Need recommendation on phase converter

[Cliff Rohrabacher]

If $$ is an issue you might consider building one. The largest expense in building one is the 3 phase motor you'd need to drive.
You can get them from a junkyard for cheap and 99 times out of 100 they are junked only because of bearings that you'll replace for less than $ 20.

[Cliff Rohrabacher]

Adding:

On a phase converter the wildleg is most often found by measuring the three voltages to ground (neutral) rather than line-to-line.

[tod evans]

4). The most relavent difference between a true three phase system and a phase converter system has to do with the neutral. A phase to neutral connection will have the same voltage regardless of which phase your one. A - B - or - C to neutral. (Of course real world systems may see a minor variation in phase voltages but its very minor). In a phase converter system, DO NOT MAKE THE MISTAKE of hooking up a wildleg to neutral connection as this voltage IS NOT (REPEAT! IS NOT) the same as say an A or B leg to neutral value. Also, your wildleg will be higher than the other two legs and this value will fluctuate as a function of loading.

Hope this background helps a bit...

dev, i`m used to three phase "pot" power and find this confusing?? are you saying that folks running a third leg generator can check voltage from the wild leg to ground and get 120 (on a 220-3ph system)?

the power that i get from pots on the pole when checked to ground is 120-120-240, not 120-120-120.

checking voltage between any two legs on "pot-power" will read 240 regardless if you include the wild leg .

please make this clear to this ol` hillbilly....tod

[Rob Russell]

Tod,

I think you're both saying the same thing.

On a rotary phase convertor that's being run off of 240v single phase power, the wild/generated leg will start at 208 volts to ground. It's 208 because that's the (square root of 3)*120 = 1.732*120=208. Based on other factors, like loading or capacitance that gets added to the system to better balance the 3 legs, that voltage on that wild leg can definitely increase.

The voltage between legs will be nominally 240v for all 3 combinations A-B, A-C and B-C.

[Steven Evans]

FYI for non electricians, most would consider that homes run on 2 phase power(no such thing) and that it is kind of mis leading to call it single phase. But 1 phase is the standard name.

3 phase has 3 hot legs and neutral/ground They come from generators with a delta or wye windings.

1 phase has 2 hot legs. Run one leg to neutral/ground gives you 1 phase 120 volts. Run Leg1 to Leg 2 and that will give you 240 volts.

So a single phase motor would have a hot leg every 180 degrees
A 3 phase motor would have a hot leg every 120 degrees.

[Rob Russell]

Steven,

There actually is 2-phase power, where the phases are 90 degrees out from each other. It was used long ago when power systems were sprouting up all over the place.

"Single-phase" power is what's supplied to our homes and is the appropriate term for it. The fact that the utility company transformers may have multiple taps so we can get both 120v and 240v is how they distribute the power, but it's still all "single-phase".

Rob

[Charlie Plesums]

...What many folks in such a dilema often do is to slave converters. For example, you may have a 5 HP, a 10 HP and a 20 HP converter. These all feed the wildleg back plane on your three phase service panel. So when its time to run the little three phase jointer, you would use the 5 HP converter only. When you wish to run your table saw, you use the 10 converter only. When you need to run the widebelt sander, you may wish to use both the 10 hp and 20 hp converters at the same time....

40+ years ago as an EE student, we did an exercise of hooking multiple three phase generators together (actually we fed 3 phase power we were generating into the Northeast power grid... before the great Northeast blackout, but unrelated...). We had to go through quite an exercise to be sure we had the same phase rotation (simple) and were in synch (exact same speed and rotational angle of our generator as the power grid, not simple). Won't you have the same issue if you hook two rotary phase converters together? Or does the bigger converter simply jerk the smaller converter into compliance?

[Dev Emch]

40+ years ago as an EE student, we did an exercise of hooking multiple three phase generators together (actually we fed 3 phase power we were generating into the Northeast power grid... before the great Northeast blackout, but unrelated...). We had to go through quite an exercise to be sure we had the same phase rotation (simple) and were in synch (exact same speed and rotational angle of our generator as the power grid, not simple). Won't you have the same issue if you hook two rotary phase converters together? Or does the bigger converter simply jerk the smaller converter into compliance?

