Rotary Phase converters

My first thought was that a flywheel should help with sudden loads on the RPC. But when people said they ran tests and found it had no effect, I revised my thinking. Without a flywheel the idler motor can slow down quickly. When it slows the back emf against the single phase input power drops , and current goes up. So I would expect that a flywheel would lessen the current spikes on the input power, but the generated three phase would be about the same, with or without a flywheel.

Dan

Hi Brian, It's good to hear from a practical situation. The fact that your motor took much longer to get up to speed shows the flywheel had a definite effect. The question is whether the load motor (lathe etc) started bettter with it? It could be that your 3 phase generator and mains supply work so well without the flywheel that it isn't needed. I suppose the only way we will ever really know is to monitor the mains when starting a large, marginally supplied motor. I also take your point that a spinning flywheel isn't the best feature to have in the shop when you don't need it. I haven't read Jerry Martes tests. Do you have any details please?

Thanks

John

jerry has conducted careful tests. I have conducted some "careless" tests. I have an Abene mill that has a 6 hp spindle motor and a 2 hp traverse motor that moves x,y,z axis. The rapids on this machine are actuated by "instantly" reversing the 2 hp motor. I have only single phase power. To make the mill start, I've added a capacitor and a relay (they fit into the junction box at the rear of the mill) - the capacitor is in the circuit only when the voltage on the missing leg is below about 60V (with respect to neutral), then the relay pulls in and takes it out of the circuit. This is about what a phase-o-matic type static inverter does, except that I took advantage of the neutral to simplify the circuit a bit. Now, some "experiment" results - 6 hp motor starts up fine by itself. this is as you woudl expect. 2 HP motor starts and reverses "instantly" with or without 6 hp motor running. Conclusion - no need for RPC, static inverter is adequate. And, a capacitor and relay, even installed in every machine is smaller, cheaper, and more efficient than a continuously running RPC motor. Now, a VFD is another thing entirely.....

The only way a flywheel can export power to the starting motor is by slowing down (my computer room at work has an online generator with super synchronous flywheel generator that uses just this principle to provide power until the

180 kW diesel starts in the event of a power outage). The speed of the idler motor is controlled entirely by the slip frequency needed to generate enough torque to overcome its losses (maybe 5 rpm at most) and the line frequency.

You aren't going to change the grid frequency even if you direct online start the 39,000hp motors that we (GEC UK) supplied to a US oxygen plant a few decades ago! That's because every single generator on the grid and every synchronous and induction motor coupled to it is stacking their inertia up against your little (or even not-so-little) motor!

Although the invertor _is_ an induction generator, its output frequency is controlled by the excitation frequency, ie. grid frequency.

Oddly enough, if the rotor of the idler motor were very lightly built, adding mass to it would have an effect because there will be a 120Hz torsional vibration caused by the fact that you are generating constant output power with three balanced phases, but only getting input power from a single phase that is producing power in a 60Hz sine wave. this means that there _is_ a flywheel effect, but the load swings from motoring to generating and back twice a cycle. The mass of the typical induction motor is more than enough to cope with this and an additional, external, flywheel would have to be tightly coupled to be able to respond at this frequency.

I spent 5 years of my life studying this at university before wasting it all by getting involved in computers :-( (I needed a program writing and no one was willing to turn my math into BASIC or FORTRAN, I did it myself and was soon doing that for other people instead of the honest electrical engineering that I'd been trained for).

Hope I haven't confused matters further.

Regards Mark Rand RTFM

Hey Mark,

You've stirred some ancient brain cells here. The motor I mentioned using to create an RPC was part of an elevator system used in the

1950's thru 1970's called "brake servo". It used a single speed AC wound rotor motor driving a worm-gear for elevator car speeds up to about 150 FPM. The extra weight of the oversized brake-coupling (this one was about 40 pounds) and heavy brake pulley were used to "flatten" the deceleration curve. Running at full RPM, the advance floor stop signal caused the brakes to be applied fully and a certain amount of fixed resistance to be introduced to the rotor circuit in steps/stages to control current flow. This allows the motor to slow, and the slip frequency goes very high of course. The slip freq is filtered at each step to produce a voltage used to LIFT the brakes if the slip freq is going TOO high (meant the car is moving too slow at a given point), which in turn allowed less deceleration. All this happens in the course of about 24" of travel. Worked pretty neat. It was a feature that allowed cars of this speed range to have an operational curve similar to and rival MG sets and DC drive motors, but at much reduced cost, and in fact better performance than with a more expensive 2 speed AC motor.

We also had a type that placed copper bars in the brake drum periphery, and along with the resistance insertions used thyratrons to fire into the brake drum effectively creating a second motor in opposition, and a lot more sophisticated in every regard to the brake-servo. These were used up to about 300 FPM.

In each of these two cases, one of the problems was always get things EXACTLY right to stop any resonance problems,. And in these cases, that was just a faint to loud audible noise vibration.

But when the industry went to Variable Voltage Variable Frequency AC (VVVFAC) motors to do away entirely with the need for MG sets, there were al lot of things to be learned empirically about harmonics. I've been in the motor test area of the our factory during the testing of one brand of motor over another, and it would scare the crap right out of you if a harmonic started. It would shake the whole building!! Everyone was in a separate room, watching on video!! I thank God I retired before we started to lose any of these motors in service in the field, and have to get them re-wound locally. I bet there will be some scary tales of woe coming then!!

Thanks for joggin' the old noggin.

Take care.

Brian Laws>>