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The classification or "family tree" of electric machines today — Transcript

by POWERSYS · 3,179 words · 537 segments · language en · Watch on YouTube

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0:00this talk is about the classification of
0:02electric machines or if you will the
0:05family of electric
0:08machines it's a personal view although
0:12it is generously hosted by jol it does
0:15not represent jol in any way I would
0:18also like to thank my friend and
0:20colleague Jim Hendershot with whom I
0:23have worked for over 30 years including
0:26the co-authoring of the green book shown
0:28here many of Jim own presentations have
0:31been an inspiration for me in preparing
0:34this
0:37one the underlying ideas are theoretical
0:41even
0:42academic I've used a number of
0:44illustrations most of them from the
0:46green book I have actually tried to
0:50avoid using the most upto-date
0:52photographs because the inventors and
0:55manufacturers provide Sumptuous
0:57illustrations of their work on their own
1:00websites and even on YouTube to say
1:03nothing of their Publications in learned
1:05Society journals and trade
1:08magazines while we can't attend the
1:11great trade shows now is a good time to
1:14study and enjoy some of that
1:17material it's not my place to steal
1:19their Thunder but what I will try to do
1:22is to show some ways in which the
1:25overall family tree makes sense for the
1:29present and into the
1:31future also I'm at pains to recognize
1:34the history of our great human endeavor
1:36in electrical engineering and so I make
1:39no apology for showing some examples
1:42from the illustrious
1:47past we will try to see the essential
1:51features of the main types of machine
1:54and try to understand some of the
1:56important relationships between certain
1:58groups of machines
2:00we will also touch on the profound
2:03effects of electronic control in
2:05variable speed drives and equally the
2:09effects of high energy permanent magnet
2:12materials in the diagram the magnets are
2:15colored red both of these
2:18developments have led to great changes
2:21not only in the machines themselves but
2:24also in the ways in which they are
2:27applied
2:29we have also seen in recent years
2:31tremendous changes in the demands which
2:34society is making on the electric
2:37machine obvious examples are the
2:40propagation of wind turbines and we're
2:43seeing the prospect of hybrid or
2:45electric vehicles on an enormous scale
2:48in Factory Automation and domestic
2:51appliances the developments are no less
2:56dramatic a particular concern in this
2:58field is the availab ility and cost of
3:01materials this is not new however it is
3:05especially interesting and topical to
3:07consider this question as it relates to
3:09permanent magnet
3:11materials the question can be briefly
3:14stated like this what would we do if
3:18permanent magnet materials became too
3:20scarce or too
3:23expensive we will see that magnets are
3:26used in several important types of
3:28electric machine if we take them away we
3:32will have to use machine types from
3:34elsewhere in the
3:36diagram in the important class of AC
3:39drives the ones colored
3:43blue fortunately there are some viable
3:46proven Alternatives the induction
3:49machine the wound field synchronous
3:51machines and possibly the synchronous
3:54reluctance machine all these machines
3:57share the same power electronic control
4:00topology and to a large degree a common
4:04control
4:05methodology this means that even if we
4:08lost all our permanent magnets we would
4:11not need to lose the pulsewidth
4:13modulated transistor
4:15inverter or the method of field oriented
4:19control this is important because of the
4:21huge investment in the
4:23manufacturer and Technology of the
4:26semiconductor drives themselves
4:31outside the family of AC drives there
4:34are other viable types of machine which
4:37serve important functions today on the
4:40left for example we have the commutator
4:43machines DC at the top and the universal
4:49AC
4:50machine at uh the at the bottom DC motor
4:54shown here has a stationary permanent
4:57magnet field but in many important
5:00applications it uses ferite magnets
5:03which are less expensive and not subject
5:06to the same availability concerns as the
5:08high energy rare earth
5:11magnets the universal motor at the
5:13bottom is common today in Mains fed
5:16power tools and some domestic
5:19appliances but it has the same topology
5:22as a DC motor with a field winding
5:25instead of a set of permanent
5:28magnets for many decades the wound field
5:31DC machine was the main stay of variable
5:35speed Drive systems and it is still used
5:38today in rail traction and elsewhere
5:41although it has been widely replaced by
5:44AC
5:47drives here is a photo more than 100
5:51years old showing the manufacturer of
5:54the rotor of a fairly large DC motor the
5:58power windings are on the rotor also
6:01known as the Armature and this feature
6:04is nowadays seen as undesirable because
6:08for one thing it limits the speed and
6:10the
6:11diameter and the rotor is difficult to
