Troubleshooting Three-Phase Electric Motors

Part 1. Equipment and initial troubleshooting techniques.

By Ed Butts, PE, CPI

This month’s Engineering your Business is the first of a two-part discussion, with the second part wrapping up in the May issue. It is also the fourth and final part of a series on why and how electric motors fail and some common troubleshooting techniques for both single- and three-phase motors.

As with all electrical troubleshooting and service procedures—but especially important for three-phase systems—the first consideration must always be personnel and equipment safety. This means individuals not fully trained, experienced, and licensed when required for electrical troubleshooting, repair, and service work should not attempt any troubleshooting or service work.

When conducting this type of work, knowledge of and adherence to the lock out/tag out, arc flash rules, and recognition and consideration of the motor style, driven system (i.e., process) of the control system and motor controller type, and potential impacts from shutting down the process must always be observed.

Required Three-Phase Troubleshooting Equipment

In addition to the typical hand tools, there are essentially four types of instruments needed to troubleshoot three-phase systems:

  • AC voltage meter
  • AC ammeter
  • Ohmmeter
  • Megohmmeter

Several of these functions can be combined into a single meter. The voltage or multimeter, if used, must be rated, insulated, and capable of measuring the full range of applicable AC voltage.

Figure 1. Voltage testing of three-phase electrical system.

However, the greatest accuracy is generally obtained when reading in the midrange of the scale. Lower voltage (less than 1000 volts AC) meters are often rated for 300, 600, or 1000 volts of AC power (VAC). Although a 300VAC rated voltmeter will satisfactorily read 120- and 240-volt circuits, they will obviously be inadequate for 480- or 575-volt circuits.

More than one voltmeter has exploded in the hands of a technician because a 300-volt rated meter was applied to a 480-volt circuit. Although a 600-volt rated meter will work on a 480-volt circuit, I recommend procuring and using a 1000-volt rated voltmeter for all low voltage three-phase troubleshooting, as the higher-rated meter will possess more insulation and be less likely to explode or fail.

The second meter, an ammeter, is used to measure the current of the motor during operation. This meter can consist of a rotary dial meter or as an element of a multimeter with an amp clip and plug-in leads.

Figure 2. Checking continuity of fuses.

Once again, it is imperative the meter is rated for all conceivable ampere ranges it may need to read. For most motors up to 200 HP, the Amprobe model RS-3 is an excellent and resilient meter for this work. It possesses several rotary scales as low as 6 amps up to 300 amps. The meter’s clip is small enough to fit over or between virtually any wire size up to 250 MCM and provides a needle read rather than an LED or LCD display that are often hard to read in illuminated areas.

A basic analog ohmmeter, such as the Simpson model 372, can be used to measure a motor’s winding and insulation resistance, but I recommend using an electronic (capacitor) or crank-style 500VDC minimum rated megohmmeter for measuring insulation resistance.

These functions can also be combined into a single multimeter, although I still prefer to stick with the individual instruments.

Initial Three-Phase Troubleshooting Techniques

Three-phase motors are surprisingly robust and versatile machines. Compared to single-phase motors that use starting capacitors and switches with two separate and distinct types of windings and horsepower limitations, three-phase motors operate using three matched windings and are available in fractional to several thousand horsepower.

As such, three-phase motors will often operate satisfactorily under extreme service conditions and harsh environments. While troubleshooting a potential issue with a three-phase motor installation, three initial steps should be conducted in the following order before moving on to the motor itself:

  1. If it is an aboveground installation and accessible, check the pump and motor for free-hand rotation.
  2. Verify the incoming power supply and voltage into the motor controller.
  3. Check the motor’s design characteristics and the starting and control equipment and control circuit.
Figure 3. Typical three-phase motor connections.

The first item to quickly check is the rotation of the motor and driven equipment, usually a pump. A bound (locked up) motor or pump is often the source of a system problem, and it is generally a brief task to verify free-hand rotation in the case of an aboveground unit.

Verifying the incoming power supply and voltage is an obvious troubleshooting step that should next be performed on all installations as this is often the only real problem. This condition can consist of a blown motor or control circuit fuse, tripped circuit breaker, or overload.

Verification of the incoming voltage should be performed, as shown in Figure 1, using line to line measurements rather than line to ground, as voltage feedback through the controls or magnetic coil can often result in erroneous and false readings.

Checking fuses can be performed on a deenergized panel by using an Rx1 ohmmeter setting on each side of the fuse, as shown in Figure 2 (although, for safety, I recommend removing the fuse and checking its continuity outside of the panel).

In a balanced three-phase system, the line-to-line phase voltages should be equal or very close to equal. Voltage unbalance or imbalance is a measurement of the inequality of the phase voltages and is a common problem, particularly with open-delta three-phase power systems.

Voltage imbalance is the measure of the voltage differences between the phases of a three-phase system. The procedure for calculating voltage imbalance was outlined in the Engineering Your Business column in the February 2022 issue of Water Well Journal. It degrades the performance and significantly shortens the life of three-phase motors.

