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100 kRPM Brushless Motor Design
4

100 kRPM Brushless Motor Design

100 kRPM Brushless Motor Design

(OP)
Hello,

I would like to see some discussion about high-speed brushless motor & controller design(60-120kRPM, 1-5kW). Our biggest challenge is preventing rotor heating and maintaining stable speed control. We currently are using two-pole segmented magnets, segmented magnet sleeves, high resistivity materials, high speed switching, and external inductors ... but, we are still having issues. Will any of the following help me?

Slotless configuration
Non-PWM based speed control
Laminated rotor yoke
Something else I'm not thinking of..

If anyone has successfully designed and tested a motor in this speed and power class I would very interested in hearing from you. Finally, do you know any companies that make motors like this in large quantities?

Thanks.
Scott

RE: 100 kRPM Brushless Motor Design

sdk,

I have had the same problem with a motor for air for fuel cells. No other solution than using pure sinewave. Check NFO Drives. They are doing just this.

RE: 100 kRPM Brushless Motor Design

You could try Precise Corp on http://www.precise-corp.com
I've used them in past lives and they are very good and reliable capable of going upto 160k rpm but it depends on the application. These are used in high speed machining applications. I used them in the semi-conductor industry.

RE: 100 kRPM Brushless Motor Design

Suggestion: It appears that magnetically levitated bearings would eliminate lubrication problems and increase Mean Time To Repair (MTTR).

RE: 100 kRPM Brushless Motor Design

(OP)
skogsgurra,

Did you have to use external inductors with the sine drive? Did you design your motor to create a sine back EMF also? I checked NFO's website and it appears that they make drives for AC induction motors. I have a DC input to the drive. I sent them a message for more info. Thanks for the suggestion and the lead!

Scott



RE: 100 kRPM Brushless Motor Design

sdk,

Actually, I did not have the problem myself. It was a company that I did some consulting for. They use PM motors with a very precisely calculated field distribution that makes the EMF of the motor sine-shaped. NFO is currently developing special inverters for them since the rotor is very small and cannot stand the heat that losses from eddy currents and hysterisis cause in the rotor. There is also some concern with the bearings being destroyed by PWM inverters as well as having the CAN-bus disturbed by the PWM HF pollution. There are no inductors or filters between motor and inverter.

RE: 100 kRPM Brushless Motor Design

We have a running design up to 90kRpm but not at that power requirement, only a hundred watts.

Tips :
>>  Slotless/toothless "large airgap" stator (toroidal)
>>  keep magnetization around 1.8 Tesla using  N40H 2 poles ring magnets

>>  Sensorless operation (ST72141) using back-emf or Field Oriented Control (not sure that DSP can handle that speed)

>>  0.05mm thick laminations of high grade silicon steel

>>  Keep bearings as far as possible from magnet & stator

>>  non-metallic shaft is a must (but expensive as you'll need stiffness)

>>  The stiffer-the better

>>  Shoot for a balancing nightmare (we have a  5µgram resolution in our application to keep it quiet)
We ended up balancing using bonding UV flashed
(see http://www.loctite.co.uk/PDF/Brochures/MagnetBonders.pd...).

>> driver side : if you can keep voltage below 40-60V then you'll have very low rds-on mosfets making things a lot easier also the wire insulation will be kept reasonnable.
>> Bearings selection : you'll need to have the Mfg involved in the design : up to 30-50 krpm standard bearings can do the job, above that, special lubrication formulations are required.
>> miscellaneous
-In the stiffness equation, bearing preload is an important parameter as well as play .
-H5 and K5 tolerances for shaft and bearing housing is what we have.
-FEM-BEM softwares can be of great help.
Good Luck
HS


RE: 100 kRPM Brushless Motor Design

Suggestion: The manufacturing accuracies or very small tolerances will reduce the motor vibration and noise. Also, any mechanical and magnetic eccentricities should be negligible.

RE: 100 kRPM Brushless Motor Design

(OP)
So far all of your comments have been very helpful.

Sounds like sinewave drive, sensorless, two pole rotor, with a sinusoidal back emf current wave shape will help.  

Cost is a big concern for me.Can a slotless motor be made low cost in high volume? If I go with a slotted stator, what are some things I can do generate a sinusoidal emf waveform?

Highspeed: How does a non-metalic shaft help me (stiffness or losses)? What material would you recommend looking at? Also, do you have any recommendation for high speed lube on ball bearings?

RE: 100 kRPM Brushless Motor Design

A nice didactic example of a complex electrical / mechanical / thermal etc. engineering problem and professional insight from Highspeed.

Let me put my couple of cents.

