Avoiding Possible Problems in
Submersible Motors
Motors
Typical agricultural, domestic and municipal systems are
excellent applications for these motors. Unfortunately, these
motors often are used in applications that unknowingly exceed
the design criteria of the motors.
- Tom Sgritta
Submersible motor use in municipal systems is not new. The
advantages of submersible motors are well known to this
industry. Since these motors are placed deep into the earth, in
a relatively stable environment, they are immune to the weather
and environmental factors that plague hollow shaft motors
driving conventional line shaft turbines (LSTs). Submersible
motors and pumps eliminate the drive shaft and bearing systems
of LSTs, thus reducing the mechanical complexity and required
maintenance. Oil does not drip into the well for bearing
lubrication. Wells can be located adjacent to housing areas.
Submersibles also do not require structures to enclose them and
do not produce surface noise.
Problems
Standard water well types of submersible motors are water
(water/glycol) filled and rely on water as the internal
lubrication for the motor. These motors are extremely reliable
when applied within their design limits of temperature,
hydraulic loading and power requirements. Typical agricultural,
domestic and municipal systems are excellent applications for
these motors. Unfortunately, these motors often are used in
applications that unknowingly exceed the design criteria of the
motors. As a result, failures occur, and the advantages of
submersible motors are lost or quickly forgotten.
The following problems are typical in municipal applications
and result in the failure of motors.
Temperature
A very common problem affecting submersible motors is
over-temperature. Causes for over-temperature include pumping
hot water, overloading of the motor by the pump, loss of cooling
flow past the motor, ochre or scale buildup and frequent motor
starts and stops.
Submersible motors somehow must cool themselves. This is
accomplished almost universally by transferring the motor's
internally generated heat to the water that is flowing past the
motor and into the pump. Most standard water well motors are
designed to do this but add little safety margin (safety margins
add cost).
In submersible motors, the thrust bearing supports the pump's
thrust weight of the water column being lifted by the pump. In
standard water well motors this thrust bearing is a water
lubricated "Kingsbury" type of bearing. A very small film of
water between the main elements of the thrust bearing provides
lubrication between the two bearing surfaces. If the motor
overheats for any reason, this water film can approach its
boiling point. If it boils, the lubricating film is lost. At
this point, the bearing surfaces come into contact with each
other and rapid heating takes place. Catastrophic failure of the
thrust bearing is likely to occur.
Stator failure is another problem that occurs when motors
overheat. Typical wet wound, water-filled submersibles use a PVC
insulation that insulates the copper windings while immersed in
water. This wire usually has a maximum usable temperature of
between 70º C for standard motors to about 100º C for higher
temperature motors. Once these temperatures are exceeded, the
insulation system is damaged and a winding turn to turn, winding
phase to phase or winding phase to ground fault becomes likely.
Once these faults develop, failure of the motor is unavoidable.
Hydraulic Loading
Another problematic area for submersibles in municipal
applications is hydraulic shock loading or water hammer. Water
hammer occurs when a rapidly moving column of water encounters
an obstacle or suddenly changes velocity. The use of multiple
pumps on a common supply manifold is a prime cause of water
hammer. When a pump turns on or off, water hammer is generated.
Additionally, when any kind of valving is actuated, water hammer
can occur. (Fast acting, electromechanical valves can be the
worst.)
Check valves in the pump discharge string and at the well
head are recommended by all manufactures to reduce water hammer.
Unfortunately, check valves may be of the wrong type (swing vs.
spring-loaded), may be simply not used or may become corroded
over time (spring corrosion is a common problem).
When water hammer occurs, there is a sudden down thrust
transmitted through the pump onto the motor's thrust bearing.
This can cause several undesirable things to happen.
Most standard water well motors use some type of rotating
carbon thrust bearing running on stainless steel thrust pads.
Carbon is an excellent material for bearings when the lubricant
is water and water hammer is not severe. Carbon, while very hard
and durable, is a brittle material. When hard, brittle materials
are subjected to shock loading they can shatter. Extreme water
hammer is known to completely shatter thrust bearings, causing
motor failure.
Standard thrust bearings in water-filled motors are
lubricated by a thin water film. Water has a low viscosity. When
put under a shock load, the water film in the bearing can be
driven from between the bearing surfaces resulting in
insufficient lubrication. This condition can lead to hot spots,
excessive wear and bearing failure.
