# Mechanical 6 step inverter for induction motor.

13473 Views 26 Replies 8 Participants Last post by  RollingCamel
Hello to everybody!

Sorry for my English, it is not my native language!

I'm trying to build 6-step rotary inverter and use it to run AC 3 phase induction motor to use it in ev.
It shoul be as lo-buck diy capable drive for those who want powerful ev but don't have money for soliton1+warp11hv or Zilla2K
+kostov11dual or something like this. Maybe not the most efficient drive... but for money saved I can buy more batteries.

How it works:
-it is a commutator and 3 brushes at 120 degrees around it.(see pictures)
-commutator is divided in 2 halfs: one connected to DC+, other to DC-(see pictures)
-small motor is used to rotate commutator at variable speed, big motor follows(I need to ensure somehow that speed difference between big motor and commutator never be more than 15%)
-since I cannot vary voltage continuosly with frequency to keep constant V/Hz ratio, I'm going to run motor always in "field weakened" mode, above V/Hz ratio. As I understand, 3 phase induction motor rated at 380v/50HZ can run up to 380V/100Hz and still have full power(twice rpm, half torque)? So, I'm going to keep motor at right rpm with gearbox, or have direct drive but stepped voltage with series/parallel switching of battery pack (like 80V/160V/320V).

Can anyone tell me what efficiency could be expected if i run AC induction motor on 6-step waveform?
In article "VFD Fundamentals" http://www.kilowattclassroom.com/Archive/VFDarticle.pdf I read
that:"For frequencies above 60 Hz the voltage remains constant. Some
AC drives switch from a PWM waveform to a six-step waveform for 60
Hz and above."(Last sentence)

I have build small commutator for test, and ran small 3 phase motor with it. To get DC I used full-bridge rectified 220V from mains with 300mf electrolytic capacitor to smooth voltage pulses. Multimeter showed 300VDC.
First I used carbon brushes, but thing sparked alot, so I submerged whole commutator in motor oil (Addinol 5W40), and replaced carbon brushes with aluminium ones.
I made brushes of aluminium because I wanted to test how metal brushes will behave when submerged in oil, but aluminium is easy to work on with hand tools.
So, it works OK, no sparks. No sight of wear after 5 minute running at 3000rpm. Motor runs fine, no heating, pretty same way how it runs from 380V 3 phase mains.
I've tried to run commutator without oil - lots of sparks! Almost destroyed the thing.
I can't find anywhere about brushed commutator submerged into some
dielectric liquid, oil for example. Why?

Sorry that so little pictures, I can make more pic and video at
weekend.

Aleksejs

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Hello, welcome to the forum.

Mechanical controllers have been discussed before, usually DC mechanical PWM but also a three phase one was suggested.

Simon Rafferty seems to be one of the initiators of the idea a while ago.
It would be neat if you could connect your commutator to the 3 phase motor though a differential. The differential could sum the speed of the 3 phase motor and an aux motor.

So commutator speed = 3 phase motor speed + aux motor speed.

This would allow you to add some trim speed to the commutator, allowing you to control "slip speed" making it easier to accel and decel the 3 phase motor.
Hello Aleksejs,
Mechanical commutator for an induction motor does not work well. Tom Edison figured this out 100 years ago. The fundamental problem is excessive arcing as you found. The windings are inductive, and interrupting current in an inductive load causes severe arcing.

You can run an AC motor on DC (as Nikola Tesla proved); but you have to do it a different way. First, the motor needs to be synchronous. Replace the inductive rotor with a wound rotor, and slip rings. That way you can adjust the field strength. Basically, this means replacing the cast aluminum windings with a set of copper windings (one winding per pole).

Your mechanical inverter should be right on the shaft of the synchronous motor. In a synchronous motor, the frequency is precisely proportional to the RPM, so this insures the frequency is correct.

You then use field excitation (the current in the DC rotor winding) to control arcing. At any given RPM and applied DC voltage, there will be a "correct" field current that just happens to cause the current in the commutator to fall to zero at the instant the brush crosses from one segment to the next. Too little current, and like an induction motor, there is still current flowing at the zero-crossing. Too much current, and the current will actually have passed through zero and be flowing in the opposite direction.

In the 1970's I built an inverter using this method. I used six SCRs for the switches of my 6-step inverter. I used a surplus aircraft 120/208vac 400cycle alternator for my motor. The field current was adjusted so the current fell to zero at the voltage zero crossings, so the SCRs automatically turned off. It was successful, and the efficiency was reasonable (about 80%). The main drawback was that it had poor starting torque, and if the load torque was too high it could suddenly lose synchronization, blow fuses, and stop.

