Why Electric Motors Fail

Part 1. Single-phase motors.

By Ed Butts, PE, CPI

As we start a new year, I thought I would begin a four-part series on electric motors. In parts one and two, we’ll cover the causes of the failures and then we’ll go over troubleshooting techniques in parts three and four.

Figure 1. Submersible motor types.

There are many potential causes for improper operation and failure of an electric motor. Some, such as direct lightning strikes or high-voltage surges, are impossible to predict and generally avoid. Others like application and design errors, excessive cycling, improper operating environment, overheating, and overloading are within the designer’s power to control and prevent.

Over the next four months, we will explore the main causes of both single- and three-phase motor operating issues and failures. We begin this month with single-phase motor problems and will turn to three-phase motor failure issues in February.

We will then move into electric motor troubleshooting with methods and techniques for single-phase motors in March and wrap up the series with three-phase motor troubleshooting in April.

So, let’s get to it!

Robust Motors

Single-phase motors are surprisingly robust, especially given the extremes of cycling and heat they often endure. Electric motors used as submersible pump motors (Figure 1) use a specialized design and operation and comprise the majority of electric motors used for domestic water systems.

The three most common single-phase motor types used for water system applications are:

  • Capacitor start-induction run motors, typically used for ⅓ HP to 1 HP three-wire single phase submersible motors
  • Capacitor start-capacitor run motors, used for 1½ HP through 15 HP three-wire single phase motors
  • Split-phase motors, often used for single-phase jet and
    centrifugal pump and two-wire submersible motors.

The operational nature and harsh environment of submersible motors dictate a robust construction in which the electrical components are sealed and isolated against the very media they must work in against heads as high as 1000 feet (440 psi).

To fully understand why a single-phase motor fails, it is initially important to understand how they work.

The induction motor is the most common type of electric motor used in the water well profession due to AC power’s higher efficiency and lower voltage drop when transmitted over any appreciable distance over DC voltage and the motor’s design simplicity.

Figure 2. Simplified schematic of capacitor start-induction run motor (1/3 HP-1 HP).

An induction motor commonly consists of two basic components: an outside stator having wound coils that when supplied with alternating current produce a rotating magnetic field and an inside rotor attached to the output or drive shaft that produces a second counter-rotating magnetic field.

When electrical energy is introduced into the rotor of an induction motor to cause rotation, it is generated through electromagnetic induction from the constantly changing magnetic field of the stationary stator windings. A single-phase AC power supply consists of a sinusoidal waveform that produces a pulsating magnetic field in the uniformly distributed stator winding. Since the pulsating magnetic field consists of two opposing rotating magnetic fields, they offset and cancel out each other—thus there will be no resultant torque produced during the motor’s starting phase.

The result is the motor stalls and will not start or run. After energizing the motor, however, if the rotor is made to rotate in either direction by an adequate external twisting motion (torque) or electromotive force, the motor will likely start and accelerate to run.

Figure 3. Simplified schematic of capacitor start-capacitor run motor (1 1/2 HP-15 HP).

This situation can be alleviated by converting the stator winding into two separate windings: one is the lower resistance winding (the main or run winding), with the other being the higher resistance winding (the auxiliary or start winding). The circuit is generally completed by connecting a start capacitor in series with the start winding of a single-phase capacitor start/induction run motor (Figure 2).

To generate the needed starting torque, start capacitors provide needed capacitance to partially negate a portion of the motor’s induction to allow starting. They possess a greater microfarad (MFD) rating than run capacitors but are designed for a brief exposure to the electrical circuit. Therefore, they must be rapidly removed from the energized circuit as longer durations will result in rapid failure of the capacitor.

With larger single-phase motors, typically 1½ HP and larger, a single or multiple run capacitor are added in series with the main winding to increase the operating power factor and tune the motor for optimal performance. Figure 3 illustrates an elementary schematic for a capacitor start-capacitor run motor and Figure 4 is of a split-phase motor. When using a motor with a capacitor for starting or running, the rated size of the applicable capacitor must be suitable to function with the motor horsepower to provide the correct starting torque, but avoid overheating of the affected winding that can cause damage. This balance is essentially an element of the motor design.

