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Flux Compensated Magnetic Amplifier Motor Starter

magdy El Kady

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We would like to use a reduced voltage starter for a centrifugal chiller motor, which is induction type, 3-phase, 11 kV, 60 Hz, 1850 HP. We received an offer to provide a “Flux Compensated Magnetic Amplifier” Soft Starter.


According to the available data, the starter consists of a coil connected in series with the motor winding so that as the motor speed increases, the impedance of the coil decrease providing smooth, stepless starting of the motor, then a bypass contactor closes to connect the full voltage to the motor.


This is the first time to know about this product. Does anybody have information/experience/recommendation about such a product?

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Hello magdy El Kady


I believe that this will be a varient on the good old primary reactance starter. An inductor is placed in series with the supply to each phase of the motor providing a series impedance and therefore an voltage reduction to the motor. Once the motor is at full speed, the reactor is shorted by a bridging contactor providing full voltage to the motor.

The value of the inductor can be altered by varying the flux in the iron, or by applying DC flux to the iron. This can be easily done by haveing a second winding on the core with an SCR across that winding and the SCR phase controlled to alter the DC bias. Increase the DC bias to increase the core saturation and reduce the inductance.


Best regards,

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Hereinafter is further information received from the manufacturer regarding the flux compensated magnetic amplifier (FCMA) soft starter. FCMA works on the principle of unsaturated core in the working zone, accordingly it does not generate harmonics, which is an advantage compared to the SCR soft starters.


FCMA is connected in series with the motor to provide stepless reduced voltage starter to ensure a constant low starting current with incremental voltage and torque to the motor to achieve smooth acceleration of the drive.


FCMA consists of main winding and compensating winding connected in phase opposition, so that the net flux in the core is difference between the main winding flux and the compensating winding flux.


The line current is limited and the motor voltage is reduced due to the impedance of FCMA.


As the motor speed increases, the compensating flux increases due to the counter emf feedback keeping the motor current constant, thus reducing the net flux and hence the impedance of the FCMA.


Due to the smooth decrease in FCMA impedance, the voltage across the motor increases in a gradual speed dependant ramp from the starting value (50-60%) to run value (94-98%). The increasing in motor voltage increases the motor torque to meet the load demand.


Then a bypass contactor closes to ensure full voltage running of the motor.

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I recieved initial qoutations from some local suppliers (not the original manufacturers) for the solid state soft starters based on their available data for similar existing projects. I am surpirised that the price of the 11 kV soft starter is about 3 times the price of 7.2 kV starter and about 4 times the price of 11 kV autotransformer starter.It seems to me that the manufacture of 11 kV SCRs is difficult or requires higher technology.


Could anybody advise in this regard? Do you think that these prices are relatively old and the price gap becomes less?

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  • 1 year later...

Dear Sir,


I want FCMA starter details for 750HP Fibirisor Motor used in sugal mills

Also, 350HP for Baggase carrier

100HP for cane cutter


Please send / suggest details along with BOQ




HS Prakash


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The main reason for the difference in price between 7.2kV solid state starters and 11kV solid state starters is the cost of the switchgear that is associated with it. The SCRs are all the same, just more in series so that means we must add additional structures to accomodate them.


The PIV (peak Inverse Voltage) rating of SCR systems must be at least 2.5 times the line voltage. For 7.2kV, we must exceed 18kPIV so we use 3 SCRs in series, each one rated 6.5kPIV, for a series rating of 19.5kPIV on each 1/2 phase. For 11kV systems, we must exceed 27.5kPIV, so we must use 5 SCRs in series for a total of 32.5kPIV. That is 5 SCRs on each 1/2 phase, so 10 SCRs per phase, totalling 30 SCRs, with associated firing circuits, isolation, heat sinks and room for the cables in all of that. It means that more steel switchgear structure is required to hold all the parts than at 7.2kV.


TThere is a change in voltage "class" in that switchgear as well. 7.2kV is usually the same design and cost as 5kV, but 11kV jumps to being 15kV class switchgear. The switches, vacuum contactors, CTs, PTs and steel structures are all typically tripple the cost or more.

"He's not dead, he's just pinin' for the fjords!"
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