Hope this helps!
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Hello Woodsmith!

I was thinking about posting there actually, but my controller is something different, it has nothing to do with PWM... it just like old type switching "choppers" they were called if I remember correctly. They converted DC to square-wave AC. Mine does 6-step waveform.

Hello Etischer!

Actually I was thinking about that commutator shouldn't be mechanically connected to 3 phase motor, just "sense" it's rpm with rpm sensor and slip should be controlled that way from 0 to 10% if A type motor is used, or 0 to 20% for B type motor. Is it possible?
I don't like differential, it's difficult to build one, i think...

By the way, Etisher, as I know you build your custom AC controller for Passat, can you tell me is it true that pwm ac controllers shift from PWM waveform to 6-step waveform at "field weakening" mode?(when voltage not need any more to be increased with frequency)?
What efficiency could be expected if AC 3 phase induction motor runs on 6-step waveform and in "field weakening" mode?

Hello Lee Hart!

I don't like syncronous motors, they aren't so widely and cheap awailable as induction motors. And I don't think I'm able to build what you described with my little skills in electrical... at least by now.

Well, maybe Thomas Edison 100 years ago didn't have high quality synthetic oils we have today.
Because when I've put my commutator into 5W40 oil, it has no arcing at all, and this is at 300VDC, and with metal brushes instead of carbon ones... As I understand, arcing is the only problem limiting use of mechanical commutators described in this http://www.diyelectriccar.com/forums...mps-31134.html thread, so maybe oil it is solution?
Or maybe there was no arcing because low amperages, but with bigger motor under load and higher amps arching will occur? Is it possible? I definetly need to try run bigger motor with it...
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Yesterday I tried to run bigger motor with mechanical inverter, and it was not sucsessful because of excessive arcing.
I think problem is that bigger motor means higher current, so oil heat up and become conductive.
Maybe I just need bigger commutator and brushes? Or better oil, like transformer oil?

Well, small motor runs quite fine at 300VDC without arcing, so maybe it is still possible to run bigger motor that way.

Interesting that in oil brushes made of solid metal (aluminium in my case) work same way as carbon brushes.

Here is video:

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Looks like video embed did not worked out well.
Do not want to be the party pooper, what about saturation?
So, the speed of the idler/inverter motor would control the frequency of the drive motor? How would you control the power?
Also, that drill being used could probably run off dc pack voltage if it has a rectifier in it.
Also, how about using the idler motor just to control some igbts or scrs, instead of the whole thing.
Yesterday I tried to run bigger motor with mechanical inverter, and it was not successful because of excessive arcing. I think problem is that bigger motor means higher current, so oil heat up and become conductive. Maybe I just need bigger commutator and brushes? Or better oil, like transformer oil?

Interesting that in oil brushes made of solid metal (aluminium in my case) work same way as carbon brushes.
Your pictures were a big help in understanding what you are doing. I'm delighted to see you experimenting with this. However, I must say that a little studying would go a long way toward improving your setup. People have been building motors and mechanical inverters (commutators) for a very long time. They have learned a lot about them. This knowledge could be applied to what you are doing.

Recognize that all motors are really AC motors. The windings always have AC in them. What people call a "DC motor" is really an AC motor with an internal DC-to-AC inverter. This inverter can be mechanical (commutator and brushes) or electronic (a "brushless DC" motor).

The kind of motor you are making is commonly found in toys. They have a rotor with just 3 windings, and a 3-section commutator to generate the 6-step 3-phase AC waveforms to run it. This design makes pretty crappy motors, which is why you only see them in cheap toys.

In these small motors, arcing isn't a problem due to the low voltages. When designers want higher voltage motors (above about 30v), they found they had to increase the number of rotor windings and commutator bars. They arranged it so the voltage between each bar was less than 30v. At 120v for example, you might have 4 windings and commutator bars in series between the brushes, so each has only 30v across it.

If you try to switch high voltage DC, particularly with an inductive load like a motor winding, you're going to have trouble. You might discover some new solution (aluminum brushes in oil, etc.) but I have a strong feeling that anything that simple has been tried and rejected. After all, we've had some very bright people working on the problem for 100+years.

