Soft Starters

7/17/2019 Soft Starters

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Soft Starters

A soft starter has Thyristor in its main circuit by which it adjusts motor voltage.Motors that run typical industrial applications employing conveyors, pumps, blowers, and

compressors all have one common quality: They draw a large amount of current when they start

up, perhaps six times what they draw during normal operation. This level of current can stressmotor components and cause power quality issues on plant electrical systems, particularly for

larger motors of hp or more. That is why motors of this si!e frequently employ soft starting.

The idea behind a soft start is to gradually allow the motor current to rise until the motor reachesits steady state. This reduces start"up current but also reduces start"up motor torque. #oft startersadjust motor voltage through use of bac$"to"bac$ Thyristor or triacs in each ac supply line to the

motor. The Thyristor are actuated during the start"up phase such that their turn"on is successively

delayed less for each ac half cycle. The delayed switching effectively ramps up the average acvoltage to the motor until the motor sees full line voltage. %nce the motor hits its rated speed, the

Thyristor switching circuit may be bypassed. &arger motors employ either soft starters or they

are controlled by variable"frequency drives that incorporate a soft"start function.



A soft starter can be contrasted with a full"voltage starter 'manual and magnetic(, which applies

full voltage directly to motor terminals on start up. )ull"voltage starters today generally areconfined to smaller motors where start"up current isn*t an issue.

#ome soft starters can also provide a soft"stop function in applications where an abrupt stop can

cause problems. +xamples include pumps where a quic$ stop can bring on water hammering and

conveyor belts where material can get damaged if belts stop too quic$ly. The soft"stop sequenceuses the same power semiconductors employed for soft starting.

Thyristor in a soft starter let part of the voltage through at the beginning of the starting sequenceand gradually increase it according to the set ramp time. The Thyristor can also typically

implement a soft stop by reducing motor voltage according to a set ramp time.

A special $ind of soft"start function called star"delta start is sometimes used on three"phasemotors. asically it starts the motor with stator windings connected in a wye configuration and

switches them to a delta configuration when the motor hits its normal operating speed. -ere, thestarter normally consists of a contactor for each of the three phases, an overload relay and a timer

that sets the duration of time in the star"position. The starting current is about /0 of that seenduring direct on"line start and the starting torque is about 120 of what would be available at a

direct on"line start. This starting method only wor$s when there is a light motor load during the

start. Motors that are too heavily loaded won*t have enough torque to accelerate up to speed before being switched over to the delta position.

#oft starters are generally used with induction 'asynchronous( motors. ut they also may provide

benefits powering synchronous motors. The reason is that many synchronous motors behave li$e

induction motors initially when they start up. That is, there is a lag between the rotating electricfield and the rotor position. There is a delay before the motor comes into a synchronous

condition. As in the case of induction motors, synchronous motors can also draw large stator

currents during start"up of perhaps five to eight times the full load current in the absence of softstarting.

3n both induction and synchronous motors, high stator and rotor current at start"up leads to a low

power factor, typically about /.1 lagging. 4ower factor and, thus, energy efficiency, rises as the

motor accelerates up to its running speed. 3n this regard, it should also be noted that some softstarters could serve as voltage controllers or voltage regulators for the motor. Their bac$"to"bac$

thyristors or triacs ma$e this possible because they can adjust motor voltage depending on the

load the motor sees, when paired with an appropriate controller. Thus, functioning this way, asoft starter can double as an energy manager: A controller monitors the motor power factor,



which is a function of motor loading. %n light loads, the power factor is low enough that the

controller reduces the motor voltage and, thus, the motor current.

Soft starter selection

Many of the applications to which soft starters are applied fall into general categories of pumps,

compressors, and conveyors. There are a few rules of thumb for each of these uses.

The start"up time for soft starters is user settable. The typical start"up time for most applications

ranges from 2 to / sec. The longer time periods tend to be found in pumping and compressorapplications where there is a high possibility of pressure waves building up in piping systems.

A typical soft starter reduces the motor torque and current during start"up. The star"delta starter

accomplishes the same thing but by means of switching motor windings from a star to a deltaconnection at the appropriate time.

The initial start voltage is settable as well, but most applications use a start level that is /0 of

the line level. #crew compressors and conveyors sometimes start at higher levels 'perhaps 5/0(depending on the load 6 conveyors sometimes have material on them when they start up so they

may need a higher starting torque, and screw compressors have don*t develop much pressure at

low speeds.

#oft starters typically have the same rating as the motors they power. The situation may bedifferent for start"up ramp times and initial voltage levels that are outside the norm. )or heavy"

duty uses, a common practice is to use a soft starter having a power rating one si!e bigger than

the motor and to use an overload relay when ramp times grow long7 ditto for applications withfrequent starts.



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Soft Starters