Problems and failures in three-phase motors: Types, Reasons and Solutions

The three-phase induction motor is one of the most popular and widely used electric motors in industry. They are used in many applications due to their simple and robust construction with almost maintenance-free operation.

It is an asynchronous motor that works on the principle of induction. But like other motors, these motors are also vulnerable to faults that can damage the motor and aggravate production. Therefore, the induction motor must be monitored for continuous operation without interruption.

There are many factors that can cause problems in three-phase motors such as heavy duty, poor working conditions, improper motor installation, motor manufacturing factors, among others, which can cause failures in motor operation. If failures in three-phase motors are not rectified beforehand, they can generate a large loss of revenue for the industry and can pose a threat to the reliability and safety of the operation.

Let us look at how to identify faults in three-phase AC motors and how we can rectify these problems. The following are possible causes of faults and their remedies.

Three-phase induction motor failures

Three-phase motor does not start

Three-phase motor makes a lot of noise

Three-phase motor overheating problems

Overheated bearing failures in three-phase motors

Problems in three-phase motors due to starts and stops

Three-phase motor operates at reduced speed

The three-phase motor runs at high speed.

Mechanical Problems in the Three-Phase Induction Motor

In general, it is found that an engine develops more mechanical problems than electrical problems. A thorough knowledge of bearings and lubrication is a must. The following are some of the do's and don'ts for mounting, maintaining, inspecting and lubricating ball bearings.

• Work with clean tools, in a clean environment, use clean solvents and washing oils.

• Do not work under the disadvantage of poor tools, a rough bench or a dirty environment, and do not use dirty, brittle or chipped tools.

• The bearings should be cleaned with clean, lint-free cloths. Do not scratch the bearing surfaces.

• Keep bearing lubricants clean when applying them and cover containers when not in use. Do not remove grease or oil from new bearings.

• To ensure that the shaft size is within the specified tolerances recommended for the bearing. Do not install a bearing on a shaft showing excessive wear.

• Use a clean, short-bristled brush with firmly embedded bristles to remove dirt, flakes or chips.

• To be sure that, when installed, the bearing is square and held firmly against the shoulder of the shaft. Do not directly strike a bearing or ring when installing, as this may damage the shaft and bearing.

• Follow the lubrication instructions supplied with the machinery. Use only grease where grease use is specified. Use only oil where oil use is specified. Be sure to use the exact type of lubricant required.

• Excess grease and oil will ooze from overfilled housings through seals and seals, accumulate dirt and cause problems. Too much lubricant will also cause overheating, particularly when the bearing operates at high speeds.

• Handle grease with clean paddles or grease guns. Store grease in clean containers. Keep grease containers covered. Do not allow any machine to remain inoperative for months without running it periodically. This prevents moisture that can condense on a stationary bearing from causing corrosion.

Troubleshooting three-phase motors

Insulation resistance testing of a three-phase motor

Once the power supply, the driven equipment and the motor controller/controls have been eliminated as the cause of the problem, the next thing to do is to follow a series of procedures for troubleshooting electric motors and how to troubleshoot a suspect three-phase motor and generally includes the following steps.

The first step is to ensure that all power is disconnected and isolated from the motor, using proper lockout/tagout procedures, as well as disabling, short-circuiting or disconnecting any power factor correction capacitors that may be present.

The next step is to carry out a insulation resistance test on the electric motorThis test will eliminate the need for further examination if the motor windings are earthed.

Whenever feasible, the insulation resistance should be tested as close to the motor as possible to eliminate possible false readings from the compensated cable or motor feeders. A grounded motor is a common winding fault and requires rewinding or replacement of the motor.

When a motor is grounded, the winding is short-circuited to the laminated core or motor structure. This situation applies to both surface and submersible motors. The problem is usually found in a slot, where the insulation of the slot has been broken.

Water is the most common cause of a grounded winding. Some causes of slot insulation breakdowns are overheating, conductive contaminants, lightning, weather, pressure from a tight coil fit, hot spots caused by lamination damage (due to a previous winding failure) and excessive coil movement.

To obtain an optimum reading in the insulation resistance test of a three-phase motor, this test should be performed with a megohmmeter with a test voltage of not less than 500 VDC (for 230-volt motors) to 1000 VDC (for 460-volt motors), although an analog ohmmeter with an Rx100,000 ohm scale is often used. When using a high output voltage megger, be aware that the devices can produce dangerously high shocking voltages; never use them by connecting the leads to people or animals.

For best results, the test should be performed immediately after switching off the motor with the three-phase motor at or just below its operating temperature. Obviously, this is not possible if the motor is not running.

Insulation resistance readings for all three-phase motor types, voltages 0 to 1000 VAC, phase and HP shall comply with IEEE 43-200/43-2013 and generally be within the ranges shown in Table 1.

Insulation resistance testing to detect problems in three-phase motors in operation should be performed at least once a year to generate a historical database and track the condition of the motor to predict an impending failure long before it occurs.

A general rule of thumb is that 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.

When checking the insulation resistance of a motor, the values will be almost identical for all readings, as the circuit is routed equally through all three windings and back to the meter.

Although an infinity (∞) reading is desirable, it is generally not achievable with most three-phase motors. The insulation resistance should be approximately 1 megohm per 1000 volts of operating voltage with a minimum value of 1 million ohms (1 megohm).

