Motion Amplification of DC Motor Brush Problem

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MotorDoc LLC provides RDI Technology Motion Amplification services along with our suite of testing capabilities to provide a visual identification of problems as part of our PdM and Time to Failure Estimation (TTFE) services.  Following is an example of the use of Motion Amplification to confirm brush issues as well as the ability to dig into the problem.

 

MotorDoc Launches IRIS-M Motion Amplification Service

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Effective January 29, 2018, MotorDoc LLC, a certified Veteran Owned Small Business, has launched our IRIS-M Motion Amplification Services.  This service using the RDI Technologies IRIS-M motion amplification hardware and software allows us to view rotating machinery, structures, driven equipment, piping, ductwork and other systems that may deflect or vibrate.  We utilize these services as part of our quality assurance, predictive and preventive, troubleshooting and forensic services.

Contact us at info@motordoc.com for a quotation.

See an example below (expand video for detail):

MotorDoc LLC Receives Recognition as a Veteran Owned Small Business

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MotorDoc LLC has received recognition from the Veterans Administration as a Veteran Owned Small Business.  Yes, we finally moved forward with it.  For those not in the know, the nickname ‘MotorDoc’ was actually given to me by Capt. Parcells when I ran the motor repair shop (EE04) on the USS Theodore Roosevelt CVN-71.  I was one of the first sailors aboard in late summer 1985 (keel laid in Oct. 1984) and was a plankowner when the ship was commissioned in 1986.  During this time, I completed the Electric Motor Rewind school being awarded an NEC EM-4615 at the same time I completed the rank of E-5 (awarded while I was at school).  Shortly after I then completed the NEC EM-4621 – IMA Electric Shop Journeyman (Motor Repair and Rewind Journeymanship) and took over as the rewind shop supervisor for the last few years I served.  I did attend additional schools related to motor controls, etc., in addition to Electrician’s Mate (EM) ‘A’ school, before and during my time on the carrier and was certified every EM conventional watch up to and including Electrical Officer of the Watch as an enlisted serviceman.  I served until November 1988 then served the following year at the Forest Park Naval Reserve Center until the end of 1989.  I took advantage of my Illinois Veterans Grant and VA to put myself through school, at that point.  The EM-4621 required that I understood vibration, balancing (including field), insulation resistance, insulation resistance, high potential testing, surge comparison testing, and other tests for electric motor circuits, in addition to shop and field repairs.  All of this before I was 22 YO.

LinkedIn Series on Electrical Signature Analysis

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This December we have started a linkedIn series on Electrical and Motor Current Signature Analysis Applications. The series will be announced and linked in the newsletter and on the http://motordoc.com website.  For notices, please sign up for the newsletter which is bi-weekly.

The first parts of the series:

 

Tech Tip: Ground Current

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Based upon several projects we have had to do a deep dive into expected ground currents in shielded, unshielded, and current loop conditions. While we have not found anything solid in standards, we have found several ‘rules of thumb.’ Note that we are discussing ground and not system neutrals.

Primarily what you should see in the ground circuit is no more than kVA/1000 of the associated ground. For instance, if I have a 480V system that carries a 500 Amp load, the leakage should be no more than ((500 * 480)/1000)/1000 Amps or 0.24 Amps (240 mA).

Causes for having higher current include using unshielded cables with unbalanced voltage, on VFDs or during startup of soft start systems. Other causes would be: leakage from contamination; ground fault; dirty (noisy) ground systems; EMI (Electro-Magnetic Induction); and a number of other conditions. This is one of the reasons you will see a requirement for shielded cabling and spacing between cables in cable trays. Finally, it is also important that you run ground cables away from individual phases in conduit and run all three phases in the same conduit so that they cancel out.

You also need to ensure that all grounds are properly bonded to the motor frame. High resistances can also create these conditions, especially as you need to run a reference ground back to the drive/soft start and one at the motor to ground. In the case of shielded cable, you must ensure that you ground one side of the shielding, as grounding both may cause a ground loop.