1. For high speed, most electromagnetical (eddy current) loss takes place in stator yoke due to fundamental flux, hence thick high grade laminations. PWM core loss rather counts for moderate speed / direct drives (www.drbrushless.com).

2. Good point is to keep bearings far away from PWM influence. PWM is not a major evil from losses perspective - chokes will not help, rather thieve AC voltage.

3. Laminated rotor yoke does not make much sense for PM machine due to large airgap (no fundamental frequency rotor losses). Slotless rotor structure helps to reduce second order PWM loss in rotor.

4. Field Oriented control helps to keep copper loss minimal(sensorless is orthogonal).

5. To prevent magnets flying off consider appropriate glue and magnets wraping.

RE: 100 kRPM Brushless Motor Design

yurim,

Yes, chokes do steal voltage. They steal room and weight as well.

Yes, most of the losses are in the stator. But that is usually not a problem with water cooled machines. The problem is to get the heat out of the rotor - so it is crucial to keep the rotor losses very small. And this is where sinewaves help.

No, it does not help to "keep bearings far away" - the problem is the capacitively coupled voltage from stator winding to rotor and distance does not help much. EDM seems to be a bigger problem with these small, high speed machines than anyone would think. Sine does not cause that kind of problem.

RE: 100 kRPM Brushless Motor Design

Some time ago in a previous company I worked on 5kW 120krpm PM motor/alternators, both slotted and slotless.  Performance of the slotted machine was superior, we used the Micro Linear 4426 driving a separate igbt module on both types, and no external inductors.  The slotless machine is potentially cheaper though, the very large effective airgap means that the stator core (which is a plain cylinder) works down at something like 0.3T, so you can use a cheap lamination material or even powder iron core.  I tried both helical and axial conductor slotless, the former has virtually no end-windings but the latter has better copper utilization in the airgap – with helical you are putting a skewed conductor in the gap, effectively more copper and lower voltage than purely axial conductors.  Because the gap flux passes through the winding, it has to be thin stranded (like Litz wire) to limit eddy currents in the copper.

The back emf for the slotless machines did have a very good sinusoidal shape, this would certainly suit the sinusoidal drive.  We did talk to NFO at the time.  With the Microlinear controller, the rotor temperature was significantly lower if controlling speed by varying the dc link voltage i.e. not allowing it to go into PWM.  According to my old notebooks we could get 5.75kW @ 120krpm without PWM compared with only 2.5kW at the same speed with PWM, on the slotted motor for the same rotor temperature (140°C).

Skogs is also right about the rotor losses, they are very difficult to remove.  The rotor eddy current problem is due to both space harmonics (due to slotting) and the time harmonics (due to harmonics in the current waveform).  In a slotted machine, the back emf isn’t too sinusoidal, I don’t know if you would get the full benefit of a sinusoidal drive, and I don’t know the cost.

There are several influencing factors that determine the rotor losses.  A metal can doesn’t help, carbon-fibre is better though it can act as a bit of a thermal blanket and it’s not so rugged mechanically – and it will fail at too high a temperature!  The slot opening/ slot pitch ratio should be minimum i.e. make the slot openings as narrow as possible and use more slots (yes, it puts the cost up).  A large airgap will also dramatically reduce rotor losses, with some reduction in performance.  This is where the carbon-fibre sleeve helps.  Cooling by air through the gap, or oil-spray at the rotor ends may help although allegedly the oil may increase the windage losses.

The losses occur in the surface of the magnets and some in the surface of the shaft where the magnets are fixed.  There is probably limited benefit from segmenting the magnets and any metal can, and even less from laminating the rotor core.

Using ceramic ball bearings will eliminate any problem with bearing currents.

One possible way forward might be to do a collaborative project with a suitable University dept, I only know the UK ones so I can’t really help there.  This is because there is a lot of complex calcs involved in predicting rotor losses.  Testing alone is difficult, it’s hard enough just measuring the rotor temperature reliably.

There are a few companies around who work in this field:
http://www.calnetix.com
http://www.levitec.de
http://www.s2m.fr

RE: 100 kRPM Brushless Motor Design

Still based on our 100W design:
Keeping bearings far away in high induction high speed motor not to put them in the flux path (1/100th of the flux  through magnets is still a lot).
Now what is far away: an initial prototype with 1.5 mm airgap toroidal stator, we had bearings 10mm away
Eddy currents in bearings resulted in a 2-5% efficiency loss at nominal speed/load (I let you evaluate the over heating ....).
Things were fixed when moving them at 25mm away (empirically determined).
When using toroidal stators, air gap is, by construction, big simply because windings are in between magnet and stator core. But with a toroidal stator it's straight forward to get a sine shaped back-emf.

