Electric motors

[OuBallieParticipant @ouballie]

State of development since 1922

[OuBallieParticipant @ouballie]

Just read ME Issue 1086 and the interest being generated in AC commutator motors, owing to the development of AC supply.

92 years on and we take electric motors so much for granted now, and think little of what our forebears had to endure, either in the hobby or industrial fields.

I just think of the VFD drives I now have on all my machines and marvel at the technology.

Makes me wonder why batteries are lagging so far behind, but no doubt vested interests have something to do with this sad state of affairs.

Geoff – Tomorrow bring more items into the Workshop, from the Shed.

[V8EngParticipant @v8eng]

I think that battery technology has made great advances over the last few decades.

If you think of something like AA cells alone, who would have dreamt of the capacity they have now compared to those of about 50 years ago.

Cars can now be powered by relatively small battery packs, then very fast charged at plug in points, tell that one to the battery powered milk float drivers from the 1950s.

I could go on but hopefully you get the drift of my thinking.

Edited By V8Eng on 29/12/2014 09:14:01

Edited By V8Eng on 29/12/2014 09:18:34

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by V8Eng on 29/12/2014 09:12:17:

I think that battery technology has made great advances over the last few decades.

Yes, in the modelling world most aircraft are now battery powered. Unthinkable 50 years ago. As for energy recovery systems in Formula 1 cars

Russell.

[Gordon WParticipant @gordonw]

The standard lead-acid car battery is far better now. One for a diesel car can be lifted easily yet still allows lights to be on, alarms etc.pre- heats fuel filters and still starts the motor on a cold morning. Downside is , when they go they go, no more charging up at night for a week or two.

[BazyleParticipant @bazyle]

We see comments on forums that modern electric motors (mains, AC, 1phase) are 'more efficient' but also that new motors are lightweight and may run hotter but that's ok since the insulation is better these days.

So does a 1HP new motor provide more power at the axle than a 40 yr old one or a 60 yr old?

And will a 1 HP 3ph produce more actual axle power than a single phase one?

[Keith LongParticipant @keithlong89920]

I think you'll find that old motors were "rated" by their mechanical work output, whereas modern units are rated according to their electrical input. So by the time you take efficiency into account an "old 1hp" motor will give you significantly more useful work out than a "modern 1hp" motor, but will be using more power to do it. Modern motors are required to have fairly high efficiencies, and there is a move in industry to replace older motors with newer ones to reduce energy costs.

I have a couple of 550w 1ph motors that the makers claim 67% efficiency, newer units to replace them would be required to have at least 79% efficiency. So to compare motors you really need to know power in for power out, not just what the rating plate says.

[colin hawesParticipant @colinhawes85982]

An older ac motor can usually cope with a big overload for a fair time whereas it seems that a modern more efficient motor with the same theoretical power rating just stalls instantly (from practical experience with 1/4 HP motors) There is a lot more iron in the older motor to make use of the increase in magnetic flux and to sink heat. Colin

[Neil WyattModerator @neilwyatt]

> Yes, in the modelling world most aircraft are now battery powered. Unthinkable 50 years ago.

Pretty much 25 years ago all you could do was flop around the sky!

Nevertheless, batteries are up against the laws of physics, which is why small hydrogen fuel cells may be the way to go.

Neil

[Phil WhitleyParticipant @philwhitley94135]

Very good point Keith , bit like everything being rounded down to the cheapest outcome for the manufacturer! As to modern motors being more efficient, that is a very moot point to which the answer can only be, some are, but most aren't. You can buy very efficient motors from British or some European manufacturers (ABB Alstom etc) and they are expensive, but many of the italian made and especially the chinese made motors are noisy and run quite hot, both indicators of inneficiency, and also they use cheap bearings. It is a mistake to believe that because it is new it must be better. Have a look at this BTH motor about 3/4 of the way through Doubleboosts video https://www.youtube.com/watch?v=3Lkb9O8Y69U It may not be quite as electrically efficient as a modern good quality (expensive) motor, but it runs quiet and cool, and is continuosly rated, and almost certainly running on windings that are coming up for 80 years old! 3 phase BTW is approximately three times as efficient as single phase due to the 120 deg spacing of the cycle peaks. Easiest analogy I can think of is like comparing a single cylinder engine to a three cylinder!

