Insulation faults in electric motors: how to measure and troubleshoot them

Electrical insulation is used to direct current along a desired path and prevent it from flowing where it is not desired. Proper electrical insulation is critical to the performance and longevity of an electric motor. Insulation breakdown is one of the most common causes of low resistance and insulation failure in electric motors. In electric generators, for example, 56% of failures are caused by damage to the electrical insulation.

Insulation systems in electric motors

There are two insulation systems in motors. One system is the ground wall insulation that separates the windings from the motor frame or casing. The second insulation system is the winding insulation that separates the conductors that are wound to create the motor windings. Studies have shown that approximately 80 % of stator electrical faults occur in the winding insulation, while only 20 % occur between the windings and the motor frame or in direct short circuit to ground.

What is insulation failure in electric motors?

Electrical insulation failure occurs when motor insulation begins to degrade over time or for other reasons. Ageing or overheating causes chemical changes in the insulation that cause the insulation to become more conductive and become less effective at preventing current from following unwanted paths between conductors or into the motor structure. Some insulation failures, particularly in the ground wall insulation system, are instantaneous due to the ingress of moisture, contamination or other unique and unusual events. These events attack voids or other weaknesses in the insulation and lead to premature failure. Failures in the insulation system of electric motor windings materialise slowly and deteriorate over time.

What are the most common faults in electric motor insulation?

Common causes of electric motor insulation failures include:

• Overheating

• Winding contamination

• Excessive current consumption

• Poor power quality

• Harmonic distortion.

In this article you have a guide that will show you the way through each stage of electrical insulation deterioration so that you can be proactive and keep track of these changes in the insulation of your motor equipment.

Stages of insulation failure in electric motors

Most insulation failures occur slowly and steadily, passing through three distinct stages.

STAGE 1: Ideal for the early detection of insulation failure in electric motors

During the first stage of electrical insulation failure, the insulation between the conductors becomes stressed and begins to change chemically. The insulation chemically changes from an insulator and starts to become a conductor. Insulation strength and capacitance begin to decrease. The insulation may begin to carbonise, which causes the current to become more resistive and less capacitive. If the ground wall insulation undergoes the change, the insulation resistance will decrease and the dissipation factor will increase. If the winding insulation undergoes the chemical change, the phase angle (Fi) and/or the current frequency response will change.

The identification of problems in the electric motor at this stage of insulation failure is extremely important for reliable operation of an industrial plant's electrical system. Undesirable current flow between conductors is not yet present at this stage, although the risk of it beginning to do so is high. Fortunately, early detection through testing of the motor windings and carrying out appropriate insulation tests on the electric motor is extremely beneficial. Early detection of insulation faults in electric motors allows a maintenance manager to address deterioration while it is still relatively minor, saving time and money and avoiding catastrophic failures.

STAGE 2: Possible intermittent engine failure

During the second stage of electrical insulation failure, the deterioration of the windings becomes more pronounced. The following are some of the characteristics of insulation failure in electric motors that can occur:

• Degradation of the insulation material increases.

• The current continues to become more resistive.

• Heat rises at the main point of insulation failure.

• The motor starts intermittently tripping the drive or circuit breaker, although it may continue to run once the insulation cools down.

STAGE 3 - Catastrophic failure

If previous signs of insulation failure have gone undetected or unaddressed, the motor is likely to experience a total failure. Below are some of the characteristics that the winding typically exhibits at this stage:

• The insulation is completely broken, creating a shortcut between the winding or a direct path for current from the winding to ground or to the motor structure.

• An explosive rupture develops at the failure point.

• Inductance and resistance changes occur.

• Copper coils begin to melt in response to excessive heat.

• The motor continuously trips the inverter or circuit breaker on start-up.

• Current flow between conductors is present.

Many electrical meters and devices should detect faults at this stage of motor failure (or when there is a complete short to ground indicating a serious safety problem). If you run motors until they fail, you may not need to know what is wrong with your motor or the health of your motor.

Causes of insulation failure in electric motors

Stressors such as temperature, contaminants and electrical stresses such as sustained surges can easily damage electrical insulation and cause breakdown. The risk of insulation failure in electric motors also increases over time as these various factors interact with each other to cause deterioration. For example, small holes or cracks may appear in the insulation due to daily wear and tear. These cracks weaken the insulation and also create pathways for moisture and chemical contaminants to enter, further degrading the insulation.

