Safe Torque Off (Sto); Safe Stop 1 Time Controlled (Ss1-T) - WEG CFW500 Series Safety Manual

Safety

1.2.1 Safe Torque Off (STO)

The STO safety function acts on the removal of torque from the motor, i.e., no power that can
generate torque is present on the motor after enabling this function. The difference between
STO and other ways to disable the motor is that the former does it with very high levels of
reliability against unexpected motor starts, even under a fault condition - refer to Section 1.3
SAFETY DATA on page 8.
Once activated, the STO safety function removes the power from the frequency inverter's
output electronic circuit, causing the motor to coast to a stop - see Figure 1.1 on page 7.
Motor speed
STO
Time
Figure 1.1: STO behavior

1.2.2 Safe Stop 1 Time Controlled (SS1-t)

Differently from the STO safety function, which, when active, must promptly ensure that there
is no torque-generating energy at the frequency inverter output, the SS1-t safety function,
when active, will wait a predefined delay time before performing the STO, in order to allow the
frequency inverter to impose a deceleration ramp to the motor before removing the torque-
generating energy. The SS1-t is especially useful in applications where inertial loads need to
be decelerated before torque is removed from the motor.
Figure 1.2 on page 7 illustrates the behavior of the SS1-t safety function, where it is possible
to observe the controlled delay time between the green bars in the figure. The safety functions
module should be configured with a time value equal to or greater than the frequency inverter's
capacity to decelerate the load; otherwise, the torque-off state may be reached before the
complete stop of the load.
Motor speed
SS1-t STO
Time
P0108
Figure 1.2: SS1-t behavior
CFW500 | 7