Generator Test Procedures: Dan Bohrer CPI Six Sigma Black Belt Griffin, Georgia

Generator Test Procedures
Dan Bohrer
CPI Six Sigma Black Belt
Griffin, Georgia
Content
• Required Tools
• Main Stator Test
• Main Rotor Test
• Shaft Current Test
Required Tools
• Insulation Resistance Tester

– To cover our full generator line (Low, Medium and High
Voltage Generators) the tool should be able to provide:
500, 1000 and 5000 VDC. Below some examples as
reference.
Required Tools
• Digital low resistance Ohmmeter / Micro-ohmmeter

– Winding resistance are very low and require a special tool
to get accurate readings. Below some examples as
reference
Main Stator Test
• Insulation resistance test
• Polarization index test
• Winding resistance test
• Surge comparison test
Insulation Resistance Test
* Taken from: IEEE Std 43-2000(R2006)

The insulation test voltage for main rotor and exciter
components should be no more than 500 VDC.
– To obtain comparable results, test should be of the same duration. The
reading should be taken after 60 seconds.
– Should only be carried out when the winding temp is above the dew point
– Winding temperature should be recorded at the time of the test in case a

temperature correction is needed (an infrared temperature gun can be
used for this purpose)
– This is a good test for insulation resistance trending
– In the past a good rule of thumb for the minimum value of random
wound coil was: “approximately 1 MΩ for each 1,000 Volts of operating
voltage, with a min value of 1MΩ”
– The insulation resistance of a random-wound stator in a new generator is
typically more than 20 MΩ
– The insulation resistance of a form-wound stator winding in a new
generator is typically more than 300MΩ
* Taken from: IEEE Std 43-2000(R2006)

The insulation resistance value

increases with decreasing winding
temperatures. (e.g., 100 MΩ at
50º C is converted to 170 MΩ at 40°C :
1.7 x 100).
For reference: see SEHS9124 Cleaning and Drying

for the Generator
• Polarization Index test
– Defined as the ratio of the 10 minute insulation resistance value
(IR10) vs 1 min insulation resistance value (IR1)
– Provides indication of insulation quality deterioration due to dirt,
moisture, cracking, aging, etc.
– The resistance of a good insulation will increase over time of test
– The min value for stators should be no less than 2.
• May not give accurate measure on new generators (>5000M
Ω)
– If not less than 2, perform drying (and maybe cleaning)
Note: C175 Commissioning A&I LEBQ8313-01 Pg 42 will be corrected to 2

Publication SEHS4892
updated to provide clearer
guidance on insulation test
value expectations for all
generator windings.
Stator Surge Comparison Test
• This is a High Voltage AC test commonly used to check stator
turn to turn coil shorts
• This test checks the integrity of the coils inside the stator,
comparing the surge waveform from three phase windings
with each other.
• This method is extremely sensitive to differences in the surge
waveforms, the surge waveforms allow for comparison of the
impedances of each of the three phase tested windings
• This test is used in factory production and rewinds, but is not
recommended for field testing as periodic High Voltage
testing may weaken winding insulation.
– Also accuracy of test is unreliable unless component being tested is isolated
from other windings
Stator Surge Comparison Test
Main Rotor Test
• Total rotor resistance test

• Individual pole resistance test
• Voltage drop test (AC and DC)
• Shaft Currents
Main Rotor Test
Troubleshooting Tips:
• Measure rotor resistance and compare with TMI values (use a
micro-ohmmeter for this test)
• Measure the excitation current and compare with TMI values
(no load and full load values in TMI)
• Compare Generator vibration levels, with and without
excitation current
• Test for short-circuited field turns (rotor pole test)
– Individual Pole Resistance Test
(use a micro-ohmmeter for this test)
– Voltage Drop Test
Rotor Pole Test
Non Drive End

(beginning of the Drive End
coil) (end of the coil)
In some cases it is not easy to get the access we

need to measure the individual pole resistance
with the rotor inside the stator.
Rotor Pole Test (cont)
Non Drive End Drive End