Phil,

[John Stevenson 1Participant @johnstevenson1]

It's a total melting pot of manufacturers out there if only you knew.

Brooks is owned by the Chinese and has been for quite a while, there are some good manufacturers in Germany and Italy, VEM and Fimec comes to mind here.

One of my customers who is a bulk importer of motors buys them from all over, most have his name on the plate as opposed to the maker. Earlier this year he arranged for the manufacturer in Bulgaria this time to drop ship 200 motors at my place to be modified to fit existing hydraulic pumps. Foreign truck pulls up, Bulgarian driver, opens the curtain side and there are my motors, the rest of Martin's load and all the rest are ABB motors for different customers, all from the same factory.

[Russell EberhardtParticipant @russelleberhardt48058]

There seems to be a lot of confusion over electric motors and a lot of old wives tales are being bandied about.

First, a motor of say 1 hp (be it single or multi-phase) doesn't necessarily produce 1 hp. It is rated such that it can be loaded mechanically to draw 1 hp of work from it continuously without overheating or stalling. A motor free running produces no useful work and thus has 0% efficiency. It will only produce useful work when it is loaded such that there is slip between the rotating field and the mechanical rotation, hence the rating plate will say something like 1450 rpm instead of the 1500 rpm synchronous speed.. The efficiency varies with the load and will typically be about 80% for a fractional horsepower motor.

All 3 ph motors currently sold in the EU must meet IEC 60034 class IE2 efficiency standards and after January 2017 must meet class IE3. There are exceptions for motors intended for inverter drive as the efficiency is reduced when the speed is reduced as the winding losses remain constant for constant torque.

There is no way a 3 ph motor can be three times as efficient as a single phase motor. If it were it would be producing more power out than you feed in. If Phil can do this he should patent it!

Why do modern motors run hot? That's easy. Modern insulation materials used in the windings can resist higher temperatures than the older ones so motors can be designed and made smaller for the same rated power without sacrificing reliability.

Russell.

[Phil WhitleyParticipant @philwhitley94135]

Hi Russell,

Without wishing to get into an argument over motors, mechanical loading does not cause the slip, the slip is always there, as an unloaded motor would not rotate without slip! The rotor must be constantly slightly behind the rotating magnetic field playing catch up, and when the load is applied, the slip lengthens, and the torque produced increases up to the point where the magnetic field shears and the motor stalls. As you have said, the speed is synchronous, ie locked to the cycles of the alternating current, and is only marginally affected by loading, thus a motor rated at 1450 rpm will rotate at this speed unloaded.

As to all motors meeting certain standards, and reading Johns very interesting comments above, who checks? Does this include all the motors where the letters CE stamped on them actually stand for (they claim) China Export!

There is nothing mystical about 3 phase motors being vastly more efficient than single phase ones, it is more to do with the inneficiency of single phase motors. What I should have said in my analagy is that it is like comparing a single cylinder and a three cylinder engine of the same cubic capacity. If 3 phase wasn't more efficient, what would be the point of it, as it is generally three times as expensive to install? BTW the patent office will have nothing to do with anything claiming to be "over unity" as physics says it can't exist, so patents would be impossible.

Heat is a form of energy created by electricity in this case, and if a motor is running hot, some of the energy going into it is being wasted as heat rather that being converted into rotational power. A motor that runs hot is inneficient…..period! Yes, there are new high temp winding protection varnishes and resins, because some motors have to work in high ambient temperatures where they will not cool as effectively as if they were running in a cooler atmosphere, but running a motor hot does not make it more efficient. (no pedants need reply!)

