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Multi-speed DC brushless motors?

Multi-speed DC brushless motors?

Multi-speed DC brushless motors?

(OP)
  I am currently using a brush-type DC motor to drive a relatively high inertia flywheel from 0 to 15K RPM in approx. .5 sec with a DC source (battery).  
  The problem I am running into is that with a single, always-on gear ratio, we spend too much time in the high current, low speed, low efficiency range (< 1/2 the max motor speed) and I'm having durability and heat related problems with the current motor.  Electrically modulating the applied voltage does not help, as it just increases the time the motor spends in this inefficient range.
  Besides finding a way to mechanically shift gear ratios, is there any way to increase the low-end efficiency of a DC motor?  I am thinking of something along the lines of 2 speed AC induction motors, but in DC form.  Any ideas?
   

RE: Multi-speed DC brushless motors?

Could you give us an idea as to the magnitude of the load?  Perhaps info on the motor currently in use or an estimate of the inrush current?

RE: Multi-speed DC brushless motors?

Hello;
     Look into field weaking a shunt type DC motor... In any case if you limit the motor current to the rated max, you should be able to control the overheating.

RE: Multi-speed DC brushless motors?

(OP)
The load is all inertial (~15x the rotor inertia), there is no applied torque outside of the flywheel during the ramp-up.  Motor is a 35mm can and we are throwing a max of ~ 100A through it at stall.  Basics on the motor: R= .15 ohm, Tmax= 8 in-LB, RPM max = 25K.  
  
  DJS... I did some calcs. and ran some tests re: current limiting in this application and found that it actually INCREASES motor heating because as the applied voltage is modulated, the motor spends more and more time in the hugely ineffiecient low RPM range.  Hitting it hard at the start gets it out of the low end and decreases the total heat generated in one cycle.  
  I'll look into field weaking a shunt type DC motor.  Thanks for the suggestion.

RE: Multi-speed DC brushless motors?

  Thanks for the motor info; it gives a good indication of the motor size.  One approach I would suggest considering is to use a three phase AC motor.  I know this sounds ludicrous, but there are very small (about the same size as your 'can' motor) permanent magnet three-phase AC motors which would be far more efficient than your 'can' motor.

  The controllers are designed to operate on anything from 6 to 36 VDC.  The controllers use a 'soft start', limiting current flow to the motor as it comes up to speed.  I don't know offhand what the ramp-up timing is but it's quite short.

  Like any small motor they don't have a great deal of torque, although more than a brushed motor.  On the other hand they're good for 50 to 60 grand so a 3:1 or 4:1 gearbox is feasible.  There are also a couple of 'rotating can' versions which offer much greater torque at lower RPM.

  The main drawback of this approach is a high initial cost, however the lifespan would be orders of magnitude greater than a 'can' motor.  If you want more details I can dig out a few websites where this equipment can be found.

Good luck, Bruce

RE: Multi-speed DC brushless motors?

Suggestion: How about getting the dc motor with appropriate parameters for your load?
Please, could you post the motor nameplate data and shaft load data?
The motor-load marriage should not be a major problem; however, it may lead to a custom built motor.

RE: Multi-speed DC brushless motors?

(OP)
  Thanks for the info.  
  FYI:  Motor performance (with a PMDC motor) is already very close to optimum sizing for this app.  (already custom built).  Making it larger (size-wise) is very difficult/ expensive because of packaging issues, but would admittedly help with some of the durability issues associated with the brush.  However, total efficiency (another big piece of the puzzle because of the battery power) is not really increased because the motor is still driven through the standard efficiency curve during the ramp-up.
  Decreasing current through the motor (either by soft start or increase dynamic resistance) decreases performance (time from 0 to 15K flywheel RPM) and does not reduce efficiency problems (motor spends even more time in the sub-50% speed range).
  Increasing current through the motor (by decreasing dynamic resistance) past its current point also leads to problems because of the voltage drop associated with the internal resistance of the battery and electronics attached.  100A is about the max. we can expect.
   Motor RPM is chosen for max efficiency (gear ratio tops the flywheel out with the motor at approx 90% of max speed).  Any lower in max speed and we won't get there.  Any higher and we spend more of our time in the low efficiency range and increase total heating during one cycle.
   The main issue is the poor effiency of the motor (ANY PMDC motor) in the low RPM range.  Because our load is all inertia and we are directly connected, there is not much way around having to load the motor heavily when it is still in the sub-50% RPM range.  I'm trying to figure out a way to increase output power at the low end above the standard parabolic output power curve. Any suggestions are appreciated.
  ve7brz...  I'll check into 3 phase AC.  Thanks for the suggestion.