6:14cool so it tends to run hot in passing
6:18we can observe that the process the
6:21craft we see here is still practiced
6:25today in specialist factories although
6:28there has been much development in
6:31materials tooling
6:33processes and of course also in
6:38workware the next picture shows another
6:42feature of the DC motor the
6:44commutator this is a small motor with a
6:47rotor diameter of about 80
6:50mm and in fact this rotor could be from
6:53a DC permanent magnet motor or from a
6:55universal
6:57motor we can see the connections between
6:59the enameled Magnet Wire and the
7:03commutator bars either by staking or
7:07soldering we can see significant signs
7:10of wear on the
7:11commutator where the brushes have not
7:14only been rubbing causing mechanical
7:16wear and noise and friction but also
7:19arcing causing radio frequency
7:22interference and electrochemical erosion
7:24of the brushes and the commutator
7:28bars it's easy to point to the
7:30disadvantages of the commutator motor
7:33but we should remember its advantages in
7:37small sizes it can be produced cheaply
7:39and its control characteristics are
7:41extremely
7:42favorable so much so that the field
7:45oriented control of AC drives goes out
7:48of its way to make the AC motor behave
7:52more like a DC motor from a control
7:55point of
7:58view let's return to the overall picture
8:01we can see most of the electric machines
8:03in common use together with the two
8:07reluctance machines which are relatively
8:10rare but
8:12interesting that classification is more
8:15than an academic
8:17exercise it reflects the organization of
8:20the electric machine industry and its
8:22applications and
8:24markets the diagram is tuned for the
8:26modern era with plenty to guide us into
8:29the future it also reflects the practice
8:32of at least the last 40 to 50 years if
8:36we go back before that the
8:39electronically controlled machines on
8:40the
8:41right would largely disappear and there
8:44would be more classical
8:46machines such as rotary converters
8:49repulsion Motors specialty traction
8:51Motors and others which are practically
8:56extinct on the left hand side we see see
8:59the classical
9:01machines which predate the modern era of
9:04power electronic control the classical
9:07machines date back to the 19th century
9:10the late 19th century in Japan yes the
9:13Magi era in Britain the end of the
9:16Victorian era and in America the Gilded
9:19Age many have become extinct but these
9:23four key ones are still with us
9:26today often with power electronic
9:30controllers when power electronic
9:32controllers started to become seriously
9:34practical and Commercial let's say since
9:37the
9:381970s they were applied in the first
9:40instance to classical machines primarily
9:43to the DC motor and then the induction
9:47motor and the synchronous
9:49motor but since roughly the mid
9:531980s the electronically controlled
9:55machines have become widespread boost
9:59greatly by two technical
10:02developments first the uh development of
10:05the pwm transistor inverter with field
10:08oriented
10:09control particularly applied to the
10:11induction motor and secondly the
10:14development of uh permanent magnet high
10:17energy permanent magnet
10:22materials over the same period the
10:24market for these machines has increased
10:28as a result of the the pressure to
10:30increase
10:31efficiency and the expansion in the
10:34range of electrically powered tools and
10:37appliances Automotive Au auxiliaries and
10:40traction and
10:43others so while we have had a look at
10:46the classical commutator machines let us
10:49take a closer look at the two classical
10:51AC machines the W field synchronous
10:54machine and the induction machine
11:02an actal starting point for this is a
11:04power station and here is an example
11:07more than a century old of course we
11:09have these installations today but
11:11they're usually much bigger over 1,000
11:15megawatts is Common Place whereas in
11:17those days one megawatt would have been
11:19a big one even wind turbines are rated
11:23upwards of three and even 6 megawatt
11:27today but if we look at the root
11:32we see a component that hasn't changed a
11:35great deal in all of those 100
11:38years
11:40apparently by the trained eye will see
11:43many improvements in materials processes
11:47tooling and of course the hidden
11:49elements of analysis used in the
11:52electrical and mechanical
11:55Design This AC synchronous machine rotor
11:58carries a DC field
12:02winding with exactly the topology shown
12:04in the overview it even has the same
12:06number of poles for as we can see by
12:10counting the wound projections or
12:12Salient poles on the
12:16rotor we can also see on the far
12:19right the ultimate source of the field
12:22current the rotor of an Exciter mounted
12:25on the same shaft
12:29now let's look at the
12:30STA uh here is a closeup but of course
12:34this one is definitely not 100 years old
12:37it represents the most modern
12:40technology and it's from WG in Brazil
12:43you can find it in the green book it's
12:46an a neat example of the art very tidy
12:50well filled with
12:51copper compact end
