PatMc Posted October 2, 2012 Share Posted October 2, 2012 John, I hadn’t noticed your reply until today & I’m afraid that I don’t agree with very much in it. My replies are in Bold italics. PAt asked the following questions a while back now...sorry, I haven't looked in on the thread for a while, so here are some answers How can the motor see AC when there is only pulsed DC ? Why would DC saturate the inductor (motor coils ?) ? I don't understand your ref to protection diodes. Where are these diodes ? Where does the 22v come from when the supply is 11v ? So, starting with the first...I don't have a circuit diagram of an actual ESC to look at, but the way this sort of thing is done is that each motor connection is a bit like one phase of a three phase power system. The motor is very like a standard star connected three phase motor, except with a permanent magnet rotor instead of a squirrel cage or a wound rotor like a big one would have. Now, what we do in the ESC is that for each phase (motor wire) we provide two switches. One can connect the phase to the positive side of the supply, and the other can connect the phase to the negative side of the supply. Obviously we must not turn on both switches at once although I have seen what happens when this occurs on a motor controller capable of 12 Amps from a 600 Volt DC supply. By switching each switch on alternately we can generate a square wave. Now, we are also doing the same on the other two phases, although at different times. Because all the waveforms are the same, apart from the timing, the star point in the middle of the windings will end up being at an average voltage which is half the supply. So if the battery was say 12 Volts, the star point ends up at six Volts. Now, as each winding is connected in turn to the positive and negative supplies, that winding will see either plus six Volts or minus six Volts applied to it. Caveat...there will probably be times when both switches are turned off, see below. I think you will find that most, if not all, our brushless motors are delta connected. It wouldn’t make sense for them to be star connected. However regardless of how the windings are terminated the ESC will supply a pulse of +12v to –0v DC across a pair of it’s outputs of the first phase, sequentially repeating this for the next 2 phases then continually repeating this cycle. I.E. +12v DC square wave pulses with a zero volts base. No AC involved. OK next, question...if the motor winding has a DC voltage applied to it, the current will rise to a value determined by the Dc voltage and the resistance of the winding. The resistance of the winding is very low, only milliOhms, so even a small DC Voltage will lead to a large current. Now, even a pulsed DC Voltage has a DC component, it can be regarded as being a DC voltage plus an ACwaveform. For instance, if we pulse our 12 Volt supply on for 1 millisecond and off for 11, the coil will effectively see 1 Volt DC plus a 12 Volt peak to peak square wave. 1 Volt DC across say 10 milliOhms will lead to a current of 100 Amps flowing, although probably not for long. The saturation in itself is not a bad thing, except it means that the efffective inductance has fallen very low so it will not take long for the current to get too big. Practically we don't have to worry about this since the ESC designer will have made sure that the circuit provides an AC waveform to the coils. This will not be a sine wave. The motor coils have low resistance (R) and high inductance (L). The current growth in an inductive circuit takes time to reach its full value. The time factor (T) is the time taken for the current to reach 63.2% of it’s full value. T = L/R seconds. The greater the inductance & the smaller the resistance, the greater is T & the time taken for to reach its final value. The pulses of DC are always at the full supply voltage, in this case 12v. But the pulses are at high frequency (In the order of several kHz) & of short duration therefore the current never gets anywhere near it’s full resistive potential final value. The inductance & resistance are fixed values. There is no AC waveform. Ok, the last question...each connection on the motor is connected in the ESC to a pair of switches in the ESC that can connect to either the positive or negative supply. So taking two of the connections, the maximum voltage one way is when one is connected to positive and the other is connected to negative. That would be 11 Volts. But at the opposite point in the cycle, the connections will be reversed. That will also give 11 Volts, but the other way around. So a scope connected across the coil would show an AC waveform of 22 Volts peak to peak. This will not be a sine wave, it will not even be square since in fact the switching devices will not be on for half the cycle. But that is how you can get an AC waveform that is a higher peak to peak Voltage than the DC supply it came from. Each phase is pulsed for 1/3 of the cycle, there is no overlap – it’s impossible with common 0v & +12v connected to each coil in turn. Therefore there can never be a time when voltages of opposing polarity are present simultaneously Our brushless motors are used as DC machines with external electronic commutators. In this usage they are no more AC machines than a simple DC brushed motor is. Pat. Quote Link to comment Share on other sites More sharing options... Erfolg Posted October 3, 2012 Author Share Posted October 3, 2012 Well Patmac I am convinced and believe you. You have confirmed in principle what I have believed for some time. Of course knowing nothing to talk about electrics or electronics, I had doubts that I was thinking along the right lines. II have in the past seen in a American magazine claimed oscilloscope traces from a brushless motor, which did say they were a square sign wave. To my mind they did not look square, certainly not as I had seen in labworks, but then again, it was purported to be a real world investigation. I do know that some brushless motors have been wire in "Star" configuration. I had just assumed it would have an effect on max torque etc. But did not really matter. Quote Link to comment Share on other sites More sharing options... John Olsen 1 Posted October 4, 2012 Share Posted October 4, 2012 OK, well, the simplest way to find out about whether the windings are seeing AC or DC would be to apply a Voltmeter across a pair of connections and see what you measure with it set to AC and then with it set to DC. My prediction is that it will measure zero when set to DC and a Voltage a bit less than the battery Voltage on AC...quite likely it will read about 70% of the battery voltage. This will depend partly on the waveform being used. One thing I had not realised is that at least some ESCs are using PWM, so in fact the current waveform through the motor should be quite a good approximation of a sinewave. This does not mean that the Voltage will be a sine wave. The armature in a simple DC brushed motor is in fact seeing only AC. This is not a sine wave AC current, it will be a pretty messy waveform since everytime a brush changes segment there will be a be step change in the Voltage. So larger motors with