3 phase Cage Induction Motor go/no go testing

How to quickly test a new three phase motor.

Many electricians have never had to determine whether a three phase motor is faulty since the majority of their work is involved with single phase work. The same can also apply to electrical apprentices since this may not have been taught.
There is nothing worse than removing an old motor to replace it with what turns out to be a faulty motor. There are some basic tests that can be carried out to try an minimise the risk of putting faulty three motors into service.

The tests below should be done prior to installation.

Test one is to verify that the shaft is free to turn and that the fan is not catching the cowl. Spinning the shaft should identify siezed bearings by listening for noises in addition to the ease at which the shaft can be turned.

Test two is to verify that all three phases are in balance. More often than not the motor will be already connected in either star or delta configuration and thus terminals UVW should be tested using an ohmmeter. U-V, U-W, and V-W should all give the same value of resistance across them. Where the motor is to be employed with a star/delta starter then the three individual coils can be tested and again should be the same.

Test three involves an insulation test of the phases to earth. Although if configured in either star or delta the three separate windings will be connected together you will see most time served electricians still test each terminal ie. U V and W to earth.

You should notice that the three simple tests outlined above will not in all cases identify faults with a new motor.

Do also investigate any anomalies in the tests above since it might be something that is simple to cure and you will not be looked upon in a good light if you condemn the new motor and increase production losses.

Transducers Part 4 - ultrasonics

Hello, is the common shout by many to see if they get an echo coming back from an object or objects. The action of shouting of hello creates vibrations of the air that travel outwards much like the action of a pebble dropped into a pool of water. If the tiny ripples of water created by the pebble hit a solid object then they change direction much like light hitting a mirror. Sound waves produce a similar effect and that is what we call an echo.

Sound is a name of vibrations that we can actually hear, whereas much higher vibrations we perceive as visible light. So there is an whole spectrum between sound we hear and the light we see. The name given to vibrations that are just above most human hearing is called ultrasonic.

No doubt your mind will have already used neural pathways to bring up how bats use ultrasonics to avoid objects and some of you may also be picturing the scan of you before you were born.

The piezoelectric effect is used for the transmitter and receiver. In the simplest form an ultrasonic frequency is sent in short bursts, then the echo received by the same sensor. Electronics are used to measure the time taken for the pulses to return. The time taken being proportional to distance between the sensor and the object reflecting the ultrasonic frequency back.

Ultrasonic transducers are not just used for distance measurments as later blogs will show

Transducer Part 3 - The Piezo-electric Effect

Piezo is derived from Greek and means to squeeze. Certain materials that are squeezed or stressed will produce a voltage that is proportional to the level stress applied. Then again your lecturer should have told you all this. All I am doing is drilling through the brain to lay down even more neural interlinks. Although many materials exhibit the piezoelectric effect most common is Quartz crystal.
Another important point is that if a voltage is applied to a piezoelectric effect material it will stress. The effect is therefore reversible and this should be noted since piezoelectric is used in transducers that convert energy into electricity or vice-versa.

If we take a diaphragm and connect to it some piezo material and subject the arrangement to air that is vibrating then electrical voltages are produced proportional to the amplitude of the vibrations. This arrangement you might know better as a microphone. Equally we could with slightly modifications to the design, namely in size, make a device that will vibrate the air in proportion to a varying electrical signal, yes, a loudspeaker.

The air is not the only thing that vibrates since there is also vibration in the earth caused by earthquakes but I doubt many electrical apprentices will get involved with those. Some though will come across devices in industry that measure the vibration of machinery. By processing the vibrations generated by machines with computers it becomes possible to identify failure in components long before total failure occurs.

You are reading this text via a computer that I entered via a computer. Tick tock, tick tock, goes the clock. No I have not lost my reasoning since the piezo electric effect is used both in computers and in certain clocks that you know as quartz clocks. Most objects will have a frequency at which they will vibrate naturally at, this is called the resonant frequency. Quartz crystal is no different, so by differing the dimensions of the crystal you achieve a different resonant frequency.

There are many more applications of the piezoelectric effect used in transducers and sensors so look for blog entries for more later

Transducer Part 2 - The Strain Gauge

I have chosen the strain gauge to start with since it uses some of the basic electrical principles that are taught to electrical students. Problem is that for many the theory will have been lost in some obscure part of the brain. Therefore it is time to trigger new neural pathways to this long forgotten collection of brain cells.

Before getting into the theory it was mentioned that a piece of knicker elastic can help understand strain gauges in Part 1 of transducers. With your knicker elastic or even rubber band in both hands, pull at either end. You will notice the the width of knicker elastic becomes thinner the more it is stretched.
No doubt the above demonstration will have triggered the eureka moment for many students of electricity into how the strain gauge works. I say farewell to those for whom the penny as dropped and continue on for the others.

Put these in order of lowest resistance, a 1 metre diameter of copper cable a meter long, 1 metre diameter of copper cable 10 metres long, 1 millimetre diameter of copper cable a meter long, or a 1 millimetre diameter of copper cable 10 metres long.

