Case Study #3 – Hidden Defect found in a VFD with Vibration Analysis

Hello Everyone,

Here is the third of the five case studies brought to you with the Reliability Training Institute. In this case study we show how vibration analysis detected a hidden failure in a variable frequency drive. Hopefully, you are finding them of use and helpful.

These case studies are to support my book ‘Enhanced System Reliability Through Vibration Technology’ and my role as an RMS Trainer with the RMS Reliability Training Institute.

Many thanks to Dr Jezdimir Knezevic from MIRCE Science for his enlightening discussion (and MIRCE Science) and to Dean and Stuart at RMS for all their support.

This case study and more can be viewed on the RMS Blog.

Case Study #1 Electrical Vibration Problem

Hello Everyone,

As promised and to follow up on my previous post on bearing failures here is the first of five technical detailed case studies presentation. This is my first voice over presentation so please excuse the English Farmer accent mixed in with some Australian twang.

These case studies are to support my book ‘Enhanced System Reliability Through Vibration Technology’ and my new role as an RMS Trainer with the RMS Reliability Training Institute.

Many thanks to Dr Jezdimir Knezevic from MIRCE Science for his enlightening discussion (and MIRCE Science) and to Austin Dunne from Infrared Training Limited for his guidance and to Dean and Stuart at RMS for all their support.

The case study can also be viewed on the RMS website.

IGBTs are the “Gatekeepers” of Current

IGBTs are the “Gatekeepers” of Current

(Case Study Electrical Defect detected thought CBM)

This is a case history brought to you with data from James Pearce – another great find! This shows how utilising multiple CBM technologies, with a certified and experience technician, can help prevent unplanned failure to assets.

 

Introduction:

Using vibration analysis and thermal imaging condition based monitoring techniques a change in condition was found and a diagnosis of electrical issue with the VFD was given. From this the variable speed drive history parameters were interrogated. This confirmed it was indeed an electrical issue. Further analysis carried out by the site electrical supervisor pinpointed the IVI card as the issue. The IVI card controls a lot of optic connections controlling the IGBT’s. This was replaced and the vibration, temperature and current reverted back to normal.

 

Background:

We have been monitoring assets at the production facility utilising vibration analysis and infrared thermography. On a routine survey a change in condition was noted and investigated.

The motor in this case study is a 4 Pole 50Hz AC motor on a Siemens Variable Speed Drive. This asset has 2 of the same motors both driving a roller each to crush and grind product.

 

On-Site CBM Recommendations:

Motor: It was reported on the day that the windings temperature has been higher in the warm weather and is 10oC warmer than the comparable motor. This survey there has been an increase in the electrical activity across the motor. Please note we can only detect indications of an electrical anomaly. Recommended actions to investigate the electrical drive.

 

Vibration Analysis Data:

The dominant change in condition in the vibration data was an appearance of running speed electrical frequency in the PeakVue data and the increase in the high frequency electrical data.

Figure 1 compares the last four PeakVue acceleration spectra taken from the motor non-drive end. This displays the normal 2xLF activity and then the appearance LF activity this survey.

Fig 1:

 

Figure 2 compares the last two Velocity spectra’s. This shows the increase in the high frequency electrical activity. The top plot is the normal activity and the bottom plot is with the defect.

Fig 2:

Normal data

Data with electrical defect

 

Thermal Imaging Data:

The thermal data below compares the suspect motor and the comparison motor. These motors are on the same asset performing the same duty at the same time.

This data confirms that the windings are indeed warmer on the suspect motor.

Normal Motor

Suspect Motor

 

Electrical Supervisors Investigation:

Below trace shows the current varying.

The below trace is the Phase 1 Current under load conditions, only reading positive part of cycle.

This compares Phases 1 and 3 motor current under load conditions.  Phase 1 only reading positive part of cycle.

On start-up temperatures all came back to normal.The IVI card in the inverter was replaced. The below plot is Phases 1 and 3 motor current equal after changing IVI card, under no load conditions.

 

NOTE:

An Insulated Gate Bipolar Transistor (IGBT) is a key component in what makes up a VFD (Variable Frequency Drive). An IGBT is the inverter element in a VFD, pulsing voltage.

