When an operation is in progress, the same question always arises: when a machine is brought to a halt via an operational stop command, how safely is the machine prevented from starting up unintentionally: What happens in this situation should a fault occur in the control system and a drive is started up unexpectedly? This is an issue which is just as important as the consideration of functional safety associated with “more obvious” safeguards. A key point to consider is the issue of converter-controlled drives. These drives are often stopped by signals such as “Zero Speed” or “Controller Inhibit”. The desire is often to avoid shutting down the power supply so as not to lose any data about the current drive status. In some cases, spontaneous shutdown of the connection between the mains and the converter or even between the converter and the drive is linked to device defects and so cannot be considered.
In cases such as these the machine designer has two options: If isolation from the energy supply is possible without damaging the unit and without initiating other hazardous movements, standstill monitoring can be used. Although the converter controlled drive is stationery it is still active, so it is monitored to check it does not move. Should any movement occur on account of an error, the supply to the whole branch is shut down via a contactor. This solution assumes that the slight drive movement which occurs in the event of an error does not cause a hazard. The movement itself consists of two parts: the part which activates the sensor technology for monitoring and the part occurring before the protection circuit has reacted and a contactor has switched. These influences must be examined in a risk assessment.
If an unintended movement such as this is unacceptable, safe drive technology must be used, which will prevent such faulty behaviour from the start.
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