Flywheel on a rotary phase convertor

I am sorry but thinking of RPC's the way I do just seems to make sense. When I learned about motors, I learned that they generate a back emf because they have a rotating magnetic field and conductors that cut the magnetic field. And if you increase the load on a motor, it slows slightly and the back emf drops and the current rises. And if you decrease the load the back emf increases and the current decreases. Now consider decreasing the load even more, so that the load is negative. ( putting mechanical power into the motor trying to make it run faster than synchronous speed ) The back emf increases and the current goes negative. That is current is being supplied by the motor to the mains. So as I see it a motor can work from locked rotor to being driven. ( Don't try locked rotor for very long unless you have a AC servo motor ) Same physics for all cases.

Now this happens whether the motor is a single phase motor or a three phase motor.

Now what happens when you have a three phase motor and run it on just one phase? After you get it started, it will run on single phase power. You still have a rotating magnetic field, and windings for three phases. So the rotating magnetic field generates a back emf in all the windings. So you consume power from the single phase, but generate three phase power.

Incidently you do not have to have an induction motor connected to the mains in order for it to work as an induction generator. It just will not work with a large variety of loads.

Another by the way. One of the better ways to make a RPC is to connect a single phase motor to a three phase motor using an adjustable belt drive. You monitor the current going into the single phase motor and adjust the drive so the current is at or below name plate current when driving the three phase load. Recommended ( by me ) for driving three phase machines as surface grinders that are sensitive to unbalanced three phase power. In that case it is pretty obvious that you have a three phase induction generator being driven by a single phase motor.

You may find this way of analysing a RPC as weird, but it works for me. I have problems understanding RPC's as transfomers that produce a voltage that is not in phase with the input voltage. And it lets me think about how a flywheel would affect a RPC.

Dan

To offer a little food for thought on the original topic, here's a picture of the rotor from the motor which I'm proposing to use as an idler:

I dismantled the motor to give it a good clean, repaint the rusty bits and check the bearings. That's a 10" ratchet in the picture and the rotor weighs 30 lbs, so it's pretty sizeable. Gunner and Jim may well be right when they suggest that the rotor itself will be plenty adequate as a flywheel.

By the way, the motor is rated at 2.5 hp at 940 rpm.

Best wishes,

Chris

In school I was taught that a motor generates a back emf because there is a rotating magnetic field whose flux cuts the windings in the motor. When a motor is running and you increase the load, the motor slows slightly and the back emf drops so the net voltage increases causing the current to increase. If you decrease the load the speed increases, the back emf rises, net voltage drops and current drops. If you decrease the load until it is negative, the back emf increases until it is above the input emf, and the current goes negative. That is current flows from the motor to the mains.

Now this works for both single phase and three phase motors from locked rotor to being an induction generator. ( Don't try locked rotor for very long unless you have an AC servo motor ) It even works for a three phase motor running on just one phase. In that case the motor consumes single phase power, but still generates back emf in all windings. Which results in generating three phase power. Because the back emf is less than the mains voltage, the voltage is not balanced. But this can be improved by either adding capacitors.

So now you can at least see how I analyse RPC's. It isn't the only way, but it works for me and maybe Don Young and Pentagrid. ( Speak up if you disagree ).

As an aside issue, an induction generator will work without being connected to the mains. You can google and find some references to this as regards using an induction motor driven by a small gas engine for operating ham field stations. It is load sensitve.

And as another aside, you can build a fine RPC using a single phase motor connected to a three phase motor via a adjustable belt drive. Two terminals of the three phase motor are also connected to the mains. You adjust the belt drive so the current drawn by the single phase motor is at or below name plate rated current while the RPC is supplying three phase power to whatever needs three phase power. I happen to think this approach is good for things as surface grinders where good three phase power is needed.

Dan

According to Don Foreman :

[ ... ]

I've been wondering why nobody has considered the motors as black-box devices, with only the three terminals leading to the outside available for consideration.

Under those conditions, there is really no way to tell a Wye from a Delta connected motor, if you allow that the windings could be special to make them work from the same voltages.

