Syncing two 3 phase PM generators.

Kevin Beitz · Jun 5, 2018

I'm wanting to sync two 3 phase PM generators. Can I use the light bulb trick to do this. I'm wanting to use a clog belt for timing. I'll keep jumping one tooth at a time until I get it synced. Would I connect one leg from each generator (T1+T1)together and put the bulb between two different legs (T2+T2)or T3+T3).

DDoug · Jun 5, 2018

I think the cog tooth spacing is too coarse.

I would rig the belt tensioner to slightly move the timing.

In the power house, the lights were replaced with a "syncroscope"
which they tell me is just an electric voltmeter (placed across both
alternators some how)

thermite · Jun 5, 2018

Kevin Beitz said:
I'm wanting to sync two 3 phase PM generators. Can I use the light bulb trick to do this. I'm wanting to use a clog belt for timing. I'll keep jumping one tooth at a time until I get it synced. Would I connect one leg from each generator (T1+T1)together and put the bulb between two different legs (T2+T2)or T3+T3).

Synchroscopes, as Doug mentioned, are "out there", new and surplused. Waaay better than the neons.

A full tooth is too coarse by half. Once you get "close" you'll only really need to put slack in BOTH sides of the belt span. Then "balance" the two idlers to shift that slack.

9100 · Jun 5, 2018

Slack in the belts will just make the generators take up uneven shares of the load. You need to have them in a tight relationship and adjust them to synch. There is a way to connect a transformer to balance the load. The basis idea is to have a transformer with two windings, each rated for the voltage of one generator. You connect them in series and take the load from the connection between them. I have done this with two different generators with different wattage ratings. Both were gasoline powered and running at about 60 cycles. When I started to draw current, the interaction between the windings sucked them into synchronization whether they liked it or not. You then adjusted the governors to balance the current drawn from each. I later found out that a similar technique was used to balance the load on paralleled Variacs.

Bill

thermite · Jun 5, 2018

9100 said:
Slack in the belts will just make the generators take up uneven shares of the load.

There IS NO "surviving slack", Bill. Toothed belts.

No slack LEFT once the opposed tensioners have changed the sharing of it so the GEAR TOOTHED segments of each pulley/shaft member of the pair rotate their shafts to a new relationship by some fraction - or full - tooth span. Easier than inlining them and clamping a coupling disc, but that works, too.

Either way, once they've been synced, the phasing shouldn't change.

Strostkovy · Jun 5, 2018

Space the generators to have slack belts, so that putting an idler on each side of the belt almost causes the idlers to touch at full tension.

Use a volt meter to get as close to zero as possible.

Electrically connect the generators together and measure the current. Shift the pair of idlers up and down to change the mechanical phase angle so that your current is the lowest.

The best way is to use three meters: one on each generator and one on the load, per phase. This will form a Y shape of meters, which lets you see how many amps the generators are producing and how many amps the load is getting. Adjust the pulleys so that the sum of the first two meters is as close to the load meter as possible.

I'm happy to draw diagrams if anyone is interested.

As far as using transformers to isolate the loads and connect them in series you will have a poor power factor instead of interphase losses, but the losses are still there. Balancing transformers can balance voltage differences quite well but don't help with phase angle.

Can you just orient both generators and shafts the same way when putting on belts?

Kevin Beitz · Jun 5, 2018

Will a PM 3 phase motor make a good RPC?

johansen · Jun 6, 2018

Yes, probably 10 times better. get it close to speed, connect it to your single phase line through say, a 10 ohm resistor, then short out the resistor after it synchronizes.

In my experience with my 2 hp diy synchronous motor, the field voltage really didn't matter all that much, plus or minus 20% didn't make much difference in the line current (no load draw from the single phase supply side)

9100 · Jun 6, 2018

Strostkovy said:
As far as using transformers to isolate the loads and connect them in series you will have a poor power factor instead of interphase losses, but the losses are still there. Balancing transformers can balance voltage differences quite well but don't help with phase angle.

Can you just orient both generators and shafts the same way when putting on belts?

Adjustable idlers will work. So would making an adjustable hub for one pulley.

Another possibility would be a mount that allows one generator frame to rotate a few degrees. Too bad generators don't come with servo mounts.

Re transformers, the phase difference between each generator and the output current will only be 1/2 of the difference between the generators. That won't be any worse than most of the common loads. To measure the error you should use a voltmeter and read the voltage at the best setting. If, for example, it is 10 volts (it shouldn't be nearly that high) you would use a transformer with a 15 volt center tapped secondary and connect it between the two outputs, say L1A and L1B just like you connected the light bulb or voltmeter and take the load from the center tap. If you have 36 tooth pulleys, the error will be less than 10 degrees, less than 5 if you are careful. The output will be a voltage and phase angle half way between the two with negligible losses. You probably could get away with transformers on two legs, but three would be better.

Bill

thermite · Jun 6, 2018

9100 said:
Adjustable idlers will work. So would making an adjustable hub for one pulley.

Another possibility would be a mount that allows one generator frame to rotate a few degrees. Too bad generators don't come with servo mounts.

Ordinarily one simply uses a single, higher capacity, generator 'head', rather than trying to sync two smaller ones when they are on the same prime-mover's shaft.

"Sync" then used only when ganging separate units, entirely.

Just for kicks and giggles, the OP could do the opposite.

Arrange his two gen heads fully OUT of phase, generate SIX phase power, match a pair of load-motors locked opposite in the same manner, and see how smoothly it all runs at the mechanical output side.

