Sealed bearing get a charge of grease on assembly at the OEM, I have witnessed this during a plant tour in Wurzburg Germany.
The standard grease in most sealed bearings is lithuim based and has a grease depletion life of aproximately 10,000 hours, this forces asset owners into an annual overhaul of continuous run motors.
Recently I ultrasonic cleaned two bearings for my wind turbine and found a shocking discovery.
I weighed the bearing before & after and found a massive difference in grease charge 5grams versus 12 grams in a 6208 2RS bearings.
The before & after weights are written on the boxes.
The total weight is within 1 gram and resolution of my scales, but once Ultrasonic cleaned the difference is 6 grams. One bearing is heavier than the other, which corresponds to the assembly. The machines match inner and outer races with balls to get the right clearance.
What I found is the heavier bearing of 368 grams cleaned got 5 grams of grease, versus the lighter bearing of 12 grams.
Here is the before cleaning photos
The lefthand one has 5 grams, and righthand one 12 grams. That is a massive difference.
It appears the grease charge is controlled by total weight, therefore a tradeoff between metal or grease.
This means bearings that are under charged will have a significant service life shortening.
Not many people would check Grease charge, if any and take what is given. You expect an OEM product to be "fit for Use" that expectation is now in question.
3Phi Reliability can extract seal without damage, ultrasonic clean and repack with a high performance grease that can see a service life of ten years. We can provide actual grease charge weights on individual bearings. Only for clients of Motor Management.
This has enormous cost benefits and precise grease charges stop Electric Motor Failures.
Capacitors are embedded in nearly all electrical circuits, be it a power supply, power factor correction, Variable Speed Drive, or a single phase motor.
One of the most common failure modes of these devices is the finite life of a capacitor capacity, but very few asset strategies cover this failure.
There is an easy way to get the health of a capacitor by a simple measurement, while uF reading may still be within specification the Phi or Phase Angle drops as the capacitor ages.
Capacitors decay due to a variety of conditions, including heat, overvoltage, fugitive gases, excessive ripple or harmonics, or mechanical damage.
example: Failure analysis of VFD drives found residual methylBromide in the insulation material. They had been fumigated as part of customs clearance in Australia. 100% failure rate within a month.
The test procedure uses a low cost portable All Test Pro Motor Circuit Analyser.
A healthy capacitor should return a phase angle of 90 degrees, and this figure decays to 0 degrees in a failed unit. The top righthand figure shows 90 degrees.
This 15uF blue start capacitor shows no physical sign of failure but the single phase motor wouldn't start. The phase angle is 0 degrees.
A simple test added to your Asset Strategies can monitor the health of your capacitors in critical equipment.
Often a capacitor will fail in a circuit long before other components but can be easily detected early and replaced.
Mark Gurney Gerant 3Phi Reliability Sarl
Summary: Electrical crimps in the elctrical circuit of a motor are surprisingly common cause of motor failure.
Poor terminations also area primary ignition cause of electrical fires, they waste energy, and cause an impedance imbalance in the motor circuit.
A motor audit makes an assessment of crimp quality and identifies defects that ultimately decay to a failure.
How to eliminate motor failures from poor termination.
When crimping training, and purchase of the correct tools was implemented at a food manufacturing site it was met with some suspicious looks. Crimping is a basic skill right!!!! We don't need that!
Well afterwards I got a number of comments that it was very informative, another words they learnt alot.
The picture above is from that site, a crimp lug overcrimped caused a high resistance termination and the motor lead blew off. It was of no fault of the site trades as it had been overhauled externally.
The motor lead (lefthand phase taped in blue insulation tape) was repaired and put back into service.
It must have been a pretty awkward position as the motor lead was very short, and hence the lug has been bent to get it to fit. This is not recommended and could lead to another high resistant joint failure.
The centre phase have also been overcrimped, and the righthand lead might be ok, but the cable seems very stressed.
Steps to stop motor failures due to poor terminations.
1. Conduct a Motor Health Assessment, whichs tests for imbalances in the circuit. Denergised motor circuit analysis can identify these defects within minutes.
2. The Motor Health Assessment includes a visual inspection of terminations and potential failure modes.
3. Implement a onsite crimp training session, and ensure electrical trades have the correct tools and consumeables.
4. Implement an acceptance testing procedure to stop defects being introduced to your enterprise.
5. Document defect elimination and use the result to lower your insurance risk. Remember these types of failure modes are the leading cause of electrical fires.
