| As we look at our submersible motors
and their usage, we keep one goal in mind. That goal is quality. Quality
products and installations equal long service life and long service
life generally equals satisfied customers. Over the years, Franklin
has reviewed many motors returned from the field. Along with looking
at the returned motor itself, Franklin examines numerous applications
and systems, looking for problems which contribute to premature motor
failure. In the next few issues of Franklin AID we will share with
you some of the ways you can avoid application related problems and
get the longest life out of your pump installation.
Although several items in this article apply to
single-phase motors and systems, the majority is on three-phase
installations. Basically there are three types of motor failures;
electrical, mechanical, and mechanical failures that progress into
electrical failures. In this issue, we will focus on the electrical
side.
Eighty percent (80%) of motor electrical failures
are a result of stator winding burnout. Most winding failures occur
due to primary or secondary single-phasing, extreme high or low
voltage, phase unbalance on three-phase motors, high voltage surges,
or direct strikes of lightning. The good news is that in most cases
these conditions are preventable.
The best way to prevent the above winding failures
in three-phase motors is by using properly sized time-delay fuses
in conjunction with Class 10, ambient-compensated overload protection
and a good quality surge arrestor. While Franklin adds overload
protection inside the motor on 4-inch, 60 Hz, single-phase motors,
you still need to use properly sized time-delay fuses and a good
quality surge arrestor for complete protection of single-phase systems.
In order for a surge arrestor to be effective, it
must be grounded to the water strata. Water strata is the actual
water underground. Any surge in the system is looking for the easiest
path to true water ground. The faster this surge is directed to
ground, the less damage it can cause to your system. Grounding the
arrestor to only a driven ground rod may not be an adequate ground
as the resistance through the soil is higher in some areas than
others. Higher resistance means the surge will look for an easier
path to ground, which may be through your motor. Connecting the
ground wire from the arrestor directly to the motor is the best
ground available. A ground wire from the motor has been a US National
Electrical Code (NEC) requirement since 1990, so the wire should
be readily available. Other potential ground sources are metal well
casings and metal drop pipes that are in direct contact with the
well water. However, not all wells are cased all the way to the
water, such as a rock well. In some situations, a metal well casing
or drop pipe may be adequate ground for a surge arrestor, but a
motor that is not grounded to the metal casing and the service entrance
does not meet 1990 and 1993 NEC requirements.
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| Voltage Effects:
High voltage and low voltage affect the operating amperage of the
motor. Franklin designs the motor windings to tolerate a voltage range
of plus or minus 10% from nameplate voltage. In this voltage range,
the amperage changes very little due to voltage fluctuations. However,
once the voltage is outside of this 10% range, the motor cannot do
its job without excessive heating of the windings. High voltage causes
the motor windings to saturate, while low voltage starves the motor
of power. Note: Both high voltage and low voltage cause high amps
in the motor. High amps are defined by an amperage reading that
exceeds the nameplate service factor maximum amp rating (S.F. max.
amps). If you envision amperage of the motor to a car's tachometer,
amperage higher than service factor maximum is like a tachometer reading
into “redline”. Nobody knows how soon the car will quit,
but everyone knows the engine is suffering damage.
Unbalance: Current
unbalance on three-phase motors is caused by unequal voltage being
presented to each winding. A 1% voltage unbalance will result in
approximately 6-10% current unbalance. This unbalance causes extreme
heat in the motor windings. When the motor is lightly loaded (amperage
at rated or full load amps), a 10% current unbalance is not harmful
to the motor. When a motor is loaded to or above service factor
maximum amperage, a current unbalance greater than 5% will cause
excessive heating. Excessive heat build-up in the motor windings
greatly affects the life of the motor. For every 10°C the internal
winding temperature is increased, the life of the motor is cut in
half. For instance, if the motor is normally designed to have an
internal temperature of 30°C with a life expectancy of 10 years,
raising the winding temperature to 40°C cuts the life to 5 years.
An increase in winding temperature to 50°C shortens the life
to 2-1/2 years. Current unbalance and the resulting winding temperature
must be avoided for normal motor life expectancy.
In the next issue of Franklin Aid we will continue
to discuss “Why Submersible Motors Fail”
Part 2
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