Brooks owned by the Heathen Chinee! John, you have spoilt my Christmas

Phil

[John HaineParticipant @johnhaine32865]

Philip, the Brook Crompton motor that I have just removed from my Myford VMB is marked on the spec plate as 80% efficiency, that's at full load I assume. Clearly the 3 phase motor that has just replaced it, also made by Brook Crompton but back in the days when they made their own motors and were also called Parkinson, cannot be 240% efficient! though I am prepared to believe that its inefficiency, or loss, may be significantly lower at least running on a 50 Hz sine wave, if not the inverter that's now driving it. Maybe even three times less…?

Yes there has to be some slip to create a magnetising current in the rotor, but the slip has to increase with loading to increase the magnetising current, therefore torque, therefore power output. The slip is a difference in speed between the stator field and the rotor rotation, not a phase difference. Actually the phase difference constantly increases at the rate of 2pi times the slip frequency. The reason the motor stalls is because the torque curve is rather peaky, it is zero at zero slip (synchronous speed), increases quickly as slip increases (I.e. motor runs slower than synchronous) and reaches a maximum a bit below the speed at which the rated power is developed, then drops back as the motor goes slower and slower, but even at zero speed there is some torque. But the slope of the torque speed curve below the peak means that as the motor runs slower the torque reduces so it runs slower and the torque….until it stops. The magnetic field doesn't suddenly "shear", or rather if you like, it's shearing all the time. With the right design, achieved by increasing the resistance of the rotor conductors, you can move the torque peak down to zero speed and then you have a torque motor, useful for things like winding tape reels in the best sound recorders. In that case the slip in normal operation is very nearly 100%.

There are complicated reasons why 3 phase motors are preferred, not just plain electrical efficiency of the motor itself. A higher power capacitor start motor with good starting torque has to have 2 windings on the core, one of which is redundant once it is running but has to be big enough to create enough starting torque. So for a given size frame a 3 phase winding can have more active copper, more flux, and give higher torque and power as well as self start. Similar considerations apply to alternators and transformers, so 3 phase generation and transmission is much better than single phase. The cost of a 3 phase installation is certainly not 3 times the cost of single phase given how much of the cost is "civils" and labour. So once you are in an environment like a factory where significant amounts of mechanical power are needed everything argues in favour of 3 phase motors. Nowadays with inverter drives the same is true in our workshops, and on some forum recently I saw an example of a washing machine motor which was a 3 phase induction motor with a VFD to run it!

A bit of investigation shows that Brooks were acquired some time back by an Austrian group called ATB that went into administration and was in turn acquired by a Chinese company. Better than going bust, don't you think? In my day job my Swiss employer is working with a large Chinese corporation in a highly interesting and pioneering project with some fine Chinese engineers.

[Neil WyattModerator @neilwyatt]

Three phase is about smoother power output, balancing the load on the supply and reduced current per phase (therefore less copper in the motor). It's also self-starting so motors are simpler and big single-phase motors would be really scary beasts with huge centrifugal switches to go wrong and large starting windings that wouldn't be in use most of the time (wasting more copper).

A bonus is that three phase motor produces constant torque at all points in its rotation, a single phase motor produces 'lumpy' torque, and that three-phase loans itself to electronic speed control..

Nothing to do with efficiency – a single-cylinder uniflow steam engine can be more efficient then a three-cylinder (simple) steam engine!

Neil4

[Ian PParticipant @ianp]

Is marking the motor efficiency (as John Haine's VMB) normal practice nowadays?

Maybe it just that I have never seen an example, all I have seen on many motors as a percentage, is the duty cycle rating.

Ian P

[John Stevenson 1Participant @johnstevenson1]

Posted by John Haine on 30/12/2014 18:06:00:

Nowadays with inverter drives the same is true in our workshops, and on some forum recently I saw an example of a washing machine motor which was a 3 phase induction motor with a VFD to run it!

.

Modern fork trucks have also moved off the expensive, complex DC high torque motors they have been on for ages and moved to semi-standard 440v 3 phase motors with special inverters taking the 48v battery voltage up to 440v 3 phase.

As well as being an inverter it can also do speed control and even take in inputs from the steering position to make one wheel go forward and one backwards to control tight turns.

With modern electrics the reliability of these units is very good.

