Author Topic: DC Electric Motors 101  (Read 4523 times)

Offline MJM460

  • Full Member
  • ****
  • Posts: 1647
  • Melbourne, Australia
Re: DC Electric Motors 101
« Reply #15 on: July 22, 2021, 01:00:42 PM »
Hi Allen, I hope others will also chip in.  For my part, difference between brushed and brushless would be great.  Personally I have very little knowledge of brushless motors so would appreciate your explanation there.

I was interested in your comment the Kt = 1/Kv.  That seemingly simple relationship must have an explanation that eludes me.

Also, I was interested in the calculation of efficiency from the basic parameters as you described.  This seems to open the possibility of using a motor as a generator to measure an engine power output, an application where an estimate of motor efficiency is clearly required.  Or do parameters change when the motor is being driven as a generator?  I probably skipped a step or two there.

No worries about the three phase.  I was thinking it might be helpful for those considering three phase motors for variable speed on their machines.  The characteristics of the three phase motors might help understanding even without too much theory.

MJM460



The more I learn, the more I find that I still have to learn!

Offline Allen Smithee

  • Full Member
  • ****
  • Posts: 1130
  • Mordor, Middle Earth
Re: DC Electric Motors 101
« Reply #16 on: July 22, 2021, 02:38:47 PM »
Hi Allen, I hope others will also chip in.  For my part, difference between brushed and brushless would be great.  Personally I have very little knowledge of brushless motors so would appreciate your explanation there.

OK, it's on the list

Quote
I was interested in your comment the Kt = 1/Kv.  That seemingly simple relationship must have an explanation that eludes me.

I was about to sit down and do the derivation from scratch (I'm currently dialled into the monthly project review on mute - they haven't said anything interesting so far today so I'm not expecting that to change), but I asked Mr google (clever chap - knows everything) and found someone had already written it out here. Note that the relationship "Kv x kt = 1" only holds true if you use the metric units (rad/sec per volt and Nm per amp).

Quote
Also, I was interested in the calculation of efficiency from the basic parameters as you described.  This seems to open the possibility of using a motor as a generator to measure an engine power output, an application where an estimate of motor efficiency is clearly required.  Or do parameters change when the motor is being driven as a generator?  I probably skipped a step or two there.

This very much is possible. In fact as I demonstrated above you can use a motor as its own generator for power measurement purposes and know directly what the efficiency is.

The fundamental concept is that:

Power in = Pi = Voltage x Current = V x I
Power out = Po = (Voltage - voltage losses) x (Current - current losses)

Voltage losses = Current x winding resistance = I x Rm

Current losses = No-Load Current = i0

Therefore

Po = (V - (I x Rm)) x (I - I0)

Efficiency = Po / Pi = 100% x [(V - (I x Rm)) x (I - I0)] / (V x I)

Now if you use a generator as a brake dyno by giving it a variable load, there is absolutely no reason why you couldn't use the same basic approach. In this case the current and voltage losses would need to be added rather than subtracted, but the principle is the same.

AS
Quidquid latine dictum sit altum sonatur

Offline Dan Rowe

  • Full Member
  • ****
  • Posts: 1170
  • Dripping Springs TX USA
Re: DC Electric Motors 101
« Reply #17 on: July 22, 2021, 03:43:27 PM »
Allen,
Very interesting topic and it made me think about my collection of DC motors. It occurs to me that that regarding the information I have about these motors there are three cases.

1) I have a good set of manufacturer's specifications.

2) I have nameplate data.

3) I have no data only a meter to measure the winding resistance.

I did a google search for DC motor specifications and found an explanation of how to use DC motor specifications.
http://gearseds.com/files/lesson3_mathematical%20models%20of%20motors.pdf

Most of my DC motors are case 3, so with only the winding resistance what is the best way to start the design process?

Cheers Dan

ShaylocoDan

Offline Allen Smithee

  • Full Member
  • ****
  • Posts: 1130
  • Mordor, Middle Earth
Re: DC Electric Motors 101
« Reply #18 on: July 22, 2021, 05:28:26 PM »
There are ways you can establish the characteristics to an approximate level.