And -- you could analyze the circuits from that basis (it is commonly done when sufficient information about the internals is not available, or when the details about the internals don't really *matter* to the functionality.

Enjoy, DoN.

I hope this message actually gets posted. My last two attempts failed. I am hoping the problem has something to do with cookies and posting through Google.

When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ).

This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor.

Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated.

This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree.

A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive.

Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power.

Dan

Robert Sw>

I hope this message actually gets posted. My last two attempts failed. I am hoping the problem has something to do with cookies and posting through Google.

When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ).

This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor.

Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated.

This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree.

A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive.

Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power.

Dan

Robert Sw>

I hope this message actually gets posted. My last two attempts failed. I am hoping the problem has something to do with cookies and posting through Google.

When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ).

This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor.

Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated.

This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree.

A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive.

Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power.

Dan

Robert Sw>

I hope this message actually gets posted. My last two attempts failed. I am hoping the problem has something to do with cookies and posting through Google.

When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ).

This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor.

Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated.

This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree.

A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive.

Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power.

Dan

Robert Sw>

I hope this message actually gets posted. My last two attempts failed. I am hoping the problem has something to do with cookies and posting through Google.

When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ).

This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor.

Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated.

This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree.

A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive.

Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power.

Dan

Robert Sw>

I hope this message actually gets posted. My last two attempts failed. I am hoping the problem has something to do with cookies and posting through Google.

When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ).

This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor.

Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated.

This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree.

A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive.

Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power.

Dan

Robert Sw>

Basically everyone one does. The fun starts when you hook the three black box wires up to something else!

Any efforts to explain rotary converters that invoke currents flowing from the motor's 'neutral' are doomed to failure I would say.

Jim

Coming through fine here Dan.

Jim

Wow, Dan! Nine posts. I am flattered (Really !) you went to such trouble to share your opinion with us. I am very sorry you apparently were responding to my first reply to you, which was in error. Maybe your server didn't deliver my second reply, the one in which I attempted to correct the error in the first. I am sorry to have thrown such confusion into this already overlong thread. Anyway FWIW, my 2nd reply in an attempt to correct the errorious one went like this:

{"Sorry, Dan - Make that overdrive via overspeed from the prime mover to make an induction generator. The induction generator (one made from a common induction motor) will generate when excited by the mains and when its rotor is driven by external means to a speed exceeding that of the motor's synchronous speed. Slip is said to be negative under these conditions."}

Please note, the operative change made here was to insert "rotor driven by external means to a speed exceeding that of the motor's synchronous speed" in place of "overdrive from the AC mains". Again, I am very sorry for that mistake and for you to have gone to the trouble of posting 9 times in order to straighten me out on the matter.

As someone once said, "I'm afraid I've already told you more than I know". Please understand, the following quote from McGraw-Hill EE Handbook, 10 Edition, Sec. 18-116 is the source of all my knowledge re. induction generators; it goes:

"""Induction Generators. Any induction motor, if driven above its synchronous speed when connected to an a-c source, will deliver power to the external circuit. This generator action is easily visualized from the motor-circle diagram. (Fig. 18-17)corresponding to the lower half of the circle in which the current vector is directed below the OV line. [pls. adivse and I can scan the figure to your direct e-mail addy, if you like] A unique feature is that the power factor of the output is fixed in value by the generator characteristics and is always leading, independent of the external circuit. The explanation is that the generator draws all its excitation from the system and so must receive a definite amount of lagging kilovoltamperes for a given voltage and load current. For this reason, induction generators alone cannot supply a power system but must always operate in parallel with synchronous machines or with capacitors. They are therefore, no more helpful in system stability than the addition of parallel reactors with a rating equal to the generator magnetizing reactance.

An induction generator delivers an instantaneous 3-phase short-circuit current equal to the terminal voltage divided by its standstill reactance, but its rate of decay is much faster than that of a synchronous generator of the same rating, and its sustained short-circuit current is zero.

