The basic steps in ascertaining the health of a three-phase AC motor are given below:
(a) General Inspections
(b) Earth Continuity and Resistance Test
(c) Power Supply Test
(d) AC Motor Winding Continuity Test
(e) AC Motor Winding Resistance Test
(f) Insulation Resistance Test
(g) Running Amps Test
General Inspections
For the three-phase motor, do the following:
(1) Check the appearance of the motor. Check for burnt, damage to body or cooling fan or
shaft.
(2) Manually rotate motor shaft to examine bearing condition. Look out for smooth and
free shaft rotation. If shaft rotation is free and smooth, bearing is possibly in good
condition, otherwise consider replacing, repair or carry out further diagnosis.
(3) As with all testing and inspections, the motor name plate provides valuable
information that will help to ascertain the true health of the motor. Examine the name
plate thoroughly and compare values of running amps test (see below) with name plate
value
Earth Continuity and Resistance Test
With a multimeter, measure the resistance between motor frame (body) and earth. A good
motor should read less than 0.5 ohms. Any value greater 0.5 ohms indicate trouble with
the motor. Further troubleshooting maybe required
Power Supply Test
For three phase motors, the expected voltage for a 230/400V system is 230V phase to
neutral and 400V between each of the three phase supply lines. Check that the correct
voltage is applied to the motor using a multimeter. Ensure the terminal for power supply
is in good condition. Check the connection bar for terminal (U, V, and W). For three phase
motors, connection type is either Star (Y) or Delta
*Missing phase (troubleshooting)
-If missing a phase, the other two phase will draw in more current which will cause the
motor to heat up and can damage the insulation. For protection an overload is used for
such case. The overload which is preset to a current value and when this two phases
current exceed this current value, it trips to protect the motor from damage. Ensure all
three phase are here and drawing the same amount of current each. Find what happen to
the missing phase, it may be a fuse, cut wire, lose connection
AC Motor Winding Continuity Test
Using a multimeter, check the continuity of motor winding from phase to phase ( U to V, V
to W , W to U ).Each phase to phase must have a continuity if winding is OK. If any
particular phase fails the continuity test, your motor is probably burnt.
Please see how to identify three phase windings for proper winding identification. U, V, W
is a European winding designation.
AC Motor Winding Resistance Test
Check the motor winding resistance or ohms reading using a multimeter or ohmmeter for
phase to phase terminal ( U to V,V to W ,W to U ).The ohms reading for each winding must
be the same (or nearly the same). Remember that the three phases have identical
windings or nearly so!
Insulation Resistance Test
Insulation resistance failure of an electric motor is one of the first signs that the motor is
about to fail. For a three phase motor, insulation resistance is usually measured between
each motor winding or phase and between each motor phase and motor frame (earth)
using an insulation tester or megger. Set the voltage setting of the insulation resistance
tester to 500V. Check from phase to phase (U to V, V to W, W to U). Check from phase to
motor frame (earth) (U to E, V to E , W to E ). Minimum test value of motor insulation
resistance is 1 Meg Ohm (1 MΩ). See how to measure insulation resistance of Electric Motor
Running Amps Test
With the motor running, check the full load amps (FLA) with a suitable meter or preferably
a clamp on meter and compare with the name plate FLA. Deviations from rated FLA could
signify problems with the motor under test.
ANOTHER METHOD
The first thing to do before testing the windings of the motor is to remove the links
linking terminals W2U2V2 and the disconnect the motor from supply (L1, L2, L3). A
multimeter terminals placed across this matrix of terminals will indicate the
following readings for a good 3 phase motor:
(a) Terminals W1W2, U1U2, V1V2 will indicate continuity for a good motor
(b) Every other terminal combinations should indicate Open for a good motor
(c) Readings between any of the six (6) terminals and the motor frame signifying
earth
(E) should indicate open for a good motor.
Ohmmeter Readings for a Bad 3 phase Motor
In the case of a burnt or bad 3 phase motor, this matrix of terminals should
indicate the opposite readings for a bad motor:
(a) If any of the terminal combinations W1W2, U1U2, V1V2 should
indicate open then
the motor is bad.
(b) If any other terminal combinations should indicate continuity instead of open,
then
the motor is bad.
(c) If the reading between any of the six (6) terminals and motor frame (E) should
indicate continuity, then the motor is dead.
ANOTHER METHOD
-set the multimeter to its ohms function. The windings (all three in a three-phase motor)
should read low but not zero ohms. The smaller the motor, the higher this reading will be, but
it should not be open. It will usually be low enough (under 30 Ω) for the audible continuity
indicator to sound.
Measuring ampere
The basic procedure is to measure the start-up and running current for any motor
while it is connected to a load. Compare the reading to documented or nameplate
specifications. As motors age, the current drawn generally rises because winding
insulation resistance drops. Excess current causes heat, which must be dissipated.
Insulation degradation accelerates until there’s an avalanche event, causing motor
burn out.
The clamp-on ammeter reading will tell you where you stand on this continuum. In an
industrial facility, as part of routine motor maintenance, periodic current readings can
be taken and put into a log posted nearby so damaging trends can be spotted in
advance to avoid expensive downtime.