Conditions such as ground loops or EMI, in addition to problems with electronics, controls, mechanical and bearing failures (fluting), cause danger to personnel due to the risk of electrocution. A quick check of current on the ground conductors will provide feedback as to problem conditions. Any value of current over 1 Amp should be investigated.

Electrical Signature Analysis for Final Testing at Repair Shops

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We were quite surprised to see a tech tip produced by an organization that stated that you required 50% load or more in order to perform motor current signature analysis (or, in our case, electrical signature analysis) and that you cannot use it for final testing in repair shops. Apparently, we have been doing the ‘impossible’ with all of the ESA/MCSA instruments for the past 20 years, or so.

Figure 1: 30hp no load with broken rotor bars – note long start time uncoupled

The 50% load statement from a few manufacturers in the 1990s, and a few that use ‘low resolution’ spectra now, has to do with the slip of an induction motor. In effect, if you have a four-pole, 1800 RPM induction motor, the motor actually runs close to the nameplate value under full load. Most newer motors will have a nameplate reading about 1785 RPM full load. At no load, uncoupled, a good 4-pole machine will run closer to 1799 RPM (low inertia motor) or, more often, at about 1795 RPM. In low resolution spectra, the peaks associated with motor defects would blend into the line frequency (power harmonic) peaks. However, most have resolutions within 0.001Hz+ at the lower frequency range (ie: <300 Hz) and about 0.01-0.1 Hz at higher frequencies (<5kHz).

Figure 2: 5hp motor with no load unbalance on test bench

When looking for issues such as broken rotor bars on an incoming motor to a repair shop or to verify the condition of the rotor and airgap following a repair, the fact that the motor is not connected to a load or coupling is better as it will eliminate outside influences on your tests. For rotor bar conditions we are looking for Pole Pass Frequency (PPF) sidebands around the line frequency, and harmonics if there are a large number of broken rotor bars with good bars in between, and the number of rotor bars times the RPM +/- 60Hz, 120Hz, 180Hz for static eccentricity (rotor off center) and sidebands of running speed around those for dynamic eccentricity (orbiting rotor). The PPF is calculated as twice the slip frequency, or 2*((1RPM/!800 RPM)*60Hz) = 0.0667 Hz, well within the 0.001Hz+ resolution available in most modern ESA devices available.

Figure 3: 1hp motor with unbalance and bad rotor bars

The peaks surrounding line frequency, and elsewhere in the spectra, are related to the peak voltage or current resulting in the ability to detect issues regardless of load. The peak values are counted down in dB from the peak current providing a value that does not vary significantly (ie: 2-3dB on average) regardless of load. In fact, the -dB (dB down) values developed by Oak Ridge National Labs, specifically when they developed ESA for the nuclear power industry, do not specify load as it was unnecessary.

In addition, one of the problems that does occur with a motor with broken rotor bars is a loss of torque and an increase in slip (multiple bars), making it even simpler to distinguish rotor bar issues. When we have run into instances where the findings are in question, we sometimes reduce the voltage to the motor in order to increase the slip to demonstrate that the PPF follows.

We perform this for quality assurance in repair shops from very small, low voltage motors to very large machines, 13.8kV loaded and unloaded, coupled and uncoupled, with great success, detecting defects before they become issues.  In most cases, however, we find the motors are in very good condition both with mechanical (vibration) and electrical (ESA) testing when following a good repair specification and processes.

At no load, current signature analysis with the correct instrumentation is far superior to vibration for rotor bar and rotor related fault detection.

The Cost of Maintenance 2 – Overview

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We are going to take a number of the issues developed in the last Blog one step at a time. This week, we are going to focus on the reason why technology has been ineffective in many motor management programs.

Maintenance Needs

The present trend in maintenance involves reduced staffing, reactive maintenance and far too much unplanned downtime. In addition, new staff is often not exposed to experienced staff before they retire, transfer or pass away. The initial purchase of technology is often done to fill the holes left behind.