I0 can be measured by simply shaft-running the motor from a battery with an ammeter in the circuit

Rm can be measured, although for the higher-current motors you would need an accurate meter that measures at the lowest possible voltage (ideally a bridge to measure under zero voltage and current conditions or a 4-terminal ohm meter).
Kv can me measured to within a few percent by either running the motor and measuring the RPM (optically with a half-coloured disk would be typical) or by turning the motor at known RPM (eg in a lathe) and measuring the voltage generated.

The trickiest one is the current rating. In theory it should be simple enough because you're just looking to ensure the armature doesn't get too hot. If you have an optical pyrometer then you can often check the winding temperature directly (if it is visible through the case). The rule of thumb is around 120-130degC. But if you haven't then you need to use another rule of thumb that suggests 80degC on the outer case is getting too high (at 80degC you can touch the case, but not keep touching it!). So run the motor at progressively higher loads to draw higher currents and let its temperature stabilise until it is too hot to touch. Find a current where you can still touch the case after 60 seconds for the short term rating, and for 10 mins for the long term rating.

AS
Quidquid latine dictum sit altum sonatur

Online Roger B

  • Global Moderator
  • Full Member
  • *****
  • Posts: 6139
  • Switzerland
Re: DC Electric Motors 101
« Reply #19 on: July 22, 2021, 05:52:40 PM »
I am using a Torpedo 850 as a starter and a load for my 25 cc horizontal engine. The engine revs to around 3000rpm (50mm stroke) and is coupled to the motor by a 4-1 step up toothed belt. It starts ok with a 12v battery and my load bank goes down to 1 ohm which gives me around 11A at 11V so 120W the efficiency of the motor at full load is given as 60% so I guess the engine is delivering around 200-220W. The spec sheet (attached) is quite detailed.
Best regards

Roger

Online Laurentic

  • Full Member
  • ****
  • Posts: 312
  • Nr Yeovil, Somerset, England
Re: DC Electric Motors 101
« Reply #20 on: July 22, 2021, 06:16:58 PM »
AS - as someone with a healthy distrust of anything that has wires coming out of it borne of long experience and ignorance I have been really pleased to have read your tutorial so far.  I did learn all this basic DC motor stuff and Back EMF nearly 60 years ago, and then promptly forgot it again once I had passed the exam; the theory was needed then far less than the practical hands on stuff, or hands off in my case!

 I must say I have found your writing exceedingly clear and am very grateful you have taken the time to write all this up.  It has been a really interesting and informative discourse and had it been explained to me thus first time round I might have been able to retain more of it.  As it is I am grateful for the knowledge gained here so far.

Now looking forward to the next instalment. (Hope I'm not presuming here, you did mention talking about brushless motors later on........)

Chris.  :cheers:

PS - AS, you mention Hughs Electrical Technology, that indeed was the text book I worked from back in the early 1960's, the 1960 edition, and I have still got it too (just checked) and was only consulting it the other week, well fancy that!
« Last Edit: July 22, 2021, 06:59:53 PM by Laurentic »

Offline gadabout

  • Full Member
  • ****
  • Posts: 234
Re: DC Electric Motors 101
« Reply #21 on: July 23, 2021, 12:55:46 AM »
AS,
All great stuff, thanks!
Tell me when I pick up an dc electric motor I always turn the shaft, some feel very notchy others almost nothing , what do these characteristics mean? The more notchy the more powerful?!
Also some are marked with their turns, say 27t to one marked 22t, the less turns more powerful or the opposite?
Regards
Mark

Offline Allen Smithee

  • Full Member
  • ****
  • Posts: 1130
  • Mordor, Middle Earth
Re: DC Electric Motors 101
« Reply #22 on: July 23, 2021, 08:12:16 AM »
OK - brushless motors, the mystery of turn count and the significance of notchiness or "cogging".

These will be included in the next exciting episode of "Everything you never wanted to know about electric motors but feared I would tell you anyway", the new internet-based insomnia treatment ...

I'll try to do something this afternoon

AS
Quidquid latine dictum sit altum sonatur

Offline Bluechip

  • Full Member
  • ****
  • Posts: 1007
  • Derbyshire

Offline Allen Smithee

  • Full Member
  • ****
  • Posts: 1130
  • Mordor, Middle Earth
Re: DC Electric Motors 101
« Reply #24 on: July 23, 2021, 04:20:14 PM »
Brushless DC (BLDC) Motors - the simplistic overview

Brushed motors
Let's look at a very simple model of a DC motor. Imagine we have some wire wrapped around a rod, and we connect the wire to a battery so that it turns into an electromagnet - North at one end and South at the other. If we switch off the current and hang this rod in front of the North end of a permanent magnet then when we switch the current on again the North end of the bar gets attracted to the magnet so it swings around until it is pointing at it. If we then reverse the wires  of the coil the South end of the bar becomes the North, so it swings around again. If we keep swapping the wires over the bar rotates, and in its simplest form we have a DC motor.

