Phase converter puzzle!

The inductance of the winding is different. The motor works. Different caps than those used would work.

That is why the caps are approx and to adjust...

Remember, "we" are finding a way to get around the normal ways. Different motors don't have to have the same specs, just have the power and voltage ratings listed on the plate.

Martin

Larger, older-style motors operate at a lower flux density and need more capacitance per horsepower than modern TEFC motors. But you say the motors are similar in design?

From my experience an unloaded motor will spin up with a wide range of capacitor values. I would expect a 7.5 hp motor to spin up with a single

92 uF capacitor.

In what arrangement are you connecting the capacitors? If you're connecting them in parallel, I reckon you most likely have two bad capacitors or a motor with a dodgy connection somewhere.

Best wishes,

Chris

What is the RPM of the idler which you are using, and the motor which you are *trying* to use? You may need a different capacitance for a different RPM motor.

Or -- you just may need more of them.

Enjoy, DoN.

Do you mean different, as in having different capacitance?

i

They look like twins, one is smaller than another but that's it. Same company, same look, and possibly same design.

That's what I would expect as well.

That makes perfect sense to me. I did check connections, though. Likely the caps are bad.

i

both my current 10 hp idler and 7.5 hp idler are 1750 or so rpm.

could be, but why can I get with 2 caps for my 10 HP RPC?

i

i

You might view this as a good time to experiment with a different method of spinning an idler. A "new in the box" electrolytic capacitor will cost about $15. That'll buy you a 250 mic capacitor that'll spin-up a good 7 1/2 HP idler in less than a second.

Jerry

The minimum single phase starting capacitor value can vary a lot between OK motors of similar HP but different design. This is a parameter of pretty well zero significance in three phase motor design so there's no incentive to control or to reduce it.

Older motors using poorer performance iron tend to need rather more capacitance. Another factor is the initial bearing friction and, to a lesser extent, armature inertia. The heat input to the motor is disproportionately large at very low speeds

- just an extra second at near stall can make a big difference.

There's probably nothing wrong with your 7.5 HP setup but if you're worried about the capacitors try the following test.

. Connect the capacitor in series with a high wattage lamp (100W or more) across 110v supply WITH CAPACITOR INITIALLY SHORTED OUT then remove the short.

No change in brightness or nil brightness - dud capacitor

Reduced brightness - functioning capacitor - compare the brightness with a ditto test on a similar value capacitor.

Jim

I definitely like this suggestion. Experimenting with this stuff is fun. What method would you suggest for turning that capacitor off after the motor spins up?

i

You see, that's fine, but what puzzles me is that the motors are made by the same company, look extremely similar, like identical twins really, but one is slightly larger and one is slightly smaller.

Yep...

I will give that a shot, indeed.

i

I

My personal favorite method of spinning up an idler is to momentarily connect the capacitor from one of the 220 lines to the "unfed winding" using a solid state switch. The switches cost $12 at Marlin P, Jones. Any big relay can be used to momentarily connect the capacitor, There is a high degree of arcing associated with disconnecting the capacitor, so 'oversized' realys are desired.

Once the idler is spinning, and the tool motor is connected and loaded, there is very little benefit derived as result of having the tuning capacitor. It is my contention that "tuning" of the RPC is best left for the experts to do. I'd advise that any RPC should be "tuned" only after it is determined that there is a need.

Jerry

Jerry sez: "> My personal favorite method of spinning up an idler is to momentarily

Do these solid state switches operate at "zero crossing" time? I am guessing they are similar to those used in high $ "switched capacitor" rotary phase converters such as the Booster brand seen on Metal Web News.

Bob Swinney

Yeah, Bob, the solid state relays do switch out the capacitor at zero crossing. That is what makes them so interesting. No arcs. In addition, the solid state relays can be controlled with 9 VDC. I use a small alkaline battery, so no high voltage is troublesome around the ON-OFF butons.

Jerry

I am quite interested, do you know of a web address where I could see these switches?

thanks

i

Iggy sez: "I am quite interested, do you know of a web address where I could see

How about it, Jerry? Is there a web site that you know of? What are some of the exact specs; and are they available from some of the chip houses, National, maybe?

Bob Swinney

The caps are trying to counteract the winding inductance. So if the motor changes between models, companies, Years then the inductance is either the same, lower or higher. One in three you have to change the cap value.

It is the same for a larger or smaller motor. (also naturally)

Martin

Bob

I had already stocked up on the solid state switches at All Electronics while they had hundreds of "take outs" at about $2.00 each. I dont see any in the most recent All Electronics

catalog. Marlin P. Jones shows some 25 amp, 280 VAC solid state switches at $11.95 each.

Jerry

Thanx, Jerry! Hey, Iggy - take a look.

Bob Swinney

Thanks, they are interesting. Using them would mean that I need to also add some DC power supply to the converter. Anyway, I will give it a serious thought. At this point, common sense suggests that the most likely cause of the trouble is bad caps.

i