Baumuller BUM64A, Manual

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Manual Single Power Unit BUM64S/A/F
Document no.: 5.01056.03
A
PPENDIX
E - S
AFETY RELAY
In this chapter we describe the safety relay.
E.1 Methods to avoid an unexpected starting
In order to avoid hazards against persons, for example operators, service- and maintenance
technics, the machine has to be kept in a secure condition (safe stop), while interfering in the
hazardous area of the machine. That is why a reliable prevention of an unexpected starting is
demanded (amongst other things Machine directive 89/392/EWG, appendix I, 1.6.3, last pas-
sage; EN 292-2, 4.1.4; EN 60204-1, 5.4). Under unexpected starting is to be understood every
starting that can cause a risk when appearing unexpected for persons (EN 292-1). Moreover,
besides the transition of the release- to the operating condition of the machine also the unex-
pected ramp-up of the machine, this means the transition from the safe stop into an unsafe mov-
ing has to be considered. This is necessary, because the unexpected ramp-up usually is to be
led back to an interruption of the control loop of the machine. In this case the drive is, because
of its control system, anxious to achieve highest speed at maximal acceleration. If an unexpect-
ed starting occurs, the operator therefore doesn’t have the possibility anymore to remove him-
self or his hand from the hazard area. This is why the drive has to be stopped and has to be kept
safe in its ’off-position’, when having opened, electrical interlocked safety devices. The motor
may not have torque and thus cannot generate a dangerous movement.
The prevention of an unexpected starting of the machine can be reached by electrical separated
safety devices, e.g. contactors. By some machine types it has to be done without the isolation
of the electrical connection of the drive to the mains, if e. g. a drive supplied by a power converter
is often stopped and started again. The constant dis- and re-charging of the DC-link represents
a big stress for the concerned parts and often leads to disturbing delays and failures of these
parts.
The requirement for the starting of a three-phase a. c. motor is the generation of a rotating field,
which drives the inductor of the motor. When having variable-speed three-phase current drives,
usually in the micro-processors a complex pulse pattern is generated, then the pulses are am-
plified and are used for the switching of the power semiconductors. If either no defined pulse
pattern is available or the amplifying connection is interrupted, e. g. by switching off of the power
supply with a relay (safety relay), no rotary field can be generated. An error by the pulse pattern
generation therefore cannot lead to a starting of the motor, as long as the second condition,
namely the interruption of the amplifying power supply is available and contrary. The protection
against unexpected starting is reached by an electromechanical method which is superior to the
electronics. It is reached by a safe isolation - elsewhere than in the load circuit.
E