multiple commutator segments will have a nicer waveform, somewhat closer to a sine wave. One thing I would like to point out is that the three motor connections do not have a common reference point with the DC supply. So the Voltage that might be measured between one side of the DC supply and a motor winding is not relevant to anything other than what is appearing across the switching devices. Ok, now, the situation with a coil with pulses of DC being applied to it at high frequency. One thing you neglect to mention is that between the pulses the current will tend to keep flowing, due to the inductance in the circuit. In fact you can predict the current that will flow in the circuit accurately by resolving the applied DC pulses into an AC component and a DC component. The AC component is a square wave with the same mark space ratio and the same peak to peak voltage, and the DC component is the mean DC voltage of the pulses. EG if the mark space ratio is 1 to 1 and the voltage is 11, then the mean DC Voltage will be 5.5 and the square wave will be 11 Volts peak to peak. The mean DC current then, after about 5 time constants has elapsed, will be given by I = V over R, and R is pretty low in a brushless outrunner. So in fact we must not apply a mean DC Voltage across a winding, and these motor controllers do not do so. Actually everything is symmetrical, so the mean DC Voltage at each winding will be the same, half the supply Voltage, so as I said above, if you measure the Voltage across a winding with a DC Voltmeter you will see zero. John Quote Link to comment Share on other sites More sharing options... John Olsen 1 Posted October 4, 2012 Share Posted October 4, 2012 OK here are the results: Motor HK2812 as supplied in their foamy Dornier Do335 18 AMP ESC as supplied in the same plane. (It has one of these at each end and is known affectionately as "Shredder". Care is needed when hand launching!) Battery 3s 2200MaH LIPO Voltages measured at full throttle with no propellor: Red to Black 0.2mV DC 10.75 V AC Black to Yellow 0.9mV DC 10.63 V AC Yellow to Red 0.6mV DC 10.66 V AC I would regard those DC Voltages as being effectively zero, in fact the meter was reading higher Voltages than that from stray effects when not connected to anything. The AC Voltages on the other hand appear quite significant, and quite comparable to the battery Voltage which since it was recently charged was about 12.3 Volts (DC of course!) There is not necessarily a very simple relationship between the AC Voltage measured as above and the actual effective AC Voltage seen by the motor since the meter is an ordinary fairly cheap digital Voltmeter and does not so far as I know do true RMS readings. So it is probably measuring some sort of mean or average and then applying a calibration factor, which would be correct for a sine wave but may not be correct for whatever it is seeing here. Also the frequency here may be higher than it is intended for. John Quote Link to comment Share on other sites More sharing options... Swissflyer Posted October 4, 2012 Share Posted October 4, 2012 Posted by Erfolg on 03/10/2012 12:53:14:II have in the past seen in a American magazine claimed oscilloscope traces from a brushless motor, which did say they were a square sign wave. To my mind they did not look square, certainly not as I had seen in labworks, but then again, it was purported to be a real world investigation. I do know that some brushless motors have been wire in "Star" configuration. I had just assumed it would have an effect on max torque etc. But did not really matter. If you look at the link suggested by Tony **LINK** You will see photos from a digital oscilloscope of exactly what the waveforms are (switched DC being mangled by inductance) I hope that helps Mark PS In the days when brushless motors cost $90 I used to wind my own. With the coils wound Delta (think of a triangle) you get speed at the expense of torque. With Star (think of a three legged Starfish) you get torque at the expense of speed. Quote Link to comment Share on other sites More sharing options... Chris Bott - Moderator Posted October 4, 2012 Share Posted October 4, 2012 The ESC output stage uses this bridge arangement. Taking any two motor connections, say A and B, it is perfectly possible to apply battery volts first one way round to the winding and then to completely reverse it. So current flows in one direction and then in the opposite direction. In my book, that's AC, I think! As far as I know, the PWM is applied to the "on" periods purely to effect a voltage control, rather than to make anything a sine wave. Quote Link to comment Share on other sites More sharing options... Erfolg Posted October 4, 2012 Author Share Posted October 4, 2012 I am sure this is a daft question, particular to those who know. At a practical level, does it matter if AC or DC. What advantage or disadvantage does either provide, if any? Quote Link to comment Share on other sites More sharing options... Chris Bott - Moderator Posted October 4, 2012 Share Posted October 4, 2012 I don't think any of it matters to us Erfolg - they just operate our motors (most of the time). Quote Link to comment Share on other sites More sharing options... PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by John Olsen 1 on 04/10/2012 08:43:01:I would regard those DC Voltages as being effectively zero, in fact the meter was reading higher Voltages than that from stray effects when not connected to anything. The AC Voltages on the other hand appear quite significant, and quite comparable to the battery Voltage which since it was recently charged was about 12.3 Volts (DC of course!) There is not necessarily a very simple relationship between the AC Voltage measured as above and the actual effective AC Voltage seen by the motor since the meter is an ordinary fairly cheap digital Voltmeter and does not so far as I know do true RMS readings. So it is probably measuring some sort of mean or average and then applying a calibration factor, which would be correct for a sine wave but may not be correct for whatever it is seeing here. Also the frequency here may be higher than it is intended for. John This does not prove anything, you are measuring the average voltage of a commutator. The DC scale is bound to register zero. The AC scale of a voltmeter will register the rms of the commutated DC pulses. The rms & average value of a square wave are identical. Quote Link to comment Share on other sites More sharing options... WolstonFlyer Posted October 4, 2012 Share Posted October 4, 2012 There is some good info on how an esc works here including oscilloscope outputs of the three phase AC and the PWM used to control the speed.Sorry I am posting from my android phone and cannot make the link active.http://www.aerodesign.de/peter/2001/LRK350/SPEEDY-BL_eng.html Quote Link to comment Share on other sites More sharing options... Peter Beeney Posted October 4, 2012 Share Posted October 4, 2012 Pat - I think I must also have to agree with John here, at least in part. Certainly as far as the AC interpretation of a DC motor is concerned. I’ve been saying this for a very long time and I’ve not seen anything recently to make me change my mind, so I will still totally stand by what I said @ 10/09/2012 14:07:01. So it would appear we have the same old standoff situation. You said - ‘In this usage they are no more AC machines than a simple DC brushed motor is.’ - I would say the the simple brushed DC motor has AC flowing in it’s armature, in exactly in the same way as the brushless motor has AC flowing in it’s stator. As I’ve asked before, how could the motor turn if this were not so? All I can say, Pat, and with the greatest respect of course, but also with some reasonable conviction, is please don’t ever have a wager on this, unless it’s with me, because I think there is every outside chance you might just lose…… Erfolg - We would need to delve into the motor principle to explain it, but the current has to reverse at 180 degrees in every 360 degree rotation. Hence we have the commutator, a mechanical revolving device for changing AC to DC, or conversely DC to AC. PB Quote Link to comment Share on other sites More sharing options... PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Chris Bott - Moderator on 04/10/2012 12:55:55: The ESC output stage uses this bridge arangement. Taking any two motor connections, say A and B, it is perfectly possible to apply battery volts first one way round to the winding and then to completely reverse it. So current flows in one direction and then in the opposite direction. In my book, that's AC, I think! As far as I know, the PWM is applied to the "on" periods purely to effect a voltage control, rather than to make anything a sine wave. The voltage is only ever varying between 0v & +12v in each phase there is never a negative voltage. If the ESC output was AC there would be an overlap between pulses of each sequentialy adjacent phase with a +ve component in one & -ve component in the next. Quote Link to comment Share on other sites More sharing options... PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Erfolg on 04/10/2012 13:07:51: I am sure this is a daft question, particular to those who know. At a practical level, does it matter if AC or DC. What advantage or disadvantage does either provide, if any? If the ESC produced AC the motor's speed would be governed by the supply frequency. Speed control would be dificult to achieve. However more than one motor could be ran from the same ESC if it had the current capacity. Quote Link to comment Share on other sites More sharing options... PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Peter Beeney on 04/10/2012 21:30:15: Pat - I think I must also have to agree with John here, at least in part. Certainly as far as the AC interpretation of a DC motor is concerned. I’ve been saying this for a very long time and I’ve not seen anything recently to make me change my mind, so I will still totally stand by what I said @ 10/09/2012 14:07:01. So it would appear we have the same old standoff situation. You said - ‘In this usage they are no more AC machines than a simple DC brushed motor is.’ - I would say the the simple brushed DC motor has AC flowing in it’s armature, in exactly in the same way as the brushless motor has AC flowing in it’s stator. As I’ve asked before, how could the motor turn if this were not so? The convention of designating AC & DC motors for about 150 years would have to be overturned for that to be true. The motors are designated by the supply & considered as a whole including the commutation be it internal or external. An AC supply always includes a variation of polarity both sides of zero. Quote Link to comment Share on other sites More sharing options... Peter Beeney Posted October 4, 2012 Share Posted October 4, 2012 Sorry Pat, but I’m afraid I reckon that’s somewhat clutching at straws. I may be old, but I’m not that old! A DC motor has DC applied to it, but the armature has AC flowing in it, which is what this whole discussion has always been about, not the motor as a whole. I certainly don’t think I’ve ever called it an AC motor. As also with the brushless motor, the current in the motor wires is AC, but it’s still a DC brushless permanent magnet motor. Lets consider the variation of polarity, I’m quite happy with that. If we connect a voltmeter to two ends of a a resistor it reads zero volts, so that’s our zero point. If we now connect it to a 12 volt battery the current will flow conventionally from pos to neg and the the meter will read +12V, but it won’t show the plus sign. If we now change the whole thing around, leaving the meter leads as they are the current will now flow the opposite way through the resistor and the meter will read -12V. Showing the minus sign. If we were able to arrange for this to change over once every half second we would have a 1 Hz 12V square AC waveform, one half cycle 12 volts positive, the other 12 volts negative. If we now connect this 12 volts DC to a brushed motor one battery pole would connect to one segment of the commutator at one end of the winding and the other pole to the other segment at the other end of the winding. The current would flow in one direction through the coil. The motor would turn and at 180 degrees the comm. segments will have changed over, they will be connected to the opposite poles so the current will have reversed and is now flowing the other way; and now we have again a 12 volts squarish AC current. In the morning I will try and do some sort of sketch, (don’t laugh!), to try and explain what I mean. And that might confuse the issue even more…… PB Edited By Peter Beeney on 04/10/2012 22:57:16 Quote Link to comment Share on other sites More sharing options... PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Peter Beeney on 04/10/2012 22:56:25: In the morning I will try and do some sort of sketch, (don’t laugh!), to try and explain what I mean. Peter now that you've invented the world's first AC battery don't bother with any further explanations on my account. This thread is starting to read like a Monty Python sketch. Quote Link to comment Share on other sites More sharing options... Piers Bowlan Posted October 5, 2012 Share Posted October 5, 2012 Presumably by now Erfolg you have tried another Rx, and so eliminated that. Trying another Keda Thumrun motor at £13 seems like a worthwhile investment. That just leaves the LiPo or at least a poor battery connection (dry joint-high resistance inside the LiPo) to consider. If, once you have changed everthing, you have another ESC fail then I think the model is just jinxed! Quote Link to comment Share on other sites More sharing options... Erfolg Posted October 5, 2012 Author Share Posted October 5, 2012 Piers I have not flown a model in some time now, due to other family issues. The new Lipo i ordered has arrived, as I think or thought at the time the volts were sagging very quickly. As for poor soldered joints, possible, although I think unlikely, those Deans type plugs take some physical hammer when disconnecting, I think a dry joint would almost certainly fail. Quote Link to comment Share on other sites More sharing options... John Olsen 1 Posted October 5, 2012 Share Posted October 5, 2012 Any convention of DC and AC motors is pretty moot now that we can change one