Electric Motor Secrets

[everwiser]

You can proof circuits in a simple form but as time goes on, efficiency and longevity will likely require some amount of circuit complexity. If Jetijs' circuit is getting warm/hot and it's not the load, then either something is wrong OR the circuit needs more refinement.

Eric's recommendations come from his strong electronics background (based on his posted comments, he DOES have a strong electronics background) and his desire to aid Jetijs AND see a viable, efficient driver circuit that will be applicable and stable for all going down this road.

In the end, it is Jetijs circuit and his decision which direction to go with it. Why not encourage the brainstorming that Eric is bringing to the subject and be inclusive in your comments?

We're all in this together; even lurkers, such as myself.

[lighty]

You probably know that old engineering saying "don't fix it if it works". Also, most of the specs are more of the general guidelines and one very quickly learn when doing EE for living that calculated values and specs often can be taken very liberaly. Also in this case driving signal is almost perfect so obviously everything works just fine.

I wouldn't get into more complicated circuitry if not necessary and judging by the driving signal it's not necessary. I understand your wish to get it as perfect as possible and I'm kind of perfectionist myself when I engineer circuitry. If I was doing it myself I would do more advanced engineering just for the fun of it but then again I know my way around EE and I can troubleshoot it myself. Jetijs is in a different situation- he is not EE professional and he has to be guided through troubleshooting procedures over the internet.

In any case its Jetijs's device and I will help him with advice as much as I can no matter what he decides.

[Tecstatic]

True, but I like the idea of operating IC's within their operating specifications. Your suggestion to use a driver with an internal schmitt-trigger could be the answer.

[lighty]

@Tecstatic

Ummm, why would he need to introduce Schmitt triggers? His driving signal is nice with fast rise times. You do realize that the more electronic circuitry is introduced the more difficult it will be to troubleshoot it from a distance.

[Tecstatic]

Jetijs,

Is it possible for you to make a scope shot showing both the current through the FET and the voltage between source and drain of the FET ? This way we can see the losses in the FET.

Is the 24V supply voltage constant ?

What is the DC resistance of your coil ?

"There is another thing that doesn't give my mind a rest. Why is it that when I adjust the dutycycle the current waveform gets elevated?"

You can operate the motor in two modes:

1. Discontinuous, the recovery current falls to zero before the next power pulse. Gives max recovery.

2. Continuous, a new power pulse is started before the recovery current is zero. You see this as an elevated curve.
This gives more torque assuming the core is not saturated.

I do not understand that the diode get so warm, I just checked the datasheet. It looks like it is meant for line frequencies (50-60 Hz). The data sheet does not tell the reverse recovery time. I have never used this diode, maybe it is too slow for this use.

I just saw you have two optical forks, you don't need more than one 40106, two inverters to drive the input for the UCCxxx is sufficient to use one input and two outputs for each channel ( total of the 6 schmitt-trigger inverters).

Eric

[Jetijs]

Lighty, I was not talking about the recovery diodes, for this test I did not use any recovery circuit. I was talking about the isolation diode that separates the power supply from the input capacitor. Anyway, your suggestion with resistors in series of the diodes will help also in this case. I will see if I can get a single powerful diode instead

About the MOSFETs, In V2 circuit I used IRFP450 MOSFETs and they worked just fine despite that their ON Rds (0.4 Ohms) is even higher than that of IRFP360 (0.2 Ohms).

[lighty]

Hm, so you have about 5W of power dissipation. Of course your signal is pulsed so it's even less. If you ask me everything works fine and the only thing to do would be to find MOSFET with smaller ON Rds.

As for recovery diodes heating up- you shouldn't connect diodes in parallel. They are not ideally same so one of them will always conduct before the other ones and thus dissipate more energy. What you could do to somewhat compensate is to put some small value resistor in series with each of the diodes. For example 0.1-0.2 Ohm will do fine, maybe even values of up to 1.5 Ohms (depending on the current because you need to get voltage drop of 0.4-0.5V). Of course since you're not dealing with sine wave but with steep rising time impulses you shouldn't use wirewound resistors because of their reactive inductivity. What you could do is to use non-inductive resistors (carbon, metal-film or specially wounded wire resistors).

Or better yet always use a single larger diode instead of trying to parallel smaller ones.

