Rotary Phase Converter-Idler Moter RPM?

[Walt]

Apologies if this has been covered, I'm not able to find an answer by searching the forums.

Does the idler motor speed matter for constructing a rotary phase converter?

I have a newly purchased lathe that needs 3-phase power and from what I've been reading, a Monarch runs better with a RPC than a VFD so I'm looking for an idler motor. So far I've been seeing 1740 or 3540 rpm. Does it make any difference?

Walt

 

[Old Iron]

Both of my 10 HP motors are 3450 and they do make a lot of noise your best be is to fine a 1750 lot better.

Paul

 

[8ntsane]

From the number of RPCs I have built, the only motors I will use is the 1750 rpm or 1200 rpm.
If you can find a 1200 rpm motor, they are my favorite. Nice and quiet, but harder to find.
All depends on how much noise you can stand. :whistle:

 

[JohnG]

I've run both 3450 and 1750 rpm, and my current convertor is a hybrid with one of each @ 7 1/2 hp. I've noticed no difference in running motors with any of those setups, but it does seem that motors which start under load, like a hoist or air compressor, have more starting torque when the convertor motor has additional mass in the armature as is typical in a lower speed motor. It's still not the instant on snap of true 3 phase, but the start up is less labored.

 

[Walt]

I've run both 3450 and 1750 rpm, and my current convertor is a hybrid with one of each @ 7 1/2 hp. I've noticed no difference in running motors with any of those setups, but it does seem that motors which start under load, like a hoist or air compressor, have more starting torque when the convertor motor has additional mass in the armature as is typical in a lower speed motor. It's still not the instant on snap of true 3 phase, but the start up is less labored.

Thanks to everyone for confirming a 1750 rpm motor will work, this will be my first choice.

In this link:

http://www.electro-tech-online.com/re-projects/100563-3-phase-converter-schematic-miller-system.html

the author states:

"On a 230 volt 60Hz system a typical load number is around 4 amps per HP at full running load. Starting load can easily be 5 times that though. A 40 amp circuit can easily start and run a 10 HP motor provided that the actual source can support the possible 200 amp start cycle load with out excessive voltage drop at the motor itself. "

1) I have to assume that the "200 amp start cycle load" refers to the total power delivered by the live current plus the power delivered by the starting capacitors. Otherwise the 220 v single phase circuit supplying the RPC would trip the breaker every time. I think this is an issue of sizing the starter capacitors correctly rather than having massive line current supplying the RPC.

2) Is the "total load number" the power of the idler motor or the power of the working machine motor?

My 220 circuit has 2x110v 20 A breakers linked together. If the guideline refers to the idler I will need 4x7.5= 30 amp breakers. If the guideline is for the machine motor I need 4x3 hp =12 amps. The 20 amp breakers I already have in my panel will work fine.

Walt

 

[Ulma Doctor]

as a point of information, a balanced RPC will have less noise than an unbalanced unit.
3450 or 1750 rpm, it makes no difference whatever you can get your hands on.
the older the motor the better, the old school motors have thicker windings and seem to put up with a lot of abuse.
i have ran 7.5 hp motors from an old 5hp RPC without consequence to either.

 

[Old Iron]

Both of my 10 hp RPC motors run on 20 amp breakers.

Paul

 

[Walt]

Both of my 10 hp RPC motors run on 20 amp breakers.

Paul

Thanks!

What HP motors do your machines have that are running off the RPCs?

Walt

- - - Updated - - -

as a point of information, a balanced RPC will have less noise than an unbalanced unit.
3450 or 1750 rpm, it makes no difference whatever you can get your hands on.
the older the motor the better, the old school motors have thicker windings and seem to put up with a lot of abuse.
i have ran 7.5 hp motors from an old 5hp RPC without consequence to either.

Does "balanced" mean "even weight distribution"? Or something else, like having capacitors in the run circuit to equalize the voltage/current produced by the wild leg?

After reading a little on motors and 3-phase electricity I'm starting to realize my knowledge is small on this topic.

Walt

 

[Ulma Doctor]

Does "balanced" mean "even weight distribution"? Or something else, like having capacitors in the run circuit to equalize the voltage/current produced by the wild leg?

After reading a little on motors and 3-phase electricity I'm starting to realize my knowledge is small on this topic.

Walt[/QUOTE]

You are correct the generated leg's voltage is lower than the other two legs.
Walt, we add run capacitors between phases to get the voltage as close to equal as possible.
a 10% difference is considered acceptable, i have been able to achieve 3% with a lot of experimentation.

if i can be of assistance feel free to message me with any questions i might just be able to help out.
mike)

 

[JohnG]

For a while I ran a 3 man shop off a phase convertor, and I became interested in how it behaved when several motors were run at once. We might be running 3 machines with an aggregate 10-12 HP off a 7 1/2 HP convertor. I noticed:
1) If you just look at a schematic, the convertor looks exactly like the motors it is running.
2) When several motors are running, they all behave like auxiliary convertors depending on how heavily they are loaded mechanically, just as a literal interpretation of the schematic would tell you. The voltage variation of the generated leg that Mike mentions diminishes with each added motor.
3) When an additional motor starts up, it draws power from all the other motors already running in addition to the convertor, which again is literally what the schematic would indicate. The convertor has to be big enough to supply the full load run amperage of the running motors plus the start amperage of the new motor, or they all bog down trying to start up the newcomer.

In my experience "balanced" motors have a dynamically balanced armature to spin without vibration. It wouldn't matter in a washing machine with a belt drive; but woodworking machines especially mount cutter heads on direct driven spindle motors to spin very smoothly, especially when driven by a high frequency convertor.