12:54windings all good and manufactured in
12:58very large numbers
13:01for our purposes here what we need to
13:04register is that this AC motor
13:07stator is absolutely the found
13:09foundation stone of the entire family of
13:13AC drives the blue ones in the overview
13:21Slide the overview slide shows this
13:23stator only as a slotted
13:26punching which doesn't do just ice to
13:29the huge variety and Ingenuity of the
13:32different winding layouts but it does
13:36underline the fact that the AC stator is
13:39universal for the family of AC
13:43drives it generally comes with
13:45three-phase winding in sizes above a
13:48kilowatt or so in induction
13:51Motors or in smaller permanent magnet
13:54brushless motors but in larger ratings
13:57we sometimes see six n or even more
14:01phases in
14:03soall uh fractional horsepower induction
14:06Motors we see many configurations with
14:08only two phase windings both supplied
14:11from a singlephase source but with means
14:14to introduce a phase shift between the
14:21currents now we can keep that STA and
14:24instead of a DC Salient pole field
14:27winding let's have a cage type induction
14:31motor rotor again although this type of
14:34rotor has been made for more than 100
14:36years this is a modern example of a
14:39fabricated rotor with end Wings welded
14:43to the cage
14:44bars and designed for operation with an
14:50inverter the thing to note about this
14:52rotor is that there are no connections
14:55it's very rugged and capable of much
14:58higher speeds than a commutator motor
15:01Armature most induction Motors of this
15:04type use cast aluminium for the cage
15:07winding but for highest performance
15:10especially efficiency copper is
15:15preferred let's go back to the overview
15:18chart we have been focusing our
15:20attention on these two AC machines the
15:23wound field synchronous machine and the
15:25induction machine Let's register a very
15:29important point the magnetic field in
15:32these machines has a definite number of
15:36poles North and South poles and the
15:40supply uh the uh and it has a and and it
15:42rotates at a definite speed determined
15:46by the number of poles and the supply
15:49frequency that is the Cardinal feature
15:52of AC
15:53machines in DC machines the field is
15:57stationary
15:59let us now remove the rotor from either
16:02one of these machines and replace it
16:05with a permanent
16:07magnet rotor like this
16:12one this is an eight pole rotor with
16:15high energy permanent
16:18magnets uh magnetized in the radial
16:21Direction and held in place in slots in
16:24the laminated rotor which also has a
16:26profiled surface to to improve the
16:29waveform of the generated EMF this motor
16:32comes from
16:33WG and again it can be found in the
16:36green book it's one of the most
16:38efficient variable speed Drive Motors
16:41that it is possible to
16:44make there is a huge Variety in the
16:47topology of the Interior permanent
16:49magnet motor or
16:51IPM this one from MST Tech in
16:55Japan uh has again eight poles but with
16:59a v-shaped configuration of the
17:02magnets this type of rotor is popular in
17:05automotive traction electric and hybrid
17:07vehicles and sometimes we see intricate
17:10Arrangements of flux barriers and slits
17:14which are designed to tailor the speed
17:16torque characteristic and help control
17:20the power
17:23factor it's also common to build rotors
17:27with magnets on the rotor surface as we
17:29see here in this Balor motor also taken
17:33from the green book this is a fairly
17:36large machine and we can see the step
17:40skewing uh which helps to eliminate
17:43so-called cogging torque and to improve
17:45the EMF waveform so that the machine
17:48runs
17:49smoothly for very high surface speeds
17:52it's common to fit a retaining sleeve
17:54which can be made of carbon fiber Kevlar
17:58or sometimes even even a metallic
18:02sleeve this surface magnet motor or SPM
18:07is here in the chart the IPM and SPM are
18:13closely related brushless AC synchronous
18:16permanent magnet machines they have
18:18different speed torque characteristics
18:21but they share the same high efficiency
18:24and of course the rotors are completely
18:26free from commutators or slip rings and
18:29don't run cool and and do run cool they
18:32do run
18:34cool shaft position sensors are often
18:37used but these don't wear out in the
18:39same way that brushes do and they don't
18:42in introduce friction heat or
18:46noise tucked in here is another
18:49embodiment of the IPM the so-called
18:51spoke motor in which the magnetized the
18:54magnets are magnetized not radially but
18:57circumferentially this again has its own
18:59characteristics and it has been used in
19:01commercial products where lower grade
19:05magnet materials can be
19:09used now a word about the reluctance
19:12machines we need to distinguish two
19:14types the
19:16synchronous and the
19:21switched this uh is a cross-section of a
19:24synchronous reluctance motor rotor with
19:27four poles it's a true AC machine with a
19:31rotating field it has flux
19:36guides which carry most of the flux in
19:40the so-called dxis or direct axis flux
19:45path and these are interspersed with