3phi-reliability.com can implement this part of best practice motor management, and highly recommend Motor Circuit Analysis with All Test Pro.
Mark Gurney Gerant 3Phi Reliability Sarl
As a follow on from the Bearing fluting blog, I had a comment that DOL (Direct on line) systems also see issues. Data shown in this example is from my electical supply at my property.
In the bearing fluting blog I explained the electrical resonance between a Variable speed drive and the motor, resulting in induced voltage across the air gap onto the shaft.
Electrical resonance is not limited to VSD's, but fortunately most resonant frequencies are audible and can be heard relatively easily. In my case I had failures of applicances, two neutrals failed on circuits before remedies inacted.
This chart shows the peak current seen on a Ir 25 amp circuit.
When an electrical resonant event occurs the supply becomes very unstable and significant damage can occur in motor circuits. (Dominantly motor circuits), and the distribution board emits audible chatter. Induced voltages from resonance can be many times line voltage and insulation material suffers.
As in mechanical resonance, a stiff strucutre with lots of mass is less prone to resonance. In an Electrical circuit a strong fault current and minimal cable capacitance is its equivalent. Unfortunately some of the these factors are controlled by the supply company.
An example of the when the damge can occur is when a "Ferranti Effect" event happens . This when a motor is stopped or supply fails and the flux feild collapses into the capacitance of the cable causing huge current spikes. My example 82 Amps on a 25 Amp circuit.
This effect is common in weak supplies, (long cable runs, small or overloaded transformers) and the natural resonance frequency of the circuit can be calculated easily.
Natural Freq = Supply Freqx sqrt (PSC/kVAr)
In my case it calculated at just over 350hz.
I collected data from a Hioki Power Analyser and the data used in this calculation being kVAr.
The negative kVAr indicates that current is in the direction of the transformer (Outgoing) from the "Ferranti Effect".
As in mechanical resonance a trigger must be present to start the resonance. So in the data collected I matched exactly the occurances of resonance with a 175hz signal.
In this example the circuit natural frequency is just over 350hz, 2x the trigger of 175hz being the ripple control of the supply company.
So I designed a high pass filter to dampen this trigger and the problem has been solved, and my power quality has improved and no more failures.
3Phi Reliability offers Electrical Supply Analysis, Harmonic Mapping and Recommendations.
Mark Gurney, Gerant 3Phi Reliability Sarl
In the age of variable speed drives bearing fluting damage is becoming a dominant failure mode in electric motors. I have experienced motors as low as 37 kW see damage in a matter of weeks. We strive to extend motor service life between overhauls to years.
In this article I'll explain why it happens and some of the good & bad remedies.
This failure mode is difficult to detect with Vibration Analysis as no defined impact zone exists but a board spread of damage. Trending the 1-20kHz alarm is by far the most effective. VA only detects the problem doesn't eliminate it.
Lots of remedies are discussed in the industry such as Insulated Bearings, Grounding Brushes, Motor Grounding, Drive chokes, etc etc.
First a little experience we had with a 400kW motor which we detected bearing fluting. I grabbed an old temperature probe approximately 500mm from the rubbish heap and ground a point on the end, wrapped the probe shaft with electrical tape and headed off with our electrical supervisor with his Crow Oscilliscope.
I jammed the probe into the fan cowl and made contact with threaded shaft end and held a good pressure on it while the motor was running. Our supervisor attached the Crow and we measured 110 Volts DC spiking 3 to 4 times per second to ground. That means the bearings were seeing welding style current, no wonder they don't last very long.
We were recommended an insulated bearing by a major bearing manufacturer, well a short time after the problem came back. The PT100 temperature probe had cracked the outer protective layer.
We installed grounding brushes which worked really well until they wore out, placing them end on the fan end shaft is way better option. All these remedies are a bandaid to the problem.
So what is the cause? When a variable speed drive is installed normally it specified with a rated cable, but I have seen drives installed without changing the cable which is a real concern.
The cable is a low capacitance type with a sheild which should be grounded both ends. EMF rated cable glands are a must which ground to the motor terminal box. Grounding the sheild lowers the capacitance if good termination applied.
The drive end termination should pigtail plait the sheild pass the gland and bonded to an ectrical earth that has been tested. I've seen cables partly burnt at the motor gland from high resistance bonds. I recommend the SKF TKED1 wand which during our Motor Health Assessments is a standard test.