to the other at will with efficient circuits. My lathe is powered by a motor with what is sometimes called an AC motor controller, which takes the incoming single phase mains, rectifies it to DC, then inverts it into three phase AC which is applied to the three phase squirrel cage motor. The frequency is varied to control the speed, and the Voltage is varied to match, since otherwise it would draw excessive current at low speeds. So is that an AC or DC motor then? Does the DC motor in a hairdryer become an AC motor just because it is supplied permanently connected to a rectifier? The Microchip application note referred to above points out that these motors will run as a synchronous motor if supplied with three phase AC at an appropriate Voltage, eg ommitting the sensing of position. Doing that reduces the torque available so is not usually done. But the motor itself, as supplied in the box, separately from the ESC, will run on AC, and will not run on DC. But yes, I would usually call them a brushless DC motor, but that does not change the fact that the current in the coils is AC. You might like to think it is just pulses of DC that happen to reverse in direction, but then that is what an AC current is. Peter you are exactly right with what you say about reversing the connections. Actually the aircraft during WWII often used to generate the high Voltage for their radios with a device called a genemotor. It was like a DC motor, with brushes, except that the armature had two sets of windings in the same slots, and one set had many more turns. The second set of turns was connected to a commutator at the opposite end to the normal one. This let the armature act as a (Multiphase) transformer and the second commutator acted as a synchronous rectifier. So the output was somewhat noisy DC at a much higher Voltage, eg you could turn a 28 V DC aircraft supply into 300 Volts DC for the anode supply to a valve transmitter or receiver. John Quote Link to comment Share on other sites More sharing options... Erfolg Posted October 5, 2012 Author Share Posted October 5, 2012 I assume any reversal of current in the motor coils is a function of the ESC, for no technical reason? If that suggestion is true, presumably any reversal of current flow in the supply wires is purely down to ESC circuit design and could be configured to supply current flow in the supply wires in a single direction? Edited By Erfolg on 05/10/2012 10:47:02 Quote Link to comment Share on other sites More sharing options... Chris Bott - Moderator Posted October 5, 2012 Share Posted October 5, 2012 It reverses because sometimes we want the coil to produce a magnet with a north pole and sometimes with a south pole. Here's a bit more reading and a nice animation. I think the animation has been simplified because in reality a third of the windings are energised at a time. Quote Link to comment Share on other sites More sharing options... Peter Beeney Posted October 5, 2012 Share Posted October 5, 2012 Pat, - With the greatest respect, I certainly have not invented an AC battery, that’s simply just the way you’ve chosen to construe it. Current from a battery only ever flows one way, as I’m sure will be attested by anyone who has connected an electrical unit the wrong way round and then watched it disappear in a puff of smoke. All I’m simply saying is that, because the current only ever flows between the battery terminals in one direction, if we change the connections over to our resistor it sees exactly the same amount of current flowing in the opposite direction. This is of little impact to the resistor, but significantly the magnetic circuit that surrounds the wire has also reversed, and this is what is crucial to the motor’s correct operation. We can see this effect partly in action all the time, if we connect a brushed motor to the battery terminals it turns one way, if we reverse the connections it runs the other way. With the same amount of power. As witness a servo……. John, - This particular debate has always been about the current flow in the motor wires of our modelling brushless motors. In the past there have been a number of references to this, invariably insisting that it is DC and it’s a myth that it is AC etc. This is often stated with some conviction, concluding that there is no argument. The science is settled, as they say in global warming circles. I’ve always thought otherwise, so this viewpoint could be the classic MP sketch, as I see it. This is just my reasoning to try and correct this, which is open to any contradiction…. … But with some proof, of course. Erfolg, - The ESC is just perform the function of the commutator, albeit remotely, it has to reverse the current every 180 degrees. That’s it’s primary task, but it also acts as a throttle control. But that’s a separate entity, the motor would run without this facility. So there is every technical reason for it to reverse the current in the motor wires. Re. the supply wires, if you mean the from the battery, it’s DC, if you mean from the ESC to the motor, it has to be AC. Hope this helps to make it all clearer. PB Quote Link to comment Share on other sites More sharing options... Erfolg Posted October 5, 2012 Author Share Posted October 5, 2012 Doesn't the brushed motor effectively reverse the current flow via the commutator? I was thinking that a segment which sees a +ve current from one brush, when rotated through 180 degrees to the other brush then become a -ve segment? Quote Link to comment Share on other sites More sharing options... Peter Beeney Posted October 5, 2012 Share Posted October 5, 2012 Exactly so, Erfolg, right on, and the current has now also changed direction and because it only ever goes one way, it’s now going the opposite way in the windings. Due to the motor effect, the magnetic field, and that’s a separate debate, it has to do this for the motor to run. Now imagine these segments changing over in the ESC, connected to the motor with wires, and operated by a electrical circuit. Thus the current in the wires also would be changing direction, reversing, that is, every 180 degrees. So it’s down hill all the way now…… PB Quote Link to comment Share on other sites More sharing options... John Olsen 1 Posted October 6, 2012 Share Posted October 6, 2012 Posted by PatMc on 04/10/2012 21:07:30: This does not prove anything, you are measuring the average voltage of a commutator. The DC scale is bound to register zero. The AC scale of a voltmeter will register the rms of the commutated DC pulses. The rms & average value of a square wave are identical. Oookay, so an actual measurement means nothing? The DC scale is bound to read zero mainly because there is no mean DC Voltage being applied to the winding. The AC scale will not necessarily measure RMS, some do but many don't, but that is a minor point. Can I point out that "commutated" is a fancy way of saying "Switched" and the ordinary mechanical commutator is in fact reversing the connections to the coils everytime it rotates through 180 degrees. If it is a really simple one like we used to make ourselves with only two commutator segments then the AC waveform on the coil will be a square wave. With more