VFD as 3Ph shop supply

If you plot motor torque versus speed you get an "S" curve which passes through zero at synchronous speed.

At all speeds below synchronism, torque is positive. The motor draws sufficient current from the supply to provide the mechanical output. In the case of an idler this is sufficient torque to overcome windage and friction.

The machine CANNOT return power to the supply unless it is externally driven to the negative torque region above synchronous speed.

These comments apply to a 3 phase motor driven from a true

3 phase supply. This differs from the case of a 3 phase motor connected to a single phase driven idler acting as a rotary converter.

In this case, although the idler is mechanically running below synchronous speed, the single phase excitation induces rotor currents at slip speed which cause the magnetic field rotation to occur at line frequency. It is THIS magnetic field rotation that generates the output at the phantom phase (at the other terminals it is of course the back EMF). The load that is delivered by the phantom phase is provided by a corresponding increase in single phase input power - it is still acting as a motor - there is no net power generation.

Jim

Not *at* synchronous speed, above synchronous speed, which is determined by the frequency of the supply, not by other machines connected to the system.

Below synchronous speed the machine is changing electrical energy to mechanical. Above synchronous speed it's mechanical to electrical.

Yes. The motor has to be slipping to develop any torque.

Yes.

Umm. The situation is that there is a motor that is turning slower than the idler. Why cannot the slower motor draw power from the faster motor, regardless of the VFD?

I'm not so sure about that, as the idler stores energy in either case, simply because the idler is spinning.

True for sure, but is it relevant? The situation under consideration is an idler driven by a three-phase power from a VFD. The question is if motors abruptly connected can draw power from the stored energy of one or more large spinning idler motors. One reason to believe that it can is the observation that it matters what order one starts motors with a RPC. The game at each step is to ensure that what is currently spinning is bigger than what one is now trying to start, the unspoken objective being to ensure that the new motor isn't able to stall the current collection by demanding too much too quickly.

Joe Gwinn

SNIP

The mechanical energy stored in the idler rotor rotation is largely useless. Useful energy can only be extracted from it if the rotor speed falls below the supply source/VFD defined synchronous speed

The reason for sequential rotary converter startups is reduction of phantom phase voltage drop due to the source impedance of the phantom phase winding. This is mainly determined by the series resistance and leakage inductance of that winding. The bigger the machine the lower the source impedance.

In a rotary converter multiple system all machines are directly connected in parallel. All machines that are spun up to speed contribute - its the total spinning HP that counts.

Jim

This part I'm not following. Generators can motor, and motors can generate, although they are usually best at one or the other role. Let's simplify the picture a bit:

Consider a number of 3-phase induction motors of similar size connected together, in parallel across a common power bus, all having been spun up while that bus was connected to commercial 3-phase power.

The motors are not mechanically connected in any way.

Disconnect the commercial power, so now the motors are all coasting, but still electrically connected to one another.

One of the motors drives an adjustable mechanical brake/dynamometer, which is then applied. How much energy is dissipated in the dynamometer? Is it only the mechanical energy of the one motor, the energy of all connected motors, or some other intermediate value? How do we know?

Said another way, can we stop one motor without affecting the connected motors?

These two paragraphs seem to contradict one another. The first implies that only the RPC motor matters, and does this by being a better transformer between the single-phase power coming in and the three-phase power going out, so the effect is that startup surges are supplied entirely by a corresponding instantaneous surge in demand on the single-phase input.

This no doubt happens, but if it's the whole story, why does total spinning HP matter, versus the HP of the RPC motor alone?

Joe Gwinn

Doesn't the back EMF of the idler motor contribute power to the motor being started?

Almost zero - as soon as the supply is removed the magnetic field which it induces into the rotors collapses. Without this magnetic field, although the rotors are still rotating, the motors can no longer generate back EMF.

Yes

The first paragraph refers to the basic case of a single idler motor starting a single load motor.

The second paragraph considers what happens if if there are multiple motors in circuit. Each of the motors that are spun up to speed are capable of supplying power to the phantom phase. Because of this the parallell connection results in a correspondingly lower phantom phase source impedance.

The customary distinction between an RPCidler motor and a load motor is entirely artificial. The setup is simply two or more motors all connected in parallel. Any of the motors that are spun up to speed act as RPCs and can supply power to the phantom phase. Equally, it is perfectly acceptable to place a mechanical load on a motor nominaly designated as an idler

Jim

Note for information. On my setup (see the post it's detailed in) the VFD output frequency drops momentarily when I switch on the 2hp grinder wheel motor and the idler motor tries to levitate across the shop.

This is an artifact of the control logic in the VFD I'm using, but it does work...

Mark Rand RTFM

This is the key. Now, you don't mean almost zero, but I know what you meant: the dynamometer will dissipate the mechanical energy of the motor mechanically connected to it, but not the mechanical energy of the other motors.

I recall discussions of people using induction motors as generators. Once started, they would generate their own field and keep on going. If one opened the circuit, the field would collapse, and peobably would not regenerate.

This needs to be tested experimentally. I would try this, but don't have enough big three-phase motors to do it, even though it is an easy experiment.

As discussed above.

