Page 76 - Electric Drives and Electromechanical Systems

Chapter 2 Analysing a drive system 69

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Example 2.5
Consider the risks associated with an individual component within a motor drive system, in
this case a motor’s tachogenerator.
An illustration of the FEMA process, which takes a bottom up approach is shown in
Table 2.5.
The rating is a subjective measure of the consequence of an undesirable event upon the
operators, company and the system itself. In practice the scale used will be company specific.
In this example the scale runs from 1 to 5, with 5 being, the highest probability, the most
serious consequence or the probability of not being identified.
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Risk assessment is the second stage of the risk management methodology. All
undesired events can be grouped into one of two categories, termed here as physical
undesired events and operational undesired events. A physical undesired event typically
involves some possibility of physical failure, for example, as a result of wear or corrosion
of part of a subsystem during use. The latter may, or may not, lead on to an operational
undesired event. An operational undesired event is defined as an event leading to death
or injury, or a near miss, in which there is no physical failure of any part of the equip-
ment being assessed. The next step requires a determination of the likelihood of each
significant undesired event and the severity of its consequences. Success is dependent
upon the comprehensive identification of possible undesired events and knowing how
these can be related back to the initiating events, which caused them. The process in-
volves determining the likelihood that the initiating event will be detected, before serious
damage can occur; determining the corresponding likelihood of recovery from, or
correction of, the initiating event; assessing the likelihood that the initiating event will
escalate to give rise to an undesired event; and finally, determining the consequences

Table 2.5 FMEA risk assessment for a tachogenerator as fitted to a motor drive
system. P is the probability, S the seriousness of the fault, D the likelihood that the
fault will reach the customer and R ¼ P S D is the priority measure. P, S and D are
measured on a scale of 1e5.
Failure
mode Cause Hazard PS D R Corrective action
Plug failure Used as a step Overspeed 3 4 1 12 Safety cover and warning label
Incorrect Assembly fault Overspeed 2 4 1 8 Revise documentation and
wiring inspection
Broken Metal fatigue due to misalignment Overspeed 1 4 1 4 Revise documentation and
coupling on assemble inspection
Wiring failure Fatigue caused by vibration Overspeed 1 4 2 8 Design and use of cable restraints