RE: Multi-speed DC brushless motors?

Hi again,

  Here are a couple of sites you might check out for brushless AC motors.  You can find motors here which fall within the constraints for direct drive.  My main concern would be the relatively high idle current given the low impedance of the windings.  

http://www.modelmotors.cz/index.php?id=en&nc=produkty_vypis&url_ps=axi_2808_16&id_odkazy=m_ac

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

  Ignoring viscous coupling which I'm sure you've considered, I can think of two other avenues of investigation:

  Use a flywheel and an electrically operated clutch.  The flywheel is driven by the motor to a velocity greater than 15 grand.  The inertia of the flywheel must be such that when the clutch is engaged the velocity of the combined mass drops to 15K.  Simultaneously the motor is throttled back to maintain the 15K until the clutch is disengaged, at which time motor accelerates the flywheel back to working speed.

  Stupid idea number two:  Use an automatic two speed gearbox.  This is an idea I came up with about 40 years ago.  A computer simulation at that time validated the concept.

  The basic two speed unit integrated a planetary gearset, spraque clutch and centrifigal clutch.  The motor drives the pinion and the planet carrier the load.  The outer (inside tooth) gear is prohibited from counter-rotating by a sprague clutch integrated into it, working against a stationary outer ring.  The centrifigal clutch located on the output or integrated into the planet carrier engages the outer gear when the carrier reaches a given velocity.  This unit could be cascaded to give any number of stepped ratios.

  I don't know if anyone has ever built such a mechanism but it looked good in the simulation.  I hope I'm not using too much space or time on this thread with hare-brained ideas.  If I am please tell me.

Bruce

RE: Multi-speed DC brushless motors?

(OP)
Bruce,
  Good input. We ARE looking at using a 2 speed gearbox with a centrifugally engaged high gear.  Calculated efficiency increases in this set-up with a 2:1 difference between low and high gear is ~30%.  FYI: similar products are sold commercially from several maunfacturers for use in RC cars.  Method of operation consists of 2 pinion gears directly connected to the flywheel.  The high gear is centrifugally engaged to the motor shaft above a trip speed and the low gear is allowed to over rotate on the shaft when the high gear is engaged by use of a one-way bearing.  Pic of the parts of one model can be seen here:

http://www2.towerhobbies.com/cgi-bin/wti0001p.pgm?Q=1&I=LXBTB6&P=7

  I'll have to think a little about the efficiency of the flywheel and electrical clutch idea.  By the way, I did model the viscous coupling and found that it just transferred heating from the brushes to the other end of the shaft, without much positive result.
  Thanks for the ideas.

RE: Multi-speed DC brushless motors?

Suggestion: This topic can also effectively be discussed under mechanical engineering since this section is reserved for electrical motors and associated electrical enginering. The interface solution in terms of gear boxes is on the mechanical side. This could lead to faster solution for you since the average electrical enginering department passes this type of integration to the mechanical department or systems department. It also indicated above that the motor is optimally sized and custom built. Incidentally, is that custom motor built low speed? What seems to be so secret about the motor nameplate data? The cooperation just is not there with volunteering expertise.

RE: Multi-speed DC brushless motors?

Hi, if you use a dc motor with a field winding you can change the "electric gear ratio" by altering the field current. You use a high field current at low speed to give you high torque and low field current at high speed.

RE: Multi-speed DC brushless motors?

(OP)
Jbartos... Thanks for the suggestion.  I'll post in the Mechanical section too.  The reason I posted here was that I already had a number of mechanical ideas in the works, but wanted to know if there were some some possible motor / controller ideas out there that may be of help also.  FYI - motor is built by Johnson (all the other particulars needed to develop the motor curve are in my Oct 30 post, except for free run current which is 1.5A @18V).