Another tip :
Ring magnets : Zn coated...sourced from china they are really cheap and you can get high tolerances within lots.
our supplier :
http://www.millennium.com.hk
due to federal regulations and patents issues if you are in the US, dealing magnets with china is a bit more complex

While centering them on shaft we have a <0.01mm excentricity but we used a centering tool rather than trying to tight fit them on shaft.
We adjust the centering tool for each lot we receive.

For the non metallic shaft, initially we had a standard 314S metallic shaft and had some derived flux through it meaning it magnetized (dunno why) going to a ceramic shaft resolved the issue but raised another: the cost so we went back to metallic shaft with a spacer between it and magnet (was good enough  and under high pressure to get done with the design, didn't go further)

Back to driving mode, we are using the 6 steps constant torque approach using STMicro back-emf topology with a 40kHz PWM, reducing commutation losses. This is probably not the best fit for your application as precise speed control could be tough.

Grease/Oil formulation : I don't have any access to it, we have our reference at NSK and we had a 3 month process to tune and validate it.
ToDo: call them, give CRS & quantities, they are very reactive.
Cheers
HS

RE: 100 kRPM Brushless Motor Design

(OP)
UKpete,
Thanks for sharing from your experience.  I wish I would have posted this question sooner as I have spent a lot of time wrestling with this problem.  

I don't understand how increasing the airgap size reduces rotor heating. Is it because it allows more cooling flow, or because of the additinal reluctance in the magnetic circuit? My airgap is currently ~1mm.

There seems to be something to segmenting the magnets ... some say it will help, others say it won't. The testing I've done shows that there is a benefit.  I wire EDM'ed radial rings into the magnet (1mm thick with .1mm gaps between) and saw significant reduction in the rotor temperature.

RE: 100 kRPM Brushless Motor Design

sdk, I worked on it for a quite a long time, I think the project got shelved eventually (how does that make me feel!)  I'm happy to help where I can.

In answer to your question, it's the latter.  It puts a large reluctance between the non-synchronous fluxes eminating from the stator, and any rotating conductor.  The 1mm you quote is fairly typical, but if you have a non-conducting rotor sleeve (carbon-fibre is best) say 2.5mm thick, thats a 3.5mm airgap that the non-desireable fluxes have to cross.  If you've got a metal rotor sleeve (what are you using?), it is very close to the stator so even though the gap reluctance is much the same, the sleeve catches all those fringing fluxes around the slot openings etc.

I don't think the airgap has much effect on the cooling, and I don't think it affects the windage losses very much.  Do you actively cool the rotor?  I'm also interested to know how you measure the temperature, we used infra-red but it wasn't easy (or cheap).

I can understand now why the magnet segmenting helps, I was assuming they were several mm thick.  I guess that's pretty labourious and expensive.  Are you using NdFeB? it has slightly higher resistivity than SmCo, every little bit helps.

One thing I didn't try was a backfilled winding, i.e. the windings are loaded from the outside of the stator so the slots are effectively closed on the stator bore.  That should reduce the rotor loss.
There is a European supplier that has done some work on this:
http://www.cogent-power.com/
(Surahammars Bruks is the Swedish supplier of speciality lamination steel).  I can probably get more info if you need it.

The problem with all these mods you can try is that it is time-consuming and expensive to evaluate it from tests alone, that is why I think it is helpful to get someone to do some accurate analysis of the rotor losses.  It's too complex for me, but there are people out there who can but mostly in Universities.

RE: 100 kRPM Brushless Motor Design

Comment: The original posting does not indicate any duty cycle for that motor. The ball bearings do produce noticeable heat unless lubricated and cooled simultaneously. The larger motor 100kW has magnetic bearing on this site:
http://www.calnetix.com/Applications/MPD100.tml
Any heat from those magnetic bearings is easier to remove since the bearing is essentially in the motor stator.
To cool the motor rotor with 1mm gap and overcome heat sources at the ends of the motor will need an effective cooling system.

RE: 100 kRPM Brushless Motor Design

jbartos the use of magnetic bearings in high-speed machines is an interesting point.

One problem I have noticed with them is that they are quite bulky, but there is a strong requirement to reduce the length of the machine for dynamic reasons.  It is also debateable whether they need to be used with touch-down bearings for those occasions when things may go wrong.

But on the positive side, if they are working correctly, at least they don't need changing every 10,000hrs or so, and no oil system is required (assuming there is no other need for one).  They also allow a degree of dynamic balancing and fine adjustment of rotor position.

As regards cooling, unfortunately there is a relatively large pressure drop through the airgap, possibly too high for a regular blower.  In other words, it may need a pump rather than a blower if the airgap is small.

