Maintenance is necessary for Electric Motors and Generators after the calculated working hours. Such maintenance aims at preventing unplanned stops of your machine and the occurrence of larger and expensive damages.

The factors effecting the maintenance time are as follows:

Nature of the machine: AC, DC, Generator

Type of the machine: squirrel cage, ringed, with collector, with bearing or with metal

Machine particularity: Protective class, operating class, revolution.

The particularity of the equipment driven by/driving the machine: Mill, pump,
compressor, fan, reducer, belt and pulley, diesel engine, turbine.

Machine’s operating environment: Ambient temperature, ambient chemistry, ambient powder, etc... are impactful.

The maintenance period of the motors are generally established by the
manufacturers that used the engine in their machinery and they indicate it in
the machine’s manual. Otherwise, maintenance periods should be established by
expert opinion and usage experience.

Workshop maintenance periods should be planned 3 years for AC ringed engines, 4-5 years for squirrel cage motors and 2 years for DCmotors in average. (These periods
can be shortened or extended with the consideration of other factors).

The onsite and atelier maintenance planning for the generators should be made
through calculation that is based on the working hours and start numbers.

In the table maintenance plan, the recommended maintenance activities have been indicated according to the equivalent working hours.

1) Equivalent hour = Total working hours + number of starts x 20 (fixed-speed
engines) = 1.2 x actual working hour (variable-speed engines).

2) Option: Isolation measurement of the stator windings

3) It depends on the placement & accessibility of the machine and the
availability of the lifting equipment.

* IR= isolation resistance

* PI= Polarization index


a) When possible, isolation and vibration measurements should be performed and recorded prior to maintenance for Motors and Generators.

b) Motors and generators should be subjected to preliminary testing in the
atelier before commencing disassembly.

The following is performed during the preliminary test with the consideration of
the machine status:

- Isolation tests are performed for the windings;

- RTD’s of the winding heaters, windings and housings are measured.

When possible, electrical and mechanical measurements are made and recorded for the
motors by rotating them in nominal voltage and speed.

During the preliminary test, the following are checked and identified:

- Status of the engine windings;

- Airspace;

- Slackness of the stator, rotor and core;

- Slackness of the ring and the collector;

- Status of the bearings;

- Status of the rotor balance.


a) In case couplings or pulleys are present in the motor, the placements in the
shaft are measured and recorded by caliper prior to disassembly.

 The structure of the coupling is examined with regard to the shaft wedge. Any locking bolts are loosened if they are present in the coupling.

b) The coupling is drawn from the shaft with hydraulic drawing that has 120 degree
legs. Its purpose is to draw the coupling over the shaft without creating any
tension and to prevent damages on the coupling and the shaft.

 In the accompany of controlled pressure, the outer surface of the coupling is
dilated by a flame heat well and thus, the coupling is easily drawn over the


a) After the coupling is disassembled, flexure measurement is performed on the shaft
using a comparator. Flexure on the coupling’s middle region should not exceed

b) Repair
of the shaft is necessary if flexure exceeds 3-5%. If load is connected to a
flexed shaft, it will deviate from the axis and vibration will occur.

Repair with Welding: This is not recommended as welding will stiffen the shaft and
render it fragile.

Coating Method: This is not recommended due to its cost and the risk for the coupling
to blister during installation and disassembly.

Lathe Method: This is the recommended method. The coupling locations is corrected by lathing using the powderly pass method and afterwards, either a coupling is
manufactured according to the new measure or the interior the existing coupling
is filled and processed.

Production of a New Shaft: Disassembling the old shaft and manufacturing a new one is necessary if the flexure is excessive according to the shaft status.


a) The shaft bearing locations of the engine rotor is measured by the external radius
micrometer with 1% sensitivity.

b) The shaft bearing locations of the trunk cover is measured by the internal radius
micrometer with 1% sensitivity.

c) The shaft coupling location and interior sections of the coupling/pulley are also
measured by micrometers.

d) In exproof motors, all rabbet joints such as greasing flange and the connector
covers are measured by internal and external micrometers in terms of engine

The bearing tolerances are established by the manufacturer. As a general
application, the shaft is between +0 and +25 micrometers and the cover is
between +0 and +45 micrometers in industrial heavy industry engines. The motor
revolution, housing dimension and the used bearing interval impact the
tolerance amount.


a) Isolation tests of the windings are performed.

b) HI-POT and SURGE tests are performed when necessary.

c) Megger test at 100 Volts is performed if housing isolation exists.

d) DC resistance tests are performed at 0,1% sensitivity.

e) Resistance tests are performed if there is a heater.