segments the Voltage wave across each coil will have steps in it and with many segments will start to approximate a sine wave quite well. While the RMS and average value of a square wave are indeed equal, this is only true for an AC square wave, eg where the peak positive value is equal to the peak negative value and the mark space ratio is one to one. Such a waveform is alternating current, and I though you were claiming that the coil was not getting alternating current? Also although the voltage applied to the coil will have equal positive and negative excursions, it is pulse width modulated to control the mean Voltage, and so there will be no fixed ratio between the mean, peak ,and RMS Voltages, the relationship will vary with the mark space ratio of the PWM waveform. I think one thing that causes this kind of conceptual difficulty for people is the idea that there must be one fixed reference point in the circuit and that all Voltages can only be considered relative to that. So pople tend to try and think about what is happening at the motor coils with reference to one side of the battery, generally the negative side. But the motor coils are not permanently connected to either side of the supply, so measuring or thinking about the Voltage between a coil connection and the DC supply only tells you about what is appearing across the switching devices in the ESC. What really counts is what is appearing across the motor coils, since that is what will drive the current through the coils. Also remember that at any given instant there is only one voltage between any two points, and only one current through a given wire, so considered at any one instant any circuit is going to look like DC. But if over time the mean DC Voltage is zero, while the actual intantaneous Voltage has been both positive and negative, then we would have to say that there is AC present. (Not necessarily a sine wave of course) John Quote Link to comment Share on other sites More sharing options... Prev 1 2 3 4 5 Next Page 4 of 5 Join the conversation You can post now and register later. If you have an account, sign in now to post with your account. Note: Your post will require moderator approval before it will be visible. Reply to this topic... × Pasted as rich text. Paste as plain text instead Only 75 emoji are allowed. × Your link has been automatically embedded. 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Erfolg Posted October 3, 2012 Author Share Posted October 3, 2012 Well Patmac I am convinced and believe you. You have confirmed in principle what I have believed for some time. Of course knowing nothing to talk about electrics or electronics, I had doubts that I was thinking along the right lines. II have in the past seen in a American magazine claimed oscilloscope traces from a brushless motor, which did say they were a square sign wave. To my mind they did not look square, certainly not as I had seen in labworks, but then again, it was purported to be a real world investigation. I do know that some brushless motors have been wire in "Star" configuration. I had just assumed it would have an effect on max torque etc. But did not really matter. Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted October 4, 2012 Share Posted October 4, 2012 OK, well, the simplest way to find out about whether the windings are seeing AC or DC would be to apply a Voltmeter across a pair of connections and see what you measure with it set to AC and then with it set to DC. My prediction is that it will measure zero when set to DC and a Voltage a bit less than the battery Voltage on AC...quite likely it will read about 70% of the battery voltage. This will depend partly on the waveform being used. One thing I had not realised is that at least some ESCs are using PWM, so in fact the current waveform through the motor should be quite a good approximation of a sinewave. This does not mean that the Voltage will be a sine wave. The armature in a simple DC brushed motor is in fact seeing only AC. This is not a sine wave AC current, it will be a pretty messy waveform since everytime a brush changes segment there will be a be step change in the Voltage. So larger motors with multiple commutator segments will have a nicer waveform, somewhat closer to a sine wave. One thing I would like to point out is that the three motor connections do not have a common reference point with the DC supply. So the Voltage that might be measured between one side of the DC supply and a motor winding is not relevant to anything other than what is appearing across the switching devices. Ok, now, the situation with a coil with pulses of DC being applied to it at high frequency. One thing you neglect to mention is that between the pulses the current will tend to keep flowing, due to the inductance in the circuit. In fact you can predict the current that will flow in the circuit accurately by resolving the applied DC pulses into an AC component and a DC component. The AC component is a square wave with the same mark space ratio and the same peak to peak voltage, and the DC component is the mean DC voltage of the pulses. EG if the mark space ratio is 1 to 1 and the voltage is 11, then the mean DC Voltage will be 5.5 and the square wave will be 11 Volts peak to peak. The mean DC current then, after about 5 time constants has elapsed, will be given by I = V over R, and R is pretty low in a brushless outrunner. So in fact we must not apply a mean DC Voltage across a winding, and these motor controllers do not do so. Actually everything is symmetrical, so the mean DC Voltage at each winding will be the same, half the supply Voltage, so as I said above, if you measure the Voltage across a winding with a DC Voltmeter you will see zero. John Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted October 4, 2012 Share Posted October 4, 2012 OK here are the results: Motor HK2812 as supplied in their foamy Dornier Do335 18 AMP ESC as supplied in the same plane. (It has one of these at each end and is known affectionately as "Shredder". Care is needed when hand launching!) Battery 3s 2200MaH LIPO Voltages measured at full throttle with no propellor: Red to Black 0.2mV DC 10.75 V AC Black to Yellow 0.9mV DC 10.63 V AC Yellow to Red 0.6mV DC 10.66 V AC I would regard those DC Voltages as being effectively zero, in fact the meter was reading higher Voltages than that from stray effects when not connected to anything. The AC Voltages on the other hand appear quite significant, and quite comparable to the battery Voltage which since it was recently charged was about 12.3 Volts (DC of course!) There is not necessarily a very simple relationship between the AC Voltage measured as above and the actual effective AC Voltage seen by the motor since the meter is an ordinary fairly cheap digital Voltmeter and does not so far as I know do true RMS readings. So it is probably measuring some sort of mean or average and then applying a calibration factor, which would be correct for a sine wave but may not be correct for whatever it is seeing here. Also the frequency here may be higher than it is intended for. John Quote Link to comment Share on other sites More sharing options...