RE: 100 kRPM Brushless Motor Design

Comment: There is more going on for magnetic bearings, namely, they provide galvanic isolation between the motor stator and rotor; therefore, there is no path for bearing currents or damages done by bearing currents, if VFDs are being applied.
Aircraft engines are often using them. They do not appear very bulky there. Visit
http://search.netscape.com/ns/boomframe.jsp?query=%22ma...
etc. for more info

RE: 100 kRPM Brushless Motor Design



So far, this has been a very interesting and educational discussion.

In high-volume applications magnetic bearings have some disadvantages. The need for a toch-down system is one. The relativly high cost is another (an active system needs sensors and coils built into the end bells).

Alternatives are air bearings, but tests and calculations have shown that air gets very "lossy" when speed goes up and the heat will be very high. So air is not an option in the dives that I have seen.

Passive magnetic bearings (no external electronics) also need touch-down and the heat generated in the induction winding gets very high if the system is to work over a wide speed range.

Regarding life: A bearing that gives 10 000 hours life (L90 is probably what UKpete is talking about) is more than adequate in an automotive system. It would mean 500 000 - 1000 000 kilometers and that is an interval that usually exceeds the life of the car the system is built into.

My personal thinking is that hybrid bearings will be used in these motors. The ceramic balls are much lighter than steel balls and this is very important since the centrifugal forces that press the balls against the outer race will be much lower and hence increase life of the bearings. Or allow for smaller and cheaper bearings.

RE: 100 kRPM Brushless Motor Design

jb, I'm not sure that Barden are saying you can do away with TDBs.  Though I have seen some wrecked mag bearings after high speed drops even with TDBs, the latter have to be rugged (and in themselves expensive) to survive.  Active mag bearing systems also need controllers and possibly custom software, which isn't cheap.  

I side with skogsgurra, for many high-speed applications ceramic ball bearings are going to win.  There are no bearing current issues with them, and unless there are special reasons for not wanting oil (eg fuel cell compressors), it is technology that most are comfortable with.

RE: 100 kRPM Brushless Motor Design

Here is a outfit that does it all pertaining to rotary equipment.  We used them and some others using their software on some very high speed equipment.  I wasn't directly involved but I know the engineers that were involved were quite satisfied.  

Though it doesn't run on electricity I have a rotary carver
that runs at 400,000 rpm on air bearings.

We had air bearings on some machines that ran 20,000 to 70,000 rpm. The touchdown was handled by the materials of construction.


http://www.rsr.com/index.html

RE: 100 kRPM Brushless Motor Design

(OP)
UKpete:
Our present motor design uses a Inconel sleeve with a 1mm gap between the sleeve OD and the stator. The stator is a six slot stator with ~3mm slot openings. It sounds like I should increase the air gap (with an appropriate increase in magnet thickness). Do you think a 3mm air gap would be sufficient?

The suggestion to look at back filled stator is interesting. I thought about making the stator such that there is no slot opening. Would this effectively reduce cogging torque even with the saturated regions between the teeth? Related: do you think skewing the stator would help reduce rotor heating?

The laminated magnet testing that we did was only a test case to verify that it does indeed have an impact on rotor temperature. Our present rotor design has magnet segments (high temperature Neo) that are ~10mm thick.  Do you think we will see any benefit with segments this thick? What is the max thickness that you would consider beneficial?

Drive question for everyone:  
I spoke with a motor controller design consultant and he questioned whether a sine drive will effectively reduce heating in the rotor because at high speeds it is difficult generate a clean sine wave. My question is this: Is there a benefit to using a sine drive even if the resultant current is not very sinusoidal? Why?

ShinEtsu has some interesting analysis presented on their website showing that the use of internal magnet rotors and  a sine drive dramatically improvement on rotor heating. You may find this interesting.
http://www.shinetsu-rare-earth-magnet.jp/e/circuit/larg...


Thanks again.
SDK


RE: 100 kRPM Brushless Motor Design

sdk

It is interesting to learn that "motor controller consultants" think that it is difficult to generate a clean sine wave. Yes, that might be so. But when you do have a clean sine wave, then the reduced heating should not be questioned.

There are drive systems that produce sine with very little voltage distortion. The secret seems to lie in the high speed switching and a "lossless" soft switch that allows switching speeds in the couple of hundred kHz region and still having low loss in the transistors.

So the answer to your drive question for everyone is Yes, but on condition that: 1, the driving voltage is sinusoidal and 2, the back EMF is sinusoidal plus 3, the motor is synchronous. This will produce a sinusoidal motor current and a flux with no harmonics and no slip which translates into low rotor losses.

RE: 100 kRPM Brushless Motor Design

(OP)
Thanks Skogs. We have a 400Vdc bus and power levels that range from 1-5kW. My understanding is that we really need to use IGBT's and soft switching to keep the losses out of the controller. We have been using 39kHz IGBT's, which I thought we're about as fast as we could go with IGBT's. (Let me state here that my background is mechanical :).

