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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

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

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