[Phil WhitleyParticipant @philwhitley94135]

I am not going to labour this point, I would suggest that anyone doubting what I have said should google it! There are many sites out there that explain the difference in efficiency far better than I can.

Neils statement that "A bonus is that three phase motor produces constant torque at all points in its rotation, a single phase motor produces 'lumpy' torque, and that three-phase loans itself to electronic speed control"

Is getting near the point. If you consider a single phase sine wave, it passes through the zero line three time in a complete cycle, and at these points, no power is developed. In a three phase system, because the cycles are 120 degrees apart there is no point where the power output is at zero. If we introduce artificial 3 phase systems to the argument, we see the power consumed by the artificial 3 phase generator of whatever type lessens the efficiency of the whole system, although with modern equipment, not by much.

John Haine, I have read your second paragraph several times, and you seem to be in broad agreement with me apart from your statement that the slip creates the magnetizing current in the rotor. If slip was required to do that then a stationary motor being that the only motor with true zero slip is stationary, and would never start. The magnetic current in the rotor is induced by the field current which then attracts the induced pole in the rotor to the pole in the stator. as the rotor moves to try and reach the pole, the pole is already decreasing in power, and the next pole increasing, so that in effect the rotor never catches up, and the difference between them is the slip. As the rotor is loaded the slip increases marginally up to the point where the loading overcomes the torque, and the motor begins to slow. As you say, once this process has begun it is self destructive of the torque. The "shear" I was talking about was the initial point where the slip has increased to the point where the rotating field first becomes unable to drag the rotor round.. I think basically we are saying the same thing in different ways. For a more thorough explanation, see http://www.engineeringtoolbox.com/electrical-motor-slip-d_652.html

"There are complicated reasons why 3 phase motors are preferred" I would love to hear them! the simple reasons are that they are more efficient, more reliable and simpler to start.

I honestly didn't know about Brooks, I knew they had become Brook_Crompton which included many other companies as well, and also knew they were a Hawker Siddely company at one time, I will take your word for it, but if they are Chinese owned, they are certainly not shouting about it on their website. The problem with this type of takeover is that the resulting group eventually becomes so big that it fails, and is snapped up by the competition, usually foreign, who are simply purchasing a share of the market. Once the home industry is destroyed in this way usually by heavily government subsidised competitors from outside the accursed EU, then the market is supplied from cheaper places to manufacture (see John Stevensons post above)and the profit leaves the UK forever, along with the unpaid taxes that multinationals are so good at avoiding. It is ok to buy out, amalgamate or otherwise do a deal with your competitors as long as you are all in the same marketplace, you have the same costs and overheads. Foreign competition however, (especially when government backed) does not have the same costs and overheads as as UK based industries, and can therefire destroy the home industry at a stroke, then dump whatever quality and price it chooses to manufacture on a captive marketplece. Problem is without strict import controls on the QUALITY of goods, there seems to be no way back. So yes, it is good that Brooks are still in Huddersfield (for now), but I wonder how many of its motors are still made there, and how many are employed compared to what there used to be. According to my "Brooks Book" (1971) 3000 employees produced 500,000 motors annually.

Phil

[Russell EberhardtParticipant @russelleberhardt48058]

Phil,

You are quite right in that there must be slip when the motor is freely rotating. I was simplifying things a little. There must be sufficient slip to enable the motor to draw enough power from the supply to cover losses such as bearing friction and aerodynamic losses, etc.

Slip, however, is defined as the difference between operating speed and synchronous speed, usually expressed as a percentage rather than an angular displacement. As the torque increases so the slip increases, proportionally at low levels but then, as the slip increases further, the torque reaches a peak and then reduces and can cause stalling. The synchronous speed of a typical motor is 1500 rpm. The rated 1450 rpm would be the speed at rated output.

In general 3 ph motors are more efficient than single phase but not "vastly more" and certainly not "three times as efficient"

Russell.

[Bill DawesParticipant @billdawes]

Hi all, I don't claim to be an electric motor expert but it caught my eye as I have worked in the industrial fan market for over 50 years (well ok 58 years) and remember many uk motor brands, all sadly gone now.

