FIELD OF THE INVENTION:
The present invention relates to the development of a Static Excitation Equipment for a
Synchronous motor with Flux Compensating Magnetic Amplifier (FCMA) based starting.
BACKGROUND OF THE INVENTION:
Static Excitation Equipment for a synchronous motor with various starting methods like
Direct Online starting, Reduced voltage starting, Variable voltage and variable frequency
using a Static frequency converter are already available in operation at many places.
Static excitation equipment which is being used for synchronous motor is modified
(both hardware and software) to meet the requirements of Flux Compensating Magnetic
Amplifier (FCMA) starting method.
The present invention proposes development of Static Excitation Equipment for
Synchronous motor with Flux Compensating Magnetic Amplifier (FCMA) based starting.
OBJECTS OF THE INVENTION:
It is therefore an object of the invention is to develop a Static Excitation Equipment to
meet the excitation requirements of Synchronous motor with Flux Compensating
Magnetic Amplifier (FCMA) based starting.

Further object of the invention is to develop a Static Excitation Equipment which is
more reliable.
Another object of the invention is to develop a simple device for static excitation of
synchronous machine.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 describes Flux Compensating Magnetic Amplifier (FCMA) softstarter line side and
neutral side.
Fig. 2 illustrates block diagram of Static Excitation Equipment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF
THE INVENTION:
FCMA soft starters work on the principle of impedance control by super-imposing two-
phase opposed alternating fluxes on a common magnetic core. Such controlled
impedance, when connected in series with the motor, provides a constant current
incremental voltage to the motor resulting in incremental torque as the motor speed
increases. In simple FCMA consists of two windings wound on a common magnetic
core. The first winding is called the main winding is connected in series with the motor
windings and carries the main motor current. The second winding is called the
compensating winding is wound with a polarity opposite to main winding. This winding
is excited with the counter EMF generated by the motor. The core is subjected to two

simultaneous sinusoidal fluxes opposing each other due to the MMF created by the
main and compensating winding. As both the fluxes are sinusoidal the resultant flux in
the core is also sinusoidal. As the motor speed increases the compensating flux
increases, thus reducing the net flux in the core. The impedance of the main winding
hence decreases with motor speed, to keep the motor current constant and increment
the motor voltage. Thus the effective motor voltage increases from a low value at start,
to nearly full value as the motor reaches full speed. As the motor accelerates to full
speed the RUN MODE (Bypass) breaker closes and bypasses the FCMA winding. The
FCMA core is always subjected to alternating fluxes and works in linear zone, thus
ensuring that the voltage and current waveforms are purely sinusoidal in nature and are
harmonic free and thereby improves the supply power quality. Reduced starting current
will limit the dip in supply voltage due to motor starting when compared to DOL
starting. The FCMA impedance varies in stepless manner resulting in a stepless
increment voltage.
In addition to the normal field winding, synchronous motor consists of an additional
winding called damper winding. This winding consists of copper bars placed in the slots
in the pole faces and are short circuited with the help of end rings. Damper winding
acts as a squirrel cage rotor winding of an induction motor. Parallely, the rotor winding
is also shorted through a contactor during starting in the Static Excitation Equipment.
Once the motor is switched ON with the 3-phase AC supply, the motor starts rotating as
an induction motor at sub-synchronous speed. DC supply is given to the field winding

when the motor attains a speed around 98% of rated synchronous speed. At this speed
as the DC voltage is given to the field, motor gets pulled into synchronism and starts
rotating at synchronous speed.
The block diagram of the static excitation equipment for synchronous motor with FCMA
based starting is shown in fig.2. On selection of FCMA mode and start command, the
synchronous motor field circuit will be shorted via breaker ‘Q34’, to allow the machine
to start as induction motor. After a fixed time interval (15seconds) the field shorting
breaker is opened and another breaker ‘Q35’ is closed, which connects the discharge
resistor across field circuit via discharge pole of the field breaker. The slip in the motor
speed is derived from the frequency of the rotor voltage sensed across the discharge
resistor. The sensed voltage is provided to slip frequency relay which is calibrated to
provide a closing command to the field breaker when the slip frequency goes below 2
Hz. As the slip frequency relay give a command to the field breaker, the discharge
contact will open prior to the closing of main contacts of the breaker. To avoid rotor
over voltage an auxiliary breaker ‘Q37’ is kept closed before closing the field breaker i.e.
before opening of the discharge contacts and opened after closing of the main contacts
of the field breaker. After the breaker is closed the field forcing is done and ceiling
voltage is applied to field from the SEE to make the rotor lock with the stator and
continue to run as synchronous motor. Bypass/Run mode breaker is closed and Motor is
switched on to AUTO Channel.

WE CLAIM:
1) A static field excitation equipment (200) for a synchronous machine (300) with
field winding (8) including flux compensating magnetic amplifier (100)
comprising:
a) Excitation transformer (6),
b) Thyristor bridge (7),
c) Field circuit breaker (Q1),
d) Auxiliary breakers (Q 34, Q 35, Q 37),
e) Rotor over voltage protector relay (OVP),
f) Discharge resistor (R),
g) Slip frequency relay (to incorporate in the Fig. if possible),
characterized by sequential closing/opening of the circuit breakers to provide
controlled power in steps to the field winding to bring the motor in to
synchronism.
2) The static excitation equipment as claimed in claim 1, wherein the flux
compensating magnetic amplifier (100), characterized by compensating winding
(4) comprising, two windings, first to carry the main motor current and the
second wound with opposite polarity to generate counter emf and flux opposing
the main flux increasing with the speed.
3) The excitation transformer (6) as claimed in claim 1 providing power source for
the field winding.

4) Thyristor bridge (7) as claimed in claim 1 to rectify AC power and providing DC
power to the field winding.
5) The auxiliary breaker (Q34) as claimed in claim 1, to short the field circuit
allowing to run the machine (300) as induction motor for a short time.
6) The auxiliary breaker (Q35) as claimed in claim 1, to close and connect discharge
resistor (R) across field circuit, having breaker (Q34) opened simultaneously.