Figure 4. Simplified schematic of split-phase motor.

A start switch is used to disconnect the start capacitor from the line voltage once the motor accelerates. This switch, used on all single-phase motor types, can consist of a centrifugal force action and an activated start switch, common to most jet or centrifugal pump motors, or a contact opened from the energized coil on an electrically operated relay.

Relays, common to submersible pump motors, can use motor generated voltage (potential), current, or solid-state sensing activation, which are commonly used in two-wire, split-phase submersible motors.

When Problems Occur

Most problems with single-phase motors involve the centrifugal start switch or start capacitor for above-ground jet pump motors or the start relay, thermal overload, or start capacitors for submersible motors. Rarely is the run capacitor the cause of system failure for motors larger than 1½ HP.

For submersible pump motors, except for two-wire motors, these components are generally found in an above-ground control box. If the problem is in the centrifugal switch or relay, thermal overload, or capacitor, the motor is usually serviced and repaired. Most above-ground jet or centrifugal pump motors are prime examples of a single-phase motor.

The principal causes of failure of these motors are capacitor, centrifugal start switch misalignment, or contact failure, as well as overheating due to inadequate airflow. As with most single-phase motors, rapid or excessive cycling will cause rapid heat buildup in the capacitor and arcing to occur on the start switch contacts, both eventually leading to component or motor failure.

As with submersible motors, above-ground motors must also receive an adequate volume and flow of cooling media in the form of air passing over the motor to dissipate and carry away the heat generated during operation.

Motors located in closets or small rooms, tight corners, or under shelves are particularly susceptible to heat-induced failure. Without adequate movement of air around and through the motor, the heat can continue to build up on and around the motor, eventually affecting the winding insulation, leading to failure.

Capacitors have a limited life and are often the problem in capacitor start motors. Capacitors may develop a short circuit, an open circuit, or may deteriorate to the point they must be replaced.

Deterioration can also change the value of a capacitor, which can cause additional problems. When a capacitor short-circuits, the winding in the motor may burn out. When a capacitor deteriorates or opens, the motor has poor starting torque. Poor starting torque may cause stalling or locked-rotor conditions and prevent the motor from starting, which will usually trip the overloads.

All capacitors are made with two conducting surfaces separated by dielectric material. Dielectric material is a medium in which an electric field is maintained with little or no outside energy supply. It is the type of material used to insulate conducting surfaces of a capacitor.

Capacitors are either oil or electrolytic. Oil capacitors are filled with oil and sealed in a metal container. The oil serves as the dielectric material. More motors use electrolytic capacitors than oil capacitors. Electrolytic capacitors are formed by winding two sheets of aluminum foil separated by pieces of thin paper impregnated with an electrolyte.

An electrolyte is a conducting medium in which the current flow occurs by ion migration. The electrolyte is used as the dielectric material. The aluminum foil and electrolyte are encased in a cardboard or aluminum cover. A vent hole is provided to prevent a possible explosion in the event the capacitor is shorted or overheated.

AC capacitors are used with capacitor motors. Capacitors that are designed to be connected to AC have no polarity. Issues with a submersible motor itself are generally the result of overheating caused by inadequate cooling flow or excessive cycling.

There are several reasons electric motors must not be permitted to excessively cycle (start and stop), particularly when driving a deep well pump.

The most obvious reason is the heat developed by the motor during each start cycle. Since a single-phase motor can draw up to 600% over its full load current during full voltage (across the line) starts, the heat buildup during each starting cycle must be adequately dissipated before another starting sequence is allowed. Otherwise, this excessive heat can result in rapid deterioration of the motor’s winding insulation.

There are many well-documented cases of recurring motor failure being addressed by increasing the horsepower rating of the motor, which generally shortens the time between failures, when the root cause of the failure was actually the frequency of starts and stops or cycles. In many cases, this will simply delay or postpone the eventual failure of the motor from the same root cause: excessive cycles.