P.S.
I re-checked your schematic and realized you're not talking about recovery diodes but about isolation diode between (-) of battery and main collector capacitor. In this case you could use wirewound resistors of small value to equalize load on smaller diodes but you would unnecessarily dissipate energy on them and it would still not be ideal.

Use one big diode. The bigger the diode the slower it is but in this case its function doesn't require it to be very fast so you can use one larger rectifier diode rated for higher currents- they will have much less conducting resistance so they will dissipate less power.

Have you tried measuring current going through that diode? Use shunt and measure RMS current on your scope. Then you'll know better what you're dealing with there. If you're going to use series shunt resistors the voltage drop on them should be at least 0.4-0.5V so it would be good to know what current goes through diode in order to select proper resistance shunt.

But again- use single high current diode.

[Jetijs]

Lighty, I am using IRFP360 MOSFETS, they are rated 400v, 28A, 410W. ON Rds is 0.2 Ohms. You know, this time when I took the gate signal scope shots, the circuit was consuming about 5A and the MOSFET's did not heat up even after 3-4 minutes, they became just warm. I did not check the transil diodes, but one thing sure did heat up - the isolation diode that separates the power supply from the capacitor. I use four 1n5408 diodes in parallel for isolation, this should handle currents up to 12A. It is late, I will do more tests tomorrow. If the results will still be confusing, I will just make another circuit board and verify everything step by step. This can be done easily because almost all the parts from the current circuit board are removable and I can use them again in the new circuit.

[lighty]

Driving signal seems OK. If voltage/div is 10 that makes it 12V signal which is more than OK.

Hmmm, what MOSFETs do you use and what is its On Rds? Also, do your protection diodes (transils) get warm as well as MOSFETs? And when you're saying MOSFETs are warm what temperature are we talking about? Is it mildly warm, can you easily touch it?

[Jetijs]

uusedman, can you post the exact circuit that you are using right now?

Lighty, this is the scope shot across the gate and source of one of the driving MOSFETs:



This is at about 8k RPM and scope set to 0,5 mS. It is a perfect square wave at all the time from low RPM's to high RPM's. I used the V3 circuit with one of the V3 motors and the coils were wired in parallel (each coil has it's own MOSFET). The input current wave from stays the same, something like this:



This is with no recovery circuit attached.
Also what is interesting, if I load the motor down, the current waveform does not change, it stays in the same proportions only gets wider and higher. Thats odd. So I guess that the problem is not in the duty cycle or RPM's.

[lighty]

Please explain how that can create a FET heating problem if the gate drive is OK ie. FET operating at the specified rise and fall times.

I got mixed up trying to follow this thread, remembering scope shots Jetijs provided while at the same time writing an article for magazine and getting pissed at my cats. You're absolutely correct, if MOSFET is fully opened then it doesn't matter what input current curve looks like.


@Jetijs
Could you please take scope shots of your gate driving signal? Just connect scope probe between gate and source and take screenshots in various situations. We still haven't seen any driving signal curves and I think it may give us better understanding of what's going on in your system.

[uusedman]

guys, i got my motor to spin work with the electricity been collected on the back to a 12 V batter.

My reed switch is buring out when i am NOT collecting the electricity. When I open circuit, the power supply injects MORE power to the motor. I think i am on the right track.

I need something better than the reed switch, your suggestions is highly welcomed and asked.

[Tecstatic]

@lighty

"To me it seems like the inherent problem of coils and armature acting as a choke not allowing for the fast rise of current."

Please explain how that can create a FET heating problem if the gate drive is OK ie. FET operating at the specified rise and fall times.

[lighty]

@Tecstatic

As I said, you're completely correct in your suggestions. And you're also completely correct in your wishing for everybody to learn something new. However, complex methods of MOSFET and IGBT switching do require some prior knowledge. People here are not stupid, they will learn about it eventually but if you look through all of the posts you will notice that people are not patient enough to spend time learning specialized topics like fast switching. In fact you would be surprised how many EE professionals are unwilling to do so.

That being said- I still doubt that driving of MOSFETs is main problem. To me it seems like the inherent problem of coils and armature acting as a choke not allowing for the fast rise of current. One can drive switching semiconductors as perfectly as possible and there will still be a problem of slower current rise. I cannot be 100% sure since troubleshooting things on a distance is somewhat tricky.

