History of the SIEG FC Series Motor Control
During the course of my repair business I have been
sent older units that provide an interesting story of the evolution
of the motor controller at SIEG. Here is what I assume was the
progression of the FC series.
A Starting Point
This board was made around 1993 and I believe it to
be among the first made by SIEG.
It was a good basic design with undersized mosfets
and heat sink.
I repaired this one July 2009. The only
problem was a burned out resistor in the circuit that generates the
control voltages.
As far as I know it is still going strong.
However, it has always seen light duty. It is
used in a lathe that repairs mouth pieces for bugles and
trumpets.
Next Stage
The first design was repackaged to provide more
free area on the main PC board. The electrical design is
almost the same except for the trim pots which were reduced in
number. The control circuits were relocated on what SIEG
called the "daughter" board.
The heat sink is bigger, but the fins are not
oriented for optimum convection air flow.
Granddaughters
The "next stage" underwent some reliability
changes.
The two power resistors on the right hand side were
doubled to increase the dissipation rating and located on
"granddaughter" (my word) boards. (Only one resistor is shown
on the first board - there are two)
The fins on the heat sink were turned and larger
mosfets were used. A heat sink was added to the bridge
rectifier. (This is the only model I know of with a rectifier heat
sink - I wish they all did)
The "J" Model
Two relays were added. One connected
power + to the motor and the other connected line power from AC3 to
the system. The system was started with power from AC3 through
the switch on the speed pot to terminal 5 which turned the
system on. This forced the operator to turn the speed pot to
zero before starting.
Control terminal P4 was added. This was
configured with the "Forward/Off/Reverse switch on lathes to limit
the maximum reverse speed to about 6o% of the maximum forward speed.
Terminals 6 and 7 were added to permit
incorporation of an external interlock (the current through the two
relay coils must go from 7 to 6).
 The
"BJ" Model
The last major circuit design change was the
addition of a third relay and adding a control power transformer
with IC voltage regulators.
The relay is activated by current overload and
causes the first two relays (see above) to open thus physically
disconnecting line power and the motor.
It adds two operational amplifiers and two trim
pots which establish the overload trip point..
The new control power system replaces two separate
circuits that used zener diodes to establish the regulated control
voltages
Current Model
I believe this represents that model that is in
current production.
The major changes were:
1. The introduction of surface mount
technology for the control circuits which permitted the location of
all components on the main board and thus eliminating the "Daughter"
board.
2. The change to a wire insertion connection
block in place of the block that accepted spade lugs.
3. The elimination of the P4 connection and
thus the elimination of the reduced reverse speed option.
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