ALONG WITH
MEGGER TEST FOR
CHECKING OF
MOTOR ,WHY WE
MEASURE FOR
RESISTANCE AND
INDUCTANCE?
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Answer Posted / stan grzegorski
Megger (or Insulation) test will test ISULATION ONLY . A "
Megger" is one manufacturer of insulation testers . This (&
most other) insulation tester also has a low resistance
range for measuring & COMPARING motot winding
resistance .Insulation resistance is normally measured at
twice the RMS line voltage minimum , (For 240 Volt rated
equipment the 500V setting is used )DO NOT USE IT IF THERE
ARE SESITIVE ELECTRONIC COMPONENTS IN CIRCUIT . ( You WILL
destroy them ). BE VERY AWARE !! The 500 V & 1000V settings
on these instruments is not what they are measuring ,IT'S
WHAT THEY ARE SUPPLYING , & IF YOU ARE PART OF THE CIRCUIT
THAT IS WHAT YOU WILL BE SUPPLIED WITH !The output current
is limited to a safe level , but if there are any
capacitors in the circuit , THIS CAN BE MULTIPLIED MANY
TIMES to a dangerous level.Specifications for insulation
resistance to earth ( Ground /Chassis ), is 1 M ohm ( 1 Meg
ohm , 1,000,000 ohm ), but in reality if isulation
resistance is 10 Mohm or under , the equipment may need
drying out , or is on its way to the bin . The low
resitance range is for measuring continuity , in this case
that the motor windings aren't broken , normally in the
order of a few ohms , & on larger motors less than 0.5
ohms .Inductance is an electromagnetic parameter which
affects AC or varying DC current & is dependent on the
frequency of the AC current or the RATE OF CHANGE of DC
current . ( Higher frequency = Higher inductance ) > In
most AC machines (Motors , transformers etc.)the Inductive
reactance is cosiderably larger than the resistance , &
measuring inductance can identify faults which would be
practically impossible using resistance measurements .
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