Brook still exists of course but as far as I know none made in the UK, an excellent company in its day.

Others that I remember are GEC, AEI, Newman, Mather Platt, Brush, Bull, English Electric, LSE, and on an.

Very many brands now made in China (TECTOP make a lot of them I believe, under different badges)

We still have ABB, Siemens etc in Europe, we use a lot of WEG, Brazil & Portugal, excellent motors.

One thing I have noticed since my early days is the physical size of motors back then compared to now, they were huge.

On the subject of efficiency IE2 rating is superseded in Jan 2015 by IE3 for motors 7.5kw and above (IE2 still ok if used in conjunction with a VSD)

When making reference to motor efficiency make sure you are looking at the load basis, industrial motors generally have figures at 50, 75 & 100% load. This applies to 3 phase motors, similar data is harder to find for single phase. EC Directives on minimum IE ratings do not apply to single phase at the moment and generally 3 phase motors are more efficient than 1 phase.

A comment also on rating, industrial motors are rated at output, eg a 3kw motor is 3kw output, electrical input power will be in excess of this.

Well Christmas all over once again, looking forward to making great strides in my building of Emma Victoria.

Happy new year to one and all.

Bill D.

[Martin CottrellParticipant @martincottrell21329]

Posted by OuBallie on 28/12/2014 20:44:34:

Just read ME Issue 1086 and the interest being generated in AC commutator motors, owing to the development of AC supply.

92 years on and we take electric motors so much for granted now, and think little of what our forebears had to endure, either in the hobby or industrial fields.

Geoff –

Geoff, more fascinating to me is the fact that, 92 years on, those "in the know" still can't seem to agree on how the things work! As one with no formal training but a lifetime interest in all things mechanical, I have always seen "engineering" as an exact science that conforms to known & proven laws of physics. As such I find it fascinating that there should be such controversy in the workings of an apparently simple mechanism that has existed in various forms for more than a century! Being a complete numbskull when it comes to all things electric/tronic this thread only serves to reinforce my belief that this electrikery stuff is all a black magic art and has no place in my understanding of the science of "Engineering"!!

Before I get shot down in flames, please don't take this as a criticism or belittlement of the previous posters. My own ignorance of the subject merely allows me to comment on the discussion from a totally different viewpoint on the sidelines, annoyed that my lack of knowledge is fuelling my confusion!!

Michael Gilligan ended a recent posting with a thought provoking statement along the lines of " individually, we all know something but together we could all know everything". A fine sentiment on the face of it but somewhat flawed when you realise that it could only come about if everybody agreed about everything!

Kind regards Martin.

[John HaineParticipant @johnhaine32865]

Philip, a stationary motor doesn't have zero slip,it has 100% slip. The induced currents in the rotor are at 50 Hz for a mains motor. Their phase angle relative to the rotating stator field is such that the motor does generate some torque, in order to get it started. I think you are confusing synchronous motor operation, where the phase lag between the rotating stator field and the rotor increases with the delivered torque ; with induction operation where the stator field rotates faster than the rotor and the rotor currents are at the difference between the rotor speed and synchronous speed. In the latter case the phase lag is continuously increasing.

It is not the case that the torque from a single phase motor drops to zero at two (not 3) points in the cycle. The field generated by a single phase driven stator can be decomposed into two contra-rotating fields. Once the rotor is moving, one of these which is rotating in the same sense as the rotor generates lower frequency emfs in the rotor conductors due to "negative slip" whilst the other generates higher frequency emfs as the rotor sees the stator field rotating faster in the opposite direction (positive slip). The impedance of the rotor windings to the latter is higher so the currents are lower as well as in an unfavourable phase relation. The result of the field interactions between the stator and rotor is that the field component rotating in the same direction as the rotor tries to generate a constant torque, but this is modulated by a small higher frequency component resulting from the interaction with the oppositely rotating field component. For example if the slip is 10% , so the speed is for a 50 Hz motor is 45 Hz, the rotor sees a field rotating at 5 Hz in its own direction but at 55 Hz in the opposite direction. The rotor currents in response to these will be very different. The second will generate I think a small torque modulation on what is otherwise a constant torque. Of course with 3 phase drive all this goes away and you get constant torque.