The manual states to measure around the bearings but from experience the root cause is the electrical circuit between the drive and motor, so measuring around the cable glands is far more effective.
What causes the electrical spikes?
Electrical circuits are very similar to mechanical resonance system. Everyone should know to get rid of resonance you need a stiff system with lots of mass that lifts the natural frequency away from running or multiple of running speed. Electrical circuits are the same. Capacitance (dominantly the cable) is the spring (stiffness) and the motor coil is the mass. Get this wrong and a standing wave of resonance will induce a voltage many times line voltage at the motor windings.
Some discussion suggests dropping the switching frequency on the drive helps and it does but its the trigger of resonance and the problem remains.
Hopefully now you can see Insulated bearings, shaft brushes don't fix the problem. The problem is caused by mutual inductance across the air gap in the motor.
The root cause is the relationship between cable capacitance and motor windings Inductance. Another option is to add Chokes (Coils) to the drive end thus increasing the electrical mass this will dampen the natural frequency. The underlying issue is cable length between the drive and the motor, which is specified by drive manufacturers. The problem is engineers like to design nice switchrooms, and push cable length specifications to the limit or disregard them.
As reliability professionals we need to introduce installation specifications for our capital peers to follow.
Electrical resonance is not limited to variable speed drives, it can occur on a DOL (direct on line) systems with low PSC (Prospective Short Current) and high KVar systems, but thats for another blog.
Our offering of Motor Health Assessments can quickly identify which electrical motor systems are at risk in your plant.
Mark Gurney, Gerant 3Phi Reliability
I joined a multisite food manufacturing company as a young engineer which had a seasonal raw material,and hence the practice was to overhaul electric motors each year. This had been the practice for many years prior, and was a dominant chunk of the maintenance budget.
There was no schedule of what had been done in previous years , only that the maintenance supervisors knew which motors needed overhauling and operational staff expected new bearings for the start of the season. Sadly motor failures were common and overhauls often rushed.
Not knowing any better I accepted the fact annual overhauls were mandatory, but introduced a Vibration Analysis programme across the nine sites. Sereval seasons of data was collected and after training our staff in precision fitting to remove common faults found sealed bearings showed a rise in the 2-4kHz band, typically lubrication depletion.
There were many motors forgotten and run to failure. A number of our maintenance staff and I attended a SKF lubrication course and it became quite apparent that sealed bearings in motors ran out of lubrication at around 10,000 hours, and that is what we were experiencing in the plant. Our annual overhaul strategy seemed to have some validity.
We slowly moved our strategy to on condition, as our vibration analysts had gained experience with numerous saves and won operational respect.
At that point we initiateda trial of sealed bearings with a higher performing grease on a selected group of pumps explaining to operations that the strategy is no worse than the current strategy.
So during our shutdown we replaced bearings but with a grease with a lower bleedrate. We monitored the pumps monthly with Vibration Analysis, and while we had one fail early of unknown cause the remaining pumps ran for many seasons.
The lubrication frequency band remained acceptable and we experienced a bearing life similar to what SKFhad reported of 3-5 times that over lithuim based grease.
As a result a working group between the sites developed a specification for new & overhauled motors which greatly improved reliability.
I made the bold decision to restock all our stores with a range of sealed bearings like we trialled made specifically for our company. The problem was our supply contractor didn'thave the bearings, but setup a regreasing service. Sealed bearing were washed out, repacked and sealed before placing into a unique box for our company.
This became our standard , and annual overhauls phased out while huge reliability gains made. What I didn't appreciate at the time was the impacton the business being able to shorten shutdownsand stay producing. The exercise took many years of learning and a bit of risk taking but paid off many times over.
Electric motors do need overhauling at approxiamtely 10,000 hours or just over a year of continuous running. This strategy is expensive and often introduces failures to the inexperienced. There are alternatives which can transform your maintenance strategies into profitable ones but do require a different approach.
What pleasing to my ear is that my experience had been confirmed halfway around the world in a paper mill, but no one told me.
Mark Gurney, Gerant 3Phi Reliability Sarl
From experience electrical workers use a "Megger" as a vertification test to ensure a circuit is safe (normally above 2 MegaOhm) before energising.
The test is carried out by attaching the test lead to the motor phase terminal and the common to the motor frame (ground),while the test is essential for safety the "Megger" can be used as a predictive tool.
