Part 2. Three-phase motor troubleshooting.
By Ed Butts, PE, CPI
We began a discussion on three-phase motor troubleshooting in last month’s edition of Engineering Your Business with an outline on the various meters, controllers, and subsystems involved with three-phase power systems. This month, we will conclude this series with a discussion on actual three-phase motor troubleshooting.
Three-Phase Motor Troubleshooting
Once the power supply, driven equipment, and motor controller/controls have been eliminated as the cause of the problem, troubleshooting a suspect three-phase motor is next and typically features the following steps.
The first step is to ensure all power is disconnected and isolated from the motor—using appropriate lock out/tag out procedures plus disabling, shorting, or disconnecting any power factor correction capacitors that may be present.
Next is performing an insulation resistance test on the motor since this test will eliminate the need for additional examination if the motor windings are grounded. The procedure is illustrated in Figure 1a. The procedure for testing the insulation resistance for an in-well submersible pump motor is shown in Figure 1b.
Whenever feasible, insulation resistance should be examined as close to the motor as possible to eliminate possible false readings from offset cable or motor feeders. A grounded motor is a common winding breakdown and requires rewinding or replacing the motor.
When a motor is grounded, the winding is shorted either to the laminated core or to the motor’s frame. This situation applies to both aboveground and submersible motors. The problem is usually found in a slot, where the slot insulation has broken down.
Water is the most common cause of a grounded winding. Some causes of slot insulation breakdowns are overheating, conducting contaminants, lightning, age, pressure of a tight coil fit, hot spots caused by lamination damage (from a previous winding failure), and excessive coil movement.
To obtain an optimal reading, this test should be performed with a megohmmeter with no less than 500 VDC (for 230-volt motors) up to 1000 VDC (for 460-volt motors) test voltage, although an analog ohmmeter with an Rx100,000 ohms scale is often used. When using a high output voltage megohmmeter, be aware that the devices can produce dangerously high shocking voltages—never use them by attaching the leads to people or animals.
For best results, the test should be conducted immediately after shutting down the motor with the motor at or just below its operating temperature. Obviously, this is not possible if the motor will not run.
Insulation resistance readings for all motor types, voltages from 0 to 1000 VAC, phase, and HP should comply with IEEE Standard 43-200/43-2013 and generally be within the ranges shown in Table 1.
Insulation resistance testing on functioning motors should be performed at least yearly to generate a historical database and track the motor’s condition to predict an impending failure well in advance of its occurrence.
A general rule-of-thumb is the insulation system of an electric motor is believed to be in good condition if the measured insulation resistance is greater than or equal to (≥) 10,000,000 ohms.
While checking the insulation resistance of a motor, the values will be almost identical for all readings as the circuit is equally routed through the three windings and back to the meter.
Although a reading of infinity (∞) is desirable, it is generally not achievable with most motors. Insulation resistance should be approximately 1 megohm for each 1000 volts of operating voltage with a minimum value of 1 million ohms (1 megohm).
However, it is important to note that the minimum generally accepted insulation resistance of 1 million ohms may not be adequate for many service conditions. This can particularly be true for submersible pump/motor installations as several variables such as water conductivity, voltage bleed-through drop cable, and motor starting inrush currents can cause nuisance tripping of circuit breakers or overloads. Therefore, greater values of insulation resistance may be required for certain conditions.
The next step is checking of the winding resistance. The winding resistance provides an indication of the condition and continuity of the windings. Testing of winding resistance is generally conducted using an ohmmeter with an Rx1 setting.
As opposed to an insulation resistance test, the winding resistance will vary with the motor’s horsepower, phase, connection (delta or wye), and voltage, and must include the two-way resistivity of the length of the cable from the motor controller or wellhead to the motor. This is an important distinction with submersible motors that may use several thousand feet of drop and/or offset cable.
Figure 2 illustrates a winding resistance test on a “wye” (Y)-connected motor. Values of winding resistance will vary but are typically available for all motors from the motor manufacturers, technical datasheets, or service manuals.
The three windings in a three-phase motor should display equal readings with low but not ohm readings of 0. The smaller the motor HP, the higher this reading will be, but it should not show an open circuit and will usually be 30 ohms or less.
When this data is unavailable, use of a rule-of-thumb can be substituted as for most three-phase motors, the leg-to-leg reading should be between 0.30 to 2 ohms. If it reads 0, there is likely a shorted circuit. If the reading is more than 2 ohms or infinite (∞), there is likely an open circuit.
