Application Notes
IEEE43-2000
Summary In March 2000, the IEEE-SA Standards Board approved a revision of IEEE Std 43-1974 by the Electric Machinery Committee of the IEEE Power Engineering Society. This revision is IEEE Std 43-2000, the "IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery". Changes made to the types of insulation used in electric rotating machines have resulted in different insulation resistance characteristics, and therefore, required a substantial revision to the IEEE standard. According to the IEEE, the standard is intended for:
- Individuals/organizations that manufacture rotating machines.
- Individuals/organizations that are responsible for the acceptance of new rotating machines.
- Individuals/organizations that test and maintain rotating machines.
- Individuals/organizations that operate rotating machines.
Megger recommends that anyone involved in testing and/or maintaining rotating machinery review this standard in detail. This article will provide some of the highlights.
IEEE Std 43-2000 recommends a procedure for measuring insulation resistance of armature and field windings in rotating machines rated 1 hp, 750 W or greater and applies to synchronous machines, induction machines, dc machines and synchronous condensers. It does not apply to fractional horsepower machines. It also recommends the insulation test voltage (based on winding rating) and minimum acceptable values of insulation resistance for ac and dc rotating machine windings.
Test Voltage
The following chart provides guidelines for the dc voltage to be applied during an insulation resistance test. Note that voltages up to 10 kV are
recommended for windings rated at greater than 12 kV.
|
*Winding
Rated
Voltage (V) |
*Insulation
Resistance
Test Direct(V) |
|
<1000 |
500 |
|
1000-2500 |
500-1000 |
|
2501-5000 |
1000-2500 |
|
5001-12,000 |
2500-5000 |
|
>12,000 |
5000-10,000 |
* Rated line-to-line
voltage for three-phase ac machines, line-to-ground voltage for single-phase
machines, and rated direct voltage for dc machines or field windings.
The standard recommends that each phase be isolated and tested separately
(if feasible) as this approach allows comparisons to be made between phases.
The two phases not being tested should be grounded to the same ground as
the stator core or rotor body. When all phases are tested simultaneously,
only the insulation to ground is tested. Insulation resistance measurements
should be made with all external equipment (cables, capacitors, surge arresters,
etc.) disconnected and grounded as these items may influence the resistance
reading. A common ground should be used to prevent stray losses in the ground
circuit that could effect the test results.
Types of Tests/Minimum Acceptable Values
The standard calls out both the insulation resistance test and the polarization
index test (PI), and recommends that both tests be made (if possible). It
indicates that testing history should be used to track changes. If history
is not available, the standard provides minimum values for both tests that
can be used to estimate the suitability of the winding. These are the lowest
values at which a winding is recommended for an overvoltage test or for
operation.
The recommended minimum values for PI are based on the thermal class of
the insulating materials and apply to all insulating materials regardless
of application per IEC 60085-01: 1984. The PI test is not applicable to
noninsulated field windings. Be aware that a very high PI (greater than
8) for varnished cambric, shellac mica-folium, or asphaltic stator windings
may indicate that the insulation has been thermally aged and may be at risk
of failure. Physical inspection can be used to confirm if the insulation
is dry and brittle.
|
Thermal
Class Rating |
Minimum
PI Value |
|
Class A |
1.5 |
|
Class B |
2.0 |
|
Class F |
2.0 |
|
Class F |
2.0 |
The recommended minimum
insulation resistance after one minute at 40 C can be determined from the
following chart. The minimum resistance of one phase of a three-phase armature
winding tested with the other two grounded should be approximately twice
that of the entire winding. If each phase is tested separately (with guard
circuits being used on the phases not under test), the observed minimum
resistance should be three times the entire winding.
|
Minimum
Insulation Resistance (M ) |
Test
Specimen |
|
kV* + 1 |
For most windings made before about 1970,
all field windings, and others not described below. |
|
100 |
For most dc armature and ac windings built after about
1970 (form-wound coils). |
|
5 |
For most machines with random-wound stator coils and
form-wound coils rated below 1 kV. |
* kV is the rated machine
terminal to terminal voltage in rms kV.
The rating of the machine determines whether the motor windings must achieve
the minimum value for either the insulation resistance test or PI test,
or must achieve the minimum for both tests.
| Machine Rating |
Evaluation Criteria |
|
10,000 kVA or less |
Should have EITHER a value of the PI test or a value of the insulation resistance test (at 40 C) above the minimum recommended values. |
| Above 10,000 kVA |
Should have BOTH a value of the PI test or a value of the insulation resistance test (at 40 C) above the minimum recommended values. |
Summary
Following is a brief summary of the highlights of IEEE std 43-2000:
- Test voltages up to 10 kV are recommended for windings rated greater than 12 kV.o Both the insulation resistance test and the Polarization Index test are recommended.
- Test results should be compared to historical values to identify changes.
- In lieu of historical records, minimum acceptable values (based on the type of equipment) for both tests are indicated.
- Depending on the machine rating, the readings for one or both tests should exceed the minimum acceptable values. o If the readings are below the minimum acceptable values, the winding is not recommended for an over voltage test or for operation.