Page 121 - Intro Predictive Maintenance

112 An Introduction to Predictive Maintenance

6.6 OTHER TECHNIQUES
Numerous other nondestructive techniques can be used to identify incipient problems
in plant equipment or systems; however, these techniques either do not provide a broad
enough application or are too expensive to support a predictive maintenance program.
Therefore, these techniques are used as the means of confirming failure modes iden-
tified by the predictive maintenance techniques discussed in this chapter.

6.6.1 Electrical Testing
Traditional electrical testing methods must be used in conjunction with vibration
analysis to prevent premature failure of electric motors. These tests should include:

• Resistance testing
• Megger testing
• HiPot testing
• Impedance testing
• Other techniques

Resistance Testing
Resistance is measured by using an ohmmeter. In reality, an ohmmeter does not
directly measure resistance; it measures current instead. The scale of the meter is cal-
ibrated in ohms, but the meter movement responds to current. The amount of current
supplied by the meter is very low, typically in the rage of 20 to 50 microamperes. The
meter functions by applying its terminal voltage to the test subject and measuring the
current in the circuit.

For practical purposes, although resistance testing is of limited value, some useful
tests may be performed. A resistance test will indicate an open or closed circuit. This
can tell us whether there is a break in a circuit or if there is a dead short to ground.

It is important to remember that inductive and capacitive elements in the circuit will
distort the resistance measurements. Capacitive elements will appear initially as a
short circuit and begin to open as they charge. They will appear as open circuits when
they are fully charged. Inductive elements will appear initially as open circuits, and
the resistance will decrease as they charge. In both cases, the actual charging time is
tied to the actual resistance, capacitance, and inductance in the circuit in question. It
still requires five time constants to charge capacitors and inductors. It is also impor-
tant to remember that when disconnecting the meter from the circuit that there are
now charged capacitive and inductive elements present, so due caution must be
observed when disconnecting the test equipment.

Resistance testing is of limited value for testing coils. It will detect an open coil, or a
coil shorted to ground. Resistance testing will most often not detect windings that are
shorted together or weak insulation.