Look at switch ratings, and see how they change as a function of voltage, current, and AC vs. DC. For example, here is power relay with a pretty complete data sheet showing its life and performance with different loads: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PB918-ND

Notice that:

• Contacts in series add to the voltage rating (2 in series have 2x the voltage rating, 3 in series has 3x etc.)
• Voltage rating for DC is far lower than AC (440v at 16amps AC, 90v at 16amps DC)
• The higher the current, the shorter the life (10^7 cycles at 1 amp, 10^6 at 4 amps, 2x10^5 at 16 amps etc.)
• DC voltage rating is strongly affected by current (one contact rated 30v at 16a, 50v at 2a, 100v at 0.6a, 300v at 0.25a)
And these are with a resistive load! Arcing is even worse with an inductive load.

This pattern is true for just about all contacts, though the exact ratios will vary. This means that the physical size of the switch you'd need to switch high DC voltage and current for a large motor would be huge! If it had to survive any length of time, it may have to be bigger than the motor itself!

You have to "beat" this problem some other way than brute force.

For example, until the 1950's they built inverters with vibrators. This is basically a relay wired like a "buzzer". Its contacts generated AC to run a transformer, motor, etc. To make it last, they added a "buffer condenser" (capacitor) to resonate the load's inductance with the vibrator's operating frequency. When you get it right, the current happens to resonantly ring to zero just as the vibrator's contacts open; thus arcing is minimized.

Another example is the one I mentioned; driving a synchronous motor with just the right field strength so its current falls to zero just as the switches open.

Still another is a current-fed inverter, where you have a large inductor in series with your DC supply. The inverter switches then momentarily short the windings rather than open them, preventing the high voltage "kick" that triggers the arcing.

These are old techniques. You'll find them in old textbooks. Start by learning what others have done to deal with the problem, and then work on your own ideas. Otherwise, you'll just be "re-inventing the wheel" (repeating things that have already been tried and failed).
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So, the speed of the idler/inverter motor would control the frequency of the drive motor? How would you control the power?
Also, that drill being used could probably run off dc pack voltage if it has a rectifier in it.
Also, how about using the idler motor just to control some igbts or scrs, instead of the whole thing.
I would control power by controlling slip between drive motor rpm and inverter rpm. 0% slip - no power to drive motor, 10% slip - full power.

About using the idler motor to control IGBTs - these things are quite expensive and I don't like to waste lots of money on something that wouldn't work...
Your pictures were a big help in understanding what you are doing. I'm delighted to see you experimenting with this. However, I must say that a little studying would go a long way toward improving your setup. People have been building motors and mechanical inverters (commutators) for a very long time. They have learned a lot about them. This knowledge could be applied to what you are doing.

Recognize that all motors are really AC motors. The windings always have AC in them. What people call a "DC motor" is really an AC motor with an internal DC-to-AC inverter. This inverter can be mechanical (commutator and brushes) or electronic (a "brushless DC" motor).

The kind of motor you are making is commonly found in toys. They have a rotor with just 3 windings, and a 3-section commutator to generate the 6-step 3-phase AC waveforms to run it. This design makes pretty crappy motors, which is why you only see them in cheap toys.

In these small motors, arcing isn't a problem due to the low voltages. When designers want higher voltage motors (above about 30v), they found they had to increase the number of rotor windings and commutator bars. They arranged it so the voltage between each bar was less than 30v. At 120v for example, you might have 4 windings and commutator bars in series between the brushes, so each has only 30v across it.

If you try to switch high voltage DC, particularly with an inductive load like a motor winding, you're going to have trouble. You might discover some new solution (aluminum brushes in oil, etc.) but I have a strong feeling that anything that simple has been tried and rejected. After all, we've had some very bright people working on the problem for 100+years.

Look at switch ratings, and see how they change as a function of voltage, current, and AC vs. DC. For example, here is power relay with a pretty complete data sheet showing its life and performance with different loads: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PB918-ND

Notice that:

• Contacts in series add to the voltage rating (2 in series have 2x the voltage rating, 3 in series has 3x etc.)
• Voltage rating for DC is far lower than AC (440v at 16amps AC, 90v at 16amps DC)
• The higher the current, the shorter the life (10^7 cycles at 1 amp, 10^6 at 4 amps, 2x10^5 at 16 amps etc.)
• DC voltage rating is strongly affected by current (one contact rated 30v at 16a, 50v at 2a, 100v at 0.6a, 300v at 0.25a)
And these are with a resistive load! Arcing is even worse with an inductive load.

This pattern is true for just about all contacts, though the exact ratios will vary. This means that the physical size of the switch you'd need to switch high DC voltage and current for a large motor would be huge! If it had to survive any length of time, it may have to be bigger than the motor itself!

