Why Electric Motors Fail

Part 3. Troubleshooting single-phase motors and controls.

By Ed Butts, PE, CPI

The past two columns in this four-part series were dedicated to an outline on single-phase motor technology and an introduction to three-phase motor technology. Now we will discuss basic troubleshooting techniques for single-phase motors and controls.

Single-Phase Submersible Pump Installation Troubleshooting

Effectively troubleshooting a single-phase submersible water pump installation is often a combination of initially identifying and fixing the problem itself and then correcting the underlying cause.

This is because the problem will usually help guide the troubleshooter to identify the primary reason for the failure. Multiple failure routes are responsible for the system failure in some instances, resulting in cascading failure modes.

As with a three-phase pumping plant installation, a single-phase submersible water pumping system possesses four components that must be separately addressed as potential causes of failure. In my case, I usually go down the line in the following order of four steps (Figure 1):

  1. Power supply and management/protection devices
  2. Motor control devices
  3. Drop and offset cable
  4. Motor.

1. Power Supply and Power Management/Protection Devices

Safety must always be the prime consideration when working with electrical equipment. De-energization of the electrical equipment when working on or in it and knowledge and use of lock out–tag out rules is necessary to ensure personnel safety.

Figure 1. Simplified one-line single-phase power diagram.

The branch circuit breaker or fuse is technically a part of the motor power circuit, but I tend to look at these devices as elements of the motor management and protection process. I generally begin the troubleshooting process with an examination of the circuit protection method and the power supply as it’s the easiest to check and verify.

A multimeter or voltage tester (Wiggy) can be used to quickly determine if the proper voltage is present at the pressure switch and control box terminals.

For 230-volt motors, it is vital the procedure involves verification of both 115-volt legs to the pressure switch and control box and not simply checking 115 volts to ground. This is because the motor and control box circuits can create a feedback loop, diverting the active leg through the motor and controls and back to the inactive leg, making you believe the proper power is feeding the motor when in fact only one leg is active.

This is particularly important when fuses or two single pole breakers without a tie-bar to create a common trip. I suggest initially checking the power at the terminals of both pressure switch and control box as pressure switch contacts can become dirty, pitted, or layered with the debris of a dead bug or rodent, interrupting the power to the control box.

In the proper circumstances, this can also cause a short circuit. I can cite numerous instances when the troubleshooting process stopped at this step, resulting only in the replacement of the pressure switch.

If the power is interrupted at the branch circuit protection device, examine the device for clues. A circuit breaker may indicate a short circuit by immediately tripping upon reset or an overload by remaining engaged for a few seconds before tripping.

If dual element plug fuses are used, examine the window into the fuse element. Is the window blackened, indicating a short circuit? Or is the spring retracted, indicating an overload? Also, check the load connections for a circuit breaker and fuse, as well as the integrity of the fuse attachment clips or plug when applicable, as these often become loose over time from repeated cycles of heat expansion and cooling contraction.

If either protection device immediately trips or blows upon power restoration with the motor or control disconnected, the likely problem resides in the external wiring between the power supply and the controls.

Once the power is verified to be available, stable, and within the high and low acceptable static (pump off) values of +/–2% of rated voltage, the troubleshooter should proceed to the next step.

2. Single-Phase Motor Control Devices

For most single-phase, three-wire submersible pump motor installations, the two most obvious control devices that generally apply are the pressure switch and control box.

Figure 2. Elemental control box schematic.

The pressure switch is a common component to virtually all domestic pump installations and often represents the single point of failure. Although a snap-action pressure switch generally functions well and evenly engages both contacts upon pressure decline, there have been instances when one or both of the switch contacts were misaligned or pitted, or the bellows or tripping device failed, resulting in incomplete closure of the contacts.

Therefore, full voltage should always be verified across both line and load terminals. This potential source of failure is doubled in failure potential for larger motors that utilize a magnetic contactor for motor control.

In addition to the possible problems that can be encountered in the pressure switch, the open nature of the magnetic contactor when de-energized can also invite bugs and rodents into the device as a hidden home. When the contactor is commanded to close, these critters can become lodged between the contacts, not only resulting in their demise, but also interfering with the electrical continuity.

In addition, the coil that operates the contactor is also prone to failure, particularly from excessive cycling or high voltage surges. A control box for 1/3 to 1 HP motors (Figure 2) typically houses at least three components:

  • The start relay
  • One start capacitor
  • One overload relay, which is usually part of an assembly as a component attached to the start capacitor.