RE: Multi-speed DC brushless motors?

Suggestion: If RPM needed are up to 15K, then the motor should be designed for RPM=15K. What about increasing the motor shaft horsepower, i.e. oversize the motor?

RE: Multi-speed DC brushless motors?

I'm kind of new to motion systems and most of my experience is in AC induction and brushless motors so unfortunately this is a question as opposed to any helpful comments.
It is stated up front in this thread that the current DC motor used in this application is having problems because it "spend too much time in the high current, low speed, low efficiency range (< 1/2 the max motor speed)".  While I have heard that brush commutated DC motors should not be run at extended low speeds due to localized commutator heating, I have never heard that they inherently generate more heat at low RPM.  Granted, the efficiency number will be low because the motor RPM is low and the mechanical energy generated is low relative to the losses.  But the thread seems to point to increased heating Is this heating due to full voltage starting and the accompanying high starting currents?  If this were the case all motor types would suffer from this loss.  is there something inherent in the brush DC design that casues this inefficiency?

Sorry for the sidetrack, but any insight would be helpful.

Keith   

RE: Multi-speed DC brushless motors?

Perhaps unlike mechanical transmissions, electric motors generally don't work at high efficiency across a wide range of operating conditions including speed - I think this is true of all motor types.  

There is a similar problem in electric traction systems.  The overall efficiency is reduced and the motor rating, determined by the winding temperature rise, is dependent on the RMS current taken over a period less than the thermal time constant (thus the repetition rate of starts can have a large influence on rating).

I don't think there is any reason to think that a different motor type to brushed dc would significantly improve overall efficiency, except that there are moderate electrical and mechanical losses with carbon brush/commutator systems.  The usual reasons for going to brushless (ac or dc, the differences are a bit subtle) are things like higher speed operation because there is not commutator, low EMC, improved power density and some improvement in efficiency.

Incidentally, you're original post says that the acceleration time is 0.5secs - I presume you mean 5secs.

RE: Multi-speed DC brushless motors?

(OP)
UKPete... No, we really do it in 1/2 sec.  FYI - flywheel has the approximate inertia of a 2" dia x .5" wide steel disk (if this helps you visualize the sizing any).
  The problem I have is that motor starts from stall and has very bad efficiency for a ~ 1/2 of the cycle.  For example, during the time from 1 to 1K RPM, I put in an average of 1000 Watts from the battery, but receive only an average of 60 Watts of useful output.  The rest is wasted as heat.  Although the timeframe is short, we're talking about a lot of heat generation.

Kamenges... You're right, any brushed vs. brushless standard designs should not make any huge efficiency or heating differences here.  What I was looking for here were any non-standard controls or motor construction ideas that might improve low end efficiency or bump up low end power.

Jbartos... The current motor is ~25K on a 1.59 gear ratio = 15.7K max possible RPM at flywheel.
  How would oversizing the motor help me?  I understand that I could get more output power, but it would come at the expense of more input power, since the same basic efficiency curve would still be in effect.  Please expand on this.  

   Thanks for all comments / suggestions.

RE: Multi-speed DC brushless motors?

Hi, I find those numbers hard to believe, if they are correct then you are misusing the motor badly.Try using a chopper to limit the current to something sensible.

RE: Multi-speed DC brushless motors?

Suggestion to RichLeimbach (Mechanical)Nov 2, 2002:
To abandon the gear, the motor should be build for about 15K RPM. One could reason the oversized motor following way, for simplicity: Supposing that one looks at motor product catalog, e.g. Baldor Motors and Drives Data Catalog 502, April 1992 "AC Motor Date Catalog"
HP   RPM   EFF   Type
1.5  3450  75.5  0516M
2    3450  78.5  0524M
1    1140  75.5  0524M
It is clearly seen on the AC motor example that the motor design makes a big difference in RPM, and efficiency varies marginally. Therefore, your DC motor custom builder or designer should go back to basics and design the DC motor of truly optimal parameters for your application or find a better DC motor designer.

RE: Multi-speed DC brushless motors?