"Selsyns" for aimng a warship's guns shipped with up to - what was it? 26 or so phases? I think 256 is out there, too, perhaps more-yet, but with some dirty beach sand and signal-foo involved so the wiring and connectors don't use up all the space in the gun turret or machine-tool. Not my job. Someone else knows.

dalmatiangirl61 · Jun 6, 2018

Kevin Beitz said:
Will a PM 3 phase motor make a good RPC?

I'm not going to say no, because I don't know, but never seen it done. I do know 3 phase PMAC motors make good wind turbine generators though.

9100 · Jun 6, 2018

Kevin Beitz said:
Will a PM 3 phase motor make a good RPC?

There is an old thread on the subject. Someone who was in the phase converter business got in the discussion and described some stability problems. I haven't been able to find the thread, probably haven't used the proper search words.

Bill

JST · Jun 6, 2018

Kevin Beitz said:
Will a PM 3 phase motor make a good RPC?

it will make a good generator...... not much independent control of voltage and speed with it being PM, though. I;d expect it to be a little low on all 3 outputs at synchronous speed.

Strostkovy · Jun 6, 2018

The issue I see with using a synchronous motor as an RPC is that it would drop below synchronous speed under high starting loads of whatever machine it is driving, then not be able to recover. I think it would do better than an induction motor up to around half of its FLA, where you risk losing sync. (Half of FLA is just a guess. It will likely go a bit more, whatever the ratio of single phase current to three phase current is).

Being permanent magnet it should provide a fairly stable voltage on the third leg, but that voltage could be a little weird and may need a boost transformer.

thermite · Jun 6, 2018

Strostkovy said:
The issue I see with using a synchronous motor as an RPC is that it would drop below synchronous speed under high starting loads of whatever machine it is driving, then not be able to recover. I think it would do better than an induction motor up to around half of its FLA, where you risk losing sync. (Half of FLA is just a guess. It will likely go a bit more, whatever the ratio of single phase current to three phase current is).

Being permanent magnet it should provide a fairly stable voltage on the third leg, but that voltage could be a little weird and may need a boost transformer.

Store-bought RPC still have enough price headroom before bumping up against Phase-Perfect costs that IF such a rig - perhaps with electronically assisted regulation - was technically feasible, I'd expect they'd already be shipping, and selling well.

As it is, I don't know if anyone is even shipping "conventional" RPC with active control of the "generated" leg to respond better to changes in load. No pressing need, here.

Anyone?

9100 · Jun 6, 2018

Strostkovy said:
The issue I see with using a synchronous motor as an RPC is that it would drop below synchronous speed under high starting loads of whatever machine it is driving, then not be able to recover. I think it would do better than an induction motor up to around half of its FLA, where you risk losing sync. (Half of FLA is just a guess. It will likely go a bit more, whatever the ratio of single phase current to three phase current is).

Being permanent magnet it should provide a fairly stable voltage on the third leg, but that voltage could be a little weird and may need a boost transformer.

Such motors often have an induction strip on the armature that makes it act like an induction motor until it gets close enough to synch to lock in. Once it does, induction ceases because there is no slip so the strip is just along for the ride.

Bill

DDoug · Jun 6, 2018

If the synchronous motor was NOT the Pm type, and rather the old type with
a externally excited field, I could see a control circuit built to
monitor the 3rd leg, and adjust the field accordingly.

The stability problems cited above might be able to be controlled with a
PLC monitoring everything, including input amperages and voltages, phase angles,
outputs, etc.

Strostkovy · Jun 6, 2018

thermite said:
Store-bought RPC still have enough price headroom before bumping up against Phase-Perfect costs that IF such a rig - perhaps with electronically assisted regulation - was technically feasible, I'd expect they'd already be shipping, and selling well.

As it is, I don't know if anyone is even shipping "conventional" RPC with active control of the "generated" leg to respond better to changes in load. No pressing need, here.

Anyone?

I'm not really seeing what you are getting at here.

If you are suggesting that since I think the PM motor would have better phase stability that they would use them in RPCs, consider that they aren't easy to start, especially not on single phase. Though if it were in a commercial RPC they would just be able to wind the two windings on the third leg (or one winding for star) to give the right voltage in the first place and would need no balancing circuitry.

I do have a schematic for a static phase converter with perfect 120 degree phase angle and microcontroller based balancing, using no active components in the power path, but it would require me to wind more transformers and inductors than I cared for. It would make a fine, very robust production unit though. I'll make a thread on it at some point maybe.

Kevin Beitz · Jun 6, 2018

I don't know if it will make a difference but the magnets in my motors is in the rotors and not in the fields.

thermite · Jun 6, 2018

Strostkovy said:
I'm not really seeing what you are getting at here.

So it seems!

Folks are constantly diddling with greater or lesser capacitance to try to hit the best performance, "generated leg", when, of course, the load ON a given load-motor is changing, or which one or which combination of several possible loads is a dynamic.

Directly variable capacitance is cumbersome, but it is not the only means of introducing a controlled dynamic to the same effect.

Only the one leg needs it. The "package" could be less costly in large HP than high-powered VFD or Phase-Perfect, and more robust, also "fail-safe", than either.

A controlled reactance and some dirty-beach sand - it could perhaps be automagicated, yah?

Phase-Perfect's big inductors don't rotate. The rest of the 'tronics might be cheaper and simpler if they did do, high HP especially, given 3-P motors are mass-produced at such high volumes and sharply competitive pricing...

Challenge in there for a younger person?

Or for an older one to dust-off fossilized experimental data?

Syncing two 3 phase PM generators.

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