Do you think we should consider MOSFETS?  Are IGBT's or FETS that switch at a couple hundred kHz considerably more expensive? Cost is a huge driver in this design.

I've asked two consultants about the benefit of using a sine drive and I've received two different responses. A motor consultant told me that there will be a significant reduction in losses regardless of the fidelity of the sine wave. The primary benefit comes from having three phases conducting at all times where the leading and trailing edges (normally seen when using a trap drive) are "stretched out" over a larger commutation window.  The other consultant (a drive guy) suggests that fidelity of the wave form is critical at highspeeds and unless you get can generate very smooth sine waveforms there is no real benefit in using a sine drive.

I need some tie-breakers on this debate.  Do you (or anyone else) have any thoughts about these competing views?

Thanks.

RE: 100 kRPM Brushless Motor Design

Yes sdk. You have no choice but using MOSFET. And they are not so bad in that power range. SiC transistors will be a lot faster and probably also more efficient, but they are not available yet - if we ever get to see them in our time. I have been involved with NFO  - and still is to some extent - so perhaps I should not say too much about their solution. But, on the other hand, I know no other system that performs like theirs.

RE: 100 kRPM Brushless Motor Design

Comment on UKpete (Electrical) Dec 7, 2003 marked ///\\\
jb, I'm not sure that Barden are saying you can do away with TDBs.
///Actually, Barden is using the TBDs. Visit the link for:
""The emergency touchdown bearings will support the shaft in case of complete power failure and allow the shaft to coast to a stop in a controlled manner.""
However, my remark below the posted link focused on:
""magnetic and touch-down bearings in one system
etc.""
Therefore, I do not see anything misleading or controversial in my posting.\\\

RE: 100 kRPM Brushless Motor Design

Hi, high voltage mosfets are quite lossy, (high conduction). High speed igbts are lossy (switching losses). The best approach is to use igbts, lower the switching speed and use a LC filter, or lower your bus voltage and use mosfets.

RE: 100 kRPM Brushless Motor Design

Yes cbarn,

And still, total efficienies above 95 percent (DC link to motor shaft) have been reached with MOSFET, soft switching to produce sinewaves for PM synchronous motors. Filters can be used in conjunction with IGBTs, but in automotive applications (fuel cells) the filter will be quite clumsy and not exactly weightless. It will also influence the dynamic properties, which are getting more and more important.

RE: 100 kRPM Brushless Motor Design

Hi skogsgurra, suitable filter shouldn't weigh more than a few pounds.

RE: 100 kRPM Brushless Motor Design

cbarn,

A sine filter for a 5,5 kW inverter weighs in at 22 kilograms, which according to my conversion table, is more than 45 pounds.

RE: 100 kRPM Brushless Motor Design

Sorry, my informer now tells me that it was for a 11 kW inverter. There was none available for the 5,5 kW so we had to use the big one. The 5,5 kW seems to be about 16 kilograms.

RE: 100 kRPM Brushless Motor Design

sdk, the increased airgap will certainly reduce the rotor heating.  You could also reduce the slot gap, 3mm is rather wide for a motor of that size.  It's a bit of a trial-and-error approach, ideally it's best to optimize the design before anything else whenever possible, more so with a cost sensitive application.

I don't think that skewing will help though, all it will do is skew the eddy currents.

jb you are correct, my misreading.

RE: 100 kRPM Brushless Motor Design

(OP)
Thanks UKpete.  I have learned a hard lesson on this project. Start with functionality, then cut the cost! Thankyou and everyone else for the input.  This has been an extremely helpful discussion for me.

SDK

RE: 100 kRPM Brushless Motor Design

Comment to posting Dec 3 2003:

"Suggestion: It appears that magnetically levitated bearings would eliminate lubrication problems and increase Mean Time To Repair (MTTR)"

Yes. It is very likely that a magnetic bearing will increase MTTR, but is that really what we want? Wouldn't it be a lot more interesting to increase MTBF?  

RE: 100 kRPM Brushless Motor Design

Suggestion to the previous posting: MTBF is more significant in applications where MTTR is insignificant. These are mutually related. Actually, MTTR can increase MTBF.

RE: 100 kRPM Brushless Motor Design

Come on jb!

MTTR = Mean time to repair = the time it takes to repair something after a failure.

MTBF = Mean time between failures.

How is your reasoning going?

RE: 100 kRPM Brushless Motor Design

Aw, come on jb!

MTTR = Mean time to repair = the time it takes to repair something after a failure.

MTBF = Mean time between failures.

How is your reasoning going?