In addition, 1½ HP and larger motors also use at least one running capacitor, and motors that are 2 HP or larger use two overloads: one to protect the start winding and the other to protect the main or run winding.

Larger HP control boxes (e.g., Franklin Electric’s Deluxe control box) are also typically equipped with one or several start and run capacitors wired in parallel and a line to load contactor used to start and run the higher load motors (Figure 3).

In various HP sizes, forms, and types, the control box has been around as long as submersible motors, and although for many smaller motors it may have been abandoned in favor of simpler two-wire motors, their use is still quite common.

There are several reasons for the continued use of a three-wire motor with a control box on domestic water systems.

Two are obvious. The first is that higher motor torque is retained with the three-wire motor design and the use of capacitors contained within the box, important for hard starting and sandy well applications. The second is easier access for troubleshooting of motor starting components that are housed above-ground, and therefore accessible.

Figure 3. Franklin Electric Deluxe control box schematic.

Single-phase motor starting and often running characteristics are vested in the use of capacitors, which are frequently the most common defective component. Capacitance is defined as the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. Thus, a motor capacitor is an electrical device that alters the current to one or more windings of a single-phase AC induction motor by storing and then releasing an electrical charge needed to create a rotating magnetic field.

A capacitor is a device that generally consists of two conducting surfaces, which are frequently referred to as plates, separated by an insulating layer usually referred to as a dielectric. An insulating oxide layer of gel or liquid is used for start capacitors and oil is generally used as the insulating media for run capacitors. The accumulation of an electric charge on the plates results in its capacitance.

For water well work, values of capacitors are usually expressed in microfarads, which are defined as one-millionth of the much larger value of a farad, and their rated voltage. Capacitors for well pump motors are designed and used as start or run capacitors. Common voltage ratings for a start capacitor are 110/125V, 165V, 220/250V, or 330V.

Checking a capacitor for functionality but not capacity can be conducted with an analog ohmmeter (e.g., Simpson model 372). After removing any residual charge by shorting the two terminals, the ohmmeter is set to Rx1000 and its leads are attached to the capacitor terminals. The needle should swing to the far right of the scale and then slowly return to its initial position.

An open capacitor will not move the needle while a shorted capacitor will move the needle to the right side of the scale and remain.

While this test can provide a basic confirmation of functionality, it cannot reliably indicate the actual capacitance. For this, use of a capacitor tester is required.

As a byproduct of their high capacity, start capacitors are only intermittently rated and can only be energized for a few seconds at a time before total failure occurs. For this reason, a start capacitor is usually one of the first components to fail when used on single-phase electric motors.

Their presence with a submersible motor represents an opportunity to often use a troubleshooting trick with a sandy well or tight motor that may be having trouble starting. During my career, I have often run across an installation where the well pump may have trouble starting due to a tight motor bearing, or maybe a small rock or grain of sand stuck under or within the first pump stage.

When reversing the motor won’t help (by temporarily interchanging the red and black wires), try temporarily swapping out the start capacitor for one twice the size or add another one of the same size and voltage to the existing capacitors by using a parallel connection. The extra, temporary torque generated by the motor will often be just enough to spin out the rock or overcome the sand or tight bearing and get the motor up to speed.

If a motor bearing is truly tight or worn, this will only be a temporary fix measure as the motor is likely doomed, but this often works to get a family temporarily back into water.

We can practically see that the motor will not rotate and remains in a stalled or locked rotor condition when the start capacitor circuit becomes disconnected (i.e., opened) from the motor since this also opens the start winding.

However, if the rotor is rotated by hand, possible with an above-ground motor, it will generally start to slowly rotate on the main winding and produce a low buzzing noise. On the other hand, a capacitor that is shorted (i.e., closed) will usually allow the motor to attempt to slowly turn and try to start since the winding is also shorted through the capacitor.

This generates a varying sound pitch, a buzz or hum, as the motor tries but usually fails to start. This is the reason an electrolytic start capacitor with a higher microfarad (MFD) value is used with a single-phase capacitor-start/induction-run motor.

An oil-filled but lower MFD rated run capacitor is used along with a start capacitor on larger (greater than 1½ HP) single-phase capacitor start/capacitor run motors to provide power factor correction and increase efficiency to the main winding as well as tune the motor and balance the phases for running conditions.

Capacitors can be used with smaller sizes and wired in parallel to provide the total equivalent capacitance. When a run capacitor is used, the motor’s operating voltage is about 1.5 to 2 times the supply voltage, which decreases with an increase of load. This is why run capacitors must be provided with a higher voltage rating than the supply voltage.