With reduced staffing occurring due to perceived profitability improvements, the shift away from planned maintenance is inevitable. The best priority for maintenance is, in order:

  1. Condition-Based Maintenance/PdM (CBM – corrections based upon actual condition)
  2. Preventive Maintenance (Lubrication, servicing, other periodic maintenance)
  3. Reactive/Corrective maintenance practices

However, it is often seen that managers, planners and maintenance personnel will often respond to reactive maintenance as a priority. I have actually observed one of the worst conditions that could be considered:

A production line was to be brought offline for eight hours to perform planned maintenance. The maintenance planner released personnel to provide planned maintenance only after they would complete reactive maintenance (that did not impact production) and pulled other personnel off of condition-based monitoring duties. In effect, the priority was: Reactive; Preventive; then, CBM. The result is a purely reactive environment, which is ineffective.

Through a focus on reactive maintenance, due to the need for personnel (pulling personnel off CBM), planned maintenance quickly moves towards breakdown maintenance. Breakdown maintenance does not involve any CBM, planned maintenance, or otherwise, and instead is an ineffective and expensive, let alone frustrating, method of performing maintenance.

MotorDoc Now Performing Shaft Current Studies

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For Immediate Release: May 17, 2017


Based upon requests from our clients MotorDoc LLC has increased our capabilities to include independent shaft current studies for motors on soft starts, variable frequency drives and motors & generators where fluting issues are suspect.

As we have previously discussed, we are seeing an increase in shaft current issues within industry.  We have added the Aegis Shaft Current inspection system to our arsenal of electric machine testing technologies.

This capability is available for troubleshooting, as part of PdM work and machine forensics.

Contact us at 800-919-0156 ext 0 or info@motordoc.com (630 310-4568 outside of the USA)

 

The Cost of Maintenance 1

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So, what is up with our industry right now? What is the future?

We are in a maintenance limbo: Technology is allowing us far greater capabilities than we have had in the past; Maintenance professionals are retiring from industry, with few following into our profession; Technical education through universities and training companies is mediocre, at best; Indecision by managers to act upon PdM/CBM recommendations; Over 93% of motor management programs fail; Over 57% of CMMS applications fail; and, Well over 90% of maintenance programs fail.

In order to understand, let’s break it down just a bit:

First, we have a love affair with new technology. Gadgets are cool to have and use. However, for the most part, they have been ineffective. Either they are not fully applied, or other maintenance actions call attention away from the use of the technology. In effect, technology has been applied as a ‘crutch’ for a rapidly ineffective maintenance and reliability industry. Coupled with a distrust of technology, the ability for technology to see developing problems far in advance of human detection (through the senses – hey, it’s still running!) and the occasional mis-call of expert systems, decision-makers will often not act upon reported reliability findings. The result is frustration by the technologist (reliability/maintenance tech’s) and lost opportunities that are often written off as the ‘cost of doing business.’ [Note: I have seen numerous instances where maintenance is placed in the expense column of a business spreadsheet and another column identifying projected production losses due to unscheduled downtime – not listed as an expense. Contradictory? I think so! If you have such a spreadsheet, look at the projected unscheduled downtime that is your reliability/maintenance opportunity.]

There seems to be a growing need, within the maintenance and reliability community, for new blood to replace the aging workforce. The problem is that many businesses are either not allowing time for mentoring or are using the opportunity to allow attrition to reduce the costs associated with maintenance manpower. Few people enter into our workforce because it is perceived as ‘grunt work,’ and not high tech. Why should they enter the maintenance and reliability world when they can get a job working with computers in a nice, clean, office?

Education has deviated from maintenance and, in the electrical world, power. Very few universities offer motor design courses (North America), distribution courses, etc. instead changing their curriculum from Electrical Engineering to EECS (Electrical Engineering and Computer Science). The reason is quite simple: Universities are businesses and they need a minimum number of people in coursework and they need to represent that they are cutting edge in order to obtain research grants. There is not a lot of money in maintenance and reliability research and development.

The high rate of maintenance and reliability failure can be attributed to a great number of reasons. In my experience, those amount to several key issues: It is determined by management that the activity is too expensive after it is initiated (false start); The activity is initiated with very poor planning; Programs are implemented in parallel; and/or The Entropy Factor. While the first few are self explanatory, the Entropy Factor is, by far, one of the most dangerous.

