Specifications

169
The insulation resistance should be measured when motor is hot, whenever possible.
The reason is that even a damp winding will give a comparatively higher reading if taken when
cold and therefore such a reading is unreliable. The reading will be lower when taken
‘
hot
’
. As
already stated, the resistance of any insulation falls as the temperature rises, which is directly
opposite to that of any metallic conductor the resistance of which rises as the temperature
rises. When recording insulation resistance of a winding, it is therefore very important to note
down the temperature of the winding by a thermometer. It is desirable to have a standard method
of recording the readings, say at 75
0
C. All megger readings may then be converted to this
common base, by multiplying or dividing them by the appropriate conversion factor.
Measurement of insulation resistance of a motor
Insulation resistance should preferably be measured by an equipment which applies a
voltage above the normal voltage. A 400 volts equipment may be tested by a 1,000 volts megger
if possible. If such a megger is not available, a 500 volts megger could be used. One test is
usually sufficient, i.e. between line and earth with all the equipment in circuit. If it is very low, find
out the reason whether it is in the motor or the starter, or in the field circuit or the armature. In a
3-phase motor, if both ends of all the 3-phase windings have been brought out to the terminal
box, the insulation of each phase winding to earth and also between phase windings should be
taken. In taking line-to-line insulation resistance of starters, it should be remembered that,
contactor coils, voltmeter, etc. may have been connected across the lines and therefore, they
should be removed/disconnected before taking readings. When testing slip ring motors, the
rotor insulation should also be taken. The rotor voltage may be as high as 400 to 600 volts.
Sometimes the rotor circuit is intentionally earthed at the rotor starter. This should be kept in
mind while doing the tests. Many motors are fitted with condensers either for power factor
correction or for purposes of starting. When megger tests are taken in such cases, special
precautions should be taken. Otherwise the readings may be misleading.
The main types of
‘
Morganite
’
brushes used are described below:
H. M. Class: These are natural graphite brushes, used for rotary convertors and
alternator slip rings
I. M. Class: These are also natural graphite brushes of comparatively high contact
resistance and therefore they suppress sparking. They are usually used in small fractional
h.p. motors, and automobiles generators.
C. M. Class: They are made up of carbon and metals like copper in the form of extremely
fine powder. These are especially suitable for low voltage heavy current machines like
electroplating generators.
E. G. Class: These are called electro-graphite class. They are suitable for all types of
industrial motors and generators.
The normal brush tension
The brush tension should be roughly 0.140 to 0.175 kg per sq. cm (2 to 2½ lb. per sq. in.)
for ordinary brushes and about 0.246 kg per sq. cm (3½ lb. per sq. in. ) for brushes with metal
content. Brush tension is measured by a spring balance.