In an actual motor we have a set of switches (actually wiper contacts) mechanically connected to the shaft that do the connecting/disconnecting automatically, and we also have more than one coil of wire around the core so that several coils can be connected in turn to give smoother, more continuous torque (we can get into poles, slots, teeth etc later if anyone really wants to - but it's not essential). We call the switches the "commutator" because calling it the "coil switcher" might give away that there isn't actually anything complicated involved and that just wouldn't do.  We also have more than one magnet - we typically have several, each one being the opposite polarity to the one before. Another little wrinkle is that there will be an odd number of coils and an even number of magnets. This means that there is not "stable" position and thus always a torque on the shaft when there is current in the coils. This makes the motor self-starting.

The commutator is almost always made of metal conductor plates on the shaft with sprung-loaded "brushes" rubbing against them. It works well enough, but there are always resistance and friction losses in the commutator even when the brushes are made from graphite or sintered bronze. There can be advantages in specific applications to varying the timing of the commutator - motors can be made more efficient if they have commutator timing optimised for a particular direction (there is a version of the Graupner Speed 480 called the "Speed 480L" which is intended for use with a single-stage geared propeller drive and so is reverse-timed). Some of the more expensive buggy motors (eg the Kyosho "le mans" series) had adjustable timing with graduated timing marks on the casing.

Brushless motors
"Brushless" DC (BLDC) motors are identical in almost all respects EXCEPT that they have no commutator. Instead of the brushgear they have a piece of electronics to do all the switching. To make it simpler they also swap over the electric coils and the magnets so that the coil stays stationary (becoming the "stator" and the magnets rotate (becoming the "rotor"). This immediately eliminates the electrical and friction losses of the commutator, and also allows complete freedom to mess with the timing at will. These motors almost invariably have their coils arranged in three "phases" (these are NOT phases and are in no way related to the 3-phase AC motor concept we are perhaps familiar with). To confuse matters further they can also be arranged in star or delta configurations - with the same three windings the Delta configuration has root three (1.732) times the Kv of the Star config. There might be anything from three to sixty or more actual coils (always an odd number, each coil having anything from 2 to 100 or more turns of wire), but grouping them into three "phases" for electrical switching purposes makes for simple, more effective electronic control. This goes back to that "slots, poles, teeth" thing I mentioned earlier, which you really don't need to know about!

The original BLDC motors had optical or magnetic timing sensors to tell the electronics unit when to do the switch-over. These "sensored BLDC" motors are still made for applications that need high levels of initial torque and high starting torque, so electric RC cars use sensored motors and the BLDC motors seen in the smaller lathes & milling machines are almost all sensored. The electronic control unit of a sensored brushless motor operates at a defined speed that produces the rotating magnetic field. When you adjust the speed it just changes the timing of the field rotation, which is why they give very large low-speed and starting torques.

But a while back someone came up with a cunning plan. If you remember back in the distant history when this whole thread was just starting and there were still people actually reading it, I talked about getting "Back EMF" when the motor was rotated. Of course this isn't constant - it's actually a pulse of volts each time a coil passes a magnet (which is actually how a Magneto works - don't you love it when a plan comes together?). This led to the thought that this pulse could be used to indicate the relative position of the coils and the magnets. And thus was born that wonder of the modern world - the "sensorless BLDC Motor". It is such a wonderfully elegant machine, having just a fixed set of windings and a shaft with some magnets attached, yet it is capable of extraordinary levels of power and efficiency.

