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  Insulation Resistance of a Motor

Introduction to Measurement of Insulation Resistance of a Motor

Insulation resistance should if possible be calculated by equipment that applies a voltage higher than the normal voltage. 400 volts equipment might be checked through a 1,000 volts megger if possible. If such type of a megger is not available, a 500 volts megger could be employed. One test is generally sufficient, that is among line and earth with all the equipment in circuit. If it is extremely low, detect 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 introduced to the terminal box, the insulation of every phase winding to earth and as well among the phase windings should be taken. In taking line-to-line insulation resistance of starters, it should be kept in mind that, contactor coils, voltmeter, etc. might have been linked beyond the lines and so, they should be removed or disconnected before taking readings. While testing slip ring motors, the rotor insulation have to be also be taken. The rotor voltage might be as higher as 400 to 600 volts. Occasionally the rotor circuit is purposely earthed at the rotor starter. This should be remembered when doing the tests. Several motors are fitted with condensers either for power factor correction or for aims of starting. While megger tests are taken in such types of cases, unique precautions should be taken. If not the readings might be misleading.

The major types of 'Morganite' brushes used are explained below:

H. M. Class:

H.M. class of brushes are natural graphite brushes employed for rotary convertors and alternator slip rings.

I. M. Class:

These are as well natural graphite brushes of relatively high contact resistance and so they suppress sparking. They are generally employed in small fractional H.P. motors, and automobiles generators.

C. M. Class:

They are built up of carbon and metals such as copper in the form of very fine powder. These are particularly appropriate for low voltage heavy current machines such as electroplating generators.

E. G. Class:

These are termed as electro-graphite class. They are appropriate for all types of industrial motors and generators.

The normal brush tension

The brush tension have to be approximately 0.140 to 0.175 kg per sq. cm (2 to 2½ lb. per sq. in.) for ordinary brushes and approximately 0.246 kg per sq. cm (3½ lb. per sq. in. ) for brushes along with metal content. Brush tension is calculated through a spring balance.

Care in fitting a new carbon brush

The surface of every carbon brush have to be thoroughly ground to correspond to the curvature of the commutator that is why the brush creates contact over its full area, thus as to be capable to take the full load current. The correct curvature is made sure through drawing a strip of extremely smooth sand paper within the brush and over the commutator or the slip ring. It has to be moved to and fro in close contact with the commutator surface. Concurrently, pressure should be applied on the brushes. The carbons should be cut preferably by sand paper only in the forward direction. Sharp edges of the carbons should then be a little bevelled and all carbon dust blown off. Emery paper must never be employed for grinding carbon brushes like the emery particles may get embedded in the carbon. If this occurs, the commutator surface would clearly be ruined.

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