RE: 100 kRPM Brushless Motor Design

Comment on the previous posting: Yes, agreed. The longer MTTR may result in the longer MTBF, since the better quality repair is probably made.

RE: 100 kRPM Brushless Motor Design

Hi skogsgura, take it from me it doesn't need to be anywhere near that weight. The filter only needs to remove the carrier frequency, 3 small chokes and some electrolitic caps is all you need.

RE: 100 kRPM Brushless Motor Design

cbarn,

We are talking about sine filters, not du/dt filters. And I would not use electrolythic capacitors in a sine filter. Would you?

RE: 100 kRPM Brushless Motor Design

I thought we were talking about an output filter for a pwm inverter.

RE: 100 kRPM Brushless Motor Design

Yes cbarn,

We are talking about the filter that sits between inverter output and motor. There are several classes of filters;

1 Motor reactors that are used mostly to avoid overvoltage due to reflected waves and also limit the charge dump that the transistors have to carry when switching on and long cables.

2 du/dt filters that serve the same pupose, only a little bit more efficiently. They are usually composed of a reactor and some capacitors. Some manufacturers depend on the cable capacitance and connect low ohm resistors parallel to the reactors to avoid ringing, which would otherwise heat the reactor cores. Other manufacturers have very elaborate du/dt filters with reactors, capacitors and feed-back diodes connected to the DC link.

3  Sine filters. They produce a pure sine wave for the motor and removes - as you say - the carrier frequency and also the EMI (mostly diverting it to PE, which then gets polluted).

4  There are also common mode filters that are ferrite or amourphous magnetic material or compacted iron powder cores (toroids) that are used to reduce the common mode voltage at the motor. The common mode voltage is the cause of bearing currents, so the CM filter is mostly used to reduce that kind of problem in VFDs.

5  Some manufacturers also use small ferrite cores to try and avoid EMI, to no avail (in my opinion).

All of these filters are output filters for PWM inverters. So we do not talk about mains filters or PFC. And since the motor voltage is always AC (except for standstill) you cannot use electrolyts. And even if you use them (putting diodes parallel to them and connecting two of these combinations in series) they would not work very well because of rather high ESR and internal inductance.

RE: 100 kRPM Brushless Motor Design

It seems we are talking about the same filter then. Electrolitics work fine (I'm not sure why you think we need diodes) as long as carrier frequencies don't go much above 15KHz ESR is not too much of a problem. Ripple current is much more troublesome but you can get around that by paralling smaller caps instead of using big ones. Of course you still may need to add small film caps for EMI suppresion if that is a problem.

RE: 100 kRPM Brushless Motor Design

Cbarn,

That is very interesting. Do you connect the capacitors from DC link to output (after the chokes)? Can you really (really) have a pure sinewave with small components? Do you have any references where I can see for myself?

RE: 100 kRPM Brushless Motor Design

Yes, they go between the choke output and the dc link, use 2 caps 1 to each side of the link for balance. These filters were used years ago before mosfets and igbts were around on inverters with thyristor output stages ( switching at around 1-2KHz). The motor current wave is quite good as the motor itself is a filter as well, you could allways cascade another stage if you wanted to be picky but it's unlikely to be needed.

RE: 100 kRPM Brushless Motor Design

OK cbarn,

I remember them. But I do also remember that the switching frequency with thyristors seldom was as high as 1 - 2 kHz. Those frequencies are IGBT territory. Thyristors were not able to switch more than once (or a few times) per output cycle and the resulting current was not very similar to a sinewave - and the voltage was, of course, even worse.

The filter I mentioned is available from one of the dominant European inverter manufacturers and it is built into a housing that adds to the weight, but I doubt that removing the housing would reduce weight or space requirements very much.

If we go back to the original question, it is obvious that much research is going on in the field of high-speed drives and it is also obvious that filters have been tried in many configurations and that they do not deliver an acceptable set of performance parameters. The problem is not only cost, weight and space, but also waveform, losses and EMI as well as getting good dynamics out of the drive.

RE: 100 kRPM Brushless Motor Design

I'm afraid your very wrong there, 1000amp thyristors with 20uS  commutation times have been around for a least 20 years (they would run at about 5KHz). Even today the thyristor is untouchable for the big inverters.

RE: 100 kRPM Brushless Motor Design

I appreciate this discussion. It brings out many aspects that I havent even touched for years and certainly not in conjunction with high-speed drives in the 1 - 10 kW range.

Yes, there are so-called "inverter grade" thyristors with "short" commutating times. But to my knowledge, which may be limited, they are nowadays only used in CSI inverters for low speed high power drives. My first inverter (around 1970) used thyristors with commutating circuits with an LC combination and an extra thyristor. This technology was then replaced by GTOs.