Swissflyer Posted October 4, 2012 Share Posted October 4, 2012 Posted by Erfolg on 03/10/2012 12:53:14:II have in the past seen in a American magazine claimed oscilloscope traces from a brushless motor, which did say they were a square sign wave. To my mind they did not look square, certainly not as I had seen in labworks, but then again, it was purported to be a real world investigation. I do know that some brushless motors have been wire in "Star" configuration. I had just assumed it would have an effect on max torque etc. But did not really matter. If you look at the link suggested by Tony **LINK** You will see photos from a digital oscilloscope of exactly what the waveforms are (switched DC being mangled by inductance) I hope that helps Mark PS In the days when brushless motors cost $90 I used to wind my own. With the coils wound Delta (think of a triangle) you get speed at the expense of torque. With Star (think of a three legged Starfish) you get torque at the expense of speed. Quote Link to comment Share on other sites More sharing options...
Chris Bott - Moderator Posted October 4, 2012 Share Posted October 4, 2012 The ESC output stage uses this bridge arangement. Taking any two motor connections, say A and B, it is perfectly possible to apply battery volts first one way round to the winding and then to completely reverse it. So current flows in one direction and then in the opposite direction. In my book, that's AC, I think! As far as I know, the PWM is applied to the "on" periods purely to effect a voltage control, rather than to make anything a sine wave. Quote Link to comment Share on other sites More sharing options...
Erfolg Posted October 4, 2012 Author Share Posted October 4, 2012 I am sure this is a daft question, particular to those who know. At a practical level, does it matter if AC or DC. What advantage or disadvantage does either provide, if any? Quote Link to comment Share on other sites More sharing options...
Chris Bott - Moderator Posted October 4, 2012 Share Posted October 4, 2012 I don't think any of it matters to us Erfolg - they just operate our motors (most of the time). Quote Link to comment Share on other sites More sharing options...
PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by John Olsen 1 on 04/10/2012 08:43:01:I would regard those DC Voltages as being effectively zero, in fact the meter was reading higher Voltages than that from stray effects when not connected to anything. The AC Voltages on the other hand appear quite significant, and quite comparable to the battery Voltage which since it was recently charged was about 12.3 Volts (DC of course!) There is not necessarily a very simple relationship between the AC Voltage measured as above and the actual effective AC Voltage seen by the motor since the meter is an ordinary fairly cheap digital Voltmeter and does not so far as I know do true RMS readings. So it is probably measuring some sort of mean or average and then applying a calibration factor, which would be correct for a sine wave but may not be correct for whatever it is seeing here. Also the frequency here may be higher than it is intended for. John This does not prove anything, you are measuring the average voltage of a commutator. The DC scale is bound to register zero. The AC scale of a voltmeter will register the rms of the commutated DC pulses. The rms & average value of a square wave are identical. Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 4, 2012 Share Posted October 4, 2012 There is some good info on how an esc works here including oscilloscope outputs of the three phase AC and the PWM used to control the speed.Sorry I am posting from my android phone and cannot make the link active.http://www.aerodesign.de/peter/2001/LRK350/SPEEDY-BL_eng.html Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted October 4, 2012 Share Posted October 4, 2012 Pat - I think I must also have to agree with John here, at least in part. Certainly as far as the AC interpretation of a DC motor is concerned. I’ve been saying this for a very long time and I’ve not seen anything recently to make me change my mind, so I will still totally stand by what I said @ 10/09/2012 14:07:01. So it would appear we have the same old standoff situation. You said - ‘In this usage they are no more AC machines than a simple DC brushed motor is.’ - I would say the the simple brushed DC motor has AC flowing in it’s armature, in exactly in the same way as the brushless motor has AC flowing in it’s stator. As I’ve asked before, how could the motor turn if this were not so? All I can say, Pat, and with the greatest respect of course, but also with some reasonable conviction, is please don’t ever have a wager on this, unless it’s with me, because I think there is every outside chance you might just lose…… Erfolg - We would need to delve into the motor principle to explain it, but the current has to reverse at 180 degrees in every 360 degree rotation. Hence we have the commutator, a mechanical revolving device for changing AC to DC, or conversely DC to AC. PB Quote Link to comment Share on other sites More sharing options...
PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Chris Bott - Moderator on 04/10/2012 12:55:55: The ESC output stage uses this bridge arangement. Taking any two motor connections, say A and B, it is perfectly possible to apply battery volts first one way round to the winding and then to completely reverse it. So current flows in one direction and then in the opposite direction. In my book, that's AC, I think! As far as I know, the PWM is applied to the "on" periods purely to effect a voltage control, rather than to make anything a sine wave. The voltage is only ever varying between 0v & +12v in each phase there is never a negative voltage. If the ESC output was AC there would be an overlap between pulses of each sequentialy adjacent phase with a +ve component in one & -ve component in the next. Quote Link to comment Share on other sites More sharing options...
PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Erfolg on 04/10/2012 13:07:51: I am sure this is a daft question, particular to those who know. At a practical level, does it matter if AC or DC. What advantage or disadvantage does either provide, if any? If the ESC produced AC the motor's speed would be governed by the supply frequency. Speed control would be dificult to achieve. However more than one motor could be ran from the same ESC if it had the current capacity. Quote Link to comment Share on other sites More sharing options...
PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Peter Beeney on 04/10/2012 21:30:15: Pat - I think I must also have to agree with John here, at least in part. Certainly as far as the AC interpretation of a DC motor is concerned. I’ve been saying this for a very long time and I’ve not seen anything recently to make me change my mind, so I will still totally stand by what I said @ 10/09/2012 14:07:01. So it would appear we have the same old standoff situation. You said - ‘In this usage they are no more AC machines than a simple DC brushed motor is.’ - I would say the the simple brushed DC motor has AC flowing in it’s armature, in exactly in the same way as the brushless motor has AC flowing in it’s stator. As I’ve asked before, how could the motor turn if this were not so? The convention of designating AC & DC motors for about 150 years would have to be overturned for that to be true. The motors are designated by the supply & considered as a whole including the commutation be it internal or external. An AC supply always includes a variation of polarity both sides of zero. Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted October 4, 2012 Share Posted October 4, 2012 Sorry Pat, but I’m afraid I reckon that’s somewhat clutching at straws. I may be old, but I’m not that old! A DC motor has DC applied to it, but the armature has AC flowing in it, which is what this whole discussion has always been about, not the motor as a whole. I certainly don’t think I’ve ever called it an AC motor. As also with the brushless motor, the current in the motor wires is AC, but it’s still a DC brushless permanent magnet motor. Lets consider the variation of polarity, I’m quite happy with that. If we connect a voltmeter to two ends of a a resistor it reads zero volts, so that’s our zero point. If we now connect it to a 12 volt battery the current will flow conventionally from pos to neg and the the meter will read +12V, but it won’t show the plus sign. If we now change the whole thing around, leaving the meter leads as they are the current will now flow the opposite way through the resistor and the meter will read -12V. Showing the minus sign. If we were able to arrange for this to change over once every half second we would have a 1 Hz 12V square AC waveform, one half cycle 12 volts positive, the other 12 volts negative. If we now connect this 12 volts DC to a brushed motor one battery pole would connect to one segment of the commutator at one end of the winding and the other pole to the other segment at the other end of the winding. The current would flow in one direction through the coil. The motor would turn and at 180 degrees the comm. segments will have changed over, they will be connected to the opposite poles so the current will have reversed and is now flowing the other way; and now we have again a 12 volts squarish AC current. In the morning I will try and do some sort of sketch, (don’t laugh!), to try and explain what I mean. And that might confuse the issue even more…… PB Edited By Peter Beeney on 04/10/2012 22:57:16 Quote Link to comment Share on other sites More sharing options...
PatMc Posted October 4, 2012 Share Posted October 4, 2012 Posted by Peter Beeney on 04/10/2012 22:56:25: In the morning I will try and do some sort of sketch, (don’t laugh!), to try and explain what I mean. Peter now that you've invented the world's first AC battery don't bother with any further explanations on my account. This thread is starting to read like a Monty Python sketch. Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted October 5, 2012 Share Posted October 5, 2012 Presumably by now Erfolg you have tried another Rx, and so eliminated that. Trying another Keda Thumrun motor at £13 seems like a worthwhile investment. That just leaves the LiPo or at least a poor battery connection (dry joint-high resistance inside the LiPo) to consider. If, once you have changed everthing, you have another ESC fail then I think the model is just jinxed! Quote Link to comment Share on other sites More sharing options...
Erfolg Posted October 5, 2012 Author Share Posted October 5, 2012 Piers I have not flown a model in some time now, due to other family issues. The new Lipo i ordered has arrived, as I think or thought at the time the volts were sagging very quickly. As for poor soldered joints, possible, although I think unlikely, those Deans type plugs take some physical hammer when disconnecting, I think a dry joint would almost certainly fail. Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted October 5, 2012 Share Posted October 5, 2012 Any convention of DC and AC motors is pretty moot now that we can change one to the other at will with efficient circuits. My lathe is powered by a motor with what is sometimes called an AC motor controller, which takes the incoming single phase mains, rectifies it to DC, then inverts it into three phase AC which is applied to the three phase squirrel cage motor. The frequency is varied to control the speed, and the Voltage is varied to match, since otherwise it would draw excessive current at low speeds. So is that an AC or DC motor then? Does the DC motor in a hairdryer become an AC motor just because it is supplied permanently connected to a rectifier? The Microchip application note referred to above points out that these motors will run as a synchronous motor if supplied with three phase AC at an appropriate Voltage, eg ommitting the sensing of position. Doing that reduces the torque available so is not usually done. But the motor itself, as supplied in the box, separately from the ESC, will run on AC, and will not run on DC. But yes, I would usually call them a brushless DC motor, but that does not change the fact that the current in the coils is AC. You might like to think it is just pulses of DC that happen to reverse in direction, but then that is what an AC current is. Peter you are exactly right with what you say about reversing the connections. Actually the aircraft during WWII often used to generate the high Voltage for their radios with a device called a genemotor. It was like a DC motor, with brushes, except that the armature had two sets of windings in the same slots, and one set had many more turns. The second set of turns was connected to a commutator at the opposite end to the normal one. This let the armature act as a (Multiphase) transformer and the second commutator acted as a synchronous rectifier. So the output was somewhat noisy DC at a much higher Voltage, eg you could turn a 28 V DC aircraft supply into 300 Volts DC for the anode supply to a valve transmitter or receiver. John Quote Link to comment Share on other sites More sharing options...