However, it is important to note that the generally accepted minimum insulation resistance of 1 million ohms may not be adequate for many service conditions. This may be especially true for submersible motor/pump installations, since several variables, such as water conductivity, cable voltage drop, and motor inrush currents, can cause nuisance tripping of circuit breakers or overloads. Therefore, higher insulation resistance values may be required for certain conditions.

Three-phase motor winding resistance test

The next step is to check the winding resistance. The winding resistance provides an indication of the condition and continuity of the windings. The winding resistance test is usually performed with an ohmmeter with an Rx1 configuration.

Unlike an insulation resistance test, the winding resistance will vary with the horsepower, phase, connection (delta or star) and voltage of the three-phase motor.

Winding resistance values will vary, but are generally available for all motors from motor manufacturers, technical data sheets or service manuals.

All three windings of a three-phase motor should show equal readings with low readings but not 0 ohms. The lower the HP of the motor, the higher this reading will be, but it should not show an open circuit and will generally be 30 ohms or less.

When this data is not available, the use of a rule of thumb can be substituted, as for most three phase motors, the branch to branch reading should be between 0.30 and 2 ohms. If it reads 0, there is probably a short circuit. If the reading is greater than 2 ohms or infinite (∞), there is probably an open circuit.

Testing the winding resistance of a motor 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 provided by the inductive reactance. The resistance of the wire in a winding is a small percentage of the total impedance of the motor (i.e. resistance plus inductive reactance). The inductive reactance makes each turn significant in the ampere demand of the motor, since each turn provides much more inductive reactance than resistance.

Only the resistance of the wire (i.e. the number of turns) within the closed loop is now removed from the phase winding. Without the ampere demand of the circulating current, the difference between the amperes of the faulty phase and those of the normal phases decreases. A small difference in resistance is all that is needed to identify the faulty phase.

Note that, if possible, the rotor should be turned during this test to eliminate its effect. Shorted turns in any AC winding are usually visible. They carbonise quickly due to the high circulating current that is transformed in them.

A phase-to-phase short circuit is caused by insulation breakdown at the coil ends or slots. This type of fault requires rewinding or replacing the motor. The voltage between phases can be high. When a short circuit occurs, a large part of the winding is bypassed. Both phase windings usually melt, so the problem is easily detected. Causes of interface breakdown include contaminants, tightly fitted slots, age, mechanical damage and high voltage spikes.

The windings forming the poles of each phase overlap in all three-phase motors. A common cause of an open winding is undersized lead lugs. Charred connections in the motor junction (terminal) box are a reliable indication of this problem.

Open windings are also caused by shorted turns, phase-to-phase shorts, ground-to-frame shorts, faulty internal coil-to-coil connections, severe overloads and physically damaged coils. These faults also require rewinding or replacing the motor.

An open winding will show several different symptoms, depending on the internal connection of the motor. A wye-connected motor with an open winding will test differently from a delta-connected motor. An open single-circuit winding will be single-phase. Its power will be halved and the motor will not start. If the internal connection of the motor is multi-circuit, it will start but will have reduced power. An open circuit will cause the magnetic circuit to become unbalanced. Therefore, under normal load, the motor will run slower and overheat.

Visual examinations of defective three-phase motors

It is always important to identify the real cause of burnt windings and not just replace the electric motor. Motor windings have different appearances to common fault situations, which include single-phase breakdown, overload, unbalanced voltage and voltage spikes.

A visual inspection of the motor windings can often help determine the cause of the fault and develop a solution. Two of the most common problems with three-phase motors are overload and single-phase.

Damage from voltage spikes occurs most frequently in three-phase motors controlled by variable frequency drives. Therefore, check the applied voltage as close as possible to the fully loaded motor to verify that the applied voltages are uniform.

Motor voltage unbalance must not exceed 5% of line voltage. For a 460-volt motor, that is up to 23 volts of line-to-line variation. If the voltage cannot be read near the motor, consider the run length and wire size to estimate the actual voltage drop across the motor. If the line-to-line voltages are the same but the current unbalance still exceeds 10 %, the winding is most likely shorted and the motor should be repaired or replaced.

Three-Phase Motor Troubleshooting and Routine Testing Guide

Routine testing of an electric motor as part of a maintenance program also reduces the possibility of failure due to excessive heat. Many motors in use today are rated for a temperature rise of 60°C (140°F). When combined with an ambient temperature of 40°C (104°F), the resulting motor temperature can rise to 244°F. This is above the boiling point of water and can lead to premature motor failure, especially in cases with inadequate cooling air circulation.

Do not judge engine temperature by simply feeling the outside surface with your hand. Touch is not a reliable heat sensor, as what feels hot to you is cool to someone else. Use proper testing methods, such as an infrared heat sensor, to find hot spots inside the motor windings, as excessive hot spots reduce the life of the motor.

Make sure that the motors have adequate protection in place. Such protection should include thermostats and overload protection. Only one element of a effective predictive maintenance plan for electric motorsThese devices ensure that the motor is not running at overload or harmful temperatures.

Electric motors are often some of the most expensive assets in a facility, but with proper maintenance and common sense, extending their lifespan becomes a little easier. If you want to learn more examples of common electric motor faults and how to fix them you can subscribe to the Technology for Industry newsletter.