19:48flux
19:51barriers which impede the flux in the
19:54so-called qais or quadrature axis flux
19:58path
19:59the d-axis path is the main flux path in
20:02all electric machines both DC and AC
20:05while the qais path is everywhere
20:08orthogonal to it in reluctance machines
20:12the d-axis path needs to be highly
20:16inductive an easy path for the flux
20:19while the qais path should allow very
20:23little
20:26flux an inherent feature of the
20:29reluctance machine is that the flux is
20:31excited by a component of the statea
20:34current there is no field winding and
20:36not even a cage type winding or any
20:39other kind of winding on the rotor it's
20:41what we call a singly excited
20:47machine because one winding must carry
20:50the current for exitation of the flux as
20:52well
20:54as uh the torque producing component of
20:56current
21:00a more or less direct consequence of
21:02this is to lower the power factor or to
21:05increase the volt ampere requirement in
21:07the inverter for a given power
21:11level over the last 30 years goodness it
21:14seems like yesterday tremendous ex uh
21:18advances have been made in optimizing
21:20the geometry of these machines to the
21:23extent that they can compete in certain
21:25markets that are normally served by
21:27induction mode
21:31it's not my place to advertise products
21:33but I do think the work of Professor vti
21:37in Italy deserves special mention it's
21:40easy to find this in i e
21:46transactions lastly we must include the
21:48switch reluctance
21:50machine this is not an AC machine it
21:53operates with a series of unidirectional
21:56DC current pulses
21:59which uh are s uh synchronized with the
22:01passage of the rotor poles past the STA
22:04poles which are usually wound with three
22:07or four
22:08phases this requires a power electronic
22:11converter circuit which differs from
22:13that of the standard AC
22:16inverter it also requires its own
22:19control theory because the principle of
22:22field orientation does not
22:26apply here is the state
22:30of a venerable
22:33example um made by liac for the uh
22:37Swedish door opener manufacturer bam and
22:41designed by the pioneering srdl company
22:45in
22:48England and here is a
22:50drawing of a switch reluctance motor
22:56rotor I can remember professor lawrenson
22:59explaining to the vice Chancellor of
23:00leed's University over a canteen lunch
23:04that because of the extraordinary
23:06Simplicity of this motor coupled with
23:08its high efficiency and other features
23:11it was going to revolutionize the
23:13electric motor industry and would
23:15probably displace even the mighty
23:18induction motor that was in about
23:221975 and I consider myself to be lucky
23:25to have been at that table as a research
23:27assistant
23:28Gathering crumbs of such portentous
23:32wisdom since that time the switch
23:35reluctance motor has served a great
23:38many specialty applications with great
23:41great
23:43distinction technically it's Justified
23:46the vision of Lawson's team and
23:49personally this is a good point for me
23:51to pay tribute to my former supervisor
23:55who passed away a couple of years ago
23:57it's worth mentioning
23:59that lawrenson also pioneered many
24:01methods in electromagnetic field
24:03analysis which of course thanks to jol
24:07is the context in which I'm making this
24:12video I think it's fair to say that for
24:15many years the reluctance machines have
24:18been kept on the sidelines by the
24:21superlative performance of PM
24:23machines it was hard luck that super
24:27high energy magnets came in into the
24:29industry at the very time when the
24:31switch reluctance motor was becoming
24:34recognized indeed looking back to the
24:371970s it was unclear what would become
24:41the preferred motor technology of the
24:44future field oriented control for
24:48induction Motors was new and untested
24:51and inverters were cumbersome and not as
24:54reliable as they are
24:56today brush's PM motor were mostly
24:59running with square wave 120° inverters
25:03which seems crude by modern standards of
25:05sinusoidal
25:07drives the race had only just
25:12begun the role of reluctance Motors
25:15could grow in future if magnets become
25:18scarce while we will always have the
25:21reliable induction motor and the wound
25:24field synchronous
25:26machine in very large machines because
25:28of the natural laws of scaling permanent
25:32magnets are not necessarily coste
25:34effective and may even be
25:37impractical it's in small motors Servo
25:40Motors and small scale Machinery where
25:43we would miss the magnets most however
25:47those Motors the ones that are smaller
25:49than a teacup don't need such a quantity
25:53of permanent magnet material so we
25:55should keep that in perspective
25:58some of them could be served by switched
26:01reluctance or synchronous reluctance
26:03Motors especially the Precision
26:05manufactured
26:07ones while in a few cases the good old
26:10DC commutator motor might come to the
26:13rescue I don't think we will have to
26:16wait too long to find
26:19out thanks for listening

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