(OP)
cbarn24050 ... I know we are abusing the motor badly.  However, the trade-offs associated with current limiting to a low current (<30A) are not acceptable with regards to performance.  The current design actually functions quite well, but runs hotter and is more inefficient than I would like.  As discussed previously, current limiting doesn't really make either of these two issues any better (although it will decrease or eliminate the slight electrical errosion of brushes from sparking at some point).

Jbartos ... This is exactly my point.  Increasing motor power does not give me any increase in efficiency.  Since I need a certain total energy to come out of the motor and transfer to the flywheel, I will generate almost exactly the same amount of heat using any of these motors.  The only difference is that with the more powerful motor, I am generating all of the heat energy in a shorter period of time.  
  Comparing this to the current situation as described above:  Doubling the motor power would roughly cut my ramp-up time in half, but would mean I am putting 2000 watts of energy in and only getting 120 watts out to go from 0 to 1K RPM.  I just doubled both sides of the equation.  No gains, it just compressed the whole event.
  I do agree that if the gear ratio was 1:1 instead of 1.59:1, our motor should be set at ~ 15K max.

RE: Multi-speed DC brushless motors?

Hi, how did you measure your input/output power?

RE: Multi-speed DC brushless motors?

(OP)
cbarn24050 ... 2 ways. Theoretical calculation and experimental verification.
  Experimental testing is done like this:  Input power can be measured by current draw and voltage across terminals. P=V*I  Output power in this timeframe can be monitored by checking flywheel RPM and calculating stored kinetic energy (since there is no applied torque during the ramp-up, all of the motor output energy has gone into kinetic energy in flywheel and rotor).  KE = 1/2 J * w2.  For the purposes of simplicity in this discussion, I averaged the instantaneuos values over the first 1000 RPM to get the numbers above.  Actually at 0 RPM, Input is ~ 1100 Watts and Output is 0 Watts.

RE: Multi-speed DC brushless motors?

(OP)
Follow-up clarification:  There is some slight applied torque from the bearings, but compared to the inertial loading this is minimal.  I've neglected this in the above method.

RE: Multi-speed DC brushless motors?

Hi Rich,

  Ive been following this thread faithfully and it appears to me that there is no electrical solution to this problem unless we invent a remarkable new motor which posseses, or can be 'adjusted' in real time to operate at maximum efficiency over an extremely wide speed range!

  A switchable winding DC motor such as you initially inquired about is obviously a step in this direction; unfortunately there seems to be no such beast.

  The problem you face seems quite clear; you need to provide a (constant) torque for 500 msec in order to impart ~60 Watts of energy to the system by the most efficient method.

  I would like a few more facts before tossing out some other ideas which I have.  What is the duty cycle for this mechanism?  How is the disk stopped?  How critical are your size/weight restraints?  Am I correct in assuming this is battery operated?

  It's an interesting problem.

Bruce

RE: Multi-speed DC brushless motors?

(OP)
Bruce,
  You're right, this is exactly what I'm looking for (although I would rather impact approx 200 Watts for .5 s for a total of 100 Joules of energy).
  Duty cycle is about 500 cycles per hour (possibly up to 1000 in future versions).
  Flywheel is stopped during the engagement of the clutch and subsequent energy delivery (we store kenetic energy for .5s and deliver in a 5-10 millisecond burst).
  Size and weight are both fairly critical.  We can add more weight than we can size.  Length of the motor is the most tightly controlled constraint (can't get longer than 3.25").  We could add .5 LB if needed and the can size could probably increase to 40mm or so.
  You're right, we are battery operated (18V Sanyo Sub-C cells with ~ 3.5 milliohms resistance each).

  Thanks for your help.
  

RE: Multi-speed DC brushless motors?

Hi Rich,

  Thanks for the quick response.  The reason I asked about duty cycle was to get a feel for the energy used (lost, wasted) while simply idling the flywheel in the approach I suggested earlier.  Obviously using a flywheel to store energy is not a new concept in this design.

  I realize this isn't an electric solution but my mind returns to the flywheel as a way to keep a much smaller motor operating within a fairly narrow (and efficient) RPM band.  Given the time frame, the two flywheels would only need to be coupled for the 500 msec acceleration window, so the current consumption of the clutch may not be a major issue (at least compared to the existing losses).  On the other hand the clutch needs to be firm enough to bring the second flywheel up to speed in the time alloted with a minimum of energy loss (heat due to slippage).  A reduction of motor speed is not required.

