Ping: Jerry Martes

Hi Jerry,

A while back you mentioned that you'd done some experiments to investigate how idler size affects the power delivering capacity of a rotary phase convertor. Did you write up the results? If so I'd be very interested to see them.

Some people say that you need an idler with twice the load power rating, some say equal, and I know of a commercial company who use idlers of half the load power rating. So I'd be interested to know what you discovered - it would shed some light on the mystery.

Best wishes,

Chris

Reply to
Christopher Tidy
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It depends on many things besides nameplate power of the load.

For example:

1) Whether it is a hard starting load (think about a compressor) or an easy starting load (think about a centrifugal blower)

2) Whether the load actually runs at nameplate power, or below (think about a lathe that's not used to the max of its capacity)

3) What amount of imbalance between legs would you tolerate. Severe duty motors, for example, may tolerate greater imbalances than CNC machines.

I have discovered that my 10 HP phase converter is not quite sufficient to run a 200A tig welder. I mean, it runs it, but the power in third leg sags considerably. I will soon add another idler (I am kind of busy lately)

i
Reply to
Ignoramus24489

Let's assume it's medium e.g., a hydraulic pump or something with a fair bit of inertia like a power hacksaw. I want to decide on a fair rating for my convertor.

In this case assume it runs at its nameplate power. As I said, I want to figure out a fair rating.

I'm guessing a voltage variation of 3% is good, 5% is okay and 10% is too much.

Did you measure the voltage drop?

I think I might just protect the motor with a thermal breaker set to about 20% above the rated motor current (4.8 A) so that it doesn't matter if I overload it.

Chris

Reply to
Christopher Tidy

I don't think Jerry is reading this group anymore, but I passed your inquiry on to him. Jerry did a lot of experiments with idlers and some pretty good instrumentation including an eddy-current brake.

Reply to
Don Foreman

Thanks very much, Don.

Chris

Reply to
Christopher Tidy

Hi Chris

Don foreman told me that you are possibly interested in the data Don and I collected with the eddy current dyno. No, I didnt write up the results. I couldnt find anyone interested in knowing the facts. I have an interesting set of curves showing how a 3 HP 3 phase motor is capable of producing power from single phase, with - and - without an idler. I'd be happy to send you the Excel plots and acquired data from the dyno.

It is my observation that the biggest idler provides the best 3 phase. That is *no* surprise. What does surprise me is; the 3 phase tool motor will deliver very nearly the same power with *no* idler as with a Big idler when loaded to no more than 2/3 the tool motor's name plate HP. And, a 3 phase motor will deliver its Full name plate rated HP when driven by single phase. There is certainly limits to how the 3 phase motor will deliver full power when fully loaded. But, the statement about a 3 phase motor being able to produce no more than 2/3 it's name plate HP, is not accurate.

E-mail me to get copies of the Excel data on the 3 HP 3 phase motor with and without the 5 HP idler.

Jerry

Reply to
Jerry Martes

Thanks very much, Jerry. I've just sent you an e-mail.

Best wishes,

Chris

Reply to
Christopher Tidy

Just a small postcript.

A pony started single phase driven 3 phase motor without any phase correction easily delivers its full rated HP at a few % lower than its rated speed.

For long term use, Brook Compton Parkinson motors data shows that rated full load current is reached in the single phase driven lines at roughly half full rated load. This is still below rated full load dissipation because the current in the indirectly energised winding is very low. For long term use roughly 2/3 rated power is a pretty fair guess.

Most of this power reduction can be recovered by a single pahse correction capacitor chosen for best phase balance near full load.

The Brook Compton Parkinson data is shown in the drop box as Single phase drive.jpg

Jim

Reply to
pentagrid

Jim

I'm think you are right on with the above comments about the 2/3 HP de-rating for 3 phase motors from single phase. 2/3 is a "good guess" My interest had been to learn why the power would *drop off* after loaded beyond 2/3 its HP rating. I was surprised to find that the 3 phase motors would actually produce full HP when fed single phase. My mind wants to guess that the actual "max HP" from a 3 phase motor on single phase will be somewhat higher than 2/3 because of the conduction of heat being related to the fans and mass of the "full HP" package. I have no expectation that I'll ever use a 3 phase motor fed single phase. I was just curious.

Jerry

Reply to
Jerry Martes

I have a 10 inch baldor pedistal grinder. It is 3ph and I have a static phase converted hooked to it. It starts and runs but I have never been happy with the amount of vibration. I am now wondering if it will run smoother with a VFD. Is it worth hooking it up to find out? or am I wasting my time.

chuck

Reply to
Chuck Sherwood

Chuch

Motor theory (and my dyno experiments) does show that the tool motor torque goes to zero while it is rotating and fed single phase. That results in some "vibration" at heavy loads. Does your grinder vibrate only when heavily loaded?

Jerry

Reply to
Jerry Martes

No, it vibrates all the time.

Reply to
Chuck Sherwood

Vibration could be either or both, wheel unbalance or single phase torque variation. Unbalance vibration is predominantly at rotation frequency. Torque variation is at twice supply frequency. Unfortunately the ear is not good at distinguishing between the two types of vibration and the only sure fire check is to monitor the vibration with an oscilloscope.

A test run with a VFD would eliminate single phase torque variation as a cause. If a VFD test shows that torque variation is the problem you may be able to eliminate it by tweaking the "static phase converter" Inverted commas are used because some so-called converters are simply single phase starters. They do not contain "run" capacitors which means that the motor is running with 100% unbalanced drive. If this is the case the addition of about 30uFd per horsepower of run capacitor will improve the phase balance enough to eliminate the torque variation problem.

Jim

Reply to
pentagrid

See if the vibration decreases immediately after you switch off the power; if there's no significant change in vibration, try running with the wheels removed.

I can switch my grinder and a few of the other tools in the shop between a Phase-o-matic static converter and a rotary. There's no discernible difference in the grinder's vibration switching from one to the other.

Ned Simmons

Reply to
Ned Simmons

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