A BLDC control unit (called an Electronic Speed Controller, or ESC) takes a DC source and feeds out on three wires to the motor. It switches these wires around in sequence so that there is a rotating magnetic field around the magnets on the motor shaft. The same wires detect the back-emf pulses and so adjust the timing of the next change depending on speed and load. Most ESCs have adjustments which vary the timing from "soft" to "hard" depending on the remagnetisation time required by the particular motor (you don't need to know about this unless you want to). along with other parameters, but in all honesty for most uses they are "fit and forget" units which can safely be left to default or self-adjusted settings. As the field rotation timing is "automatic" the speed control of sensorless brushless motors is done by chopping the current just like it is for Brushed motors. That inherently means that at very low speeds they spend less time with the current "on" and so have poorer low-speed & start-up grunt.

And that is really all there is to Brushless motors. There is one feature about their construction which I could touch on. As I said above while a brushed motor has a rotating armature carrying the coils inside a can lined with permanent magnets, a brushless motor has a fixed stator carrying the coils with a rotating shaft carrying the permanent magnets. But these come in two configurations called "inrunner" and "outrunner".

The Inrunner is exactly what you might expect - a shaft with magnets fixed to it, rotating inside a can lined with coils of windings. The magnets are usually bound with epoxied kevlar thread to stop them making a break for freedom. These are now quite rare because they are expensive to make and tend to have a high Kv - the Czech "Megamotor" range are mostly inrunners.

The outrunner is a different animal. It has a set of fixed windings in the middle and a rotating can around the outside, with magnets fixed to the inside of the can:



This is both easier (and thus cheaper) to make and has an inherently larger magnetic moment, so they are far and away the most common type of brushless motor around today. They are extremely good - for example I have a Czech  Axi4120/18 which is the size of a satsuma, weighs under 12oz but on a 6s (25v) battery turns a 14x8.5" prop at over 9,300rpm. That's 1.3kW or 1.7bhp, roughly the usable power of a 4stroke .91 glow motor.

I think that's all I have on brushless motors, but as ever do ask if you have any questions. Next episode will be on turn counts and cogging.

Stay safe,

AS
Quidquid latine dictum sit altum sonatur

Online Laurentic

  • Full Member
  • ****
  • Posts: 312
  • Nr Yeovil, Somerset, England
Re: DC Electric Motors 101
« Reply #25 on: July 23, 2021, 06:43:27 PM »
Allen - many thanks for that explanation, that was great, and in addition to explaining brushless dc motors you have also answered answered a question that has been bugging me for ages but haven't asked - the difference between inrunner and outrunner motors. Now I know!

I occasionally read a model flying mag and go on their website - sister to the ME website - and get baffled by the shorthand used, for IC engines terms like 60-70engine - is that 0.6 to 0.7cc?, why not say so and make it clear - and for the rapidly upcoming electrical engines which now seem to be the engines of model aeroplane choice, again baffling, the sizes of engines, the need for an ESC (you have explained, an Electronic Speed Controller, so know that bit now too) and the batteries; ah yes, the batteries, another baffling bit. 

Say someone quotes a 40C/80C 2s 2200mAh lipo is required for a certain motor.  I deduce lipo is type of battery and 2200mAh is battery 'capacity' for want of a better word, but 2S?  What the devil is 2S? And 40C/80C - what does that mean?  All well and good when you are into it and know what it all means but when you don't - and I certainly don't! - I really don't know what anyone is really taking about. 

I know I am digressing from your DC motor subject in a way, and apologise for that, but batteries are an integral of the dc motor set-up.  If only there was an idiots guide to model radio control which also encompassed the battery/ESC/motor/servo bit I might be a happier bunny!

But thanks again for your input to date - really good, and I am sure I am not the only one still reading on!!

Chris

 :ThumbsUp: :ThumbsUp: :ThumbsUp:


Offline Allen Smithee

  • Full Member
  • ****
  • Posts: 1130
  • Mordor, Middle Earth
Re: DC Electric Motors 101
« Reply #26 on: July 23, 2021, 10:23:01 PM »
OK the scope is getting bigger, but I'm happy to continue this as long as (a) people are interested and (b) people don't get upset about talking about this stuff in this forum!

Just as a quickie on the engine sizes. The tradition used to be that small diesels were sized vaguely in CCs, so a PAW249 is 2.5cc and an AM10 was 1cc. But glow motors tended to follow the American system and were sized in cubic inches. So a Webra 61 was 0.61cu in or 10cc, an OPS 40 is 0.4cu in or 6.6cc and a Cox TD010 is 0.01cu in or 0.16cc. More recently people got lazy and just talk about a 40 or a 60, but they mean the cubic inch sizes and all the modern 4-stokes follow the same scheme. The big glow motors are mostly sized in cubic inches, so an Irvine 150 is a 1.5cubic inch (25cc) engine, but the Supertigre 2000, 2500 and 3000 are 20, 25 and 30cc respectively even though all the other Supertigres are sized in cubic inches - because the really great thing about standards is that there are so many to choose from... 

