Why Motors Fail: Part 1

When you think of Franklin Electric, we hope that quality and durability come to mind. That is what we strive for everyday, and after decades of service to the water industry, we are proud to continue providing quality products and reliable support. However, even in the most reliable applications problems can occur, causing a motor to prematurely fail. In the next two issues of the Franklin AID, we’ll examine the most common reasons for application motor failures.

Motor failures come in three basic types: electrical, mechanical and mechanical that progresses into electrical. We’ll first focus on electrical motor failures and next issue will look into mechanical failures. Both will share tips for avoiding application-related problems and getting the longest life out of your pumping installation.

About eighty percent (80%) of electrical motor failures are a result of winding damage in the motor stator, due to either overheating or voltage issues. Electrical motors take electrical energy and convert it into mechanical energy, producing heat as a by-product. In submersible motors, that heat is removed by the flow of the water past the motor. Overheating is always either caused by a lack of cooling flow, or the generation of more heat than can be displaced.

Electrical current (amperage) that is higher than normal poses a problem to the motor. As amperage in the motor increases, the amount of heat on the windings increases. This build-up of heat in the windings affects the life of the motor. For every 10 °C (18 °F) the internal winding temperature is increased, the life of the motor is cut in half. For example, 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 five years. This rule-of-thumb applies to all motors, not just submersibles.

High amperage can be caused by several conditions creating bound pump, dragging impellers, and high, low or unbalanced voltage. It may seem counterintuitive, but voltage that is either too high or too low causes high amps. Franklin Electric designs voltage windings to operate up to plus or minus 10% from nameplate voltage. For example, a 230 Volt motor needs to see voltage between 207 and 253 Volts. Once the voltage exceeds this range in either direction, the motor cannot run without excessive heating of the windings.

A three-phase motor is designed to operate with three equal voltages and any voltage imbalance will lead to an even higher current unbalance. For example, a 1% voltage unbalance will result in approximately 6-10% current unbalance, causing overheating in the motor windings. At service factor amperage, the current imbalance in a three-phase motor should not exceed 5% (see page 34 of the AIM Manual for details and how to correct). Any current that significantly exceeds service factor amperage (found on the motor nameplate and the AIM Manual) is considered high amperage.

The extreme case of three-phase imbalance is single-phasing. That is, when one of the three phases is lost. There are two types of single-phasing: primary and secondary. Primary single-phasing occurs when one line of the high voltage (primary side) of the transformer is opened. A tree limb falling or a car hitting a power pole can cause this. Single-phasing of the primary can cause the motor amperage on two of the three lines to increase by 115%, while the third line increases by 230%. Secondary single-phasing occurs when one line on the motor side (secondary side) of the transformer is opened. This can be caused by storm damage, loose connections or insulation problems in the wiring. 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.

High voltage surges or spikes are usually the result of close proximity lightning strikes, power line switch gear or the removal of large inductive loads from power lines. These spikes and surges can travel to the motor windings and break down insulation resistance. Franklin Electric motors can handle momentary voltage surges up to 10,000 Volts; however, power surges are not limited to that magnitude of voltage. That is why surge arrestors, capable of handling multiple hits, are recommended for submersible motors without internal arrestors. Surge arrestors are provided with most Franklin 4-inch, single-phase motors. Surge arrestors will need to be provided as a separate component with three-phase motors. Keep in mind that proper grounding is key to the effectiveness of a surge arrestor. Any surge is looking for the easiest path to the ground and there is little advantage to an arrestor unless it is grounded to the water strata.

Proper overloads are critical in protecting a submersible motor electrically. Overloads protect motor windings against heat damage caused by high current. The job of the overload is simple-detect the condition and take the motor off-line. All Franklin Electric 60 Hz, single-phase motors have overload protection built-in, either in the motor itself or in the control box (see the May / June 2011 edition Franklin AID for more details).

Franklin three-phase motors do not come installed with overload protection; you will need to supply the protection in the three-phase panel. Three-phase panels from Franklin Electric come supplied with the overload protection you need. These overloads can be one of three types:
1) the traditional heater type
2) adjustable overloads such as the ESP100
3) the electronic type such as SubMonitor.
No matter which type of overload is used it must be a Class 10 overload or capable of providing Class 10 protection. Improper overload selection and adjustment is one of the leading causes of three-phase motor failures. Overloads are also designed to protect the motor from extreme cases of overcurrent. For the best protection against a complete range of scenarios, an electronic protection device such as Pumptec (single-phase) or SubMonitor (three-phase) is ideal. For more information on three-phase overloads see pages 29-31 of the Franklin Electric AIM Manual.

Even when the unexpected happens, Franklin Electric encourages you to be prepared by understanding the cause and effect of electrical application motor failures. With a Franklin motor you can feel confident in your installation and by taking the proper steps you can better provide a long motor service life.