Three Phase question

[Bob Cooper]

I have a 3 phase rotary converter. It has a switch on top which I need to flip when starting but I need to make sure it's off when I cut the power off. So this requires me to be standing beside the phase converter in order to start it. I'm building a new shop and I want to have this thing in the basement part of the shop so being near it when I start/stop it isn't practical. Is there a way to eliminate this manual step?

[David Kumm]

Bob, Is the switch magnetic or manual? Why do you need to cut the power to the switch? Dave

[Bob Cooper]

It's manuel...if i don't turn it off, and i cut the power, the phase converter makes a grunting sound -- kind of like the sound a car motor makes when you show down w/o engaging the clutch -- almost like it's trying to start. The motor stops rather quickly iirc. I'm guessing it's kind of like back emf resisting the motor spinning or something similar.

If i turn the switch off prior to cutting the power, the motor will free spin for probably 5 minutes.

It's possible i never need to dis-engage the switch but i always have.

lastly...i'm assuming the switch has something to do with startup capacitors to get this motor moving? just a guess as wll.

[David Kumm]

With a manual switch you would need to bypass the one on the unit and wire one in the shop. If you went to a mag starter you could wire a low voltage start stop in the shop and leave the enclosure down by the motor. Dave

[Bob Cooper]

from reading a little, it looks like this switch is for the start capacitor(s). Do you know if the startup capacitors should be bypassed as soon as it's started or can they stay in circuit up until i turn the power off to the idler? If the latter then i shoudl be able to use some type of magnetic switch if not then it seems like i'd need to keep it as a manual momentary swtich.

[David Kumm]

Bob, My RPC is a Kay and did not come with a switch. I first added a disconnect and used that and later went to a mag starter. With mine there was no other requirement. If yours is purchased I would check with the company to be sure but can't imagine needing more than the starter. I used a three phase starter on mine and ran the manufactured leg back into the starter on the load side. Allows the third leg to get back to the panel when on but isolates it from the power in the panel when off. Otherwise you will have power running back from the panel L1 or L2 on that wire. Doesn't hurt unless you forget when you mess with it. Dave

[Stephen Cherry]

You could use a timing relay to switch in the start capacitors when the unit is powered for a limited amount of time. It's important that to switch them out after a few seconds so that they don't blow up.

[Rick Christopherson]

To your question, yes, you could use a voltage-sensing relay that engages the start capacitors during startup, and disconnects them when the motor reaches about 80% full speed. Some RPC manufacturers will sell these to you as spare parts, or you could hunt around eBay.

But that's not why I took the time to respond. If this switch you currently have is for the start capacitors (which I strongly suspect it is) and you have been leaving this engaged the whole time your converter is running, then you are doing significant damage to the windings of the idler motor. When the start capacitors are engaged, the current through the windings is very high. You should be shutting this switch off as soon as the idler motor reaches operating speed. Most people use a momentary push button switch simply to avoid forgetting to turn the switch off once the converter is running.

[David Kumm]

Bob, I'm missing something here. If your RPC comes with a manual switch that turns the converter on and off there must be something internal to the machine to shut the start capacitors down or the system makes no sense. It would eventually kill itself if you need to turn off the capacitors but have no method to do so. Dave

[Jerome Stanek]

Some of the RPC have a timer on the start Caps.

[Bob Cooper]

Rick...thanks for the reply. I don't leave the switch on but i have once or twice accidentally. Actually a momentary push button would make sense.

David...my writing was probably not the clearest. There's no switch to turn on the RPC...rather i just throw a breaker to turn it on. So the normal procedure is (1) engage the start capacitors via a switch (2) throw the breaker (3) dis-engage the start capacitors.

when i relocate this RPC i guess i'll wire a 240V contactor or relay to a swiitch and put a momentary switch right next to it...20' from the RPC.

[Bruce Wrenn]

Several years ago, I built my neighbor a RPC that used a pony motor to start it. We were lucky enough to find a Herbish Dryer motor that has one lead that line voltage appears on when motor reaches full speed. (It was to control gas solenoid.) We use it to pull in a two pole contactor which controls voltage to RPC. Line voltage fails, contactor drops out.

[Rick Christopherson]

when i relocate this RPC i guess i'll wire a 240V contactor or relay to a swiitch and put a momentary switch right next to it...20' from the RPC.
There is really no reason to mess around with a contactor for this. Simply use a disconnect at the RPC. You can find simple disconnects on eBay and the like. They are very common. It doesn't need to be fused or anything fancy.

[Stephen Cherry]

Rick...thanks for the reply. I don't leave the switch on but i have once or twice accidentally. Actually a momentary push button would make sense.

David...my writing was probably not the clearest. There's no switch to turn on the RPC...rather i just throw a breaker to turn it on. So the normal procedure is (1) engage the start capacitors via a switch (2) throw the breaker (3) dis-engage the start capacitors.

when i relocate this RPC i guess i'll wire a 240V contactor or relay to a swiitch and put a momentary switch right next to it...20' from the RPC.

Another way is to have either a timing relay that switches in the start caps for a few seconds when first turned on, or else to have a second contact on the start button for the contact so that when the start button is held depressed the start capacitors are switched in. You would want to duplicate this through a contactor, because of arching on the start switch.

For my phase converter, I used a start button with a second contact for the start caps.