You have to "beat" this problem some other way than brute force.

For example, until the 1950's they built inverters with vibrators. This is basically a relay wired like a "buzzer". Its contacts generated AC to run a transformer, motor, etc. To make it last, they added a "buffer condenser" (capacitor) to resonate the load's inductance with the vibrator's operating frequency. When you get it right, the current happens to resonantly ring to zero just as the vibrator's contacts open; thus arcing is minimized.

Another example is the one I mentioned; driving a synchronous motor with just the right field strength so its current falls to zero just as the switches open.

Still another is a current-fed inverter, where you have a large inductor in series with your DC supply. The inverter switches then momentarily short the windings rather than open them, preventing the high voltage "kick" that triggers the arcing.

These are old techniques. You'll find them in old textbooks. Start by learning what others have done to deal with the problem, and then work on your own ideas. Otherwise, you'll just be "re-inventing the wheel" (repeating things that have already been tried and failed).
Thanks for great info!

Looks like I will not be able to "beat" arcing problem with brute force, or I will need huge commutator.
I need to study this subject more... it's hard to find any good info about this on the internet, such things aren't popular today.
Thanks for great info!
Looks like I will not be able to "beat" arcing problem with brute force, or I will need huge commutator. I need to study this subject more... it's hard to find any good info about this on the internet, such things aren't popular today.
True. The internet tends to ignore things that are older than the internet.

The situation is difficult; but not hopeless.

Old books will describe the older solutions. You can find them in good libraries or at universities. Or, look at old equipment. Make friends with people that work in power plants, railroad yards, mines, etc.They will have 50-year-old equipment (that is still in service!) to illustrate some of these older techniques.

Start collecting parts to experiment with. Tom Edison said, "To invent, you need imagination and a pile of junk."

Try scaling down the problem, to make it cheaper and easier. Your final EV motor will almost have to be 3-phase, but you can experiment with single phase for simplicity. And, you don't need to drive a motor; a transformer with light bulbs for a load is a reasonable substitute. The light bulbs clearly show your power output level.

See if you can find a case of inexpensive power relays with visible contacts. Use them as your "commutator". You'll burn up a bunch of them, but you can see how much arcing you're getting with each test circuit, and quickly replace them.

1. Wire a relay as a buzzer, so it turns itself on/off. Also wire a SPST contact to switch the primary of a transformer. Use a light bulb on the secondary as your load. You'll see a huge amount of arcing!
2. Now add a capacitor across the transformer winding. You'll note that it immediately reduces arcing. Experiment with the value to find the optimum. It should occur when the L and C resonate at the frequency of your "buzzer".
3. Now try using a SPDT contact to drive the transformer. Common to one lead of the transformer primary, Normally Open to +V, Normally Closed to -V. Put your capacitor in series with the other transformer primary lead to ground. This will have even less arcing. Resonating it with the capacitor will get it even lower.
4. Next, you can connect "snubbers" across each contact. A snubber is a resistor and capacitor in series. Start with something like 10 ohms and 0.1uF, and experiment again. A snubber further reduces arcing.
5. You can also add "electronics". Use a diode across each switch contact, oriented so it doesn't conduct (cathode toward +V, anode toward -V). Again, it further reduces contact arcing. a properly sized R-C-D (resistor capacitor diode) snubber is pretty effective.
This should get you started. Once you figure out how to minimize arcing at the switch contacts, you can replace your relays with your rotating switch (or SCRs or IGBTs or other devices).
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True. The internet tends to ignore things that are older than the internet.

The situation is difficult; but not hopeless.

Old books will describe the older solutions. You can find them in good libraries or at universities. Or, look at old equipment. Make friends with people that work in power plants, railroad yards, mines, etc.They will have 50-year-old equipment (that is still in service!) to illustrate some of these older techniques.

Start collecting parts to experiment with. Tom Edison said, "To invent, you need imagination and a pile of junk."

Try scaling down the problem, to make it cheaper and easier. Your final EV motor will almost have to be 3-phase, but you can experiment with single phase for simplicity. And, you don't need to drive a motor; a transformer with light bulbs for a load is a reasonable substitute. The light bulbs clearly show your power output level.

See if you can find a case of inexpensive power relays with visible contacts. Use them as your "commutator". You'll burn up a bunch of them, but you can see how much arcing you're getting with each test circuit, and quickly replace them.