The following are some of the most common causes of electrical insulation failures in a motor as well as the most common causes of motor insulation failures. procedures for troubleshooting electric motors and how to resolve them:

• Pollutants: winding insulation is weakened by contact with contaminants such as machine tool coolant, oil and other chemicals. These contaminants often have a corrosive effect, breaking down the insulation over time. Wet contaminants are often conductive because they contain impurities, so they decrease resistance as they seep into the insulation through small cracks and pores.
• Poor power quality: windings can overheat due to power quality problems, including unbalanced voltage and current levels. Even a modest increase in temperature due to these problems can create a thermal hotspot that leads to a substantial decrease in insulation resistance and cause electric motor failure.
• Overloading: the windings can overheat due to high current consumption caused by excessive loads. Overloading can also cause a voltage surge that breaks the insulation.
• High ambient temperature: A common cause of insulation failure in electric motors is that the windings can also overheat due to high heat in the operating environment. Especially in an area with limited ventilation, the heat generated by the equipment can place excessive stress on the insulation of a motor winding.
• Transient voltages: Transient voltages can develop from internal or external sources and often occur during motor starting. The frequency of the transient current can be several times higher than the typical current in the windings, resulting in extreme insulation stress.

Because the risk of insulation failure in an electric motor is relatively high over time, maintenance technicians must have the tools and training they need to detect signs of insulation failure and address them quickly.

Basic tests, Methods and Instruments for detecting insulation faults in electric motors

To the question How to measure the insulation resistance of an electric motor, here is a guide to follow.

To prolong the service life of electric motors and systems, periodic insulation resistance tests are required. Over the years, after many operating cycles, electric motors are exposed to environmental factors such as dirt, grease, temperature, stress and vibration as discussed in this article. These conditions can lead to failures in the insulation of the electric motor, which can result in production losses or even fires.

An effective motor insulation resistance system has a high resistance, usually (as an absolute minimum) greater than a few mega ohms (MΩ). A poor insulation system has lower insulation resistance. The optimum insulation resistance for an electric motor is usually determined by the manufacturer's specifications, the criticality of the application in which the motor is used, and the environment in which the motor is located.

It is practically impossible to determine rules for the actual minimum insulation resistance value of an electric motor because resistance varies according to method of construction, condition of insulation material used, voltage rating, size and type. A general rule of thumb is 10 megohms or more. The insulation system of an electric motor is said to be in good condition if:The measured insulation resistance is greater than or equal to 10 MΩ.

Typical insulation resistance level for electric motors

There are no rules for determining the minimum insulation resistance value of a motor. Most of the available data is empirical.

How to measure the insulation resistance of a motor

The insulation resistance measurement is performed by means of a Megohmmeter, a high resistance range ohmmeter. To measure the insulation resistance, a DC voltage of 500 V or 1000 V is applied between the windings and the motor ground.

During and immediately after the measurement, do not touch any motor terminals as some of them carry dangerous voltages which can be fatal.

The minimum motor insulation resistance measured to ground at 500 V can be measured at a winding temperature of -15 °C to 20 °C. The maximum insulation resistance can be measured at 500 V at a winding operating temperature of 80 to 120 °C, depending on the motor type and efficiency.

How to calculate the minimum insulation resistance of motors

The minimum insulation resistance of any motor, Rmin, can be calculated by multiplying the rated voltage, VR, by the constant factor 0.5 MΩ/kV:

08Rmin = 0,5*VR

Periodic checks of motor insulation resistance

The key to prolonging the life of any electrical device is regular checks and maintenance. The insulation resistance of stored and active motors should be checked regularly:

1. If the insulation resistance of a new, cleaned or repaired motor that has been stored for some time is less than 10 MΩ, the reason may be that the windings are wet and must be dried.
2. For a motor in operation, the minimum insulation resistance may drop to a critical level. If the measured value of the insulation resistance is greater than the calculated value of the minimum insulation resistance, the motor can continue to operate. However, if it falls below this limit, the motor must be stopped immediately to avoid injury to personnel due to high leakage voltage.

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