Condition Monitoring

Major Components and Failure Modes

 

Some of the major issues from the various components of the motor system shall be reviewed in order to provide an understanding of the types of faults and the technologies used to detect them. As an overview, this may not encompass all of the modes of failures you may experience.

 

Incoming Power:

 

Starting from the incoming power to the load, the first area that would have to be addressed is the incoming power and distribution system. The first area of issue is the power quality then transformers.

 

Power Quality:

 

1)    Voltage & Current harmonics: Voltage limited to 5%THD & current to 3%THD. Current harmonics carries
the greatest potential for harm to the electric motor system.

2)    Over & Under voltage conditions. Electric motors are designed for +/- 10% variation from the name plate
voltage.

3)  Voltage unbalance is the difference between the phases. The relationship between the voltage & current unbalance varies from a few times to many times current unbalance as related to voltage unbalance based upon motor design. This can be as high as 20 times.

 

Power factor:

 

The lower the power factor from unity, the more current the system must use to perform work.Signs of a poor power factor are the dimming of lights when heavy equipment starts. Overloaded system: Based upon the capabilities of the transformer, cabling and motor.

Primary tools used to detect problems with the incoming power are power quality meters, Electrical Signature Analysis (ESA) and voltage and current meters. Knowing the condition of your power quality can help identify a great many "phantom" problems.

Transformers:

Transformers are one of the first critical components of the motor system. In general transformers have fewer issues than other components in the system. However, each transformer usually takes care of multiple systems both in the electric motor as well as other systems. Common transformer problems include: (oil filled or the dry type)

1. Insulation to ground faults
2. Shorted windings
3. Loose connections
4. Electrical vibration/mechanical looseness
5. Contaminated oil

Test equipment for monitoring the health of transformers includes the following:

1. Motor Circuit Analysis (MCA) for grounds, loose/broken connections and shorts
2. ESA for power quality and late stage faults
3. Infrared Analysis & Ultrasonic for looseness and severe faults
4. Insulation testers for insulation to ground faults
5. Oil analysis/ purification

MCCTs, Controls & Disconnects

The motor control or disconnect provides some of the primary issues with electric motor systems. The most common for low and medium voltage systems are:

1. Loose connections
2. Bad contacts including pitted, damaged, burned or worn
3. Bad starter coils on the contactor
4. Bad power factor correction capacitors which normally results in significant current unbalance.

The test methods for evaluating the controls include infrared, ultrasonic, volt/amp meters, ohm meters and visual inspections. Infrared, MCA and ESA provide the most accurate systems for fault detection and trending.

Cables - before and after the controls

Cabling problems are rarely considered and, as a result, provides some of the biggest headaches. Common cable problems include:

1. Thermal breakdown due to overloads and age
2. Contamination which can be even more serious in cables that pass underground in conduit
3. Phase shorts can occur as well as grounds
4. Physical damage is often a problem in combination with other cable problems

On the Motor supply side the problems can be broken down as follows

1. Poor power factor-3 9%
2. Poor connections-36%
3. Undersized conductors-10%
4. Voltage unbalance-7%
5. Under or over voltage conditions-8%

The most common equipment that covers these areas includes MCA, Infrared and ESA.

Electric Motors- Include mechanical & electrical components

Primary Mechanical problems

1. Bearings- General wear, misapplication, loading or contaminants
2. Bad or worn shaft or bearing housings
3. General mechanical unbalance, looseness and resonance

Vibration Analysis is the primary method of detection of mechanical problems in electrical motors. ESA will detect late stage mechanical problems as will Infrared and Ultrasonic.

Primary Electrical problems

1. Winding shorts between conductors and coils
2. Winding contamination
3. Insulation to ground faults
4. Air gap faults, including eccentric rotors
5. Rotor faults including casting voids and broken rotor bars

MCA will detect all of the faults early in development. ESA will detect late stage stator faults and early rotor faults. Vibration Analysis will detect late stage faults, insulation to ground will only detect ground faults which make up less than 1 % of motor system faults, surge testing will only detect shallow winding shorts and all other testing will only detect late stage faults.

Coupling (Direct and belted)

The coupling between the motor and the load provide opportunities for problems due to wear and the application.

1. Belt or direct drive misalignment
2. Belt or insert wear
3. Belt tension issues are more common than most think and usually result in premature bearing failure
4. Sheave wear

The most accurate system for coupling fault detection is Vibration Analysis. ESA and Infrared Analysis will normally detect severe or late stage faults.

Load (Fans, pumps, compressors, gearboxes etc)

The load can have numerous types of faults depending on the type of load. The most common are worn parts, broken components, unbalance and bearings.

Test instruments capable of detecting load problems include Vibration Analysis, Infrared Analysis, ESA and Ultrasonic.

Application:

There are two common applications for electric motor system testing. These include:

1. Commissioning components or the complete system as it is newly installed or repaired. This will help you avoid infant mortality disasters.
2. Trending of test results for system reliability. Using test such as MCA, Vibration Analysis & Infrared Analysis, potential faults can be trended over the long term, detecting many faults months in advance.

The equipment available from Postma & Postma is listed in the following table.

POSTMA & POSTMA

(PTY) LTD.