablycarry a heavy penalty –i.e.; higher cost per unit- for consumption at low power factors. Further, the costcharged is increased if demand exceeds a maximum limit.

The AC induction motor with a permanently coupled load, when it is operated without the benefit of asoft starter draws a high starting current. Typically, more than 7 times it’s rated full load current.“Reduced voltage-starting decreases the demand”. A Soft starter makes use of this principle.

The mechanical shock delivered to the rotor or to couplings or to any intermediate gearing and thedriven load by a high starting current is most severe when the motor is started directon-line. Evenreduced voltage starting devices like the star delta starter or Auto transformer starters still imposeshock loads because of the very current peaks which cause severe transient torques at the moments ofintermediate switching.

The effect of such repeated shocks is to decrease the life of the motor and increase the cost of maintenance.

Soft Starter Construction

The Flux compensated magnetic amplifier (FCMA) Soft starter essentially consists of predominantlyinductive impedance in series with the motor windings to control the motor current through a potentialdivider.

Finally when the motor has reached full speedand the motor terminalvoltage has reachedaround 90-96% ashorting device cuts theMag Amps out of themotor circuit and themotor runs under

normal operatingconditions.

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Inductive impedance in series with the motor windings with shorting device

Constructionally theFlux compensatedmagnetic amplifier isan extremelyrugged devicecomprising of magneticcore and amply sizedpower windings. It

should be emphasizedthat there is no externalcontrol loop on thesystem and impedancevariation is the naturalcharacteristic of thesystem achieved through suitable winding design and geometry.

This leads to 100% reliability of the operation.

Working principle

Conventionally the magnetic amplifiers have been equated with saturable core reactors, which work onthe principle of superimposition of magnetic fluxes in the additive mode thereby saturating themagnetic core & varying the impedance .These have found limited usage in alternating current circuitsbecause they require D.C. excitation & generate a heavy percentage of harmonics in the currentwaveform due to core saturation.

The Flux compensated magnetic amplifier (FCMA) is an antithesis of the saturable core reactor. TheFCMA works on the principle of flux opposition instead of flux addition & hence operate always in thelinear nonsaturable zone of the magnetic circuit. Oppposing flux linkages are impressed on the same

magnetic circuit, thus reducing total flux & hence reducing the impedance.

Total flux linkage = main flux linkage – opposition flux linkage

N I = N1*I1 – N2*I2

The system is designed to provide a predetermined constant current in the motor windings in thestarting zone. The main flux is generated by the current in the motor winding and the opposing flux isgenerated by the counter emf of the motor as the motor speed increases. The net flux thus reduces,allowing the motor voltage to rise gradually form as low as 50% to 96% while keeping the current withinthe prefixed limit, thus efforting a smooth start. In fact the reduced start current allows closer calibration

of the protective relays.

The FCMA Soft starter achieves this through a unique system of flux compensated non saturatedmagnetic amplifier for controlling the motor current and hence the motor torque during acceleration.

Main features of Soft Starter

These are the main features and benefits of soft starter:

1. Fastest payback period


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Soft starter operating stages

2. Avoids fluid hammer

3. Easy to install

4. In built protection

5. Designed for extreme operating conditions

6. Programmable current and torque limits

7. Programmable acceleration and deceleration ramps

8. Two set point current limits

9. Instantaneous protections

The soft starting sequence

A Soft starter normally operates in three distinct stages:

Ramp

Dwell period

Energy Optimizing

These operating stages are as shown in figure below:

TheRampstage

A. For Openloop

Configuration

From the instant that the start command is given, the system first imposes a fixed delay (3 to 300/500ms) for self monitoring, and then causes a voltage to be applied to the motor terminals to achieve thebreak-away torque.


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Pedestal voltage; Top: Long ramp; Bottom: Short ramp

This is the pedestal voltage in the figure below. This voltage is then ramped up linearly providing a stepless increase of motor terminal voltage from the pedestal upto the maximum voltage. At the end of theramp period the motor terminal voltage is equal to the input power supply voltage. As a standard thepedestal voltage is 40% of the supply voltage. This can however be changed to suit desiredapplications.

The current drawn by any motor accelerating from the rest position in an open loop configurationdepends upon the selected ramp. Shorter the ramp time, greater the peak starting current. Why choose

one ramp time rather than another?

For everymotor anddrivenload

system, there is a natural ramp at which the acceleration of the load and the motor matches the rate of increase of the output voltage of the soft starter. Selecting a short time will increase the rate of acceleration and the starting current drawn; a longer time will prolong the time before the load reachesthe full speed, and will reduce the starting current.

This may be an advantage depending upon the particular application and perhaps on whatis convenient for matching with related process equipment.


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B. Closed loop Configuration

This configuration of the Soft starter is used in applications where the driven load has a particularly highinertia else it is difficult to start. For example, in a stir application where the medium is glutinous andstiff at the beginning of the process.

The acceleration time of such loads is apt to lag behind the rate of rate increase of ramp voltage even ia long ramp time is chosen. With such loads it may however be necessary to select short ramp time toensure sufficient starting torque. A short ramp will raise the voltage quickly but the nature of the loadprevents the motor speed matching it. The result may be excess current demand.

This problem is over come by a current limit feature which stops the ramp when the current outputreaches a pre-selected limit. The ramp is then held until the current falls naturally. The current limitingfeature is active only during the ramp period of the soft start sequence. Closed loop control requirescontinuous measurement and feed back of the current delivered to the motor terminals by using a CT.

The Dwell period

The dwell period begins when the output voltage reaches maximum and this period is equal to theselected ramp time, approximately equal to 10 seconds.