I would be very grateful if you could point me to any URL where thyristors are used for high-speed drives (30 000 RPM and above) with good sine voltage output since that would be a great contribution to the high-speed technology. I have searched the web for this, but have not found anything. Have you?

RE: 100 kRPM Brushless Motor Design

I don't recall saying thyristors were suitable for this design.

RE: 100 kRPM Brushless Motor Design

Well, but the subject for this thread makes one temted to think that you were talking about high speed drives. Thanks for the views, anyhow.

To round off: 1 -10 kW 30 000 up RPM drives do need sine wave drives and PM synchronous motors. Especially if efficiency, space, cost and weight are important.

RE: 100 kRPM Brushless Motor Design

If I got it correctly soft switching was mentioned in the context of clean sinusoidal output voltage. I think soft switching itself rather reduces semiconductor switch losses and has nothing to do with voltage harmonic contents.

Soft switching does allow for higher switching frequency (cleaner high frequency voltage sine waves). Is there any additional motivation for switching frequency increase like PWM loss reduction?

RE: 100 kRPM Brushless Motor Design

100kRPM=1.66kHz,
A clean sine wave needs at least 32 steps driving to PWM minimum time = 1/53kHz = 18.75µs this will also allow a reduced size filter to avoid bearings currents
based on a 8 bits resolution, smallest PWM element = 73ns
it will be tough to get that with "soft" switching specially if using a full bridge MOSFET configuration where some dead time is required to avoid cross conduction.

Another tip that we used in our design is to have a "dynamic" balancing :
1st step : Each rotor is balanced (mass & excentricity) on a reference stator.
2nd step : when rotor in the final device, PWM generation is tuned during final test to compensate windings unbalance
HS

RE: 100 kRPM Brushless Motor Design

(OP)
avigodner,

I've done some back-to-back testing at 19kHz and 39kHz soft-switching. Higher switching frequency will reduce rotor temperature. The testing I performed was at 30 kRPM, 1kW, running at partial duty cycle condition, 4-pole motor, with a trap drive. The magnet temperature reduction was less than 10degC by going to the higher switching frequency. I'm sure that this finding is highly dependent on motor and operation variables. Adding external inductors did far more to reduce the rotor temperature than did increasing the switching speed.

sdk  

RE: 100 kRPM Brushless Motor Design

Suggestion to skogsgurra (Electrical) Dec 10, 2003 ///\\\

We are talking about the filter that sits between inverter output and motor. There are several classes of filters;

1 Motor reactors that are used mostly to avoid overvoltage due to reflected waves and also limit the charge dump that the transistors have to carry when switching on and long cables.

2 du/dt filters that serve the same pupose, only a little bit more efficiently. They are usually composed of a reactor and some capacitors. Some manufacturers depend on the cable capacitance and connect low ohm resistors parallel to the reactors to avoid ringing, which would otherwise heat the reactor cores. Other manufacturers have very elaborate du/dt filters with reactors, capacitors and feed-back diodes connected to the DC link.

3  Sine filters. They produce a pure sine wave for the motor and removes - as you say - the carrier frequency and also the EMI (mostly diverting it to PE, which then gets polluted).

4  There are also common mode filters that are ferrite or amourphous magnetic material or compacted iron powder cores (toroids) that are used to reduce the common mode voltage at the motor. The common mode voltage is the cause of bearing currents, so the CM filter is mostly used to reduce that kind of problem in VFDs.

5  Some manufacturers also use small ferrite cores to try and avoid EMI, to no avail (in my opinion).
///It appears that the above filter classes tend to overlap in the operating frequency range. For example, if a filter is designed to attenuate frequency above certain cutoff, then the higher frequencies do not have to be filtered by another class of filter for the higher frequency since those frequencies are supposed to be attenuated by that low pass filter. Ideally, a notch filter would pass 50Hz or 60Hz only, leading to a pure sinusoid. It would suffice for all unwanted frequencies attenuation.\\\

RE: 100 kRPM Brushless Motor Design

Yes jb,

Many things "appear" to you. And a lot of your "suggestions" are difficult to understand for us technical people that live in the real world and have first-hand experiences.

Has it never "appeared" to you that real world components have parasitic properties? Like an iron core having hysteris and eddy currents that can be represented by a parallel resistor and that windings have parasitic capacitance that also are in parallel to the winding.

These parasitic properties act as parallel roads for the high frequency signal components and need to be reduced by dedicated HF filters. So even if the different filters overlap in theory, they do not in practice.