S "To further confuse you, you can build a very nice RPC by using a single phase motor to drive a three phase motor via a belt drive adjusted so mechanical power is going into the three phase motor. Now if you apply single phase power to the three phase motor, it will act as a three phase induction generator. If you do this use an adjustable pulley on one of the motors and measure the current drawn by the single phase motor. Adjust the pulleys so the current drawn by the single phase motor is close to but below rated nameplate current when the RPC is driving whatever load you are going to drive. As you might suspect such a RPC produces voltages that are very closely balanced."

It seems to me you may be describing an idler driven by a pony motor, similar to systems in which the pony is disconnected after the idler has come up to speed. Could this be the case you're describing, except that the pony motor is not disconnected after the starting interval?

Bob (easily confused) Swinney

<

Yup - All six of 'em, and the other three you thought didn't, too... Ain't Technology wonderful? ;-P

"There is nothing that can go wrong can go wrong ..."

-->--

I am very sorry for the multiple posts. I kept getting messages that said " Server Error try again in 30 minutes " or it would seem to accept the message but would not show up when I would read the thread. Apparently the messages were buffered somewhere and all of them eventually got posted.

Bob, the first reply was before your post that clarified what you meant to say.

I am more or less describing exactly what you said above. Except when you use a pony motor you may have the pony motor only bring the three phase motor to say 80 % of its rated speed. If you apply power to the three phase motor while leaving power on the pony motor, the pony motor will be driven faster than is synchronous speed until you disconnect it. I think this is how Jim Rozens RPC is done, where his sytem throws the belt when you apply power to the idler.

You might have the pony motor directly connected to the three phase motor. In that case there would be little or no mechanical power transferred between the two motor when power is applied to both.

So I am describing a subset of an idler driven by a pony motor, where the drive is adjusted so that the pony motor turns the idler at a speed which is faster than the idler would run by itself. The pony motor may or may not be large enough to drive the idler above its synchronous speed but mechanical power is still going to the idler ( actually not an idler anymore, but a driven motor ).

Again I apologize for the multiple posts. None of my posts showed up here until today.

Dan

Dan's comments there are about deliberatly driving the idler motor at above slip speed. It will behave like an alternator at that point and deliver the mechanical power that is entering the shaft to the electrical load.

Jim

You have a pony motor driven RPC, connected as in other RPC's described in this thread, where the single-phase pony and 2 terminals of the 3-phase idler and 2 terminals of the 3-phase load are all connected across a single-phase source. The pony, an induction motor, is belted such that it mechanically drives the idler ( 3-phase motor) at a speed greater than synchronous speed. If I "see" it correctly you'd have a RPC (includes load) which runs at a speed somewhere beyond normal synchronous speed of the

3-phase load motor.

Jim's comment, "It will behave like an alternator at that point

Well! Knock me over with a rubber dick, err, make that rubber duck. Seems to me that ought to work. Id'd bet it is kind of tricky to get the right belt, or gear, ratio between the pony and idler. I suppose, though, it would just refuse to "generate" if everything was not properly sized. BTW - how would you know if it was functioning as an induction generator ? If the load motor was running at greater than its normal synchronous speed - it'd be generating, right??

Bob (learns something new every day) Swinney

PS: Hey, why not use a 3-phase pony to get things started and then, and then, then . . .connect the 3 terminals of the load back to the 3 terminals of the pony. Vee-I-ola! Perpetual motion, Naw!

According to snipped-for-privacy@krl.org :

It has something to do with posting through Google, at least.

Google is sometimes *very* slow at getting articles posted through its interface back onto its own display of the newsgroups. The

*do* go out to other news servers, however.

I think that this is number six of your tries with the same message. (And I think that there are two or three more following this.)

Just be patient. Or sign up with a *real* news server.

Good Luck, DoN.

That's why dan suggested a varispeed pulley setup in his post.

Yes, if it weren't line excited. But it is.

Generating on the third leg. I think the idea would be to make a rotary converter that somehow would have better balance or lower impedance by deliberatly driving the rotor above synchronous speed.

Jim