  The motor must bring the faster flywheel back up to speed in the time between machine cycles.  This will be the limiting factor unless the mechanism is to become even more complex.  

  Another thought that came to mind would be to use two motors coupled in such a way as to both a low speed/high torque section and a high speed/low torque section.  This is conceptually the same as your two speed motor, and you've probably considered it already.

  The other thing that pops into my head is an air or CO2 motor for torque but that creates a whole new design approach.  Besides it isn't electrical and definitely does not belong in this thread.

Bruce

RE: Multi-speed DC brushless motors?

(OP)
Bruce,
  Thanks for the input.  I'm going to take your suggeston for a 2 flywheel system and do some simulation and calculation with it.  I'll post results when I get a chance to do it right.  
  We ARE looking at the 2 motor concept (packaging will be tough).  Preliminary simulation shows we could get about the same benefit as with the 2 speed auto-shifting transmission.  This was the original idea behind starting this thread (wanted to know if anyone had heard of getting two motors in the space for one).  
  I still feel that it is feasible for one motor to be produced with 2 different sets of windings that could be powered or unpowered depending on the current speed, giving 2 effective motors in the same package.  Unfortunately, like you said, I have been unable to find a manufacturer that makes such a thing.  Anybody have any leads on a manufactuer that would be willing to give this a shot or does something similar already?

RE: Multi-speed DC brushless motors?

  I'm beginning to feel very guilty about proposing mechanical solutions in this forum.  Perhaps this approach could be moved elsewhere?

  In the meantime, I forgot one  - engage a cog on a heavy flywheel (momentarily) to transfer an appropriate amount of energy to your load.

Bruce

RE: Multi-speed DC brushless motors?

(OP)
added a thread for mechanical ideas here:

  Thread404-35775 Rich

RE: Multi-speed DC brushless motors?

Suggestion/Comment to RichLeimbach (Mechanical) Nov 4, 2002
marked by ///\\\
Jbartos ... This is exactly my point.  Increasing motor power does not give me any increase in efficiency.
///Disagree. See the Baldor motor 2hp has higher efficiency than Baldor motor 1.5 HP.\\\
  Since I need a certain total energy to come out of the motor and transfer to the flywheel, I will generate almost exactly the same amount of heat using any of these motors.
///Heat corresponds to RI**2. The larger the motor, the smaller the R and higher the I may be. Now, the motor designer can exaggerate the smallness of R to make the RI**2, and therefore the heat amount, extremely small. If it happens to be feasible, the superconductor would have the R negligible. In that case, the heat would be negligible.\\\
  The only difference is that with the more powerful motor, I am generating all of the heat energy in a shorter period of time.
///Very simplistic reasoning. See my comment above.\\\  
  Comparing this to the current situation as described above:  Doubling the motor power would roughly cut my ramp-up time in half, but would mean I am putting 2000 watts of energy in and only getting 120 watts out to go from 0 to 1K RPM.
///This implies efficiency 100% x 120/2000 = 6% and 96% in losses, approximately.\\\
  I just doubled both sides of the equation.  No gains, it just compressed the whole event.
  I do agree that if the gear ratio was 1:1 instead of 1.59:1, our motor should be set at ~ 15K max.

RE: Multi-speed DC brushless motors?

(OP)
Jbartos... Yes, I understand that a marginal increase in efficiency (which really has very little to do with rated power) would help me marginally.  Moving from 75.5% to 78.5% would surely increase my overall efficiency by ~ 3% or so.  This is the kind of marginal improvement I would expect to get by changing rotor bearings, reducing eddy current losses,  or improving brush design slightly.  It is not the kind of increase that will make a noticable impact on the issues we are discussing.

  However, I agree that I have been overly simplistic (for the purposes of clarity and focus) so far in this post.  Since the motor is not an isolated component in our actual system, but has to coexist and function efficiently with the battery described above, I should say that what I am most interested in is Total System Efficiency.  For the purposes of this discussion I have tried to keep it confined to the motor.  

