116
Installation and user guide
Rev: 4.0
NTT 240/460
S.T.O. safety circuit
Ch. 7 S.T.O. safety circuit
S.T.O. safety circuit (Safe Torque Off) is realized with IEC EN 61800-5-2 law reference conformal, 0 stop category, and with
IEC61508 law reference for SIL3.
The a.c. induction motor requires a rotating magnetic field to produce torque. This requires a threephase source of
alternating current at the connections. The drive has a single internal d.c. power supply available which is converted to
a.c. by a continuous active switching action of the six power semiconductor devices.
The Safe Torque-Off (STO) feature of NTT 240/460 drive is made of a redundant electrical circuit designed to bring a
drive to a state of safe torque absence. It is a feature used to prevent unexpected motor rotation in case of emergency
without the necessity to interrupt power supply. When STO function is active, the servodrive and the motor are in a
state of functional safety, which means that is impossible to cause an active rotation of motor shaft or, if it is alredy
rotating, it stops by inertia.
The safety stop category 0 is achieved with the immediate disconnection of electronic components (IGBT) capable of
system energization, that cause an uncontrolled stop of the axis, by inertia.
It is usual, in the applications where there isn’t a drive equipped with STO, to secure the system interrupting the power
supply using a contactor of adequate capacity. Using a STO it is possible to eliminate the contactor allowing economical
advantage, space saving in the cabinet and achieving an higher level of integrity.
Principle diagram for drive without STO safety circuit is shown below:
Hardware enable
Interface
µ
P
PWM
Control
Opto
(6)
Power
(6)
Stop
Start
User
I/O
To
motor
The commutation signals are transmitted from the complex control circuit of the PWM to the IGBTs by optocouplers
that use light-emitting diodes (LED) for the transmission of simple on/off commands through the electrical isolation
barrier. In the Secure Power Disable system shown in Figure_2, the LED power is provided by a safety circuit from the
two enable inputs (STO IN 1, STO IN 2). The switching sequence can reach the IGBTs only if the two enable inputs are
present, or if therehas been a highly improbable combination of undetected errors, which allowed both the two enable
inputs to receive power supply. If the STO signal is inactive, the drive is certainly disabled through the loss of power of
the LEDs and by the microprocessor.
!
NOTES:
•
Failure of an individual IGBT or their drive circuits either into the on or in the off state cannot generate
torque, but when a permanent magnet motor such as a servo motor is used, a single torque transient
could be produced by a multiple IGBT failure. The motor can rotate at the maximun of 360°/p, where p
is the number of poles.
Summary of Contents for NTT 240
Page 129: ......