Run capacitors also assist in compensating for using single-phase power with larger HP motors. For example, assuming a capacitive reactive power of about 75% of the nominal power of the motor, the comparison of input power is slightly lower than that of a three-phase motor of equal size. The proper sizing of a start and run capacitor is a critical element of the motor design.

Typically, sizing of each should be within +/–5% of the calculated size. If the wrong start capacitor is used, the motor may not develop the torque needed to rapidly accelerate the motor or it may stall.

Conversely, if the wrong size of run capacitor is applied, the motor will not function with a stable magnetic field. This will cause the rotor to hesitate at those specific locations that are uneven in magnetic attraction. This brief hesitation will cause the motor to become noisy, increase energy consumption, incur a performance drop, and result in overheat of the motor.

The single-phase start capacitance (C) in microfarads (MFD) is equal to the rated power in amps (I) × 1 million ÷ the product of 6.28 × the line-to-line voltage (V(V)) × the frequency in hertz (Hz).

Determining the run capacitor size requires using the input wattage × the motor efficiency × 1000 ÷ the product of line-to-line voltage squared (V(V))2 × the frequency in hertz. For situations when capacitor ratings are unavailable (always use the manufacturers’ recommended sizes when available), use the following equations to approximate the required size:

Start Capacitance (MFD @ +/–10%) = (I × 1 million) ÷ (6.28 × V(V) × Hz)

Run Capacitance (MFD @ +/–10%) = (Input Power in watts) × Motor Efficiency × 1000) ÷ ((V(V))2 × Hz))

The required voltage rating of the capacitor (VC) equals the product of the voltage measured at both ends of the main winding in volts (VP) × the square root of 1 plus the ratio between the main/start winding turns (n) squared:

VC = VP × √(1+n2)

Example: Calculate the size and select the start and run capacitors for the following 5 HP, 4-inch single-phase motor: rated for 23 FLA at 230VAC; running watts = 5000; efficiency = 75%; power source = 240VAC, 60 Hz

Start = (23 amps × 1 million) ÷ (6.28 × 240VAC × 60 Hz) = 23 million ÷ 90,432 = 254.33 MFD (+/–10%)

Run = (5000 watts × 75% × 1000) ÷ ((240)2 × 60 Hz) = 375 million ÷ 3,456,000 = 108.50 MFD (+/–10%)

Start Capacitor: As the motor requires 254.3 MFD, use the common range of 216-259 MFD at 330VAC rating

Run Capacitor: for a 108.5 MFD calculated size, use (2)-55 MFD = 110 MFD at 370VAC ~ 108.5 MFD required

Although all starting relays are designed to perform the same basic function, voltage (potential) relays are the oldest method and the most common. They are used to initially send power to the motor’s starting winding through the relay’s normally closed contact from the start capacitor and then open the contact to disengage the start winding and start capacitor once the motor has started.

A higher voltage (approximately 300 volts to ground on the red lead with a 230-volt motor) is induced from the start winding due to a transformer effect in the winding upon motor acceleration and applied from the motor through the red (start) and yellow (common) conductors to the control box to energize the relay and open its contact.

Starting relays have gone through several versions and variations since the voltage relay was introduced as the original method. Current and solid-state relays (i.e., Triac type) have also been used with varying success.

In my opinion, not only is the voltage relay the most robust and best overall starting relay ever made for submersible motors, motor manufacturers should have never gone away from them. Various arguments from manufacturers such as longer life, less cost, and better relay performance have been offered for the alternatives, but you simply cannot beat the performance, life, reliability, and ease of troubleshooting and servicing the original voltage relay for my money.

A voltage relay has only two primary operating components subject to potential failure—the contact (use Rx1 ohmmeter setting to check) and the coil (use Rx1000 ohmmeter setting to check)—and both are easily checked for proper values using an ohmmeter.

However, even if the relay checks out, the troubleshooter will often need to open the relay to examine the contacts. I have run across numerous instances when the contacts were either welded shut or the opening mechanism was worn or broken, not allowing the contacts to open, even though the relay coil functioned.

These are both indications of excessive cycling that should be investigated. Forget the idea that you only need to stock three relays in your service rig—a 120-volt (GE #101) and 230-volt (GE #102), and a slightly larger contact thrown in for the larger motors (GE #103). The other types of start relays require the troubleshooter to carry replacement relays for each HP and voltage. Conversion kits are also available to allow retrofitting a voltage relay in place of a current or solid-state relay. I simply never had the recurrent problems with the voltage relays we have seen with the other generations of current and solid-state relays since then.