  All of the calculations, tests and simulations I have done show my statement "doubling motor output power, just compresses heating into shorter period of time" to be true.  My calculations, based on the standard P=I^2*R, P=T*W, KE= 1/2J*w that we all know to define the situation, are a bit more lengthy than I would prefer to put in a post like this.  If you want 'em, I might be able to find a way to post the simplistic version in Excel format.

  Regarding the loss calculation above 120/2000 = 6%, this is true.  We actually experience 94% loss as heat to get from 0 to 1K RPM (of course this goes down as the motor starts to move faster, but I have used this as an illustration).  Taking this example, if I changed my overall efficiency upwards by 3%, I would expect to get 123.6 watts (120*1.03) out for the same 2000 watts in.  The new efficiency moves to 6.18% for the same 0 to 1K RPM timeframe.  This is of course an almost negligible gain at the low-end (the 3% won't really start helping much until the higher RPMs).  I'm looking for a lot more.

  

RE: Multi-speed DC brushless motors?

Hi, I still dont belive your 94% loss, perhaps you would be so kind as to explain HOW you MEASURED these values.

RE: Multi-speed DC brushless motors?

(OP)
cbarn24050 ...  As I have explained, the 94% loss is the loss from 0 to 1K RPM, there are of course different average efficiencies from 0 to 5K or from 14K to 15K (this efficiency is ~70% by the way).

  The values were measured as explained above.  I'll elaborate:

  Input:  Current and Voltage across terminals was measured.  For this example (from 0 to 1K), avg current = 90A and average voltage = 11.1 (not 18V due to voltage drop across battery resistance).  P=V*I = 11.1V * 90A= 1000 Watts.

  Output:  Flywheel RPM is recorded vs. time during one event (at the same time the above current and voltage readings are being measured).  At 1000 RPM, Flywheel has a known and measured kinetic energy = .73J.  Average power out of the motor is then P avg=Energy input / time.  In this instance, it took the motor .012s to get from 0 to 1K RPM, so average power transfer from motor to flywheel= .73J / .012s = 60 watts.

   60/1000 = 6% efficiency in this range.  The reasons for this should be intuitively obvious if you look at the 0 to 1K RPM efficiency of any 15K RPM max motor (or the lowest 1/15th of any other motor chart).  This is the efficiency that ANY standard motor would get in this range under these conditions.

RE: Multi-speed DC brushless motors?

Hi again, still a bit mysterious! however i'll asume that you used a storage oscilloscope to measure voltage/current waveforms,it goes without saying that using a meter would be madness but i'll say it anyway.Your figures indicate an armature resistance of 100mohm, which would normaly be way too high for a motor with a emf of only 1.2v/1000rpm.Did you say that this was a PM motor? if so this sort of current is going to destroy the magnets sooner rather that later, I shan't tell you what it's doing to the battery I'll let you guess.Did you say that DC motors are inefficient at low revs? I don't think so, DC motors are used extensively because they are very efficient througout their operating range.Your problem is that to get high speed you need a weak field, but to get high torque you need a strong field, or to put it another way you need a motor with a field winding and a field control circuit of some sort.

RE: Multi-speed DC brushless motors?

Comment to RichLeimbach (Mechanical) Nov 5, 2002 marked ///\\\
cbarn24050 ...  As I have explained, the 94% loss is the loss from 0 to 1K RPM,
///If I understand you correctly, your intent is to either avoid the 94% loss or convert/store this loss in some energy storage.\\\
 there are of course different average efficiencies from 0 to 5K or from 14K to 15K (this efficiency is ~70% by the way).
///Naturally, the avoidance of the loss or the "would be energy loss" storage will wary.\\\

RE: Multi-speed DC brushless motors?

(OP)
Jbartos....  Yes. This is exactly what I am looking to do.  Trying to get more for less.  Either more low-end output for the same power or the same output for less power.

RE: Multi-speed DC brushless motors?

(OP)
cbarn24050 ... Now you've got it.  The whole purpose of this post was to find a good way to get multiple fields at different RPMs.  Still looking for any good suggestions / ideas on how to get this done.

  Incidentally, I agree with your statement "DC motors are used extensively because they are very efficient througout their operating range."   However,  we need an operating range that is much wider than required in most applications.  DC motors are not efficient through MY operating range.  Trying to find a way to change this.