The dwell period maintains a constant voltage at maximum to allow the time for the motor and its drivenload to settle to a steady state after acceleration is complete and before the Soft starter automaticallyenters the energy optimizing mode.

The optimizing stage

The energy optimizing stage is the normal operating condition of the Soft starter when the motor is upto rated speed and driving the load at whatever torque output is demanded. The energy optimizingstage is continuous after the dwell period until a stop command is given.

During the ramp period the system computes a reference value of the power factor. For energyoptimizing this value is continuously compared with the running power factor. From the comparator output the system continuously computes, adjusts and updates the firing points of the thryistors so thatthe total energy delivered to motor corresponds to the load torque demand without wasting energy inoverfluxing the motor. The power factor at the supply terminals is in this way maintained at the highest

possible value for every condition of the load demand.

Management of the power factor in no way detracts from the capability of the motor to respond to theload demand .The motor performance is unaffected. This feature of the Soft starter is a purely electricfunction which has the effect of ensuring that at all times the motor delivers the torque demanded, but isallowed to draw only the precise amount of magnetizing current required to support the torque output.

Without this feature, the motor would draw a maximum magnetizing current regardless of the load.

The effect of the energy optimizing function is a cumulative economy in energy consumption.

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The Soft-Stop function

This is a unique feature of the Soft starter. Its effect is to prevent an undesirably sudden deceleration of the load. It is useful in various mechanical handling and conveyor systems and in many hydraulicpumping operations where a sudden removal of the drive input can induce undesirable effects, such asthe fluid hammer in the pipelines.

The soft stop ramp operates to reduce the motor terminal voltage down to 40% of maximum, at which

time, the thyristors are disabled. The motor and load will coast to standstill.

Comparison: Soft Starter and Auto Transformer Starter

A. Operational differences

Soft Starter Auto Transformer Starter

Easy starting-by a push button. Similar method of starting.

Motor starts without any jerks i.e. smoothly andgradually picks up to the synchronous speed. Thelife of the motor is thus increased significantly andthe maintenance problems are drastically reduced.

Motor is subjected to heavy jerksduring starting and a bit later because of the sudden inrush of current .This causessevere damages to the motor windingwhich ultimately burn off.

The valve on the delivery side need not

be operated while switching the motor ON or OFF.

This is mandatory on both occasions i.e.

ON or OFF.

Soft starting and soft stopping avoids back thruston the NRV or the pipe line which is thereforeprotected against the back thrust during switchingOFF.

Such a protection is not providedand therefore, proper operation of thevalves is absolutely essential.

No electrical moving contacts and no oil hence nosparking and hazards.

Circuitry consists of several movingparts and hence sparking is unavoidable.

No limitations on the No. of starts and No. of stopsper hour.

Limited No of starts and stops per hour because of: motor windings gettingheated.

Acceleration time for motor to speed up from restto rated full speed can be adjustable from 2 secsto 90 secs. This will facilitate selection of appropriate ramp times to suit thedesired load/torque requirements.

Such a facility is not availablewith conventional timers used in thismethod of starting.


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In rush (starting) current restricted to 200% of ratedfull load current (FLC).

Inrush current restricted to maximumof 400% of the FLC.

B. Protections


Phase reversal, single phasing, overload, over voltage, under voltage and short circuit are available.

Phase reversal and shortcircuit protections notavailable.

Current limit facility is available which limits the current drawnby the motor plus the load to a desired safe value.

This facility is not available.

C. Economics


Approximately Rs. 15000/- to Rs. 50000/- per

annum, per equipment is saved by virtue of energysaving. The exact amount of energy saved dependsupon the loading conditions.

Energy saving not achieved.

By using this the power factor can be improved by.05 which will cost further saving and will also avoidKEB penalties.

Provision for power factor improvementnot available.

Reduction of maintenance costs on account of:

- No necessity of transformer oil.- No moving parts present (like contactors, relaysetc.).- Motor fully protected and hence no rewinding costs.

This method of starting is prone toall these problems occurring very

frequently and hence involvestremendous expenses on maintenance.

Minimum down time. Down time too large.

Advantages of Soft Starter


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Soft starters are used on high tension motors for the following advantageous features:

1. Smooth starting by torque control for gradual acceleration of the drive system thuspreventing jerks and extending the life of mechanical components.

2. Reduction in starting current to achieve break-away, and to hold back the currentduring acceleration, to prevent mechanical, electrical, thermal weakening of the electricalequipment such as motors, cables, transformers & switch gear.

3. Enhancement of motor starting duty by reducing the temperature rise in stator windingsand supply transformer.

4. The microprocessor version of the Soft starter has a soft ware controlled response at fullspeed which economizes energy, what ever may be the load. Because of the tendency to over specify the motor rated power, this feature has benefits for most installations- not only thosewhere load is variable.

5. The power factor improvement is a self monitoring in built feature. When the motor is runningat less than full load, the comparative reactive component of current drawn by the motor is unnecessarily high due to magnetizing and associated losses. Hence the voltage dependentlosses are minimized with the load proportional active current component and as a result the

power factor also improves simultaneously.

Areas of Soft Starter application

These are main applications of Soft starter:

Steel industries (Rolling mills and processing lines)

Cement industries

Sugar plants

Paper and pulp

Rubber and plastic

Textile industries

Machine tool applications

Power sector

Water supply scheme

And various process control applications…

Conclusion

The starting performance of the squirrel cage induction motors using Soft starters provides valuableeconomics of electrical energy .Optimum benefits are gained when a motor duty involves frequent startor stop cycles but is still likely to be worth while in systems which are in continuous operation.

The word Soft starter has almost become synonymous to conventional starters.

Reference: Significance of soft starter in industrial applications by N.Mahesh and P.Vishnu (Aurora


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Technological and Researc Institute)

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