http://stearns.rexnord.com/pdf/Cat_902/18-19.pdf
Brief Operating Description: Longer Motor Life Starts with a Switch
1
For over 75 years, single-phase
motors have utilized a mechanical
centrifugal switch to switch the start
circuit. Inherent characteristics of a
mechanical device have made these
switches prone to various problems,
including tolerances, tolerance buildups,
mechanical fatigue, vibration
and a host of others that can lead to
switch failures and/or performance
inconsistency.
Our challenge was to design a
reliable solid-state switch to replace
the mechanical switch and actuator
mechanism that would duplicate
the function of connecting and
disconnecting the start circuit at
particular speeds with the additional
benefits of a solid-state device. After
considerable research, we decided a
successful electronic motor starting
switch could be created by sensing the
voltages present in the main and start
windings.
Until the rotor of a single-phase motor
begins to rotate, there is no coupling
between its start winding and main
winding. When the rotor begins to turn,
the main winding induces flux in the
rotor, which then induces a voltage
in the start winding. The voltage
induced in the start winding is directly
proportional to motor speed.
In Stearns SINPAC Electronic
Switches, the voltage across a motor’s
main winding and the voltage across
its start winding are sampled and fed
to a comparator. The logic circuitry is
designed so that the electronic switch
interrupts the start circuit current after
the motor has accelerated to the speed
at which cut out voltage is developed,
generally 75 to 80% of synchronous
motor speed. The logic circuitry then
shuts down the switch’s power stage,
which consists of a triac or inverse
parallel SCR’s. This function is referred
to as “cut out.” When the start circuit
is disconnected, the main winding
field then drives the motor’s rotor to its
running speed.
If the motor encounters an overload,
and the motor speed falls to
approximately 50% of its synchronous
speed, the SINPAC Switch
automatically reconnects the motor’s
start circuit. This function is referred
to as “cut in.” Cut in detection circuitry
constantly monitors start winding
voltage. When the motor’s speed falls
to the cut in point, the detection circuit
causes the control logic to energize
the SINPAC Switch’s power output
stage. The motor then goes through its
normal startup procedure, with the start
circuit being switched out at a motor
speed approximately 75 to 80% of
synchronous speed.
SINPAC Switches are potted
and completely sealed, making it
impervious to dust, dirt and moisture.
The unique speed sensing circuit
provides a universal design which
allows a few switches to work on most
standard motor designs regardless of
manufacturer.
Acceptance by Motor
Manufacturers
US and foreign motor manufacturers
have tested and retested the SINPAC
Switch for reliability and quality. Today,
many of these manufacturers have
begun installing SINPAC Switches
on their standard motor lines with
more companies ready to make the
changeover.
UL Recognition
Many SINPAC Switches have already
been recognized under the Component
Program of Underwriters Laboratories,
Inc. (E-71115). In addition, all switches
have internal surge protection which
is tested according to IEEE C62.41 –
1991 Category A3.
CSA Certification LR-6254.