Maintenance Technology June 2017 - 42

SOLUTION FOCUS

test picked up on the problem, but had been discounted.
This failure showed that tests for insulation strength are
real, and to be ignored at a motor user's peril. If vibration
monitoring indicates that a bearing is starting to fail, it is
replaced. The same should happen with insulation.
Case #3 (dynamic testing): U.S. utility company Pacific
Gas & Electric was facing a potential $23,000 bill to replace a
motor on a 125-hp screen-refuse pump, which was overheating and drawing excessive current. Rather than simply
replace the motor, the company looked into the
reason for the high current, as there were no
signs of bearing problems, current imbalance, excessive harmonics, or rotor-bar
problems.
Real-world payback
Dynamic testing with SKF Baker
Motor-insulation testing can have
Exp3000 technology revealed that
an enormous impact on a plant's
the load was running higher than
bottom line. Consider these
the motor's rated value. Looking
real-world examples:
back through the maintenance
Case #1 (static testing):
history, the personnel found that
Technicians with a leading pulp
a 15.75-in. impeller on the pump
and paper company began using
had been replaced with a 17-in.
SKF Baker AWA-IV testers to
impeller. Once the correct-sized
identify problems in about 800
impeller was installed, the current
motor systems. Among the many
returned to normal values.
problems this testing found-and
This testing helped prevent a
solved-were:
costly mistake, given the fact that the
■ blown holes in insulating boots that
oversized pump impeller would also
covered cable lugs in junction boxes
Tests to detect insulation
have overloaded a new replacement
(identified through step-voltage
motor.
testing)
weakness aren't used as
Case #4 (dynamic testing):
■ a bad lug connection in a motor
On-line
testing also prevented a huge
junction box (found after a failed
widely as techniques that
loss at a Progress Energy power plant
resistance test)
check out bearing health.
in the United States. Technicians
■ a stator coil turn-to-turn short on
were investigating why one of three
a booster fan (identified by surge
submerged circulating water pumps
testing)
was requesting less input power and,
■ a cable shorted to ground in starter,
as a result, running faster. An SKF Exp3000 captured the
and a pinhole in the cable (found after failed surge and
torque signature of all three motors, giving a snapshot of
step-voltage tests).
the load demands of each unit. The pump in question had
In all, the company reportedly was able to reduce its
a torque of about 75% of a healthy pump. The torque band
annual motor costs by nearly a third through the use of the
was also too wide, and varied dramatically. A diver sent to
insulation testing rigs.
examine the underwater pump discovered that its end bell
Case #2 (static testing): A steel mill in Australia had a
had fallen off.
6.6-kV pump motor that required maintenance-including
The unit was quickly repaired, which helped to maintain
a rewind. The motor was rewound at a motor shop, then
output when one of the other pumps failed soon afterward.
transported 500 mi. (aprox. 800 km) to a second facility for
The company estimated that it would have lost $3.5 million
vacuum-pressure impregnation (VPI). A subsequent surge
in revenue if output levels had fallen.
test, however, indicated that insulation strength was still not
These examples (and many others) show that a simple,
right. The motor was then put onto a test stand and run.
inexpensive insulation-testing regime can generate
Motor currents and vibration tests were acceptable so, since
significant benefits for an operation. One example also
the mill was in a hurry to resume production, the motor
demonstrates that ignoring insulation-testing results can cost
was put back into service. Three days later, the unit failed
an operation dearly. MT
catastrophically and ignited a fire. The cause was traced to
the windings, which had probably been damaged when the
motor was transported to or from the VPI facility. The surge
For more information visit skf.com.
While the first five tests assess the health of a motor's
groundwall insulation, it's important to keep in mind that a
unit's winding insulation is more prone to failure. The last
procedure on the list, the surge test, focuses on winding
insulation.
All of the six listed insulation tests produce clear, unambiguous results that require little interpretation. Those results
can also be trended over time. This allows operators or
maintenance managers to assess the progress of a potential
condition over time. For instance, increasing
non-linearity in step voltage could suggest
weakening of the groundwall insulation.

42 |

MAINTENANCE TECHNOLOGY

JUNE 2017


http://www.skf.com

Table of Contents for the Digital Edition of Maintenance Technology June 2017

Editorial
Uptime
On The Floor
IIoT
ISO 55000
Products
Ad Index
Showcase
Final Thought
AZ Puts Proactive in Reliability
Glimpse The Future of Predictive Maintenance
Automation-System Updates
MRO: Counterfeit Parts
MRO: Reduce Your Lube Inventory
MRO: Optimize Belt Conveyor Systems
Manage Used and Waste Oils Wisely
Motor Insulation Test Payback
SAP: Tips and Tricks
Cool Off Compressors
Be A Safety Leader
Maintenance Technology June 2017 - Cover1
Maintenance Technology June 2017 - Cover2
Maintenance Technology June 2017 - 1
Maintenance Technology June 2017 - 2
Maintenance Technology June 2017 - 3
Maintenance Technology June 2017 - Editorial
Maintenance Technology June 2017 - 5
Maintenance Technology June 2017 - Uptime
Maintenance Technology June 2017 - 7
Maintenance Technology June 2017 - On The Floor
Maintenance Technology June 2017 - 9
Maintenance Technology June 2017 - AZ Puts Proactive in Reliability
Maintenance Technology June 2017 - 11
Maintenance Technology June 2017 - 12
Maintenance Technology June 2017 - 13
Maintenance Technology June 2017 - Glimpse The Future of Predictive Maintenance
Maintenance Technology June 2017 - 15
Maintenance Technology June 2017 - 16
Maintenance Technology June 2017 - 17
Maintenance Technology June 2017 - Automation-System Updates
Maintenance Technology June 2017 - 19
Maintenance Technology June 2017 - 20
Maintenance Technology June 2017 - 21
Maintenance Technology June 2017 - MRO: Counterfeit Parts
Maintenance Technology June 2017 - 23
Maintenance Technology June 2017 - 24
Maintenance Technology June 2017 - MRO: Reduce Your Lube Inventory
Maintenance Technology June 2017 - 26
Maintenance Technology June 2017 - 27
Maintenance Technology June 2017 - MRO: Optimize Belt Conveyor Systems
Maintenance Technology June 2017 - 29
Maintenance Technology June 2017 - 30
Maintenance Technology June 2017 - 31
Maintenance Technology June 2017 - SAP: Tips and Tricks
Maintenance Technology June 2017 - Cool Off Compressors
Maintenance Technology June 2017 - Be A Safety Leader
Maintenance Technology June 2017 - 35
Maintenance Technology June 2017 - IIoT
Maintenance Technology June 2017 - ISO 55000
Maintenance Technology June 2017 - Manage Used and Waste Oils Wisely
Maintenance Technology June 2017 - 39
Maintenance Technology June 2017 - 40
Maintenance Technology June 2017 - Motor Insulation Test Payback
Maintenance Technology June 2017 - 42
Maintenance Technology June 2017 - Products
Maintenance Technology June 2017 - 44
Maintenance Technology June 2017 - 45
Maintenance Technology June 2017 - Ad Index
Maintenance Technology June 2017 - Showcase
Maintenance Technology June 2017 - Final Thought
Maintenance Technology June 2017 - Cover3
Maintenance Technology June 2017 - Cover4
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