IGBTs have become highly reliable devices that can handle high voltage devices and are able to switch in less than a nanosecond.

The IGBT acts as the switch used to create Pulse-Width Modulation (PWM). An IGBT will switch the current on and off so rapidly that less voltage will be channelled to the motor, helping to create the PWM wave. This PWM wave is key to a VFDs operation because it is the variable voltage and frequency created by the PWM wave that will allow a VFD to control the speed of the motor. Therefore, without the IGBT switching the current on and off so rapidly a PWM wave—and the speed control that comes with it— could not be created.

The IVI card in the drive controls a lot of optic connections controlling the IGBT’s

 

 

A reliable plant is a safe plant

…..an environmentally sound plant

….. a profitable plant

……a cost-effective plant

Electrical defect found with Velocity data – Case Study

Electrical defect found with Velocity data – Case Study

Has anyone found many electrical defects though vibration analysis? We know that VA will show the indications of electrical activity but not necessary the severity. This case study shows that the Velocity vibration data can indicate what the cause of the vibration problem is, this will enable the engineer to target the investigation.

Thanks to James Pearce for the data. linkedin.com/in/james-pearcevibrationanalysis

 

Background:

A routine client called after the operators noticed an increase in noise and vibration from a main plant drive motor. This is a DC motor and usually operates around 400-500 RPM. This is a rather old motor and drive system.

 

 

Initial Vibration Survey:

On attending site vibration data was collected, analysed and before leaving site recommendations were given.

Figure 1 is the Velocity Spectrum collected from the motor. This showed a 1 Order amplitude of 0.07mm/s RMS, with a dominant peak at 49.95Hz with an amplitude of 3.2 mm/s RMS with many harmonics. The motor was operating at 384 RPM during data collection.

Fig 1:

 

 

Figure 2 is the PeakVue Spectrum. This displayed a dominant peak at 149.86Hz, 3xLf. This was also sidebanded by running speed.

Fig 2:

 

The recommendations was to check all supply cable connections and inspect the variable speed drive components for condition.

 

 

Maintenance Inspection:

The site electrical engineer was dispatched to inspect the drive for this variable speed motor. Upon inspection 2 Thyristors were replaced and all electrical connections checked for security.

The operator then reported that the vibration magically disappeared.

 

 

Post Maintenance Vibration Survey:

Vibration data was then collected after maintenance. The motor was running at a higher speed of 456 RPM on the follow up survey.

Figure 3 is the Velocity overall trend from the initial survey and post maintenance survey. This trend shows the reduction on motion from 4.301 mm/s RMS to 1.162 mm/s RMS.

Fig 3:

 

 

Figure 4 compares the before and after maintenance Velocity Spectra’s. From this you can see the dominant 49.55Hz and harmonics have disappeared. The only activity left is a peak at 299.74Hz again sidebanded by 1 Order.

Fig 4:

 

 

 

Summary:

This again shows the benefits of sending a certified, experienced and correctly mentored Vibration Engineer and not a data dog to investigate vibration issues. James quickly pinpointed the cause of the excess vibration that enabled the client to efficiently target the area of concern and quickly rectify the issue saving time and money.

 

 

A reliable plant is a safe plant

…..an environmentally sound plant

….. a profitable plant

……a cost-effective plant

What happens when recommendations are not followed – “when things are left to burn”

What happens when recommendations are not followed – “when things are left to burn”.

How often have you performed a reliability survey and issued a report of findings and recommendations to reduce the risk of unplanned system failure… and the client does not follow the recommendations.

This is one example of an infrared thermal imaging survey that highlights the importance of following the recommendations and also that a thermal survey should be performed by an experienced/qualified reliability technician who does not just rely on the thermal camera to rush round the site but also uses the human senses and experienced to assess system condition.

 

 

Initial Survey:

One panel unfortunately had Perspex in the way of the cable terminations, so this could not be surveyed with thermal imaging. Through the perspex cover it was noticed that the cable sheath has split, probably due to excess heat and exposing the copper cable.