Transients can result from switching of power lines or harmonics from VFDs, fluorescent lighting ballasts, and other electronic and capacitive equipment. The impact of transients on motors can also be severe, as motor winding insulation can steadily degrade, leading to costly early motor failure and unplanned downtime.

The recent trend towards retrofitting high or premium efficiency motors with existing pumping installations can also present problems with existing motor controllers. High efficiency electric motors are often designed for a greater inrush current during the starting phase than older motors. This can result in instantaneous tripping of the circuit breaker, especially with full voltage starting methods.

This can often be corrected by adjusting the inrush dial setting on the circuit breaker. However, replacement of the circuit breaker or switching to dual element fuses may be needed in some cases.

Improper connections to the motor, particularly with new installations and dual voltage motors, is a common problem with three-phase motors. Most three-phase motors are provided with nine leads, although some types use only six or three leads. These are pulled from the stator and must be combined and wired to the incoming power supply in accordance with the type of winding and supplied voltage.

Each motor type, delta and wye, use a standard wiring configuration as illustrated in Figure 3. However, the installer or electrician must always verify the type of motor and its applicable wiring diagram before performing the junction box connections.

Inadequate knowledge of the type of motor starter is often the single greatest impediment to effectively troubleshooting a three-phase motor. Troubleshooters must make the effort to familiarize themselves with the function and complexities of the applicable motor starting and control method as the problem often lies in the motor controller or control circuit rather than with the motor.

Problems with the motor controller or control circuit can be as simple as a blown fuse or tripped circuit breaker to as complex as burned-out diodes or capacitors in a variable frequency drive. A working knowledge of electrical schematics and understanding of the specific type of motor controller and its function is necessary to effectively troubleshoot three-phase motors, particularly those with complicated starters,
extensive wiring, external devices and circuits, or numerous components.

Figure 4. Common methods of motor control.

Often, correcting an issue with the motor controller or control circuitry will repair the entire issue. Three-phase motors are capable of starting and running motors using various styles of full and reduced voltage equipment. The typical types are shown in Figure 4 as one-line diagrams, including:

  • Direct-On Line (DOL) (across the line or ALS): Applies direct full voltage to motor at starting
  • Star-Delta: Reduces voltage during starting using a star connection, then transitions to a delta connection
  • Primary Resistor or Reactor: Appropriate resistance is introduced in series with each winding to start.
  • Autotransformer: Uses winding taps to reduce start voltage to 50%, 65%, or 80% of full voltage
  • Part Winding: Starts motor on one half of the motor’s windings, requires specialized motor design
  • Electronic Soft Starter: Electronically reduces the voltage to the motor during the starting phase
  • Variable Frequency Drive (VFD): Functions the same as the soft starter, but is capable of variable speed.

With the exception of a VFD, all of the above starting methods are designed to transition to full voltage values once the motor has started and began to accelerate to full speed.

Many of these motor starters, including part winding and autotransformer, use time delay relays to perform the transition from reduced voltage start to full voltage running. These time delay relays are often the source of the problem, particularly if the motor does not successfully ramp up to full speed after it has started.

The troubleshooting protocol associated with each controller is specific to the method and type of system and cannot be easily simplified. It is imperative that anyone contemplating troubleshooting these various motor control methods possess a full understanding of the controller type, the system it controls, and possible problems.

Electronic soft starters and variable frequency drives require even more specialized knowledge of electronic circuits beyond most motor controllers. Additional test meters, such as frequency and sine wave meters, are often required.

When troubleshooting a system as complex as a drive, sometimes it’s difficult to know how and where to start. By initially checking the supply voltage, current, and frequency, the troubleshooter can rule out problems that might affect the motor drive or breaker circuits. If operable, examining the output voltage and frequency to the motor with the reference value or systems that employ speed control feedback, such as an analog control loop, is a valuable troubleshooting step.

This can save valuable time and lead to a faster resolution to the problem. In addition, by identifying over- or under-voltage conditions, nuisance tripping of soft starter or drive fault circuits can often be avoided and prevent eventual damage to the motor and device.

The basic procedure for troubleshooting electronic soft starters and drives is as follows:

  1. If an aboveground installation, always check the pump and motor for free-hand rotation.
  2. Check the input voltage from the AC power supply entering the soft starter or drive.
  3. Check the device’s components for burned solder joints or loose connections, including the AC to DC converter, circuit board, DC filter, and the DC-to-AC inverter that provide the power to the motor.
  4. Check the motor itself.

Once the power supply (three-line phase to phase voltage), motor controller, related control circuit, and junction box connections have been verified for correct values and properly function, the troubleshooter can turn to the motor itself.


This concludes this first of our two parts on three-phase motor and electrical system troubleshooting. We will wrap up this topic and series in May with a discussion on three-phase motor troubleshooting.

Until then, work safe and smart.

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 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