Ag42em PDF
Ag42em PDF
Ag42em PDF
Turn-to-turn Failures
Turn-to-turn failures are one of the most common Insulation type failures. In the case of random
wound motors, placement of the conductors is not consistent. The first turn, for example, can be
adjacent to the last turn and therefore, be subjected to maximum voltage stress. Magnet wire is
allowed to have a certain number of pinholes for a given length and wire gauge. It is possible for
two pinholes to line up. If a pinhole exists where the magnet wire is bent, such as in the end turn,
the hole can be stretched bigger than it was at time of manufacture and further increase the
opportunity for two pin holes to line up.
Mechanical rubbing can cause failures to occur as well. If the mechanical strength of the end turn
bracing is not sufficient, movement can occur during starting. The magnetic forces exerted on a
winding are proportional to the current squared. If there is any movement, individual turns can rub
against each other and result in a turn to turn failure.
The mechanism of turn to turn failure is varied. Recently the most common cause of failure in low
voltage motors is attributed to variable frequency drives that use devices with very fast rise times.
Very similar stresses can occur on medium voltage motors though drives are usually designed with
different topologies and/or slower devices, which reduce the problem. Large HP medium voltage
motors that are switched by vacuum breakers can be subject to very fast rise time multiple pulses
caused by prestrike on breaker closing or stresses caused by re-strike on breaker opening. This is
a separate topic unto its own but this issue can be very effectively dealt with by adding CR surge
suppressors at the vacuum breaker that switches the motor or by adding a damping resistor to the
surge capacitor at the motor.
When a turn to turn failure occurs, the shorted turn acts like the secondary of an autotransformer.
If for example, a coil has 60 turns per phase and the shorted turn has one turn, the current in the
short will be 60 times larger than the normal current. The heating effect is current squared times
resistance (I2R). This example would result in (60)2 or 3600 times the normal heating effect. It
doesnt take very long for this heat to start melting
copper and insulation. The resulting loss of insulation
manifests into a phase to ground fault, usually within
seconds. Typically this type of fault is recognized by a
lot of melted copper.
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Monthly Informative Application Guidelines, with respect to Motors & Drives to keep you better INFORMED.
Use a LOW-OHM resistance meter and check resistance between the phases. Check phases 1
to 2, record the reading, then 2 to 3, record the reading, and 3 to 1, record the reading. A regular
Fluke meter or similar meter will not work. It has to be a LOW- OHM resistance meter. Toshiba
can provide typical Ohm readings for our motors by rating and voltage. This test can also be
used to determine if nameplate voltage rating is correct, you can make comparisons with 460v
typical values and 575v typical values. Also, this test can be performed to determine if the motor
is the cause of an imbalance situation.
1
f =
o 2 LC
where,
fo = Oscillation Frequency
L = Inductance of the motor winding
C = Capacitance of the tester
If each phase has 100 turns, and 1 turn is shorted on one of the phases, the current changes
negligibly. L goes down 1% therefore fO increases by 0.5%. This is a very small change and only
becomes apparent when the two waveforms are superimposed on top of each other.
The switch above closes once the capacitors have charged up. This provides a fast rise time, high
voltage pulse to the windings. L1 and L2 represent two windings that are being compared.
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The Figure above shows the scope patterns of two windings. The pattern on the left shows the
oscillating frequency of a good coil. The pattern in the middle shows the oscillating frequency of a
coil with a shorted turn. They look very similar until they are superimposed on top of each other.
Note that the coil with the shorted turn has a higher oscillating frequency. This test is only effective
for detecting problems in the first few turns of a coil, which is typically where damage will occur
due to surges and switching transients.
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