RE: Multi-speed DC brushless motors?

Suggestion: Some motors have designed their characteristics to the motor anticipated loads, e.g. the squirrel cage induction motors have A, B, C, D, and E designs. D design has substantially different characteristics from others. It is good for motor operated valves. What about if the dc motor has its characteristic designed to your peculiar needs?

RE: Multi-speed DC brushless motors?

Hi Rich,

  I've come back to some of the basic principles of this application.  The ideal solution for a 500 mSec acceleration would be to apply a constant torque to your flywheel for that period.  Since motor torque is a function of motor current, in theory a constant current source would provide a constant torque to the system.

  It also reasons that since individual motors have a current range in which they operate most efficiently, you would look for a motor which exhibits its best efficiency at about 17 A from your 12 V supply (using the numbers you've given).

  Perhaps I have those two paragraphs out of order, but I'm sure you'll follow the reasoning nontheless.  The RPM of the motor is irrelevant in regard to the quest for best motor efficiency.  As long as your motor stays within allowed RPM you gear the motor up or down for your 15K flywheel RPM.  At high motor RPM the motor armature itself becomes a more significant portion of your 'stored' energy but that can be corrected at the flywheel.

  I've not followed the reasoning any further than this yet, but I'll continue to try to find flaws in it.  By the way, although it doesn't apply to this concept, I don't know if you're aware that controllers are available for motors of this size which are not load sensitive.  They are purely a speed control without regard to load, similar to large VFDs.

Bruce

RE: Multi-speed DC brushless motors?

(OP)
Bruce,
  This is just like current limiting as discussed above.  The problem is that the output power is P= T*w, so you still have poor efficiency at the start, you have just constrained your current flow. In this example, output power would then rise linearly when graphed vs. RPM (from 0 to say 250 watts) while input power would stay constant (at say 300 watts) in all RPM ranges.
   Clipping the current draw will help the battery efficiency and heating, but will not help the motor much since it will increase the motors residence time in the low-efficiency area.

RE: Multi-speed DC brushless motors?

Sorry Rich, I just came back to withdraw that notion.  The approach I was following led me into the trap of looking only at the last instant of acceleration.  I'll try not to let it happen again.  (I'm so ashamed ~/).

Bruce

RE: Multi-speed DC brushless motors?

I got the smiley wrong (I can't do anything right today).  I'll try again.

RE: Multi-speed DC brushless motors?

(OP)
Bruce,
  Happens to the best of us.  By the way, the multi-speed auto-tranny from the RC car seems to be working.  Right now it is shifting too soon (~2600 RPM) and not giving us the full benefit, but it looks like it will give us some efficiency improvements.  I've had to dig back into the physics books to figure out how to change the shift point upwards, but we should be able to get this going.  I'll let you know if we do actually see the 30% efficiency increase that was predicted next week, but now it is time to go out and have a beer and forget all about efficiency and motors and batteries.  Have a good weekend.  Thanks for your help.

RE: Multi-speed DC brushless motors?

Thats good news Rich.

  As a suggestion, skip the C. shift mechanism and use a servo to accomplish the physical shift.  Response time < .1 sec..  If you like I'll try to talk you into my constant mesh trany.  It can easily use an externally triggered shift.

Bruce

RE: Multi-speed DC brushless motors?

Suggestion to the original posting. If you are thinking of two speed AC motor in DC term, this DC motor would need either two different windings or two different amounts of poles. This is what I meant by customizing the motor characteristics. Apparently, it is still in the making.

RE: Multi-speed DC brushless motors?

Hi Rich,

  Have't seen anything on this thread for a while.  I'm curious as to how the two speed transmission turned out.

  Thinking on the lines of the motor concept suggested on the other thread as well as multi-speed motors suggested on this thread, I'll toss out an old idea for a (miniature) wheel motor which I never pursued to the prototype stage.

  The design was inspired by the ready availability of 6 to 20 VDC three phase AC controllers.

