3450 vs 1760 RPM for rotary phase converter

Other than being somewhat harder to start than when using a 1760 RPM idler motor what are the issues with using a 3450 RPM as an idler?

This question may have been asked before but I can not find a clear answer in the forums. I have a new Baldor 3450 RPM motor that I would like to use. The only other concern is that my BP runs a 1730 RPM motor.

Thanks

Mark

My idler is 1740 RPMs but I talked to several people who said it doesn't matter. This website has some info on it www.RotaryPhaseConverters.com but I called the guy when I set up my converter and he was more than helpful at answering all my questions without even wanting to try and sell me something. I believe his name was Jim. I would give him a call, seemed to really know his stuff. Hope this helps

My 3450 was considerably easier to start than the 1750 it replaced, but that was just my experience.

John Oder

John,

What did you have to change to use the 3450 RPM motor?

Mark

What did you have to change to use the 3450 RPM motor?

Click to expand...

Nothing - it was an idler for home brew "RPC".

It had a few capacitors across two legs (7 1/2 HP and had 105 micro farads of run caps). The 3450 would start on these by rolling the shaft with your foot. The 1750 took much more to get it going.

Since the 3450 still had its fan, it was way too noisy - I replaced the whole shebang with a 10HP Phase Perfect - the best money I ever spent.

John Oder

"new Baldor 3450 RPM motor" exact description of mine. Starts easily with a 1/4HP pony motor.

Post #11 here; http://www.practicalmachinist.com/vb/showthread.php?t=170299


Bob

Noise.

There's no difference between using a high or low-speed motor... in reality, even the magnetism comes out identical, because the low-speed motor has twice-as-many poles.

The high-speed motor, when fitted with a heavy sheave or flywheel, might provide a little more 'stability' due to it's increased polar inertia, but you'd be hard-pressed to actually notice it.

The high-speed motor WILL be somewhat noisier, on account of the motor's cooling fan roaring away... and depending on the motor, some may exhibit a higher start-surge than others, but not necessarily a function of the motor's speed, as it is the design and your choice of start capacitors.

You can directly interchange one idler for another, provided you re-check the voltage balance and start capacitance.

Idler flywheel

DaveKamp said:

.......The high-speed motor, when fitted with a heavy sheave or flywheel, might provide a little more 'stability' due to it's increased polar inertia, but you'd be hard-pressed to actually notice it........

Click to expand...

Dave's note above reminded me of a question that has rattled around in my head regarding the free spinning idler with no physical load and the "flywheel effect", which has been mentioned before.

While the idler is often said to "generate" (and I agree with that term) the missing third leg of 3Ph, in actuality, the idler does no generation of total feed, it simply redistributes the available amperage from two legs into three legs with some parasitic loss.

With the frequency of 60HZ (USA) being the speed controller and the total amps drawn having no effect on the frequency, how could a flywheel benefit the system, whose frequency and therefore RPM is governed by the utility?

Obviously if the amperage drawn should constitute an overload, then the small amount of amperage required to maintain the idler could be reduced enough to cause a reduction in idler RPM but the driven machine slowing from overload would be the controlling factor, not the idler and adding a heavy flywheel to the idler could not remedy the overload, (remember, it's not a generator) it's only effect would be to slow the recovery to synchronized 60HZ, right?

Bob

FWIW, I see on Phase-a-matic's website, specifically their "Method No. 2" for using an idler motor with the static converter that they recommend a 3450 rpm motor to be used as the idler. I also have one of their 3 hp rotary units that, incidentally, comes equipped with a 3hp Baldor "1725" rpm motor.

The bearing life is twice as long on a 1725 rpm motor, all else being equal. Not an issue for most users unless the RPC runs 24/7/365. Most better motors are rated for 17500 hours belted, or 100k hours coupled, which may be two or three lifetimes in a small shop. Especially since the RPC motor is neither belted nor coupled

To R. Cambell's comments regarding a idler motor NOT acting as a generator.

It is true what you say regarding the idler motor "redistributing" the power from two incoming lines to three, but that does not mean that the motor is not acting as a generator.

Induction motors act as generators all the while they are turning. If this were not the case, Running current would be the same as locked rotor requirements. Counter EMF is (generator action) the mechanism.

Think of an idler motor as a packaged motor/AC generator. Two windings drawing power and providing the motion to get something from the third leg. In fact, if the RPC wiring is looked at carefully, the incoming lines pass directly to the load, While the "generated" third leg comes only from the idler (oh! that makes a phased AC generator!)


A flywheel on an idler motor helps to limit droop of the third leg due to voltage drop during starting the load motor.

Great stuff this!

CalG

Err, not so fast. There is considerable controversy
about this exact issue.

There's a school of thought that says, a rotary converter
delivers power to the manufactured leg by inducing slip,
and that putting a large flywheel on the rotor makes this
harder to do.

There was at least one well-respected converter guru
(not fitch) on RCM who claimed this, and it was a very
pluasible argument.

To the best of my knowledge *nobody* that I ever
interact with has done a real experiment to check
the ability of a rotary converter to provide transient
start currents, as a function of flywheel mass on the
converter rotor.

Maybe somebody like peter H. could shed light on this.

Jim

Jim

Help me understand what is transient start current?

My RPC puts out real voltage to the generated leg and makes real current available whenever it is running (Voltage drives current after all). I have never measured output to this leg under locked rotor conditions.

When the load motor is powered up, the spinner slows down a bit with the voltage droop. The flywheel effect of the spinner gives some of that momentum energy back to the generated leg.