The Entropy Factor exists when a program is becoming successful. The initial cost avoidance and payback appear during the growth stage of a program. The continuing, crucial, portion of the program exists after that point: Maintaining the success of the program. However, this is also the point where some enterprising manager will determine that expenses can be reduced in maintenance and reliability because they have not been having equipment failures and there are expenditures without obvious payback. The program is cut, then there is a lag-time before equipment starts failing (usually 12-24 months) at a high rate. In effect, the success of the program ensures its failure: The cause – poor management and training of managers and poor goal setting. Management and Sales (typical MBA stuff) goals are forced upon the maintenance community. This is bad practice as the concept of MAINTENANCE is to MAINTAIN the FUNCTION of EXISTING RESOURCES. It cannot, by its very nature, be a growth part of the company. The fact that maintenance and reliability of machinery is able to produce a return on investment by reducing unplanned downtime is actually a measure of historical MANAGEMENT FAILURE and POOR PLANNING. Sinking in yet?

The primary problem that we have in the Maintenance and Reliability industry is that the metrics used to evaluate the success of our programs is built around the WRONG MODEL. This is exacerbated by the myriad (thought I would use a few big words so I sound like I know what I am talking about – basically, made worse by a lot of) consultants who try to massage maintenance cost avoidance into the profit/loss models of sales and business. In effect, the MBA programs, management training programs, and all the other BS that has been passed around over the past few decades, has focused on just one part of the business, but not the underlying structure.

This is the first in a series related to this topic.

Electrical Signature Analysis Series – Part 1

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In this lecture series, we will be discussing Electrical Signature Analysis (ESA), which is a method for evaluating electrical machinery while energized. The topic will be quite broad and is to include an analysis of supply power through the driven load.

While we will rely upon some of our previous discussions to provide information and definitions for some of our new information, we will start this series by providing some definitions unique to ESA:

  • Voltage: Electrical pressure, is also termed as electromotive force. Voltage is generated.
  • Current: Defined in classical physics as electron flow. Current is demanded in order to produce work and is a result of the load.
  • Upstream/Downstream: Upstream refers to the electrical system in the direction of generation or distribution from the point of test. Downstream is towards the motor and load from the point of test.
  • FFT: Fast Fourier Transform (FFT) is a mathematical method of separating the frequencies of a ‘sine wave’ and presenting them as frequencies and amplitude.
  • Spectra: Is the graph of frequencies and amplitudes resulting from an FFT.
  • Voltage and Current FFT: Spectra of voltage and current.
  • Motor Current Signature Analysis (MCSA): A method of viewing demodulated current and current FFT’s to evaluate the condition of machinery downstream of the point being tested.
  • Voltage Signature Analysis (VSA): A method of viewing voltage FFT’s to evaluate the condition of machinery upstream of the point being tested.
  • Torsional Analysis (TA): A method of viewing the current resulting from the load and its torsional effect (pulsating loads, etc.).
  • Inrush Analysis: A method of viewing the inrush effects on voltage and current when electrical machinery is started.
  • Power Quality: The industry has defined this as reviewing voltage and current. Voltage unbalance, over/under voltage, voltage and current harmonics and current unbalance.
  • Power Analysis: This is defined as viewing power quality as well as surges, swells, transients, interruption, etc. and requires datalogging capabilities.
  • Electrical Signature Analysis (ESA): A method of evaluating the motor system, which includes supply, control, motor, coupling, load and process, utilizing MCSA, VSA, TA, Inrush Analysis and Power Analysis.

The purpose of ESA is to obtain enough information, concerning the circuit being tested, to evaluate the health of the electrical system from supply through load.

ESA has been successfully applied in these applications:

  • AC induction motors
  • Variable Frequency Drives (VFD’s)
  • Wound Rotor Motors
  • Synchronous Machines
  • DC Motors
  • Alternators and Generators
  • Machine Tool Motors and Servos, including robotics
  • Driven equipment including Belted, Direct Drive and Geared
  • Transformers
  • Traction Equipment
  • And numerous other applications

What it comes down to is the ability to evaluate the information provided by ESA. That is the purpose of this lecture series.