Motor Seals
The seals at the motor's shaft keep the well fluid from
getting into the motor. Both water-filled and oil-filled motors
need to do this in order to keep abrasives out of the internal
motor bearings. Additionally, in aggressive water, the low pH of
acids or high salt levels can cause internal corrosion if
allowed into the motor.
Standard water well duty submersible motors use a single
mechanical seal. This seal can be of carbon/ceramic or
silicon/carbide. Silicon carbide is considered a premium seal
and is a harder seal than carbon ceramic. It is very useful in
sandy well applications.
Voltage
Submersible motors, like any electric motor, require a good
voltage supply at the motor terminals. A leading cause of
submersible motor failure is under-voltage or voltage spikes.
Under-voltage typically is caused by sizing the drop cables
(supply cables) too small or by the utility grid supplying low
voltage to the site. Ohms law is always in effect in electrical
circuits (V=I ¥ Z). Voltage equals the current flowing in a
circuit multiplied by the impedance (the resistance of a cable
is usually the majority of the impedance).
As smaller diameter drop cables are used (they are cheaper),
the resistance increases (less copper = higher resistance). As
the resistance of the lead goes up, the voltage drop in the lead
goes up as well for a given operating current.
For a fixed supply voltage from a transformer, as the voltage
drop in the lead goes up, the terminal voltage at the motor must
go down. As the voltage at the motor terminals fall, the motor
must have more current to produce the same horsepower (which
induction motors want to do). Eventually, too much current is
flowing for the size of winding wires used in the motor and
internal overheating begins to take place. This leads to failure
of the motor.
In large motors commonly used in the municipal industry,
large power cables typically are required and used. If the
setting of the motor and pump are deep, very large and very
expensive cables must be used in order to provide the rated
voltage at the motor's terminals.
An economical alternative to this is to design the motor to
operate at higher voltages. If a motor is designed to operate at
higher voltages, the required full load current will go down. As
the current required goes down, the size of the drop cable can
be reduced as well. Smaller drop cables use less copper and are
less expensive-sometimes much less expensive.
Consider a 200 hp motor. If the motor is designed for 460
volts, the full load current is 247 amps. If, however, the motor
is designed to operate at 2,300 volts, the full load current
falls to 49.4 amps.
In this situation, if a 500 foot setting depth is used, the
recommended drop cable goes from a size 250 MCM cable for the
460 volt motor to a #8 size cable for the 2,300 volt motor.
Higher voltage cable and components with better insulating
values must be used, but the cost of these typically is more
than offset by the savings in the copper of the drop cable.
Smaller drop cables also are much easier for the installation
crew to handle.
Voltage Spikes
A very serious problem for all induction motors is voltage
spikes. These spikes typically are very short in duration and
high in voltage. These spikes can be generated by lightning,
other motors turning off or utility switch-gear opening.
In municipal well fields with multiple pumps and motors
running, this can be a serious problem. Each time one of these
motors is switched off, the stored inductive energy in the
magnetic circuit of the motor is dumped back onto the power
lines. This creates a very short but very high voltage spike.
These spikes often are of a greater voltage level than the
motor's insulation system can tolerate and will burn through the
insulation. Once this occurs, the motor has a potential site for
an internal short and current can flow in a path that it was not
designed for. Typically, this will raise the temperature of the
windings near the shorted spot and further burning will take
place until a catastrophic failure occurs.
If the voltage spike is large enough and has enough energy
(e.g., lightning strike), it can completely burn a hole in the
case of the motor.
There is little that motor manufactures can do to protect
against severe voltage spike problems. For this reason, external
surge protection mounted near the motor starter is recommended.
This tends to "clip" the voltage spike before it can travel down
the cables to reach the motor. These surge protectors do nothing
until the voltage reaches a certain critical value. At that
point, they begin to conduct current and continue to do so until
the voltage falls below the critical value. Large power
distribution transformers that feed these motors also should be
protected because these large voltage spikes can damage them as
well. Warning: Consult with local suppliers or engineers for
properly selecting and applying these devices.
These are the typical problems that occur in municipal
operations using submersible motors. Failures never seem to
occur at a convenient time. They are always a nuisance at best
and more often a major problem.
Tom Sgritta is the global
submersible marketing manager-industrial for Franklin Electric,
Bluffton, Ind. He also is a member of the faculty of the Belk
College of Business Administration at the University of North
Carolina at Charlotte.
Source: Water Engineering &
Management January 2002 Vol: 149 Num: 1
Copyright © 2005 Scranton Gillette Communications
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