The key is to closely monitor the number of hourly starts. The common rule of thumb states a typical insulation life is cut in half (50%) for every 50°F (10°C ) of additional heat to the windings. For example, if a motor that would normally last 20 years in regular service is running at 104°F (40°C) above its rated temperature, it would likely have an adjusted operating life of about one year.

There are five primary issues associated with the life of a submersible pump motor:

  • Avoiding motor overload
  • Maintaining a stable and adequate starting and running voltage
  • Ensuring proper cooling flow is passing the motor during operation
  • Avoiding excessive cycling
  • High voltage or lightning strikes.

Although significant overloading of any motor, particularly a submersible style, can result in premature failure, submersible motors used with domestic water pumps are typically designed and applied for the maximum duty or load of the attached pump.

This is performed by the pump manufacturer to create a pump and motor unit, which generally prevents continuous operation of a domestic pump outside of its service factor ability. Obviously, in cases with sand, low or high voltage, or other anomalies, overloading remains a real potential, but this factor generally does not apply as a detrimental life element for general design considerations.

Windings and Wiring

The most common type of single-phase submersible pump motor for residential use, the basic three-wire type, uses two separate motor windings to operate.

The first, the start winding, is a relatively high resistance motor winding designed to provide ample torque to the motor to overcome the head and pump inertia resistance to begin rotation of the locked rotor or to even get the rotor to turn in stalled conditions. The second winding, the running or main winding, is a lower resistance circuit intended to operate as the sole winding during normal operating conditions.

The commonality of these windings typically is vested in a distribution between the three legs that constitute a three-wire submersible pump motor. (Do not confuse these three wires with the three wires also used in a three-phase motor.)

Normally, the three leads of a single-phase, three-wire submersible motor are color coded to enable the installer, and future troubleshooter, to distinguish which wire is dedicated to which winding. The red lead is usually part of the starting winding, the black (or sometimes blue) lead is dedicated to the running winding, while the yellow motor lead provides the common connection between both windings.

Most submersible motors used for domestic water systems are designed to operate on 115- or 230-volt alternating current (VAC), single-phase electrical power with a ±10% voltage allowance each way (although three-phase power is occasionally used for larger motors and PMM service).

This means a 230-volt nominally rated motor is designed to operate on a voltage as high as 253 volts or as low as 207 volts. The allowable voltage drop during starting that is required to provide the motor’s minimum starting torque is generally recognized as 35% of the rated voltage.

This translates to a minimum starting voltage at the motor of 230 VAC × 0.65 = 150 VAC. Most utility single-phase power supplies are now provided with a nominal voltage of 120/240 VAC, which provides a built-in reserve of 10 volts or 4.35% as an allowance for voltage drop in the circuit.

Providing adequate voltage for starting and operating a submersible motor is largely a mathematical relationship. In some cases, the use of a boost or buck transformer may be needed to maintain the operating voltage within the specified limits. Motors are usually referred to as “three-wire” or “two-wire” motors, which also describe the method of motor starting and running.

A three-wire motor operates as a capacitor start-induction run (or capacitor run for more than 1½ HP) motor, while a two-wire motor is either a split-phase or capacitor start-induction run motor with the start capacitor and relay often located within the bottom of the motor itself.

Although two-wire motors are popular in many locations, they are limited in available starting torque and may have problems reliably starting in severe applications with sand or low voltage. They are often referred to as one-time motors, as the first time the capacitor or relay happens to fail is also the first time the motor must be pulled from the well.

Most motors of current design use exchanged and filtered well water for motor cooling and lubrication, although there are still a few submersible motor manufacturers who use oil-lubricated and cooled motors, especially when using the two-wire capacitor-start method.