This was reported on the day to the site supervisor, and in writing in the report. Site confirmed that they were going to schedule in repair at the soonest opportunity due to the high unknown probable risk.

This is the thermal image of the panel, note no readings as infrared energy doesn’t pass through Perspex.

This is the digital image of the panel showing the Perspex cover and damaged cables.

 

 

Unplanned Failure:

This was not inspected/repaired and the panel caught on fire. This caused shutdown of the plant and a huge costs to the company in downtime and reputation due to unfilled orders to their customers.

Images of the failed component.

 

Repair:

Image of the repair. Here you can see the burn fire marks on the back panel.

 

 

Conclusion:

Sometimes we try our best to ensure our clients do the right thing for reliability on their plant. Unfortunately they don’t always action what we recommend, not matter how much we try to convince them. In this instance all we can do is keep spreading the word of how important it is to know the condition of your system and then to actually action any risks. This in turn will reduce the risk of unplanned failure.

A special thanks to James Pearce for sharing his experience.

 

Recently I saw a post from Terrence OHanlon of Reliabilityweb.com, that I feel summed up Reliability.

A RELIABLE plant is a SAFE plant

…..an ENVIRONMENTALLY SOUND plant

….. a PROFITABLE plant

……a COST-EFFECTIVE plant

Electrical Motor Defect

This was a first for me in 17 years! It was one of those with the right experienced people at the right place at the right time beauties!

We were called to a site when the operators reported an unusual sound from a 4 Pole 1500RPM fan motor. We attended and through vibration analysis and temperature measurements recommended stopping the motor and checking the connections in the motor terminal connection box. This enabled the site electrical team to quickly pin point the defect.

The vibration data indicated a dominant 100Hz electrical noise, the motor felt like it was hunting or pulsing and in addition the whole motor was hot with the motor DE housing was over 100oC.

It would be interesting if anyone else has ever found a defect like this?

Vibration Data:

The overall velocity Increased from a normal level of 1mm/s RMS to 3.9mm/s RMS and then in two days increased to 8.88mm/s RMS.

The velocity spectrum below displayed a new peak at 100Hz:

This is a single frequency 100Hz Trend, and shows a marked increase at 100Hz:

The PeakVue spectrum displayed dominant 100Hz and harmonics.

The acceleration 10 KHz spectrum displayed two mounds of activity with 100Hz sidebands:

We then decided to look at the auto correlation data and found the following.

10 KHz Acceleration time waveform circle plot with four main peaks and inner lower peaks:

 

PeakVue Acceleration Time waveform circle plot showing 4 clear peaks:

And this is what was found. (Note cable broke when it was lifted for photo.)

We were surprised the motor was still running (very badly with high vibration) in this condition !!!!!

What is the hottest electrical defect you have found with thermal imaging?

What is the hottest connection you have found with thermal imaging in a LV electrical control panel? This little beauty was found by a good friend of mine James Pearce [linkedin.com/in/james-pearce-89053520].

This would have been detected with a routine visual inspection as the red cable is going a black colour! but she’s a beauty.

Shows the benefit of a CBM electrical survey, if not for insurance but to reduce the risk of production downtime.

 

EDM on the outer diameter of the bearing

I have come across many bearings with bearing fluting (EDM) with the damage on the inner or outer bearing raceway. But this is the first bearing where I have seen it on the outer diameter of the bearing outer raceway. Has anyone else come across this?

This was on a AC soft start.

Images of the outer diameter.

This was the inner raceway.

Thoughts?

 

Update:  8th August 2017.

Site Inspection: This motor drives a process blower and is isolated from the ground via rubber isolation mounts, there is no earth bonding on the motor or blower. The blower pipework is also isolated by expansion joints.

The motor  supply cables are three seperate SWA with a seperate earth cable that has 28.5 Amps on it, probably due to back EMF.

After onsite shaft current discharge tests also revealing no current discharge when direct on line this looks to be Static generated Electrical discharge damage.

 

Update:  13th August 2017.

Responding to some comments here are some more photos:

Image of the motor and blower.

DE Shaft.

DE Rolling element.

DE Zoom into rolling element.

DE Bearing Outside diameter.