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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

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

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