This experiment is the key to the rest of the discussion as well.

And all motors are doing an instantaneous phase transformation, but not storing much mechanical energy?

Given that the single-phase supply is connected to one but not the other, one would think that there has to be a distinction.

Although I understand in general how RPCs work, I've never read a real theoretical analysis of this. One thing that nags me is that there has to be energy buffering by the RPC, or there would be no way to get from pulsating power (singlephase) to continuous power (polyphase). And the rotation of the rotor is essential, as no static transformer or collection of transformers can do this, while there are transformer configurations to convert between N-phase and M-phase, where M and N are all at least two. Something is smoothing those pulsations out, filling the gaps in. Can you suggest some learned articles for me to read?

Thanks,

Joe

How do you connect the single phase to one and not the other?

Dan

I think of a RPC as a three phase motor running on single phase and connected to a three phase induction generator. Where the three phase motor and the induction generator are one and the same motor. Try thinking of it first as a single phase motor mechanically coupled to a three phase motor. Once the single phase motor is running, there is no capacitors in the system.

With a sine wave single phase power in, there are not what I would consider " pulsations".

Dan

Joseph Gwinn sez:

". . . .Can you suggest some learned articles for me to read?"

You've already seen the best description of RPCs in Jim's comments. Go back and read them again. The key here is Jim's reference to the currents flowing in parallel. Try to get past preconceived notions re. "generator and load". A RPC is a very complex network consisting of currents flowing through 2 or more 3-phase motors connected in parallel with each other and with

1 set of those legs in parallel with the input mains.

Bob Swinney

Very interesting. On pretty well all VFDs the output frequency is uniquely defined by low level electronic circuitry that should not be affected by output load. I don't know of a parallel to your case.

It looks as if your VFD frequency is dropping to the extent that the idler is instantaneously operating above above the new synchronous speed. Quite a small change would make a drastic difference - about 3 Hz would be sufficient to extract the full nameplate rating of the idler!.

Jim

SNIP

All motors can phase transform. They also store a significant amount of kinetic energy but this is not available as a useful electrical output

ALL motors are parallel connected so every motor receives single phase excitation.

The torque pulsations of the single phase drive are smoothed out by the rotor inertia.

Nothing really appropriate comes to mind but you might find my own book "Electric Motors" ISBN 13 978 185486 246 4 reasonably helpful..

Jim

I've set the "motor power" parameters on the VFD to a value that will not take out the breaker if the VFD tries too hard (4kW in this case). It seems to see the DOL starting of the grinder motor as an overload and drops frequency to shed load. The speed of the whole system then ramps up at the rate that I set for VFD startup.

This VFD is obsolete, but seems to be a fairly serious piece of kit in terms of configuration options and capability. Definitely worth the $15

Only another 4-5 years before I can afford 3 phase from the utility...

Mark Rand RTFM

So, where is the energy buffered then?

Ahh. Right.

I suppose we could posit a 3-phase 220/440 motor, with single-phase power supplied to the 220-volt inputs (center taps), but with the

440-volt terminals of the various motors connected together.

So the energy is stored in the rotor's mechanical energy. That's the fundamental meaning of "inertia" in this context.

The electrical parallel of inertia is inductance, which is the ratio of energy stored in the magnetic field to the current causing the field. The higher the energy, the higher the inductance.

I'll get a copy. To be continued.

But I'm still interested in learned articles on how RPC work, at the physics level.

Joe Gwinn

I think you are overthinking this. Think of a single phase motor. How are the torque pulsations smoothed out. Now think of a three phase motor running on single phase. Pretty much the same thing.

Dan

Not a good parallel. The fundamental problem is that in single-phase (and assuming a resistive load), no power is transferred whenever the current passes through zero. In a single-phase motor, the rotational energy (and momentum) of the rotor largely fills in these gaps, although the torque under load does ripple a bit.

In poly-phase, while each phase passes through zero periodically, it is never the case that they pass through zero at the same time, and things are arranged so the energy is delivered at a constant rate.

This is the reason that some lathes produce a better finish when powered by a 3 phase versus a single phase motor.

In math:

Power is proportional to the square of voltage or current, so:

For single phase, we get (Sin[t])^2= 0.5 (1-Cos[2t]), so the power comes in pulses at twice the power frequency.

For 2-phase, the simplest polyphase case, we instead get (Sin[t])^2 + (Sin[t+Pi/2])^2= (Sin[t])^2 + (Cos[t])^2= 1, so power delivery is constant, regardless of power frequency.

For N-phase, one gets the same answer, 1, so long as the phases are uniformly spaced around the phase circle, regardless of the value of N or of the drive frequency.

This is why there is no configuration of static (copper wound on steel, nothing moving) transformers that can convert from single phase to polyphase, while there are configurations to convert between different kinds of polyphase power.

So, the fundamental question in RPC theory is how pulsating power becomes steady power. There has to be some mechanism that is storing and releasing energy in exact synchronism, to ~exactly fill in the gaps.

Joe Gwinn

A wild guess. If your 4KW setting is close to the point at which your supply breaker drops out it's possible that there's a large transient supply voltage drop - maybe enough to affect the VFD frequency generation.

Jim