L.T. control panels


FCMA Soft Starter means Flux Compensating Magnetic Amplifier

In case of motor driven pumps, while starting, the motor normally takes 6 or 7 times full load current when started Directly on Line (DOL). Such high values of current have following implications. 

a) Weakening of insulation and affecting motor life adversely.
b) Sudden jerks on incoming line, resulting into large voltage dips, which affect the other sensitive equipment operating on the same line. 
c) High size of transformer / Generator, as a backup supply system.
d) Large jerks on the pumps and associated piping etc.

To eliminate these harmful effects, Soft Starters are used which reduce the starting current considerably and allow the motor to accelerate gradually. 

The Soft starters provide gradual increase in voltage up to rated voltage ensuring even acceleration. 

Benefits of Soft Starters 

The provision of Soft Starters ensures following benefits.

a) Smooth starting by gradual acceleration of the drive system, thus preventing jerks and extending life of system components.
b) Reduction in starting current preventing mechanical, electrical & thermal weakening of electrical equipment such as motors, cables, transformers and switchgears.
c) Enhancement of motor starting duty by reducing temperature rise in stator winding and supply transformer, (subject to thermal curves).
d) Increase in Circuit breaker as well as contactor life.

To summarize, Soft Starters can optimize the starting performance of motors and motor manufacture can concentrate on improving running performance to ensure lower life cycle costing. 

Types of Soft Starters 

Various reduced voltage starting options have been available. The prominent among them are Star delta, Auto transformer, Electronics (Thyristors) and Flux Compensated Magnetic Amplifiers (FCMA) for LT motors. For HT motors predominantly FCMA Soft Starters are employed.

Salient Features of FCMA Soft Starter :

Advantages :

Neutral Side Connections :

The reactor is of multiple tapings. It has an adjustable air gap to vary the permissibility. The coil is wound on CRNGO core with 3 limbs with terminals bought out for connections.

The core will not saturate during the starting of the motor. The coil has an high X/R ratio, which will ensure less power consumption and have less heating of the coil.


In the enclosed circuit the core reactor connected in series with motor winding by RYB respectively as shown in reactor drawing the winding constructed in three individual core limbs with separate top core. In any reactor circuit it appear the current flowing through it, because of impedance of its own (impedance = (resistance)2 + (reactive)2.

In working condition of the above system the applied voltage (V) leads the current by an angle / i.e. current lags behind the applied voltage by an angle 900 when current flows through the inductor the motor will get start with reduced starting current and the sense will get accelerated due to frequency induced in the rotor as same as the line voltage applied to the winding. With this result the starting current will slowly reduce directly proportion to the speed. Because decreasing in current IR drop across reactor will also reduce. Naturally the reactor will allow flow of applied voltage to the motor side. Due to increase of voltage at winding terminal rotor will develop more rotating flux. Because of this, result the motor will reach 100% speed within 10 – 15 Sec, then the reactor will short circuited by the contactor will result in 100% line voltage flow to the motor.

In reactor air gap will be maintained to increase or decrease the reactor, with tap and at winding to split the starting torque.

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