Induction motors are wonderful things, and are used in preference to brushless dc motors in the Tesla Roadster, driven by rather advanced VFDs that also allow energy recovery from them when braking.

[John HaineParticipant @johnhaine32865]

Philip, a stationary motor doesn't have zero slip,it has 100% slip. The induced currents in the rotor are at 50 Hz for a mains motor. Their phase angle relative to the rotating stator field is such that the motor does generate some torque, in order to get it started. I think you are confusing synchronous motor operation, where the phase lag between the rotating stator field and the rotor increases with the delivered torque ; with induction operation where the stator field rotates faster than the rotor and the rotor currents are at the difference between the rotor speed and synchronous speed. In the latter case the phase lag is continuously increasing.

It is not the case that the torque from a single phase motor drops to zero at two (not 3) points in the cycle. The field generated by a single phase driven stator can be decomposed into two contra-rotating fields. Once the rotor is moving, one of these which is rotating in the same sense as the rotor generates lower frequency emfs in the rotor conductors due to "negative slip" whilst the other generates higher frequency emfs as the rotor sees the stator field rotating faster in the opposite direction (positive slip). The impedance of the rotor windings to the latter is higher so the currents are lower as well as in an unfavourable phase relation. The result of the field interactions between the stator and rotor is that the field component rotating in the same direction as the rotor tries to generate a constant torque, but this is modulated by a small higher frequency component resulting from the interaction with the oppositely rotating field component. For example if the slip is 10% , so the speed is for a 50 Hz motor is 45 Hz, the rotor sees a field rotating at 5 Hz in its own direction but at 55 Hz in the opposite direction. The rotor currents in response to these will be very different. The second will generate I think a small torque modulation on what is otherwise a constant torque. Of course with 3 phase drive all this goes away and you get constant torque.

Induction motors are wonderful things, and are used in preference to brushless dc motors in the Tesla Roadster, driven by rather advanced VFDs that also allow energy recovery from them when braking.

[John Stevenson 1Participant @johnstevenson1]

Posted by Phil Whitley on 30/12/2014 20:16:05:

I honestly didn't know about Brooks, I knew they had become Brook_Crompton which included many other companies as well, and also knew they were a Hawker Siddely company at one time, I will take your word for it, but if they are Chinese owned, they are certainly not shouting about it on their website. According to my "Brooks Book" (1971) 3000 employees produced 500,000 motors annually.

Phil

But how efficient were they at their peak?

Watch these two video's

**LINK**

**LINK**

[John OlsenParticipant @johnolsen79199]

Posted by John Haine on 30/12/2014 23:24:10:

Induction motors are wonderful things, and are used in preference to brushless dc motors in the Tesla Roadster, driven by rather advanced VFDs that also allow energy recovery from them when braking.

Hi John, Professor David Byers built an electric car as a testbed for AC motor controls back in 1977/1978 at Canterbury University. It had a pair of standard induction motors driven by the same inverter driving the front wheels, giving a limited slip differential effect….the wheel that resists turning the most gets the most torque applied. It also had regenerative braking. The first travel of the foot brake pedal activated regenerative braking, by reducing the AC frequency. Extra pressure on the brake would bring the mechanical brakes into play. The inverter used SCRs, this being before IGBTs were available. That meant that the design of the commutating arrangements was quite critical, and errors tended to lead to expensive failures. (I refer to commutating the SCRS, there were of course no commutators in the motors.) So these features are not new to the Tesla. Dave Byers was one of the pioneers of the VFD field. I had the experience of driving the car about 1979 while a student there. My final year project was a high speed inverter using gate assisted turn off SCRs. These were a device that was quickly overshadowed by IGBTs and high power FETs, but I did manage to build an inverter that switched at 18kHz, quite fast for SCR technology.

John

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