[Tecstatic]

Almost got it

@Jetijs

No need to excuse for your electronic skills. A friend of mine says that a sound foundation for success is 15% ingenuity and 85% hard work. You surely qualifies for that and have my full respect. (no intended judgement on our intelligence)

I know several persons being excellent doing something although they have no formal education in the field. Again good internet search skills and hard work is the key.

And you got it almost right.

Let the optical fork be connected as it has been before, just move the resistor on pin 1 from the fork emitter to fork collector.

Add a decoupling capacitor on the 40106. thats it :-)

@Tecstatic

While everything you wrote is more than true I suspect that most of it is way over head of average experimenter. Also, there are some variables that may occur that could confuse people if they do happen. For example gate series resistor value can be easily calculated but that calculation will be valid only for one value of current. In practice one would have to find the value of that resistor oneself. The easiest way would be to use multiturn non-inductive trimmer, adjust it and then observe input signal on the gate until one achieve as steep rise time as possible.

Also, SMD caps and resistors will help quite a bit with fast transients but I suspect most of people won't know what to do with them and how to solder them and not destroy them. Same goes with PCB- one surely must observe the length, width and geometry of gate tracks. As you very well know there are simple ways to compensate for the tracks inductivity but even the more experienced amateurs have problems grasping that. Also, I agree that ground plane would reduce some of the problems with transients. However, there are no visible transients on the driving side of the MOSFETs so I don't see any point of bothering with it. It simply doesn't look to me as noisy environment worth the effort of all of the precautions one would employ in commercial projects.

Negative voltage on the gate would help getting steeper faling edge but it matters only when MOSFETs are getting hot and when they are passing larger currents. Also, introducing negative gate signal would complicate schematic even further.

Of course I will again sound ominous and arrogant (as some people called me in the past) but I think all of the above is way above knowledge of average amateur. What you could do if you have time is to engineer everything and then simply offer people finished solution that they could copy. If you do that there is a simple solution for most of the driving problems you mentioned- you could use advanced drivers like ST series TD35x- they have Schmitt trigger input, adjustable delay, active Miller clamp (no need for negative drive) and ever desaturation protection (can also be used as overcurrent protection). Of course they can sink "only" 1A so you may add totem pole driver stage and negative voltage drive in order to be able to controll larger MOSFETs conducting larger current. I used TD351 and TD352 in comercial projects and they're amazing at what they can do when switching fast IGBTs (and MOSFETs) that are working near their maximum power ratings.

Also, optical switch rising time can be solved by using integrated reflective optical switches rather than slotted ones. Because one can use reflective tape it's also easier to adjust timing without any additional mechanical contraptions.

I'm not sure I agree that working solutions should be avoided just because they maybe take more than 2 minutes to understand and implement. We are here to learn something new. Considering the mechanical work Jetijs has accomplished, I think this is manageable also. But I will follow the wishes of Peter if he thinks it is too much.

I have seen many circuit layouts in my time from terrible to excellent. Imho we have a hen-egg problem here, I can't say whether Jetijs has made a good layout.

The FET does spend too much time in the linear region to get this heating problem, so presently the circuit has no sharp pulses.

With a 1,5 ohms gate resistor they will become much sharper and circuit noise may become a problem, this is the reason for the more complicated stuff presented in steps. For me it is the removal of gate charge thats the issue, in this case the faster the better as long as we stay within specifications. When the gate charge has been removed, the resistor value is not significant anymore. And yes you can overdo everything, but simply changing a resistor to hopefully get rid of the heating problem is not the worst problem I can think of.

And may I add I have gone from a small heat sink to using a PCB copper area as "heatsink" by using negative gate turn-off although the driver data sheet states it is not necessary. And the cost of the negative turnoff is less than the heat sink, not to speak of reduced consumption during the lifetime of use.

The document AVR040 located here explains some more in case someone is interested.

Atmel Products - Application Notes

No need to dis-pare if some parts are difficult, just browse the document, every understood detail is a step forward.

Having said that from a world I have worked in for many years, I often feel a bit depressive about all the new stuff I have been though the last 18 months.

The suppression that makes most people make incomplete or hard to understand descriptions, and the internet harassment from which I have had my fair share, makes it a difficult subject to learn fast.

I wish I had all the insights of you Lighty and a lot of other experienced and gifted people in this forum. Again hard (and fun) work is part of the key.

Eric