Need help engaging starter capacitor for rotary phase converter Remotely

[Joseph Spallina]

I'm attempting to add a remote controlled magnetic start switch (PSI Long Ranger Heavy Duty Pro Remote Starter Switch) to my rotary phase converter and have hit a snag regarding how to engage the motor start capacitor.

The idler motor is currently uses a double pole light switch from L1/L2 to get current flowing and then a momentary push button switch to engage a starter capacitor and get the motor running. I'd like replace the light switch in the circuit with the magnetic starter and then add whatever component is necessary (relay?) to perform the function of the push button momentary switch for the capacitor. Does anyone have any advise as to how this could be accomplished?

thanks in advance,
Joe Spallina

[Charles Lent]

You're going to need a relay to replace the push button and a time delay to limit the period that the relay connects the capacitor into the circuit.

Home brew converters that I have worked on used a large 3 phase motor to do the phase conversion, and a 1/2 or 3/4 hp single phase motor connected by a belt to the shaft of the larger motor. A relay and a time delay was used to run the little motor for 20-30 seconds to get the big motor spinning, and then the little motor was powered off by the time delay and relay. Once the big motor is spinning, it creates the pseudo phase for 3 phase power, and it will continue to spin on it's own, dragging the un-powered little motor along with it until the converter is shut down. The little motor was used to get the big motor turning, just like the capacitors and push button in your converter serve to get your big phase converter motor turning.
The relay and time delay would replace the push button circuit, but you would still need a large contactor controlled by start/stop switches or a wireless controller to remotely start and stop the converter.

Charley

[Joseph Spallina]

Thanks so much for the reply, Charlie. Makes a lot of sense.

Joe

[Mike Schuch]

You can either use a time delay relay like Charles said or you can use a potential relay. I use an adjustable time delay relay with a 240v coil and have it set to about a second and a half.

The other option is to use a potential relay. A potential relay cuts out when a particular voltage is reached.

What size idler motor are you driving? Good idea on moving from a light switch to a contactor, a light switch isn't made to handle much current.

312375

Here is my RPC. The upper right is the main contactor energized by the remote start/stop switch. The upper middle orange box is the adjustable time delay relay and the black knob is the time setting. The gray contactor directly below the orange timer relay is the starting capacitor contactor. The timer relay wasn't rated to handle enough current to handle the starting current which is why I added small contactor for the starting capacitors. The oval capacitors are the run capacitors to balance out the legs. The bank of round capacitors are the starting capacitors. I used 440v rated starting capacitors after I blew out my first set of 240v rated starting capacitors. I have been running this rotary phase convertor with the 10hp idler motor for about 15 years and have never had a problem with it after upgrading the starting capacitors.

All of the contactors and the timing relay have 240v coils... so I would not have to bring in a 120v (neutral) leg or use a step down transformer.

312376

[Joseph Spallina]

That's a great setup. After reading up a bit, it seems that potential relays are pretty common for this application. Whether it be timed or potential, I suppose getting the relay specs right for my setup will be the challenge (a challenge for me, at least). My idler motor is 7hp and uses a single 280-330 mfd starter cap. Under the current setup, I only need to press the button for an instant to get the idler running. Any idea what specs would be appropriate for this setup?

[Bruce Wrenn]

If you haven't, go over to "practical machinest" web site. There is tons of info on building RPC's there.

[Mike Schuch]

That's a great setup. After reading up a bit, it seems that potential relays are pretty common for this application. Whether it be timed or potential, I suppose getting the relay specs right for my setup will be the challenge (a challenge for me, at least). My idler motor is 7hp and uses a single 280-330 mfd starter cap. Under the current setup, I only need to press the button for an instant to get the idler running. Any idea what specs would be appropriate for this setup?


Uhhh.... I am thinking that the timer relay I am using is 0-3 sec. And looking at the dial in the photo it is set to under a second. Like I kind of implied I built the RPC and set the timer relay 15 years ago and haven't had to do anything but use it since. When I built my RPC there wasn't quit the wealth of information available that there is today... this is why I went with the timing relay. I think a potential relay would have been more efficient but I did not know what cut off voltage I should order for the potential relay? I recall my guess at the time being 240v as a logical choice but did not have anything to back up this hunch.

Next time I go out to the shop I will see what the exact delay time on the relay is and let you know. Like you said though it is under a second so if you get a variable timing relay some where in the 1 to 3 second range you should be able to adjust it to the perfect timing. If you ordered a timer relay with a fixed delay I am pretty confident a half second would serve you well.

Edit - It appears that a potential relay with a latching voltage of 166v-180v is pretty commonly used on phase converters. Another source stated the start capacitor should be disengaged when the motor is at about 3/4 of full speed. I see several relays in this range on ebay for around $8. Seems like a potential relay also simplifies the circuit.

[Joseph Spallina]

Thanks so much for the help. I had read that 3/4 of full speed figure as well but wasn't sure if that directly translated to a percentage of voltage. I'm going to go ahead and order a potential relay with the specs you mention and see how it goes. I've already hooked up the mag switch in place of the light switch so it should just be a direct swap of the relay for the momentary switch.

Bruce, I have been reading a lot in practical machinist, and that info has been very helpful as well.

[Mike Schuch]

Thanks so much for the help. I had read that 3/4 of full speed figure as well but wasn't sure if that directly translated to a percentage of voltage. I'm going to go ahead and order a potential relay with the specs you mention and see how it goes. I've already hooked up the mag switch in place of the light switch so it should just be a direct swap of the relay for the momentary switch.

Bruce, I have been reading a lot in practical machinist, and that info has been very helpful as well.

Your welcome.

I am pretty confident the 3/4 speed and 3/4 voltage are purely coincidental. I did find the 166v-180v range mentioned multiple places. I am confident the 166v-180v range does not have to be exact. A few volts either way on the upper or lower end should not be an issue.

[Charles Lent]

In my experience, you really just need to get the converter motor spinning somehow, immediately after power is applied, and it will magnetically pull itself into synchronization with the incoming power sine wave. Most any starting method will work fine if it can get the converter motor spinning, but attaining 60-85% of it's rated speed as quickly as possible will result in less starting current being required. It will still run fine if you can get it started spinning at most any speed, but it will draw more starting current if it has to bring itself up to speed.

Charley