Rockwell Automation 808 SPEED SWITCHES - Manual

BULLETIN
INSTRUCTIONS
808
SPEED SWITCHES
Rear View of Speed Switch Front View of Speed Switch
Less Lockout Solenoid and Less Lockout Solenoid and
Mounting Bracket Mounting Bracket
NEMA Type 1, 4 and 13 Enclosure NEMA Type 1, 4 and 13 Enclosure
DESCRIPTION
The Bulletin 808 speed switch is a pilot device
which is used with an automatic starter arranged for reversing
or plugging duty to provide plugging orantiplugging of squirrel
cage motors. This device can also be used as a speed sensing
switch, or to indicate direction of rotation of a driven shaft.
Plugging is a control function that provides braking by
reversing the motor. The phase sequence is reversed so that
the motor develops a counter-torque that exerts a retarding
force. Antiplugging protection is the effect of a control device
that operates to prevent the application of counter-torque by
the motor until the motor speed has been reduced to an
acceptable value.
PLUGGING When applied in a suitable plugging application
such as the circuit shown in Figure 1, the switch with normally
open contacts is used. The Bulletin 808 speed switch is
designed to automatically interrupt reverse braking power as
the motor approaches zero speed. The speed at which the
contacts operate can be adjusted so as to avoid coasting or
reverse rotation of the motor. The contacts can be wired for
plugging in either or both directions.
TYPICAL PLUGGING CIRCUIT Bulletin 808 speed switch used
with a reversing starter to bring a motor to a fast stop.
NOTE:
See applicable standards, codes, and laws for all
applications.
FIGURE 1
Forward
Reverse
PRECAUTIONS IN PLUGGING APPLICATIONS
T h e r e a r e m a n y
advantages in plugging a motor to a stop, however, care must
be taken that these advantages are not lost through misapplica-
tions. The following information should be considered before
deciding to use a plugging control.
1 . When plugging a motor to a stop, full load motor current
may be greatly exceeded. The electrical supply must be
able to handle the excess load.
2. The resultant forces of a plugged stop may be too great for
the driven machine to withstand. For example, a twisted or
broken shaft may result.
.
3. The motor may not withstand braking duty. The duty cycle
and load must be considered and the motor manufacturer
should be consulted to determine if a particular motor is
adequate for plugging duty.
NOTE:
See applicable standards, codes, and laws for all
applications.
ANTIPLUGGING For this application, the switch with normally
closed contacts is used. The speed switch is designed to keep
the reverse circuit open until the speed of the driven machine
has decreased to a predetermined value. At this speed the
contacts are designed to close, permitting reverse energization
to be applied manually, or automatically when using a second
speed switch arranged for plugging duty. Mechanical braking
can also be used. The speed switch can be used for
antiplugging control in either or both directions. See Figure 2.
PRECAUTIONS IN ANTIPLUGGING APPLICATIONS
The speed switch
must be adjusted so that contact closure does not occur before
the speed of the machine is reduced to a safe level for
secondary braking.
TYPICAL ANTIPLUGGING CIRCUIT
This circuit is used in cases
where damage would result from a sudden reversal of power.
The speed switch contacts are designed to keep the reverse
circuit open until the motor has coasted to a preset speed.
NOTE:
See applicable standards, codes, and laws for all
applications.
L1 L 2
FIGURE 2
Publication 808-5.1 June, 1980
Supersedes Publication 8084.1 Dated July, 1978