Been there and done that. We used hunkin copper knife switches mounted to marble control panels along with the three phase lights. On the pelton wheels, we also had feedback loop controlled needle jets to adjust the turbine shaft speed by throttling the two main jets. On the francis turbine (made by worthington) we have a more complex mechanical set of vanes controlled by a hydraulic/electric governnor and a mechanical linkage. Stability and lack of rotational transients is controlled by huge flywheels located between the turbine and the alternator.

The grid is massive and a poly phase machine driven mechanically has momentum. When the two diverge, you can get a multi ton machine to try to bounce around like a out of balance washing machine. This would be very bad!

An induction motor can function as a generator however with one serious drawback or caveat. Induction rotors are cammillions in the sense that they can change pole counts etc. depending on the particular stator used. If I run an induction motor as a generator (alternator), because of its slip, it has no regulated means of maintaining 60 cycles. You also have to run faster than 60 cycles (i.e. faster than sycnro speed) to get it to function as a generator. How much faster? That varies depending on the applied load. So when this is done to provide small aux back up power to field stations, stock tank water pumps, etc., the system is usually wired up to the grid. It does not take much of a hook up and your not back feeding power onto the grid either. But the grid connection uses the 60 cycles of the grid to sync up your induction generator. What a pain and that is why they are not used much and when used, for only small aux or backup applications.

In the case of a phase converter, a converter is not an alternator. It is a converter. The idler functions as a rotating transformer using the two secondary unused windings of the stator. The first winding is used to drive the idler. This is called single phasing and you can run a three phase motor on single phase albeit you cannot start it nor can you get much power out of it. In the case of an idler, you often have a start up scheme such as a phase shifting start capacitor and cutout switch or even a secondary pony motor. Because the idler stator is built like an alternator, you pick up the mechanical 120 degree phase separation.

But the sycnronous nature of the grid drives and locks the idler to speed and provides the AC coupling frequency for your fields. So converters are very much driven by and sycned up to the grid inputs.

The only time one is really concerned about sycnc frequency is when your doing something on your own with say a freq or your own alternator in which case you have manage the frequency yourself. Either way, I dont want this stuff back feeding onto the grid. This is esp. true of a freq which is used to drive older direct drive shapers and moulders to 7200 RPM and they do this by changing the 60 cycle input to a 120 cycle output.

There are two types of converters used in woodworking shops. The phase converter and the frequency converter. One converts single phase to three phase and the other converts 60 cycles to 120 cycles.

[Dev Emch]

FYI for non electricians, most would consider that homes run on 2 phase power(no such thing) and that it is kind of mis leading to call it single phase. But 1 phase is the standard name.

3 phase has 3 hot legs and neutral/ground They come from generators with a delta or wye windings.

1 phase has 2 hot legs. Run one leg to neutral/ground gives you 1 phase 120 volts. Run Leg1 to Leg 2 and that will give you 240 volts.

So a single phase motor would have a hot leg every 180 degrees
A 3 phase motor would have a hot leg every 120 degrees.

This IS NOT TWO PHASE POWER! This is playing tricks with the pole pig. Many older turn of the century woodworking machines had true two phase motors and the OWWM guys are always checking to make sure their not hooking up three phase to a two phase motor. This would be very bad!

In a two phase system, your alternator has two independent winding sets in place of the standard three. You have two power supply conductors on the pole instead of three and your phase shift if solid at 180 degrees instead of 120 degrees.

Transformers are special versions of inductors/coils. Inductors/coils and capacitors both shift phase by 90 degrees but in opposite directions. But phase shift can be modified at run time by haveing inductance and capacitance partially cancel out any effective shifts. Mechanical shift cannot be cancelled out the same way.

So in a standard pole pig, you have a single primary rated at about 7200 volts and dual center tap secondaries rated at 120 volts each. The secondary tap is at ground level so you have two discrete systems at work here if you will. That is why you want your service center breakers balanced out. One half is being fed by one secondary and the other half is being fed by the other secondary.

But because one secondary is effectively polarity reversed, you have 240 volts accross both hot leads of your secondaries. This is called differential potential as opposed to single ended potential (black and white). Now if I switch polarity on a sine wave, what was positive is now negative, etc. This is exactly what happens if I phase shift a sine wave 180 degrees which is where the notion of a two pole system came from.

Hope this clears a few things up....

[Mike Henderson]

Dev, maybe I'm misunderstanding your comments on two phase power. I think two phase power is offset 90*, as mentioned by Rob in an earlier post. The two legs of single phase power in a residential installation are 180* offset (when referenced to ground).

Mike