Rotor Pole Test (cont)
In some models you can

access the beginning and
the end of each pole
winding from the rear
end.
Individual Pole Resistance Check
Procedure
– Record rotor winding temperature
– Check resistance on individual poles using a Micro-
Ohmmeter.
– The resistance difference between poles should be less
than 5%. Calculated as follows:
%Deviation = 100 x (High – low) / Low
Voltage Drop Test
The object of this test is to detect field coils with short-
circuited turns.
– DC Voltage drop test: This test can be misleading and not
reliable.
– AC Voltage drop test: This is a more sensitive test, the
voltage across a coil with a short-circuited turn will be
substantially less (due to reduced magnetic flux) than the
voltage across a good coil.
AC Voltage Drop Test
P1 P2 P3 P4
250mV 250mV 250mV 250mV
Apply 2A Current Source (example is 1 ohm Rotor Winding)
• Individual Pole Voltage should not vary more than 10% (The
preferred method is to inject a known current value (such as 2A) then measure
the voltage drop on each individual pole)
• A rotor needs to pass the pole resistance check and the voltage
drop test to be considered a good rotor
AC Voltage Drop Test
Please note: Performing this test with the rotor inside the stator
will give you inaccurate results. The rotor should be outside of the
stator and isolated from steel table, for better results take the
readings with the rotor hanging from a hoist.
Shaft Current
• Caused by irregularities in the magnetic circuit causing a small
amount of flux to link the shaft
• An electromotive force is generated between the shaft ends
• This electromotive force may cause a current through the
shaft, bearings, bearing supports, and machine framework
and back to the other end of the shaft, unless the circuit is
interrupted by insulation.
• The current flowing through the generator bearing will
produce an arcing discharge creating marks similar to a bar
coding pattern.
Shaft Current
Fluting on bearing raceway is classic evidence of shaft currents

Shaft Current Testing
• Test kit P/N 147-5562 is available in the system for single
bearing generators
• There are different methods to test shaft currents Caterpillar
Customer Services is investigating alternate methods to
measure shaft currents
• If you have evidence of shaft current problems please contact
DSN for additional support
Equipment Required:
1 - Valhalla Model 2100 Digital

Power Analyzer
1 – Insulated copper test probe
(copper rod: 28” long, 3/8”
diameter)
Probe insulation: A shrink tubing
first then Grade A 7000 Volts silicon
sleeve.
1- 72” of 14 gauge wire with a ring
terminal in the probe end, and a
fork terminal on the meter end
Details for the construction

of the insulated copper rod
• Procedure
Connect the N terminal to the main stator frame, connect the

probe wire to the adjacent terminal (the internal shunt
resistor). The meter should be set as follows: Current:2,
Display: Amps, Voltage 600
Remove the coupling screen, start the unit and run at operating speed no
load. Place the probe against the machined surface of the generator fan and
capture reading.
* Caution – When holding the probe to the fan hold at an angle so the
direction of rotation will be pulling the probe instead of pushing the probe
back
Eliminating the effects of Shaft Currents
Insulated Bearing Sleeve
Bearing Retainers bars

insulator
NDE bearing bracket from

1846 frame generator shown
in this picture
Questions?

Generator Test Procedures: Dan Bohrer CPI Six Sigma Black Belt Griffin, Georgia

Uploaded by

Copyright:

Available Formats

Generator Test Procedures: Dan Bohrer CPI Six Sigma Black Belt Griffin, Georgia

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Generator Test Procedures: Dan Bohrer CPI Six Sigma Black Belt Griffin, Georgia

Uploaded by

Copyright:

Available Formats

What are the main tests performed on a generator stator?

What are the main tests performed on a generator stator?

What tools are required to perform insulation resistance and winding resistance tests?

What tools are required to perform insulation resistance and winding resistance tests?

Generator Test Procedures

• Insulation Resistance Tester

• Digital low resistance Ohmmeter / Micro-ohmmeter

* Taken from: IEEE Std 43-2000(R2006)

– Winding temperature should be recorded at the time of the test in case a

* Taken from: IEEE Std 43-2000(R2006)

The insulation resistance value

For reference: see SEHS9124 Cleaning and Drying

Note: C175 Commissioning A&I LEBQ8313-01 Pg 42 will be corrected to 2

• Total rotor resistance test

Non Drive End

In some cases it is not easy to get the access we

Non Drive End Drive End

In some models you can

250mV 250mV 250mV 250mV

Apply 2A Current Source (example is 1 ohm Rotor Winding)

Fluting on bearing raceway is classic evidence of shaft currents

1 - Valhalla Model 2100 Digital

Details for the construction

Connect the N terminal to the main stator frame, connect the

Insulated Bearing Sleeve

Bearing Retainers bars

NDE bearing bracket from

You might also like