EVALUATION PROCEDURES FOR ROTATING MACHINES

It may be necessary to conduct preliminary processes prior to removing a piece of equipment for repair or overhaul. In these circumstances it is recommended that as much operational information as possible prior to shutdown of power is obtained. The following pages provide some scenarios.

5.5.2 Before power shutdown (typically for large machines only, i.e. ≥1 MW

In order to establish the correct operating conditions of a rotating machine that is to be repaired/overhauled it is strongly recommended that ‘before’ shutdown (on-load or no-load, excited or unexcited, as applicable) the following conditions and parameters are established:

(a) Operating voltage, current, power factor, speed and frequency.

(b) Operating characteristics needed for re-design if applicable within certification/approval (for example, open-circuit or short-circuit curves for d.c.

machines and generators).

(c) Voltage unbalance and harmonic distortion (for example, motors for run-up of gas turbines and pumped storage machines).

(d) Current unbalance or fluctuation, or spectral analysis.

(e) Leakage flux spectral analysis (for example, industrial motors).

(f) Ambient temperature, humidity, air pressure or altitude.

(g) Ozone emission of machine windings (windings with high electric stresses).

(h) Brush sparking inspection (commutator and slip ring machines).

(i) Commutation test (IEC 60034-19), d.c. machines.

(j) Bearing temperatures (where sensors are fitted).

(k) Bearing and shaft vibration (where sensors are fitted).

(l) Coolant temperatures, flows, pressures (where sensors are fitted).

(m) Winding temperatures (where sensors are fitted).

(n) Winding vibration (where sensors are fitted).

(o) Lubricant temperatures, flows, pressures (where sensors are fitted).

(p) Core vibration (where sensors are fitted).

(q) Frame vibration.

(r) Shaft voltages and currents.

(s) Stator winding insulation partial discharges or dielectric loss analysis (bars or form-wound coils with high dielectric stress).

(t) Rotor cage integrity.

(u) Rotor winding inter-turn insulation (impedance measurement) for cylindrical rotors.

(v) Rotor winding ground insulation resistance for wound rotors.

5.5.3 After shutdown

In the event that the equipment is to be removed to a service facility it is strongly recommended that footprint positions are clearly recorded to aid in alignment checking when the equipment is to be repositioned. This task is to be completed before any work is undertaken to remove hold-down bolts. It should also be noted that in many instances shims are used to assist in achieving correct height alignment. Where these are found to have been used, correct recording of the quantity, position and thickness of each shim must be undertaken.

5.5.4 Before dismantling

Upon receipt of the equipment at the repair facility an assessment should be undertaken to determine the following:

(a) Reference to documents from owner/operator.

(b) Cause or contributing factors of failure or work to be undertaken, as instructed by owner/operator.

(c) Visual external condition (for example, damage noted).

(d) Missing items.

(e) Full nameplate data including certification references.

(f) When applicable, water jacket flow test.

(g) Shaft straightness—Run out.

(h) Electrical tests—Insulation resistance of bearings, couplings, pedestals and shaft seals (where insulated).

(i) Stator winding insulation d.c. leakage currents (form-wound coils or bars).

(j) Stator winding insulation resistance and polarization index.

(k) Dismantle noting condition of bearing caps, bearings, end-shield spigots, stator flamepaths, etc.

(l) Dimensional checks of commutators and sliprings, journals, seal faces, shaft run-out couplings and bolts, shaft extensions, keys and keyways, and other parts subject to wear.

(m) Dimensional checks to stator housing ovality, spigoted joints, terminal boxes, terminal blocks and terminals including bushes where applicable.

(n) Rotor condition.

(o) Bearing fits on shaft.

(p) Bearing fits in housings.

(q) Bearing housing fits on frame.

(r) Rotor retaining ring crack testing.

(s) Rotor slot wedge crack testing.

(t) Rotor cage integrity (crack detection).

(u) Shaft forging and end-ring integrity (ultrasonic, dye penetrant, magnetic particle, as appropriate), solid cylindrical rotors.

(v) Shaft, spider, rim and through-bolt integrity (hydro machines).