  The concept is to wrap a permanent magnet (armature) linear motor into a circle, or wheel.  Two forms are possible; with magnets on the rim of the wheel aligned radially or with the magnets aligned parallel to the wheel axis.  Obviously, the radial alignment would require radially aligned stator coils, whereas the axially aligned magnets would provide much greater torque (and efficiency) using laminated iron 'U' shaped stator coils.

  For this application you could distribute stator coils (in three's) around the periphery of the 'armature' at will (on a two to three spacing, obviously) within the constraints dictated by size.

  This motor would have considerable torque at low RPM and might still offer reasonable efficiency at high RPM.  Significant losses might be encountered at the controller at very high RPM.  The control system could be designed to use only some of the stator coils at high RPM.  If you wanted to become really creative you design your own controller which would only energize selected coils at selected times based on the back EMF from the other coils.

 Just food for thought,

Bruce

RE: Multi-speed DC brushless motors?

Rich,
I did not looked at your problem from the electrical point of view. From what I remember from electrical machines, you should use a Series DC motor, with brushes in which the armature is in series with the rotor. That kind of motor has a very high torque at low speeds, which decreased with the increase of speed. This is the kind of motor that tramways used, and they were able to get electrical energy back from their kinetic energy.
I think that at this thread there should be more people who know best about this, and the way to get the electrical energy back. Their construction is similar to that used in small AC appliances, (blender, small drills, etc).
sancat

RE: Multi-speed DC brushless motors?

Suggestion: The multi-speed motors can achieve the higher efficiency by the principle of their operation and associated characteristics rather than by changing electrical variables only.

RE: Multi-speed DC brushless motors?

(OP)
Guys,
  Sorry I haven't osted for a while.  Experiments with the 2 speed tranny have been positive.  We've seen efficiency increases of ~ 20% with the 2 stage gear system (based on a modified RC car trans).  Because of the high speed that we need the shift to occur at (and the associated impact energies), we would need to redesign this significantly to increase durability of the production set-up.  This is in the works.

  Bruce...   Interesting idea.  not sure how I could fit all of that into the package size that I have, but I'll think about it some.

  Sancat...  Doesn't a DC series wound motor just replace the field that you get for free from the permanent magnets with one that you have to pay for (with electrical energy)?  Maybe there is some hybrid permanent magnet / wound motor combo that could help us?  

  Jbartos... Not sure what you're getting at with your post.  Can you expand on that?

RE: Multi-speed DC brushless motors?

Rich,
Well, I opened after 15 years my DC machines book, and this is what I found.
No it is not the same to use a magnet instead of a series winding. A permanent magnet will have the same behavior of a derivative winding.

The torque equation for a DC motor is
T = k . phi . I,  (1)
where I is the current, phi is the armature field, and k is a constant.

The product phi.I balances with the torque, so a higher torque demands a higher intensity at a constant phi (a permanent magnet).

When the motor is in series, this current circulates through the armature windings too,
Phi = I . k3  (2)
reinforcing the field and therefore increasing the torque in a much higher factor.
However, on the other hand
V=E + I .r + other losses
where V is the applied voltage, and E is the counter electromotive voltage generated at the rotor, I is the current circulating, r is the total resistance of the motor, and other losses account for brush losses etc
And E= k2 . n . phi,
Where n is the motor speed and phi is the field again.
From there,
I = ( V - k2 . n .  phi – other losses) / r
so each time the motor accelerates its current decreases,
And when the current decreases, it weakens the field (on 2) and thus decreases its torque with the second power (on 1).

I think you should not worry too much about a loss of power at the armature field since normally this would be a small percentage of the total power, because the main power is consumed producing work by means of this E counter electromotive voltage. The armature field is there just to help, but you need it to be very hard on the start, and weaker on the run. Indeed a series motor is very comfortable at 15k speeds, as any vacuum cleaner motor will tell you.
Well I am mechanic not electrical, so I hope I have not spoken too much sht
sancat

RE: Multi-speed DC brushless motors?

Suggestion/comment.  The motor set of parameters associated with a speed range given by winding connection has its efficiency range. Another motor set of parameters, of the same motor, at different speed range given by winding connection has its efficiency range that could be approximately close to the previous efficiency range. This would mean that the reasonable efficiency could be achieved by the winding connections that would discretely vary the motor speed.

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