Resistance tests are performed if there are heat detectors.

Core tests are performed by passing the magnetic field.

Faults such as slack, ruptured bobbins and bandages are checked visually.


The windings are cleaned of all dirt by using a machine that sprays pressurized hot water and mixes adjusted cleaning detergent into the water. The application is performed in a special cabinet by/under the supervision of experienced masters that use appropriate protective equipment. b) All motor parts as well as equipment such as covers, rotors, couplings, etc.. are washed and cleaned in the same manner. c) In particular, if the generator or the engine contains special equipments, circuits, etc... mechanisms that can negatively be effected by washing, these are disassembled prior to washing.


a) The motors are kiln-dried following washing. The kiln temperature is adjusted and
kept steady by heat control relay.

b) The kiln should include fans and gas exhaust chimneys that homogeneously circulate
the air inside.

c) As a standard, the engines are kiln-dried in 100 degrees Celsius for 24 hours, and
in 130 degrees Celsius for 15 hours.

d) Following kilning, the winding isolations are measured by the megger device and
they are subjected to varnishing if the measured values are good as per the

e) During varnishing, the varnish applied on the hot winding thins down and absorbed
within the winding. Application of the appropriate kiln varnish as per the heat
class is very important. When not in use, the varnish is kept in a specially
equipped and vacuumed container and during application, it is sprayed on the
windings by pressurized air under a certain pressure. The motos are kilned
for the same periods also after the varnishing operation.


The outer surfaces of the fiberglass or ceramic materials such as brush holders and
connectors are covered by conductive or insulator oxide layer and dirt. Water
and chemical materials are insufficient to clean these. Sandblasting method
provides a perfect cleaning by removing such dirt from surfaces by a mechanic force.


According to the ISO 1940-1 standard, the matters requiring attention during the
balancing operation are the being of revolving parts at the top, their
installation in conformity with their post as well as the statuses of the wedge
and the coupling/pulley.

The status of the wedge has been marked with the H (Half) or by the F (Full) sign
on the surface of the coupling in the rotor shaft.

The balance of the coupling/pulley should also conform to this. The balance of the
coupling/pulley should be full wedge if that of the rotor is Half wedge; and
the coupling/pulley balance should be half wedge (without wedge) if the rotor
balance is Full wedge.

The rotor should be full wedge if the coupling/pulley length is shorter than the rotor
wedge and the coupling/pulley should be without wedge. In cases where the rotor
is half wedge and the coupling/pulley are with wedge, than the protruding
section of the wedge, which is located inside of the coupling/pulley on the
rotor, is cut and rendered half wedge.


The bearings are heated by magnetic method. Current is passed through the bearing
by using devices that have automatic or manual adjusted power and the bearing
is heated in a controlled manner using a heat detector. For ease of
installation, the temperature to be applied on the bearing should be 80 degrees
Celsius plus the ambient temperature.

 Higher temperatures may cause the bearing to breakdown.

Upon arriving the appropriate temperature, the device automatically decreases the
voltage gradually by a demagnetization directive, thereupon removes magnetism
and gives a signal for its attachment. As a sudden interruption of energy may
cause redundant magnetism on the bearing steel, there is breakdown risk owing
to the attraction of ambient metal powders and parts.

Sinus check is performed by simultaneous commencement of transmission and
interruption boundaries, that emanate and rise in both alternans.


a) The motors are subjected to rotation tests after the finalization of the standard
isolation tests and visual examinations.

b) It is important to provide nominal voltage for revealing breakdowns pertaining to the electrical and magnetic field.

c) It is important to rotate with nominal revolution for the mechanical and
bearing-related breakdowns. Our system can give voltage until 6000 Volts.

d) Load test particularly attains importance in DC engines. Our system can make loading until 250 kW.

e) To be sue on rotor windings and ring connections in ringed motors, one should first
give nominal voltage to the stator without short-circuiting the rings, read the
rotor voltage over the rotor rings and compare the label information.

f) An FFT vibration analysis to be performed during idle running and gE Envelop tests can provide detailed information on vibration and bearings.