The "big" petrol engines are nearly all sized in CCs, so a Zenoah 62 is 62cc, and a DA150 is 150cc.

Of course Turbine engines are mostly sized in thrust rating, measured in newtons. But that's a different skillet of mullet altogether.

Batteries - these days most electric models use "Lithium Polymer" (actually Lithium-cobalt, but that's just being picky) cells. They are nominally 3.7volts per cell at mid-discharge, or 4.2v/cell when fully charged and these days they are both very good and very cheap. The standard nomenclature would be something like a "3s2000/30C". This means a 2,200mAh capacity with three cells (nominally 11.1volts or 12.6volts off a fresh charge).

The "30C" is the current rating - it means the cell is nominally rated to be discharged at 30-times it's one-hour rate which (for the 2,200mAh cell) would be:

30 x 2,200mA = 66Amps

The "C-rating" is really a sort of "quality indicator" relating to the technology and construction of the cells which why they use the generic C-rate rather than the specific current rating for the cell. They often quote two numbers for the rating, the second being a "burst" rating. It's also true that most C-ratings beyond 40C are hopelessly optimistic marketing B/S! It doesn't really matter, because no one actually flies a model pulling a 60C current - you'd drain the pack in under a minute!

In a previous post I mentioned a setup I use with an Axi 4120/18 chucking out 1.7bhp at full throttle. The motor actually pulls 58.5Amps from a fresh 6s3300 battery so at full chat it should being out of juice after 3 minutes. I actually get 12 minute flights out of it because it doesn't even need full power for a vertical climb (let alone take-off)! But I digress...

AS
Quidquid latine dictum sit altum sonatur

Offline Charles Lamont

  • Full Member
  • ****
  • Posts: 321
Re: DC Electric Motors 101
« Reply #27 on: July 24, 2021, 12:57:21 AM »
Allen, I reallly appreciate the time you are putting into this. It would have taken me all day write as much as you have already today.

At the risk of trying your patience, I would like to take you back to my original question, because, although you have probably answered it, I have not understood the answer.

My engine cranking tests were carried out with a Graupner Speed 320 motor, specified as 7.2V, 21,000 free rpm, 0.3A free current, and one site says max current 12A. I can't find a resistance figure, and don't seem to be able to get a sensible answer from my multimeter.

By trying different gear ratios I found that the greatest cranking speed I could achieve, and therefore the maximum motor power*, was at about 17,000 motor rpm, drawing about 3A. With a smaller reduction ratio the motor speed dropped away dramatically, despite drawing a much larger current. At higher ratios, the motor was more lightly loaded and ran, as you have said, somewhat faster, but not enough to compensate for the increased ratio.

Now, I am happy with these results and the motor appears to suit what I want to do.

The problem I have is with understanding the motor power curve. From what little I understand, and so far as I can see your explanations confirm this, the torque should drop as a straight line from 0 to max speed (cos the back-EMF is proportional to speed), and that this should result in max power at half of max speed, at least in theory. So why am I getting max power at around 80% of free running speed?

* that might not be immediately obvious - I can explain   
« Last Edit: July 24, 2021, 01:02:46 AM by Charles Lamont »

Offline Ginger Nut

  • Full Member
  • ****
  • Posts: 133
    • Woolnwood
Re: DC Electric Motors 101
« Reply #28 on: July 24, 2021, 04:41:04 AM »
What an amazing insight thanks

Sent from my SM-T580 using Tapatalk


Offline Admiral_dk

  • Full Member
  • *****
  • Posts: 3752
  • Søften - Denmark
Re: DC Electric Motors 101
« Reply #29 on: July 24, 2021, 11:19:54 AM »
A VERY informative thread and explained in a (for me at least) very understandable language  :praise2:

Please continue  :cheers:

I admit that as an electronic technician, I might have a headstart on the tech stuff you explain.

Per

 

SimplePortal 2.3.5 © 2008-2012, SimplePortal