"A lot of life can be reduced to momentum transfer"

CalG

"Maybe somebody like peter H. could shed light on this"

A LOT of work has been done on transient models for synchronous three-phase machines.

A lot of work has been done on induction machines, too.

However, little work has been done on modeling machines of the RPC type, where induction within the two real phases is transferred to the one manufactured phase across the squirrel cage rotor.

I can state with reasonable certainty that "slip" is an important factor, and, within reason, the more slip, the more power which can be transferred, and that with zero slip, essentially zero power is being transferred.

However, slip can be a bad thing, particularly if it exceeds 90 electrical degrees, a violation of the so-called "equal area criteria", whereupon the system becomes unstable, perhaps for a cycle or two, or possibly for an extended period.

Ideally, one wants the RPC system to be reasonably stiff, yet flexible enough to respond to significant changes in load, without violating the stability criteria.

Having no load at all on the rotor, save windage losses, presents one extreme.

Having a substantially immovable load on the rotor, say, as a consequence of a very large flywheel, presents another extreme.

Pole-slipping cannot occur in the first case, but is almost guaranteed to occur in the second case.

So, as is usually the case of these empirically designed systems, one performs some experiments, and then settles on that case which appears to work best.

My intuition says that adding a reasonably sized flywheel can improve transient response in some cases, but not in all cases, and for each case where such a reasonably sized flywheel results in an improvement, a more significant improvement is possible by eliminating the flywheel altogether and increasing the size of the idler.

Ah...

"My intuition says that adding a reasonably sized flywheel can improve transient response in some cases, but not in all cases, and for each case where such a reasonably sized flywheel results in an improvement, a more significant improvement is possible by eliminating the flywheel altogether and increasing the size of the idler."

Ah... but of course! And I can see this being for more than one reason:

Larger idler equates to:
a) More intertia
b) More inductance
c) Lower resistance

(and probably a few other things I haven't considered)

"Larger idler equates to: a) More intertia b) More inductance c) Lower resistance"

The most significant effect is the lower impedance, which is certainly a function of resistance and inductance, but primarily inductance.

Think of the manufactured phase as being an ideal voltage source behind a significant impedance, the equivalent series impedance of that phase.

Then, recall that the equivalent series impedance of the other two phases is very low, and may be assumed to be almost zero.

The composite system therefore has A and C phases with very low (almost zero) equivalent series impedances, and a B phase with a comparatively high (and definitely non-zero) equivalent series impedance.

Which explains why the manufactured phase may be a good source of power at one load point, only, and also why the manufactured phase is a very poor sink for regenerated power.

Regarding "slip" in induction motors

To Simplify the image in my mind:

If the rotor turned synchronous with line frequency, the reverse EMF (generatd power) would equal the applied EMF (line power), and no current would flow. No current, No work! No work, and the rotor will slow down, but always would try to catch up ;-)

Not to the point of rotor inertia really, But just thinking out loud.

"Everything important came to me when I was very young"

CalG

"If the rotor turned synchronous with line frequency, the reverse EMF (generatd power) would equal the applied EMF (line power), and no current would flow. No current, No work! No work ..."

The current can never be zero as the machine's windage losses have to be provided for.

However, the current is indeed at a minimum whenever the rotor is running at synchronous speed, which is significant for so-called "pony" starters.

Basically, you accelerate the idler to synchronous speed, then you place the idler across the line, and finally the idler slows down to its normal sub-synchronous speed.

This is how one achieves minimum starting current, and may be the only way to implement a very large idler in a residential or light industrial (or rural) premises.

The other starters require more than 7 times the idling current.

Here's a proposed "rule of thumb":

All other things being equal, if a small flywheel improves the transient response of an RPC somewhat, then the best, but not the least-cost solution is the next larger-sized idler and no flywheel at all.

The transient response of the idler alone will be better with the least possible mass, which generally means no flywheel.

One cannot eliminate the "flywheel effect" of the idler itself, due almost entirely to the mass of the squirrel cage rotor and its related structures.

convertor hyjinks

About 15 years ago I was building phase convertors up to 30 hp and 600 volt. A few thoughts.
3600 rpm is the most desireable for the idler motor. This is a two pole 3 phase motor. It is the purest supplier of manufactured leg. The reason: multiply 60 cycles per second current frequency times 60 seconds per minute and you get 3600. In actual practice rotor slip gives no load shaft rpm of about 3450 rpm.
Most rotary phase builders use lincoln 3 phase motors. These motors often have no shaft visible. Lincoln by far has been the least efficient industrial motor. Factors including large air gap between rotor and windings, winding precision, wire quality, bearing quality, housing precision all come into play. When the US Dep of Energy rated motors, some Lincoln series were as low as 79%. The best-Toshiba world energy series and GE high efficiency series. Around 96%. For a no load low hp home shop you will not be hit too hard with higher utility bill for less efficiency. For a 1000+ hp instillation real money is involved. Suprisingly, high hp 3 phase motors are more efficient than low hp.
Capacitors. These are the heart of a convertor. If you make your own convertor, never wire more than 4 starting or running capacitors in series. I built a 30 hp 480 volt system and exploded several capacitors because I had as many as 13 in series. A very bad thing to do. A 4 pound oil filled capacitor blowing is about 1/4 sitck of dynamite. now you know.
Electrolytic capacitors are placed in most commercial convertor starting circuits. They are good for about 15 seconds energizing. I never used these. I used only oil filled constant running caps. I had a system of switches that allowed caps to be taken off line after starting but incrementally be brought back to achieve the highest quality power factor.

read about KVAR and power factor. this will really help your understanding of converted power.

jh