1. Measurement of winding resistance
Table of Content:
1 Objective..........................................................................................................
2 Scope...............................................................................................................
3 Referenced Documents....................................................................................
4 Definitions and Abbreviations...........................................................................
5 Conduction of test.............................................................................................
6 Proceeding in case of deviations .....................................................................
7 Copyright ..........................................................................................................
1 Objective
- Verification of fault free manufacturing of machine windings, particularly the winding
connections, by means of measuring the electrical winding resistances at defined
temperature (cold resistance measurement)
- Determination of the mean temperature rise of windings after current load (e.g. at the
end of a heat run) by means of measuring the increase of the winding resistance as
compared to the cold resistance measurement.
2 Scope
Machine windings in stator and rotor, auxiliary windings of any kind
3 Referenced Documents
IEC 34-4, 56-57
EN 60034-4, 56-57
4 Definitions and Abbreviations
None.
5 Conduction of test
- Winding resistances are preferably measured by volt-amp (U/I)-method, alternatively
by resistance bridge method.
- The measured resistance is obtained from the quotient R = U/I. Phase resistances for
star or delta winding connections are evaluated acc. to a.m. standards.
- For resistance measurent a DC current is fed phase per phase to the terminals and/or
accessible winding ends.
- The DC voltage supply must have sufficient power, the measuring current should be
several amps (preferably 10 A if adequate constant DC-voltage supply or welding
machine is available).
- With rotor windings that are excited for the very first time via brand-new slip rings not
yet having patina (e.g. turbo field after mechanical zero excitation heat run) considerably
higher DC current (e.g. up to 100 A) might be required to get reasonable results. In this
case readings must be taken very quickly so as to avoid remarkable heating of the
winding during the test.
- For windings with high inductance wait sufficient time for the transients after switching
on the DC supply to decay and the measuring current to become stable, in order to
Page 1 of 2

2. avoid erroneous measurement due to induced series voltages (e.g. up to 2 min for field
windings of large turbo generators).
- During cold resistance measurement the winding temperature must be recorded, too.
Because of the high thermal time-constant of the machine at standstill, the cold
resistance measurement should only be performed after sufficient time of rest (days if
possible). When the winding has carried load current prior to this test, w ait until
temperature distribution over the entire winding has nearly equalized. Temperature is
measured with built-in resistance detectors (ETD) or thermometers, sensing blank
copper parts (Thermophil).
- Accuracy of all measuring instruments must be of class 0,1 at the min. Special care
must be taken during cold resistance measurement, since these values are referenced
for evaluation of all temperature rise measurements (1% resistance error corresponds to
approx. 3 °C temperature error).
- The formula for correcting resistance values measured at temperature T1 to another
temperature level T2 (resp. solving the equation for determining temperature T2 from
resistance measurement R2 and reference measurement value R1 at T1) is:
R2/R1 = (T2 + 235)/(T1 + 235)
6 Proceeding in case of deviations
Any nominal value, minimum value, maximum value, tolerance etc given in an instruction
is intended to serve for quality control purposes only. In the case of deviations, a
nonconformity report has to be written according to PNG Power.
7 Copyright
This Document is properly of PNG Power internal use only and must neither be copied.
Neither is it to be handed over nor in other way communicated to a third party.
Infringement will lead to prosecution.
Page 2 of 2