To facilitate motor cooling, most submersible motors are constructed using stainless steel for the outer shell. As with  4-inch submersible pumps, the efficiency of 4-inch submersible pump motors is also less than above-ground motors of comparable horsepower or larger submersible motors. A ½ HP to 1 HP single-phase motor will exhibit an efficiency between 55% and 65%, while larger (1½ HP to 5 HP) submersible motors will generally be between 65% to 75% efficiency.

The second common issue with submersible pump motors is maintaining an adequate cooling flow past the motor during operation. Certainly, an important factor for all electric motors, maintaining proper cooling and lubrication is a critical design and application factor for a submersible motor.

If the requirements for providing the appropriate flow rate and velocity past the motor exist as an initial design consideration, most of any last-minute changes made in the field can be avoided. Cooling of the motor is a relatively simple process of forcing the entire (or partial) pumping flow rate to go past the motor before entering the pump suction.

In most cases, this flow rate will be adequate to provide the minimum water velocity that is needed to remove and carry away the heat generated from the motor during operation to the pump. Even though this value may vary slightly between manufacturers, as a rule of thumb, a minimum annular (area between the motor and well casing) velocity of 0.25 feet per second (FPS) is required for a 4-inch-diameter motor and 0.50 FPS for a 6-inch motor.

These values can be met in several ways. The simplest is to divert and force the pumped water to travel by the motor on its journey to the pump inlet in a bottom-fed application or by using a shroud for a top-feed installation or where the differential area between the motor and well casing sizes is too great to provide the minimum velocity.

Table 1 can be used as a source of the flow rate needed based on the annular area between 4-inch and 6-inch motor diameters versus well casing or shroud sizes.

For example, if all water is fed into an 8-inch well through a screen below a 4-inch motor, all flow will obviously have to pass the motor on the way to the pump suction. Therefore, maintaining the minimum flow rate is the only factor.

Using Table 1, the minimum motor cooling flow rate required for this annular area to maintain 0.25 FPS of velocity is 30 GPM, (plus all water is fed below the motor as illustrated in Figure 5), ensuring the entire cooling flow is passing the motor before entering the pump suction screen as long as the sustained flow rate is 30 GPM or more.

Figure 5. Submersible motor cooling options.

The next common issue with a domestic submersible well pump motor is to design the system to prevent excessive cycling between the starting and stopping setpoints. Whether the system is set up using conventional pressure tank water storage or a constant pressure system, the goal should be a minimum cycle time of one minute, with one-and-a-half to two minutes preferred.

With a pressure tank system, this is simply a matter of providing an adequate volume of pressurized water storage to necessitate one minute of minimum pump running time between pressure setpoints by matching the midrange pump capacity.

With a constant pressure system using a control valve or variable frequency drive, the system must be fine-tuned to ensure the pump operates at a minimum flow rate without shutting down and quickly restarting. This may require a greater pressure tank volume than originally thought, but it is better to spend $500 on a second pressure tank now than to shell out three times or more of this amount in a year because the motor failed from rapid cycling.

Finally, high-voltage electrical surges through the motor, primarily those caused from utility surges or induced lightning strikes, must be avoided if at all possible. Although a direct lightning strike on a well pump circuit will likely result in motor failure since the voltage value and needed response time are generally outside the equipment’s ability to react fast enough, rerouting a low-intensity, high-voltage surge or partial lightning strike is theoretically possible if properly performed.

This requires the connection and grounding of a gap-type (thyristor) lightning arrestor, usually provided as an element of a standard control box, to a well-grounded, low-impedance electrical path such as steel well casing to a minimum depth of 100 feet or a series of buried ground rods or plates.

Connecting the lightning arrestor only to the utility’s ground grid is generally not adequate to totally bypass a typical electrical surge away from an operating submersible motor; thus, the motor can easily incur a partial hit to the windings, often resulting in weakening of the insulation and leading to premature failure. The key is to locate and securely connect the grounding conductor of the lightning arrestor to a reliable and low-resistance ground path and the arrestor’s line connections to both sides of the AC circuit.

Until next month, as always, work safe and smart.

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.