1. Wire a relay as a buzzer, so it turns itself on/off. Also wire a SPST contact to switch the primary of a transformer. Use a light bulb on the secondary as your load. You'll see a huge amount of arcing!
2. Now add a capacitor across the transformer winding. You'll note that it immediately reduces arcing. Experiment with the value to find the optimum. It should occur when the L and C resonate at the frequency of your "buzzer".
3. Now try using a SPDT contact to drive the transformer. Common to one lead of the transformer primary, Normally Open to +V, Normally Closed to -V. Put your capacitor in series with the other transformer primary lead to ground. This will have even less arcing. Resonating it with the capacitor will get it even lower.
4. Next, you can connect "snubbers" across each contact. A snubber is a resistor and capacitor in series. Start with something like 10 ohms and 0.1uF, and experiment again. A snubber further reduces arcing.
5. You can also add "electronics". Use a diode across each switch contact, oriented so it doesn't conduct (cathode toward +V, anode toward -V). Again, it further reduces contact arcing. a properly sized R-C-D (resistor capacitor diode) snubber is pretty effective.
This should get you started. Once you figure out how to minimize arcing at the switch contacts, you can replace your relays with your rotating switch (or SCRs or IGBTs or other devices).

Thanks again for great info!
Now I have something to think about.

Actually oil is reducing arcing a lot (without oil commutator arcing heavily even when connected to small motor which is running in video).
So maybe if I combine it with methods you described it will give some good results.

Actually, I think I did wrong thing connecting motor in wye.
In picture 1, when brushes A and C are in "+" position, but B in "-" position, current flows in way arrows show (I assume current flow from "-" to "+").
When commutator rotates and brush A comes into neutral area, winding OA is suddenly interrupted, and as it has high inductance, this causes high voltage kick that triggers the arcing.

Now motor is connected in delta (picture 2).
When commutator rotates and brush A comes into neutral area (picture 3), no winding remains open, and current in winding AB still flow in same direction as before, but at half voltage (AB becomes in series with CA and these two in parallel with CB).

So I think if I connect motor in delta there should be less arcing than in wye. Am I right?

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1. Thanks again for great info! Now I have something to think about...
2. Actually oil is reducing arcing a lot... maybe if I combine it with methods you described it will give some good results.
3. I think I did wrong thing connecting motor in wye... I think if I connect motor in delta there should be less arcing than in wye. Am I right?

1. You're welcome! (PS: you don't need to quote an entire long post; it's already available anyway. It's easier to read if you only include the bare minimum to maintain continuity.
2. Oil helps because it is a better insulator than air. Oil is used in many really large switches, contactors, and circuit breakers for this reason. But it's messy and can catch fire. There are other gases that also work. The same measures that reduce arcing in air will also reduce them in oil.
3. It's a trade-off. A motor wired in delta can have circulating currents (that flow around the ring). These are bad because they reduce efficiency and create "drag" (oppose the motor's torque). Wye is more common for motors powered by inverters.
Commutators are normally built so the brush shorts adjacent bars as it crosses over them. When the load is inductive, this provides a path for the current to keep flowing, minimizing arcing. But you have to arrange it so that momentary short does not short the power supply!

Here's how it should work. Assume a wye-connected motor with three phase wires, A B C. The positive brush connects to the windings in the following sequence: A, A+B, B, B+C, C, C+A, A etc. The "+" connections are when the brush is bridging two windings together.

The negative brush is doing the same thing. Things are arranged so only one of the two brushes is crossing at a time.

When arranged like this, if you look at the voltage and current in each winding, it is a rough approximation of a sine wave. If the supply voltage is 3V, then each winding sees +1v, +2v, +1v, -1v, -2v, -1v, +1v etc.
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1. It's a trade-off. A motor wired in delta can have circulating currents (that flow around the ring). These are bad because they reduce efficiency and create "drag" (oppose the motor's torque). Wye is more common for motors powered by inverters.
I would trade a little efficiency to reduce arcing. It's not going to be the most efficient controller anyway.

Here's how it should work. Assume a wye-connected motor with three phase wires, A B C. The positive brush connects to the windings in the following sequence: A, A+B, B, B+C, C, C+A, A etc. The "+" connections are when the brush is bridging two windings together.
This is just same way how it works in my case, only I supplied DC to commutator bars, and took AC from brushes (I thought it's easier to build this way).

What I don't like in wye connection - in any case one winding will remain open for short time just to prevent shorting of DC supply.
But with delta connection I can safely disconnect one point from DC supply, but windings still remain connected. This should reduce arcing!