Firstly, tests must align with common failure modes of a component, in this case the three phase motor windings in the region of the crown where multiple phases cross and the voltage differential is the maximum,orin the stator slot where different phases exist (usually each third slot).
The former is quite common as mechanical movement can fatique the insulation causing a phase to phase failure.
The test requires the removal of the terminal links and supply leads and conducting a phase to phase insulation resistance test.
To ensure you have each winding isolated conduct a continuity test across each terminal winding, test only those which have open continuity else your reading will be 0 megaohm.
u1 to u2, v1 to v2, w1 to w2 should have all should have continuity with the links removed.
A standard megger test will not identify a phase to phase fault, but will find a stator slot fault which is less common and likely to be late stage failure propagation.
Stator slot failure are common with rotor rub from bearing or housing problems.
The crown test (Phase to phase) can propagate like a bearing fault therefore trending the result is recommended. Typicall a new motor without pre existing fault will test off scale or above100 megaOhm. Any fall or trending lower is a concern, and a rule thumb cutoff of 40 megaOhm for replacement.
The crown fault areais prone to failure due to high current starts (DOL or soft starter) inducing mechanical stress to the winding. Windings in the slot area are mechanically supported and less prone.
Crowns are also subject to dV/dT steps from variable speed drives and reflective voltages from cables where the insulation is the thinnest. VSD rated motor have thicker wire insulation to combat this problem.
The less common stator slot fault canbe trended but different test is recommend to be discussed in a further blog.
Remember: Meggering with the links in only testsfor safety as the total three windings see the DC charge.
DC motors can be tested the same but ensure stator & field windings are isolated and tested separately.
If you get a lower than expected result visually inspect the terminal block, cracks, corrosion, debris, or burnt material that will cause a low result with the winding ok.
It is recommended to conduct this teston acceptance, usually the links are out on delivery.
Mark Gurney Gerant 3Phi Reliability Sarl
The sole purpose of holding electric motors on site is to reduce the response time in the event of a motor failure.
Often electric motors held on site are managed in a way that impedes response time, to a point that questions thebenefitsof a store. If your Asset Strategy recommends holding an electric motor it must be "Fit for Use" ALWAYS, easily accessible, and have the correct consumables.
The common impedients to response time are:
1.Identification of the motor specification.Nameplates take forever to identify in a breakdown situation.
2.Not having one storage location. If motor are poorly managed it promotes squirelling away of motors by tradesmen.
3.Poor storage conditions that aren't dry, underlit, no dewpoint control, subject to vibration, or poor access to regularly rotate shafts.
4.No system for managing failed motors, new deliveries, and acceptance tested motors. Motors can easily be mixed up and unfit motors returned to the store,causing disastrous consequences.
5.Lifting and Moving motors under a breakdown situation can lead to back strain, or trying to shift for identification purposes is a waste of time.
6.Not booking out a motor under breakdown event, because the CMMS is in the office and it's 2am. Motoris never reordered.
7.Unique motors with gearboxes or special shafts not having "Where Used" or "BOM" information.
8.Not having the correct consumables leads to a substandard job that never gets reworked. Acceptance testing including links, emf rated glands, stefa seals,gaskets, and shaft keys are the basics.
9.Limited access to tools and sealant.
10.Stores cluttered with spares from redundant assets.
Motors are the most common asset type and require unique storage conditions & procedures.
A well managed store ensures a fit for use spare which can be depended on.
The recommended approach togain control is a motor management programme, in which the store is a key player.The programme consists of Motor Audits, Motor Health Assessments, Asset Strategies, rationalsation of spares, purchase & Overhaul specification, acceptance testingof motor windings,identification & movement procedures,and storage requirements
This shipping container motor store has been installed with lights, racking on wood, lifting trolley,ventilation and dew point control. Motor shafts face inwards taped for corrosion control and rotated quarterly.
Motors are sortedby speed, kW rating, and frame size for easy identification. The inspection label shows the motor has passed a winding test and fit for use.
A tradesman requiring a motor writes the details on a bookout sheet hanging inside the door. The storeman checks the log each morning.
The container is placed on wooden bearers for vibration isolation. Each electrician has a key and are responsible for following procedures.
Once implemented no one wants to return to a poor store.
Mark Gurney Gerant 3Phi Reliability Sarl