Testing a motor’s winding resistance can often reveal several motor problems, including a shorted or grounded winding or turns. Shorted turns are caused by nicked coil wire, high-voltage spikes, conductive contaminants, overheated windings, aged insulation, and loose and vibrating coil wires.
Most of the resistance to current flow in an AC motor is furnished by inductive reactance. The resistance of the wire in a winding is a small percentage of the motor’s total impedance (i.e., resistance plus the inductive reactance). Inductive reactance makes each turn significant in the motor’s ampere demand as each turn supplies much more inductive reactance than resistance.
Only the resistance of the wire (i.e., number of turns) within the closed loop is now eliminated from the phase winding. Without the ampere demand of the circulating current, the difference lessens between the amperes of the faulty phase and those of the normal phases. A small difference in resistance is all that is needed to identify the faulty phase.
Please note, if possible, the rotor should be turned during this test to eliminate its effect. Shorted turns in any AC winding are usually visible. They become charred quickly from the high circulating current that is transformed into them.
A phase-to-phase short is caused by insulation breakdown at the coil ends or in the slots. This type of fault requires rewinding or replacing the motor. Voltage between phases can be high. When a short occurs, a large amount of the winding is bypassed. Both phase windings are usually melted open, so the problem is easily detected. Among the causes of interphase breakdown are contaminants, tight slot fit, age, mechanical damage, and high-voltage spikes.
Coils that form the poles for each phase are placed on top of each other in all three-phase motors. A common cause of an open winding is undersized lead lugs. Charred connections in the motor’s connection (terminal) box are a reliable indication of this problem.
Open windings are also caused by shorted turns, phase-tophase shorts, ground-to-frame shorts, faulty internal coil-tocoil connections, severe overloads, and physically damaged coils. These faults also require rewinding or replacing the motor.
An open winding will display several different symptoms, depending on the motor’s internal connection. A wye-connected motor with an open winding will test differently from a delta-connected motor. An open single-circuit winding will be single-phased. Its power will drop to about half and the motor won’t start. If the motor’s internal connection is multi-circuit, it will start but will have reduced power. An open circuit will cause the magnetic circuit to be unbalanced. Thus, under normal load the motor will run more slowly and will overheat.
Visual Examinations of Defective Motors
It’s always important to identify the real cause of burned windings and not just replace the electric motor. Motor windings have different appearances from common failure situations, including single-phase burnout, overload, unbalanced voltage, and voltage spikes.
A visual inspection of the motor windings can often assist in determining the cause of failure and developing a solution. Two of the most common problems with three-phase motors are overload and single phasing.
Each burnout condition displays different appearances. Figure 3 illustrates a burned motor winding from overload while Figure 4 illustrates a burned-out motor from a single-phase condition.
Voltage spike damage occurs more often in motors controlled by variable frequency drives. Thus, check the applied voltage as near as possible to the fully loaded motor to verify the applied voltages are even.
Motor voltage unbalance should not exceed 5% of line voltage. For a 460-volt motor, that is up to 23 volts of line-toline variance. If the voltage cannot be read close to the motor, consider the length of the run and size of wire to estimate the actual voltage drop at the motor. If the line-to-line voltages are the same but the current imbalance still exceeds 10%, the winding is most likely shorted, and the motor should be repaired or replaced.
Routine Motor Testing and Troubleshooting Guide
Routinely testing an electric motor as part of a maintenance program also reduces the possibility of failure due to excessive heat. Many of the motors in use today are rated for a 60°C (140°F) temperature rise. When combined with an ambient temperature of 40°C (104°F), the resulting motor temperature can rise to 244°F! This is over the boiling point of water and can result in premature motor failure, especially in cases with inadequate cooling air circulation.
Don’t judge the temperature of the motor by simply feeling the outside surface with your hand. Touch isn’t an excellent or reliable heat sensor, as what feels hot to one is cool for somebody else. Use appropriate testing methods such as an infrared heat sensor to find hot spots within the motor windings because those excessive hot spots reduce the motor’s lifespan.
Make sure that motors have the proper protection in place. That protection should include thermostats and overload protection. Just one element of an effective maintenance plan, these devices ensure the motor is not operating at an overload or damaging temperatures.
The four most likely issues with a three-phase motor with possible causes are listed in Table 2.
Electrical motors are often some of the most expensive assets in a facility, but with proper maintenance and common sense, extending their useful lifespan becomes a little easier.
This concludes this edition of Engineering Your Business and this series on troubleshooting electric motors. I hope the information is helpful and will be valuable to you in the future.
Until next month, work safe and smart.
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 firstname.lastname@example.org.