And also if this disconnection length will be correct it will give one more step in waveform, like 0v, +1v, +2v, +1v, 0v, -1v, -2v, -1v, 0 This should be good for efficiency (less harmonic distortion).

Anyway this weekend I will try to run motor in delta and see what will happen.
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I would trade a little efficiency to reduce arcing. It's not going to be the most efficient controller anyway.
You might be surprised. As I said, all motors are really AC motors. Note that you can easily get 90-95% efficient brushed motors. And they have mechanical inverters!

What I don't like in wye connection - in any case one winding will remain open for short time just to prevent shorting of DC supply. But with delta connection I can safely disconnect one point from DC supply, but windings still remain connected. This should reduce arcing!

And also if this disconnection length will be correct it will give one more step in waveform, like 0v, +1v, +2v, +1v, 0v, -1v, -2v, -1v, 0 This should be good for efficiency (less harmonic distortion).
Suppose you have two inductors in series, with a connection to the tap between them. One inductor is carrying 1 amp, and the other is carrying 2 amps. What happens if you open the tap with a switch? What voltage does that tap go to?

It heads for infinity (i.e. the switch arcs)! That's because both inductors insist on maintaining their same currents (1a and 2a). The only way this is possible is if the tap also has 1 amp flowing. The voltage will go to whatever it has to to force this 1 amp to flow. (I.e. you get a 1 amp arc across the switch).

Thus, the amount of arcing you get is the same, whether wye or delta.

An inverter circuit is always set up so it never tries to break current in an inductor. It always provides some path for the current to flow immediately after a switch turns off. This is true regardless of whether the switches are real mechanical switches, or electronic ones (transistors, etc.)

So, your "mission" is to arrange your mechanical inverter switches so they a) never short the supply, and b) never open-circuit an inductive winding.

The switching sequence I describe will do this. I'll put the connections made by the + and - brushes one above the other, so you can see they never connect + to -, and never leave a winding open.

(+) A AB B BC C CA A etc.
(-) BC C CA A AB B BC etc.

Also, remember that each winding has AC in it. So, it has zero-crossing. You can switch that winding open at the instant that its current happens to cross zero. There won't be any arcing at this time.
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1. Next, you can connect "snubbers" across each contact. A snubber is a resistor and capacitor in series. Start with something like 10 ohms and 0.1uF, and experiment again. A snubber further reduces arcing.
2. You can also add "electronics". Use a diode across each switch contact, oriented so it doesn't conduct (cathode toward +V, anode toward -V). Again, it further reduces contact arcing. a properly sized R-C-D (resistor capacitor diode) snubber is pretty effective.
I have a question regarding snubbers. As I understand, these should be connected in parallel with each coontact. So, 6 for 3-phase.
In case of commutator, I have 2 wires of DC supply and 3 wires of AC output. So I should connect snubbers from +DC wire to each of 3 phases and from -DC wire to to each of 3 phases, total 6?
Suppose you have two inductors in series, with a connection to the tap between them. One inductor is carrying 1 amp, and the other is carrying 2 amps. What happens if you open the tap with a switch? What voltage does that tap go to?

It heads for infinity (i.e. the switch arcs)! That's because both inductors insist on maintaining their same currents (1a and 2a). The only way this is possible is if the tap also has 1 amp flowing. The voltage will go to whatever it has to to force this 1 amp to flow. (I.e. you get a 1 amp arc across the switch).
If I have delta connection (see picture, pos.1), BA and AC are in series, and I disconnect switch S from point A (pos.2), isn't it will be easier for winding BA to pull its current (it want to maintain) from winding AC instead of pulling it from switch? When I disconnect switch S from A, current will start to go in way C-A-B anyway.

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So, your "mission" is to arrange your mechanical inverter switches so they a) never short the supply, and b) never open-circuit an inductive winding.

The switching sequence I describe will do this. I'll put the connections made by the + and - brushes one above the other, so you can see they never connect + to -, and never leave a winding open.

(+) A AB B BC C CA A etc.
(-) BC C CA A AB B BC etc.
The thing I worry about is: To change state from
(+) A
(-) BC
to
(+) AB
(-) C
I need to open contact 2 and close contact 1 simultaneously (see picture 1). If contact 2 will open a little before contact 1 closes, winding B will be open-cirquited. But if contact 1 will close a little before contact 2 opens, DC supply will be shortened.
In case of commutator it will look like on picture 2.
How precise should this be?

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