RE: 100 kRPM Brushless Motor Design

jb,
any reference or drawing of a notch filter that is efficient for EMI for the kind of load we are talking about?
I'd like to test that
HS

RE: 100 kRPM Brushless Motor Design

Eng-tip to the previous posting: The notch filter is meant to be for the 50Hz or 60Hz fundamental frequency pass not for EMI only. I have never posted such a statement.
Please, exercise care what you are writing about.

RE: 100 kRPM Brushless Motor Design

Eng-tip to Highspeed (Electrical) Dec 13, 2003 marked ///\\\
jb,
any reference or drawing of a notch filter that is efficient for EMI for the kind of load we are talking about?
I'd like to test that
HS
///Attached is a low pass filter with a cut-off 10xfundamental frequency. If there are not subharmonics, then the low pass filter is adequate.  This should filter out EMI on the output too. However, the technial details may be proprietary.
Visit
http://transcoil.com/documents/I_O_M_Manual.pdf
for:
Figure 3: Wiring diagram

Visit
http://www.transcoil.com/#klc

for:
Sine Wave Filter
KMG High Performance Output Filter

The KMG filter is designed to attenuate the carrier components present in the output waveform of typical PWM output power supplies. The general filter topology is an L-C-R low pass circuit. The circuit input is a three phase reactor of sufficient impedance to control the capacitor charging below the inverter peak current fault point. The filter cutoff frequency is set approximately ten times the max allowed fundamental frequency of the inverter to minimize the fundamental KVAR absorbed by the filter while attenuating the carrier components at the rate of 40db per decade. This allows carriers greater than 2KHz to effectively be eliminated from the output of the filter. The purpose of the damping resistor is to control the over voltage excursion at the cutoff frequency to a reasonable level and keep the peak capacitor currents within design limits. An added benefit of the low pass configuration is the capacitive reactance at the load will provide motor power factor improvement thereby improving the overall filter insertion loss to that of the resistor and inductor thermal levels, typically about 2-3% of the inverter full load level.\\\

RE: 100 kRPM Brushless Motor Design

Eng-tips to skogsgurra (Electrical) Dec 13, 2003 marked ///\\\
Yes jb,
Many things "appear" to you.
///Yes, especially, when presented popularly without references or more rigorous way. What is wrong with that?\\\
 And a lot of your "suggestions" are difficult to understand for us technical people that live in the real world and have first-hand experiences.
///I do post many references, links and analytics. I do not see that in your postings.\\\
Has it never "appeared" to you that real world components have parasitic properties? Like an iron core having hysteris and eddy currents that can be represented by a parallel resistor and that windings have parasitic capacitance that also are in parallel to the winding.
///Yes. However, they tend to be neglected in many engineering and conceptual phases. Often, a detail design phase consider them. Are you by any chance a designer?\\\
These parasitic properties act as parallel roads for the high frequency signal components and need to be reduced by dedicated HF filters. So even if the different filters overlap in theory, they do not in practice.
///Not quite true. Visit
http://www.transcoil.com/#klc
where KMG filter has the same feature as KLC among other features, i.e. KMG is somewhat overlapping the KLC functionality.\\\


RE: 100 kRPM Brushless Motor Design

First, I must say that I find this forum most interesting.  I am spending far too much time reading the interesting discussions.

I have one suggestion.  Search for IEEE papers on copper sleeving (rotor)for this application.  I don't recall the exact figures, but the rotor heat (from eddy current) was drastically reduced by employing this techinque.

RE: 100 kRPM Brushless Motor Design

Clyde, I believe you are correct.  If the electrical conductivity of the sleeve is high (i.e. copper or aluminium) then the rotor losses go down.  The magnet is effectively screened by the sleeve.  One problem though is the hoop strength of the sleeve - there are high strength copper alloys but they have a creep (or stress relaxation problem) at temperatures above about 150°C.  Maybe aluminium alloy is best.  There was a GE patent on a copper screen some years ago.

RE: 100 kRPM Brushless Motor Design

Suggestion: Visit
http://www.emoteq.com/images/bdhnew.pdf
for:
The new range of BDH brushless/brushed servo drives has been designed to drive high velocity Servo-Motors at speeds
in excess of 100,000 rpm. These motors have virtually no inductance making them difficult to drive with normal PWM servo amplifiers. The BDH uses a high frequency proprietary PWM technique together with a high-speed MOSFET output stage and other proprietary circuitry to achieve precise control of the motor. The second and probably the most important problem is that of controlling
and monitoring the significant energy stored in a high speed rotating mass. The BDH is designed and built to do both with additional monitoring for loss of hall sensors and an Over-speed limit.

RE: 100 kRPM Brushless Motor Design

hello SDK

as I see it a 120kRPM 2 pole motor needs a 2kHz sine wave - it is quite easy to achieve this up to a few kW with small filters. I have done something similar. ask if you want more help.
BR
Ian

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