3450 vs 1760 RPM for rotary phase converter

"I built a 30 hp 480 volt system and exploded several capacitors because I had as many as 13 in series"

Capacitors placed in series do not form a perfect potential divider.

Slight differences in measured capacitance can dramatically affect the voltage division, and with electrolytics being sized essentially for the line voltage, and with no safety factor (example: 250 VAC caps on a 240 volt converter) failures are more common with series capacitors.

In order to equalize the voltage division, place a resistor across every capacitor. This can dramatically reduce the incidence of capacitor failures.

15 K ohm, 5 watt, wire would is fine, and this will also act as a "bleeder".

Bleeders should always be used on capacitors which are not also shunted by a motor winding.

Using run capacitors for starting is OK, but it is an unnecessary expense, particularly as run capacitors have such a low capacitance/volume ratio.

Run capacitors should be rated 1.56 times the RMS voltage, which is roughly 370 volts for a 240 volt converter.

The 1.56 factor includes both the conversion from RMS to peak, plus a ten percent safety factor.

On a machine which is constantly undergoing starts/stops and/or reversals, run caps should be used instead of start caps, as start caps have a limited number of cycles overy their entire lifetime, and a relatively low number of cycles per fifteen minute periods.

IOW, start caps are designed for a very low duty cycle, whereas run caps are designed for a 100 percent duty cycle.

RPCAn old thread but a newbie here. I read through this post & didn't get an answer to my question, which is, Can I use a 1750 rpm idler motor to efficenetly run a 3450 load motor on my capacitor started RPC??

WXP said:

RPCAn old thread but a newbie here. I read through this post & didn't get an answer to my question, which is, Can I use a 1750 rpm idler motor to efficenetly run a 3450 load motor on my capacitor started RPC??

Click to expand...

Idler motor rpm is irrelevant, it runs on 60hz, and outputs 60hz, and the load motor wants 60hz regardless of the fact that it is 1750 or 3450 rpm.

Thanks for the info.

The only thing that I have noticed with my RPC's is that higher RPM RPC's will start a heavier load.

I have a 30 hp 3450 rpm Phase-a-matic RPC, and had a 40 hp 1075 rpm Cronin Converters RPC - both running on 240. One of my pieces of 240 3-phase equipment is a 10 hp 35 cfm air compressor, which would spin up acceptably by the 30 hp RPC but not the 40 hp RPC. When I queried Cronin Converters about this, they told me that the problem was the low RPM of my 40 hp converter. Apparently higher RPM RPC's handle current inrush demands better than lower RPC versions.

That is possible, since the higher rpm stores more energy. That should translate to less slowing during a peak load such as starting.

That may translate to a bit better "kick" given at the beginning of the start. The stored energy is not a ton, but more is likely better, as voltage is directly related to RPM in a generator, which the RPC is acting as.

Warning: old tread from 2009.

I read all of it, will echo some earlier points: for a given hp, the higher efficiency, higher power factor motor will be better. Rpm is irrelevant, except that due to geometry constraints, 1200 and 900rpm motors are generally 10 points lower on power factor. Linear induction motors btw (infinite pole count) are about 50% power factor. 8 pole about 60 to 70, 6 pole 70 to 80% and 4 and 2 poles about the same maxing out at 80 to 90%.

Flywheels don't do anything. I measured the slip of a 4hp motor at settling at 3580 rpm after just 7 line cycles or 50ms after a substantial overload was placed on the rpc.

Sure a larger motor has more inertia, but a 40hp motor does not have 10 times as much as a 4hp. Hard starting loads need several seconds, hundreds of times longer than the rpc slows down by just a few rpm.

johansen said:

................................

Sure a larger motor has more inertia, but a 40hp motor does not have 10 times as much as a 4hp. Hard starting loads need several seconds, hundreds of times longer than the rpc slows down by just a few rpm.

Click to expand...

The difference, if any, is probably in the energy available to give the initial "kick" that starts the load motor going. The point about total available energy is perfectly correct, and I have made that point before.

At the instant power is applied, the most current from the source is required, because there is no back EMF at all. There is also the "sticktion" of all the moving parts, which needs good torque to get them going.

If there is any effect at all, it is probably at that moment of starting that a difference is noticed from the stored energy. After any significant time, that is all dissipated. But a few cycles of more output can make a difference in starting.

The difference between flywheels and rpm is a matter of energy. A change in the moving mass scales energy directly, the rpm (velocity of moving mass) scales as the speed squared. So at double rpm, the stored energy (MV^2) is 4x.

Also, for a given number of rpm drop, the back EMF of the idler drops less for a 2 pole motor than for a 4 pole. Since the idler back EMF is what produces the generated leg, there is a possible advantage there.

I don't intend to make a strong argument either way. There are a number of differences in 2 pole vs 4 pole motors, any or all of which might make a difference (or not).

As others say, rpm really makes no difference in an RPC but in general, 3450 rpm motors make a lot more noise than 1750 rpm motors do. All things considered, I'd opt for the lower rpm rating.

as far as the noise goes ... if the motor is TEFC (totally enclosed, fan cooled) you can place just the motor (not the control and caps cabinet) outside of the shop. put a small lean-to roof over it if you choose
you could also do this with an ODP (open drip proof) but i would suggest a screen around it to keep bugs and mice out of it
and of course if you have fire ants you would have to check it every couple of days to ensure they dont move in
the love the generated heat source and always ruin electrical eqpt.

if you are concerned about forgetting to turn it off, bring the control power from the output of the light switch that you will turn off when you leave the shop

Just as a point of reference,
I run two 6 hp 1200 rpm idlers
that easily start a 10hp motor.
I can run 6hp or 12hp worth of
converter, depending on my
needs, and they are quitet as
an added bonus. In my shop
attic, and I never hear them.
My idlers weigh 220 lbs a piece.


----Doozer

I would always use a 1,750 rpm motor over a 3,450 rrpm for an PC idler. Although the two RPC's that I built (5 hp and 7.5 hp) are automatic start, both of them easily started with a rope when I was testing snd assembling them. That's harder with a higher rpm motor.
Then there is the issue of noise. My first RPC was store-bought on eBay and came with a 3,450 rpm motor. It was incredibly loud. I qquickly got rid of it and made my own. Slower is better in RPC idlers. Slower motors are relatively quiet.

Not related, but....
Same applied to motor driven hydraylic pumps.
If you have the space and extra weight,
use a 1800 or 1200 rpm motor and a larger pump
over a 3600 rpm motor and smaller pump any day.
Much more quiet and longer lasting, less cavitation.
A loud whine from a hydraulic pump is very annoying.

-D

I had a 10hp 2 pole motor that I capacitor started. I had zero issues (that weren't due to my own miswiring of the potential relay initially).
I measured the RPM and it never got lower than around 3580 with a 6.5 hp motor at full load.