Why Submersible Motors Fail – Part 1

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.

Single-Phasing:

Single-phasing on a wye-delta three phase power distribution system can be disastrous to a three-phase motor, unless it has excellent overload protection. There are two types of single-phasing; primary and secondary. Primary single-phasing (see figure 1) occurs when one line on the high voltage or primary side of the transformer is opened. This can be caused by a tree limb falling across the lines or a car accidentally hitting a power pole. Single-phasing of thz primary causes the motor amperage on two of the three lines to increase by 115%, while the third line increases by 230%.

Secondary single-phasing (see figure 2) occurs when one line on the motor side or secondary side of the transformer is opened. This can be caused by storm damage, loose connections or insulation problems in the wiring that blow fuses. Single- phasing of the secondary causes the motor amperage on the remaining two lines to increase by 173%, while the third line drops to zero.

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.