(w) Winding condition.

(x) Fretting at winding, core, core/frame junction.

(y) Loosening of end-winding supports and bracing system.

(z) Stator—rotor air gaps.

(aa) Signs of electrical corona and tracking.

(bb) Flashover.

(cc) Single phasing.

(dd) Surge damage.

(ee) Cooler gaskets erosion marks on the heat exchangers.

(ff) Burn damage between core plate dovetails and dovetail beams.

(gg) Insulation resistances of bearings, couplings, pedestals and shaft seals (where insulated).

(hh) Insulation integrity of up-shaft leads, radial connectors and slip rings (solid cylindrical rotors).

(ii) Insulation integrity of winding connectors, leads, and terminals.

(jj) Dielectric loss angle (DLA) (tan delta) testing of terminal bushings (high voltage machines).

(kk) Gas-tightness of up-shaft leads and radial connectors (hydrogen cooled machines).

(ll) Core insulation integrity (power flux test, low flux test), before and after winding removal.

(mm) Diagnosis of the cause of the machine failure.

(nn) For water-cooled motors de-scale the water jacket and conduct volumetric, pressure, flow and wall thickness tests to ensure compliance with certification/approval documentation.

NOTE: In the event that a rewind is required for an Ex ‘e’ motor, the manufacturer’s original winding data is required or a full locked rotor test is required to ensure compliance to the ‘tE’

time.

5.5.5 Rewinding

Before undertaking a rewind the following information is necessary:

(a) Type of winding, e.g. single layer, double layer.

(b) Winding diagram.

(c) Number of conductors per slot and parallel paths per phase.

(d) Interface connections.

(e) Conductor size.

(f) Insulation system, including varnish specification.

(g) Resistance per phase or between terminals.

NOTE: Winding data should be available from the manufacturer or the certified holder. The whole of the winding should be restored to the original condition, except that partial winding replacement may be possible on larger equipment. A partial winding replacement should only be undertaken after reference to the equipment manufacturer or the appropriate certification/approval authority.

5.5.6 Removal of windings

The process for softening the impregnating varnish of damaged windings with solvents, before stripping, is acceptable. The application of heat in a controlled manner, via a temperature controlled burn-out oven, is acceptable provided that the operation is carried out with caution so it will not adversely affect the insulation between the laminations of magnetic parts. If in doubt, the advice of the equipment manufacturer should be sought regarding the inter-laminar insulation material used in a rotating machine classified as utilising the increased safety technique.

An increase in core loss resulting from degradation of inter-laminar insulation can significantly affect the motor operation or cause the temperature class to be exceeded.

As part of the motor rewind process the service facility should conduct a core-loss test following the removal of windings.

Burn out ovens should be equipped with accurate temperature control and monitoring systems. They should also be fitted with water spray devices that activate automatically in the event of an over temperature alarm or ignition within the oven.

5.5.7 Before winding

(a) Casing and frame weld quality (certification) (welder qualification, if necessary).

(b) Frame and mounting integrity (weld integrity, crack detection, distortion).

(c) Shaft straightness.

(d) Shaft forging and end-ring integrity (ultrasonic, dye penetrant, magnetic particle, as appropriate), solid cylindrical rotors.

(e) Shaft, spider, rim and through bolt integrity (hydro machines).

(f) Core insulation integrity (power flux test, low flux test, acceptance criterion for rewinding.

(g) Core loss evaluation (refer to Clause 5.11).

(h) Core lamination stack tightness tests (stators, rotors, hydro machine rotor rims), acceptance criterion for rewinding.

(i) Winding material quality checks (to winding designer’s requirements).

(j) Winding support components fits and dimensions (in overhangs and slots) to winding designer’s requirements. Winding connector, lead and terminal insulation integrity.

(k) Bearing fits on shaft.

(l) Bearing fits in housings.

(m) Stator winding dielectric loss angle (DLA) (tan delta) tests on replacement bars and coils (HV windings only).

(n) Stator winding partial discharge tests on individual bars and coils (HV windings only).

(o) Stator winding dielectric tests with high voltage on each bar or coil (HV windings only).