Did you get:-

1. metre diameter of copper cable a meter long,
2. metre diameter of copper cable 10 metres long,
3. millimetre diameter of copper cable a meter long,
4. millimetre diameter of copper cable 10 metres long.

So for the same material the resistance is proportional to both it's cross sectional area and length. Those neural pathways are now being formed, I can sense it.

Back to the knicker elastic in which when pulled gets longer and narrower (the csa reduces). Copper does have a very very tiny amount of stretch property like the elastic but so small you are unlikly to notice it, however as with the knicker elastic the more it is stretched the longer in becomes increasing the resistance and the less csa so again increasing the resistance. Once released the resistance returns back to its previous value.

The actual strain gauge that uses copper or another metal will be either a wire looped and bonded onto an insulated material or metal foil again bonded to a non metalic surface.

So hopefully you will now have the gap between your knowledge bridged with neural network links from basic theory of resistance to strain gauges and thus transducers.

Rather than filling up your head with more information about strain gauges at this point I will return later in another blog.

Electrical Transducers Part 1

This is again another blog entry to give electrical apprentice the link between the theoretical world taught at technical college and the practical world in which a few will be working in.


So what is a transducer?

Your lecturers will no doubt give you the answer that a transducer is a device that converts one form of energy into that of another. For the electrical apprentice this either means converting from or to electrical energy from another source of energy.

For many electrical apprentices that may later work in industry it is important to understand how transducer devices work, since having this knowledge may help to understand the operation of a particular process or machine. Knowing a process or machine operation is a key to effective diagnosis. Later I intent to, at a later date write a blog on fault diagnosis.

With so many different transducers available I thought I had better split the topic up into many different blogs so that you will not have to search through loads and loads of words on a single page.

Where I can think of an analogy, this will be included so as to assist in the explaination a specific transducer. I always find that electricity, because it cannot be seen is best explained using something more familar that is visible or at least easier to understand.
For example, what happens when you slowly close a tap/faucet on a water pipe? The action of closing the tap causes resistance to the flow of water, thus the flow slows down, eventually stopping. The tap/or faucet can therfeore be likened to the variable resistor found in electrical circuits in that it too offer resistance but this time to the flow of electricity and not water.

In part 2 I am going to be looking at knicker elastic and strain gauges.

Variable Speed Drives or AC Inverters

Variable Speed Drives or AC Inverters are a common subject that I get questions asked by apprentices and thus thought I might tell others in my words how these are used in an easy and fairly non technical way.

50HZ (60Hz in the US) is the cycles per second of the mains frequency which most apprentices can tell me. Now we look at a 3 phase squirrel cage induction motor which reveals that it will rotate at a given speed. The fastest motor speed found by the apprentice in the UK will be slightly less than 3000 rpm. So the formula is given N (rpm) = f * 60/P where f is frequency in hertz and P is the pair of poles.

So now asking the apprentice what we can change to alter the speed they are most likely to answer either the pair of poles or the frequency and both are correct. Pole change motors are possible but not very practical and are expensive so we are left with changing frequency.

The black box is now brought into play since it is best at this stage to only give the the very basics. Three phases go into the box and another three come out and then we add a speed control potentiometer. Explained is that the mains three phase frequency is now converted to a different frequency. Now many apprentices at this stage begin to ask what is inside the box since it is a little bit simplistic.

So now the black box is opened so to speak and on the drawing additional boxes are drawn. First there is a rectifier a storage capacitor and a box marked chopper. At the stage of asking about speed control most apprentices will understand how the rectification of AC into DC takes place and also how a capacitor can store electricity. The chopper box internals may or not be understood by some. However the simple explanation is that it chops up the DC to produce AC.

It is at this stage that most will accept the explanation of the basic of how inverters or Variable Speed Drives work?

Basic Three Phase AC Theory Understanding

Another subject that often baffles electrical apprentices is in the use of three phase AC. At technical college they are given the theory and little else by the lecturers. Confusion thus remains which can for some inhibit or at least slow down further learning on the subject.

Now at this point I have to mention that most of my experience is in industry and thus these apprentices are likely to be familar with the equipment found there. For those of you reading this that wishing to gain a better understanding I should point out that electrical motors form most of the direct use of three phase AC since lighting is single phase.

I ask the apprentice to demonstrate their understanding of single phase AC by having them draw the sinusodial waveform. I then see if they can draw the three phase sinusodial waveforms which hopefully they can and show that there is a 120 degree displacement between the three. Most have no problem in drawing out the waveform. A question is asked to the apprentice "When one phase is at a maximum in the postive direction what is the value of the other two at that instant." With a little bit of help they point out or can see that this happens

With two magnets it is demonstrated for the purpose of refreshing their knowlegde that two like poles repel each other and that two differing poles attract each other. From this I draw a circle and put on three windings and in the centre is a magnet. I then ask them to look at the waveform and show the direction on the circle that each phase would become postive. I then ask them what happens with the magnet and most indicate that this would turn in the direction that they indicated on the circle.
Have you ever seem the light bulb on someones head glow bright because for most apprentices in my experience it is at this moment that you can almost hear them shout "Eureka".