Erfolg Posted October 5, 2012 Author Share Posted October 5, 2012 I assume any reversal of current in the motor coils is a function of the ESC, for no technical reason? If that suggestion is true, presumably any reversal of current flow in the supply wires is purely down to ESC circuit design and could be configured to supply current flow in the supply wires in a single direction? Edited By Erfolg on 05/10/2012 10:47:02 Quote Link to comment Share on other sites More sharing options...
Chris Bott - Moderator Posted October 5, 2012 Share Posted October 5, 2012 It reverses because sometimes we want the coil to produce a magnet with a north pole and sometimes with a south pole. Here's a bit more reading and a nice animation. I think the animation has been simplified because in reality a third of the windings are energised at a time. Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted October 5, 2012 Share Posted October 5, 2012 Pat, - With the greatest respect, I certainly have not invented an AC battery, that’s simply just the way you’ve chosen to construe it. Current from a battery only ever flows one way, as I’m sure will be attested by anyone who has connected an electrical unit the wrong way round and then watched it disappear in a puff of smoke. All I’m simply saying is that, because the current only ever flows between the battery terminals in one direction, if we change the connections over to our resistor it sees exactly the same amount of current flowing in the opposite direction. This is of little impact to the resistor, but significantly the magnetic circuit that surrounds the wire has also reversed, and this is what is crucial to the motor’s correct operation. We can see this effect partly in action all the time, if we connect a brushed motor to the battery terminals it turns one way, if we reverse the connections it runs the other way. With the same amount of power. As witness a servo……. John, - This particular debate has always been about the current flow in the motor wires of our modelling brushless motors. In the past there have been a number of references to this, invariably insisting that it is DC and it’s a myth that it is AC etc. This is often stated with some conviction, concluding that there is no argument. The science is settled, as they say in global warming circles. I’ve always thought otherwise, so this viewpoint could be the classic MP sketch, as I see it. This is just my reasoning to try and correct this, which is open to any contradiction…. … But with some proof, of course. Erfolg, - The ESC is just perform the function of the commutator, albeit remotely, it has to reverse the current every 180 degrees. That’s it’s primary task, but it also acts as a throttle control. But that’s a separate entity, the motor would run without this facility. So there is every technical reason for it to reverse the current in the motor wires. Re. the supply wires, if you mean the from the battery, it’s DC, if you mean from the ESC to the motor, it has to be AC. Hope this helps to make it all clearer. PB Quote Link to comment Share on other sites More sharing options...
Erfolg Posted October 5, 2012 Author Share Posted October 5, 2012 Doesn't the brushed motor effectively reverse the current flow via the commutator? I was thinking that a segment which sees a +ve current from one brush, when rotated through 180 degrees to the other brush then become a -ve segment? Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted October 5, 2012 Share Posted October 5, 2012 Exactly so, Erfolg, right on, and the current has now also changed direction and because it only ever goes one way, it’s now going the opposite way in the windings. Due to the motor effect, the magnetic field, and that’s a separate debate, it has to do this for the motor to run. Now imagine these segments changing over in the ESC, connected to the motor with wires, and operated by a electrical circuit. Thus the current in the wires also would be changing direction, reversing, that is, every 180 degrees. So it’s down hill all the way now…… PB Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted October 6, 2012 Share Posted October 6, 2012 Posted by PatMc on 04/10/2012 21:07:30: This does not prove anything, you are measuring the average voltage of a commutator. The DC scale is bound to register zero. The AC scale of a voltmeter will register the rms of the commutated DC pulses. The rms & average value of a square wave are identical. Oookay, so an actual measurement means nothing? The DC scale is bound to read zero mainly because there is no mean DC Voltage being applied to the winding. The AC scale will not necessarily measure RMS, some do but many don't, but that is a minor point. Can I point out that "commutated" is a fancy way of saying "Switched" and the ordinary mechanical commutator is in fact reversing the connections to the coils everytime it rotates through 180 degrees. If it is a really simple one like we used to make ourselves with only two commutator segments then the AC waveform on the coil will be a square wave. With more segments the Voltage wave across each coil will have steps in it and with many segments will start to approximate a sine wave quite well. While the RMS and average value of a square wave are indeed equal, this is only true for an AC square wave, eg where the peak positive value is equal to the peak negative value and the mark space ratio is one to one. Such a waveform is alternating current, and I though you were claiming that the coil was not getting alternating current? Also although the voltage applied to the coil will have equal positive and negative excursions, it is pulse width modulated to control the mean Voltage, and so there will be no fixed ratio between the mean, peak ,and RMS Voltages, the relationship will vary with the mark space ratio of the PWM waveform. I think one thing that causes this kind of conceptual difficulty for people is the idea that there must be one fixed reference point in the circuit and that all Voltages can only be considered relative to that. So pople tend to try and think about what is happening at the motor coils with reference to one side of the battery, generally the negative side. But the motor coils are not permanently connected to either side of the supply, so measuring or thinking about the Voltage between a coil connection and the DC supply only tells you about what is appearing across the switching devices in the ESC. What really counts is what is appearing across the motor coils, since that is what will drive the current through the coils. Also remember that at any given instant there is only one voltage between any two points, and only one current through a given wire, so considered at any one instant any circuit is going to look like DC. But if over time the mean DC Voltage is zero, while the actual intantaneous Voltage has been both positive and negative, then we would have to say that there is AC present. (Not necessarily a sine wave of course) John Quote Link to comment Share on other sites More sharing options...
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