Submersible Motor Megger
Testing
Meggers, or Megohmeters,
are often used by service personnel or sophisticated end users as an immediatetrouble shooting device.
Unfortunately, Megger results are not appropriate for use as a pass/fail
determinationdue to the many
variables and judgments involved. Some of the variables to be aware of are:
length of cable, size of conductors, ambient temperature and humidity, status of
the motor (new, submerged, previously submerged), with or without cap attached
and even coiled cable vs laid out cable. Consideration for all these variables
should be made when using a Megger reading to evaluate the condition of a
submersible motor.
Meggers are best used as
a trend analysis tool. To accomplish this, a test procedure should be
established andused consistently over
time. The above variables should be controlled as much as possible to promote
accurateinsulation resistance
measurement data. After a benchmark is established, periodic measurements should
betaken and recorded along
with notation of temperature, humidity and duration of the Megger test. This
proceduremust be continued for
subsequent tests to accurately determine if the rate of insulation breakdown is
occurring at an abnormal rate. This determination is also very subjective and
should be based on sound judgment andexperience.
The reason why the term
"subjective" is used above, is that often electrical equipment manufacturers and
industry standards give vague information and/or general rules of thumb that do
not necessarily apply to submersible electric motors where the cable actually
becomes an integral part of the motor. These guide lines normally have large
plus or minus tolerances which makes trend analysis the only logical approach
for the use of Megohmeters readings.
There are two
characteristics that make submersible motors substantially different from
standard electrical motors or other electrical devices that can be Meggered. The
first is that the electrical cable for a submersible motor is an integral part
of the motor as a whole. Most other electrical apparatus is measured exclusive
of a cable. The other big difference is that the motor and a portion of the
cable is normally submerged in water or wastewater and is subject to ambient
moisture. Both of these conditions greatly affect Megohm readings. Both
conditions must be given allowances for lower Megger readings when compared with
other electrical equipment or when using standard guidelines for motors or
equipment that is operated "in-air" or are not subject to the above factors.
The cap & cable
assemblies of a submersible pump motor are generally 25-50 feet long and of a
relatively largediameter because the
contain both power and control wires. These larger cables provide a large
surface area that is exposed to wastewater and ambient moisture. Because they
are also submerged in an extreme environment, they are more susceptible to
damage. This unique environment and operating conditions contribute to two
physical phenomena: 1. changes to the outside insulation resistance by exposure
of a greater surface area compared to standard in-air cable and 2. increased
dielectric effect expands overall capacitance. The following is an explanation
of the effects that these two phenomena have on producing a lower Megger
reading.
- While
the cable jacket is a very good insulator, it is not perfect. There are no
perfect insulators. A good conductor requires less surface area for electrical contact to conduct more
amperage with minimal loss. Conversely, a poor conductor requires more surface area to conduct
the same amperage when compared to a good conductor. When a cable is submerged in
wastewater, there is a great amount of its outside surface area in contact with water. Water is a good
conductor when compared to air. This overall coverage of water on the jacket of the cable acts
like hundreds of resistors connected in parallel, reducing the total resistance as demonstrated
by Ohms Law (4), where Multiple Resistors in Parallel in Ohms = (1/ ((1/R1)
+ (1/R2).......+
(1/RN) a
division of resistance. This overall reduced resistance will be manifested in a lower Meg Ohm reading. This
lower reading does not necessarily mean the cable insulation has electrically broken down or that
there is moisture inside the cable or motor.
- The motor and cable act as a large capacitor. When the motor and part of the
cable are submerged, the water acts as an additional dielectric component to increase the electrical
potential as a capacitor. When a Megger or a multi meter is connected to take readings, a voltage is
actually being supplied. At initial contact, a low resistance reading will be observed. When the meter is
connected for a duration, it will gradually and continually exhibit increasing Ohm readings. What is
actually happening is the "charging up" of the effective capacitor. Resistance times a capacitance
constant is involved. It may take six to eight hours before a full charge is achieved and a "true" reading is
obtained.
Megger testing is not an
exact science, and should not be used as an immediate conclusion of whether a
motor is good or no good. It is a useful tool if it is used to measure against a
benchmark in an effort to analyze the quality of insulation over time.
The Submersible
Wastewater Pump Association (www.SWPA.org) has published an excellent manual on
theselection, installation,
operation and maintenance of submersible pumps. This book includes some valuablereference data such as a
guide to Insulation Resistance Readings. This data is restated below:
Yeomans Chicago Corporation manufactures and
markets submersible pumps under the Yeomans Pump and Chicago Pump
Company product brands. We also manufacture
submersible motors on an OEM basis. Submersible motor sizes range from
fractional to 200 HP and are available in a
variety of speeds and voltages. Motor frame sizes use standard mounting flange
sizes ranging from 140TY to 360TY. For more
information, please contact Yeomans Chicago Corporation or visit our website
at
www.YCCPUMP.com
Revised 11/08/00 © 2000 Yeomans Chicago
CorporationUsed with
permission from Yeomans Chicago Corporation., Copyright 2000. All rights
reserved
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