Tips for Servicing Marine & Land Based Induction Motors.
Safety & Isolation of supply of induction motors.
Taking a casual approach to electricity can prove fatal.
This is especially true when we are talking about three-phase motors, as they operate in the UK & EU at 400 Volts Alternating Current (400 VAC).
Marine installations typically operate at an even higher 440 Volt Alternating Current (440 VAC).
Never work on a piece of three-phase machinery, such as an induction motor unless you are both qualified to do so, and have authorisation.
People able to give authorisation include senior managers, with appropriate responsibilities, in the case of onshore factory installations.
For work to be carried out aboard Ships, permission from someone such as the Chief Engineer is appropriate.
Once permission has been gained, and the appropriate paperwork issued, only then can work commence.
Certainly in the marine environment, and normally onshore as well, ‘locks and tags’ will be issued.
The lock is to ensure that once an isolator switch has been turned off, no one can switch it back on accidentally.
The ‘tag’ details who has isolated the supply, and is working on that circuit.
Only the person who has been issued with the lock and tag set, can remove them.
Double check that circuit is dead.
Don’t assume that just because you have locked and tagged the appropriate electrical isolator, that you are safe to work on a circuit.
The isolator may be incorrectly labeled, or even worse, you have taken someone else’s word for it.
Before you stick your fingers in, and potentially kill yourself, you need to use an appropriate device to check that the circuit is safe to work on.
There are three possible devices that can be used:
Multimeter / Voltmeter
Firstly lets look at the test bulb as an option.
A test bulb with appropriate leads and clips attached, can provide indication of a live circuit, but has a flaw.
If the bulb filament breaks, then you could falsely assume that the circuit is safe to work on, with possibly fatal outcomes.
The second option is the Multimeter / Voltmeter which these days will probably be a ‘solid state’ digital type, rather than the older analogue types, which are commonly referred to as ‘AVO’s’ in the UK.
The Multimeter / Voltmeter being ‘solid state’ is more likely to be a bit more reliable than, a filament bulb tester. However it still may be broken, and you would not necessarily know. An example being the test probe wires may be ‘Open Circuit’.
The third option, the ‘Line Tester’, will provide the most reliable indication of whether a circuit is safe. Therefore this is the preferred option.
The reason that a line tester is safer is because it contains four separate Neon bulbs (some modern ones are LED).
The bulbs light up according to how high the voltage is, for example a 400 VAC supply would light not only the 400VAC light, but the lower voltage indicator lights as well.
So imagine that the 400VAC indicator bulb has broken.
The lower voltage indicator bulbs will still light up, for example the 230VAC and 110VAC indicator bulbs.
Therefore the engineer will still have an indication that there is voltage in the circuit, and can investigate further.
Before using a Line Tester you should use a ‘proving unit’. A proving unit is a small hand-held device capable of producing a voltage such as 250 Volts.
The Line tester can thus be tested using the proving unit, prior to testing a real live circuit.
To test the Line Tester the two probes are pushed against the Proving Unit which then produces a voltage.
This will be indicated by an indicator LED lighting up on the proving unit itself.
The Neon or Led indicator lamps of the Line Tester should also light up at the same time, to indicate the voltage being supplied.
Tips when changing bearings on Induction Motors
Importance of identification code facing outwards.
When refitting bearings to an induction motor you will notice that the bearing itself has a code written on the one side of it.
This code is the product identification code, and is what you need to quote in order to order the correct replacement bearing.
Once the correct replacement bearing has been obtained, and is ready for fitting, ensure the following.
Firstly, that the bearing identification code is facing away from the Stator, and outwards towards the end of the motor shaft.
This will help you in the future, if you ever have to replace the bearings again.
The reason for this is that you can just remove the end plate of the induction motor, and read the bearing code easily, provided it has been fitted with the code facing outwards.
If the bearing code was facing inwards, then it is harder to read the bearing code, and might mean that the motor shaft has to be disconnected from its mechanical load.
This adds to the motor downtime, and hence has financial and productivity implications.
Ways to remove bearings from induction motor shaft.
The ideal way to remove an old bearing from the induction motor rotor shaft is to use a bearing puller tool.
Removal is then just a matter of fitting, the tool into position, and winding in the screw thread in a clockwise direction.
As this happens, the bearing is slowly pulled up and off the shaft.
If however you don’t have a puller, other methods, such as using a metal bar to leverage between the bearing and the end of the shaft can be tried.
However this is not the way I recommend, and you do it at you own risk of injury and damage to the motor shaft.
Methods for fitting a new induction motor bearing.
Ideally you will have a hydraulic bench press, that you can use to put massive pressure down onto the bearing to ‘press it’ onto the shaft, in the correct position.
When using such a press, a number of precautions should be observed.