Overload relays also play a prominent role in the function and operation of a control box as well as motor protection. Although not necessarily true for all makes, older (pre-1995) Franklin Electric QD control boxes generally incorporated the overload in the control box as an attachment of the start capacitor. The single overload, formerly used in control boxes for motors from 1/3 through 1½ HP, is used to provide dual protection to the start capacitor and winding and the motor by opening the black lead on an overload.

Due to issues surrounding UL approval, Franklin Electric relocated the single overload to the motor during the 1990s. In accordance with UL listing mandates, this move effectively allowed the overload to be an integral part of the motor, allowing the motor to thereafter be UL recognized.

Overloads for smaller motors are designed as automatic reset. Larger control boxes utilize two manual reset overloads, a main and start overload, still in the control box.

The main overload is designed for the main winding and start winding. The motor is protected by opening the black motor lead upon an overload; this effectively shuts down the motor by opening one of the two 240VAC power circuit leads to the main (run) winding.

The start capacitor and winding are protected from the main overload, which is fed from line 1 and feeds the start overload in series. Therefore, either is capable of opening the circuit to both run and start capacitors. The other line of the main winding (yellow lead) is directly fed from line 2 in the control box.

3. Drop and Offset Cable

Although the drop and offset cable are usually not associated with failure of a pumping system, I have encountered multiple instances where the drop or offset cable was the culprit. The primary cause of pumping system failure due to the drop or offset cable is caused from abrasion of the insulation with direct buried cable, permitting the entrance of water, resulting in a dead short to ground.

However, this is far more prevalent with submerged drop cable than direct buried offset wire. In some cases, water leakage through the insulation or cut can be enough to lower the insulation resistance, but not result in a complete dead short. This is known as bleeding leakage and the type of problem usually presents itself as intermittent tripping of the circuit breaker or fuse.

This situation can be difficult to identify or locate unless the offset cable is isolated from the drop cable and the wire checked for a ground fault, ohms, or megohms using two of the conductors tied together at the wellhead.

This often requires the use of a ground fault detector or a high voltage DC megohm meter rather than a conventional ohmmeter. When checking the winding resistance of a submersible motor, it is necessary to add the wire resistance for the size and length of cable from the point of examination to the motor. The values in Table 1 can be used to determine the loop resistance of drop and/or offset cable.

4. Motor

For the most part, an electric motor used in a domestic pumping system is not unique and follows the same basic requirements as any other type of pumping system. Generally, the motor either runs or it doesn’t.

The only effective troubleshooting that can be conducted on the motor is to verify the insulation resistance and winding resistance. Of the two, the insulation resistance is the most important and often determines if the motor is salvageable or not.

Although most troubleshooting guides indicate the minimum insulation resistance in which a motor will still operate is usually between 10,000 to 50,000 ohms to ground, I have witnessed numerous installations with an insulation resistance of less than 10,000 ohms or even a dead short when the motor would still run, albeit not for long.

Although this contradicts both logic and electrical theory, I have seen it happen too many times to say it’s not possible, so be careful of writing off the motor by using this parameter alone without checking other functions.

When checking the winding resistance, don’t forget to add the drop cable and offset cable loop resistance to the reading for accuracy. Defective or worn thrust bearings in a motor can also impact current readings by drawing higher or undulating running current on the black and yellow leads.

Occasionally, single-phase motors will experience noise from induced harmonics. This is usually associated with an objectionable high pitch whine. Adding a single (for smaller motors) or additional (for greater than 1½ HP) 5 to 10 MFD run capacitor across the red and black leads at the control box can often lower this noise to acceptable levels.

Electronic Motor Controllers

The recent trend towards the use of constant pressure systems affords the expanded use of three-phase motors on a single-phase power supply. Rather than using a conventional control box, a variable frequency drive is used as a combined phase converter and variable speed controller to convert the single-phase power supply into three-phase power to enable operating the motor at variable speeds.

There are simply too many available versions and options from the various manufacturers to include a universal troubleshooting procedure into one column. Therefore, troubleshooters must obtain the data for the specific model of control and adhere to the instructions for each device.


Troubleshooting a single-phase motor is a simple matter of understanding how the motor and its controls work together to operate in harmony. In addition, always using the troubleshooting manual and data provided by all motor manufacturers will often prevent errant or ineffective troubleshooting techniques and save valuable time.

By reviewing the control box schematic associated with the specific motor horsepower, troubleshooters will also gain a better understanding of the way single-phase motors operate and the role the controls play.

Until next month, 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.