Firstly, ensure that you are fully competent to use the hydraulic press. Even fairly cheap versions are capable of exerting many tons of pressure, which can be dangerous to human health.
Secondly, ensure that the tube or sleeve that you fit over the shaft of the motor is only just wide enough.
The reason for this is that a wide metal tube (or sleeve) put over the motor shaft in order to push against the bearing, can damage it.
This is because too wide a tube will make contact with the plastic middle of the bearing, or the outer metal edge.
Both of these two scenarios are bad, because pressure applied to anywhere but the centre metal part of the bearing, will cause damage.
This damage can result in the replacement bearing being ruined, which defeats the object of replacing it.
Using a hydraulic press is the method that we would recommend, however this option is sometimes not available.
In particular to engineers working at sea in a marine environment, such as a cargo ship.
If you find yourself in this situation, then there are other ways to re-fit a replacement bearing to an induction motor.
One method is to take advantage of the fact that metals contract and expand due to cold and heat.
This method involves carefully wrapping up the Stator part of the induction motor in a polythene bag, and putting it in the freezer overnight.
This will very slightly shrink the size diameter of the bearing shaft.
The second part to the operation involves gently heating up a pan of engine oil, so that it is warm.
Obviously extreme care needs to be taken, so that either a fire is not caused by the oil igniting, or the engineer receiving burns while trying to handle the hot bearing.
Once the bearing is warm, the Stator can be removed from the freezer, and the warm oiled bearing should slip fairly easily onto the shaft.
The oil can then be wiped off the bearing with a non fluffy cloth, and motor reassembly can begin.
[bctt tweet=”Tips when changing bearings on Induction Motors #training” username=”@acraigmiles”]
Safety is important in any working environment, but at sea, do often can literally mean the difference between life and death.
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[bctt tweet=”Article about casing design considerations for marine environments.” username=”yeswayradio”]
Developing environmental monitoring systems for the marine environment has additional challenges when compared to designing systems for inland shore side operation.
One reason is the fact that the sea is salty. This means that the casings for the monitoring equipment must not only be watertight, but also be made from a material that will not easily corrode, such as marine grade stainless steel, or plastic (a good resource for more information on marine plastics is here
Ingress Protection (IP) Ratings of ‘off the shelf’ casings should be checked and considered before purchasing an pre-made solution. For example an IP54 gives: Limited protection against dust ingress. (no harmful deposit), and Protected against splash water from any direction.
Therefore IP54 would not be suitable for a device or product that is going to be submerged for periods of time. IP68 which gives: Totally protected against dust ingress, and Protected against long, durable periods of immersion in water.
Choosing to have a custom made casing for your product has a number of advantages:
Firstly, with product branding. Having a custom designed and manufactured casing distinguishes and differentiates your product from the competition, and a well designed casing can give your product competitive advantage.
Secondly, by designing a custom made case you can create a more efficient product.
The reason for this is:
a) Space and weight saving can be achieved by eliminating extra space that is available in a pre-designed casing. This could well have cost savings, due to less materials being used in its manufacture. This is especially true in mass production, due to production economies of scale factors.
b) Greater product usability. The user experience, (or UX experience) is important in all products, but particularly in the marine environment where harsh conditions demand easy handling and operation in challenging conditions. A custom casing can be made easier to hold (for portable equipment), and not drop overboard!
c) Product efficiency. By designing a custom enclosure, it can be made to further protect the electronics within, from dust and moisture.
For example using inspiration from nature, a product casing could incorporate a sloping top (a bit like a pine tree shape, so that water naturally runs down the sides.
This design would have practical advantage over an off the shelf rectangle casing, due to the fact that conventional casings rely on gaskets to seal between the lid and the main casing body. The gaskets can fail due to the harsh environment, and even changes in air pressure, which can create a vacuum or pressure differential between the inside of the case and the outside environment.
Another consideration that needs to be taken into account when developing products for the marine environment is Maintainability.
All electronic parts have what is known as an MTBF, or Mean Time Between Failure. This is a statement by the electronic component, or system manufacturers of how long their product is likely to last. Therefore the product design should take the MTBF into account.
By analysing the MTBF data for all parts of the design, the ‘weak link’ can be identified. This will be the component with the lowest MTBF.
Once the component, or system part with the lowest MTBF has been identified (by using manufacturers data), the criticality of that component should be considered.
For example, if the component part in question failed, what effect would it have on the operation of the whole system?
A failed indicator lamp may not affect the operation of the equipment, however a failed thyristor or Micro-controller board would stop the design from functioning.
There is always a balance to be struck between design reliability and cost.
For example, you could design a kettle to last 100 years without maintenance, but the cost would be to great for most consumers, and it would be a financial failure for the manufacturer.
I would suggest that the first question to ask is, how long is the equipment that is being designed going to be deployed for?
Article to be continued, and expanded in near future (when I have the time 🙂