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Rotary phase converter that only runs when the compressor runs

AdamPrince2

Plastic
Joined
May 10, 2011
Location
Honeoye Falls, NY
I have a 5 hp 3 phase industrial air compressor with 90 gallons of capacity. I built with Bill Cosby a crude 10 hp 3 phase rotary converter. I want to wire it so that the rotary phase converter will only run when the compressor runs to save power. I also want to have it wired, so that I can run it constantly when I add a lathe to the shop. I've made a basic wiring diagram, but I am not sure if it is correct. Or if there is a better way.

Can someone look over the diagram and give me advice?

-Adam Prince
Crisp Line Metal
3 phase converter for compressor.jpg
 
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I have a 5 hp 3 phase industrial air compressor with 90 gallons of capacity. I built with Bill Cosby a crude 10 hp 3 phase rotary converter. I want to wire it so that the rotary phase converter will only run when the compressor runs to save power. I also want to have it wired, so that I can run it constantly when I add a lathe to the shop. I've made a basic wiring diagram, but I am not sure if it is correct. Or if there is a better way.

Can someone look over the diagram and give me advice?

-Adam Prince
Crisp Line Metal
View attachment 107170

Your diagram looks like you have the basics thought out.
Give the RPC time to come up to speed, and then engage the air compressor motor.
With the HP rating of your RPC, I would have expected to see a magnetic starting circuit, but you may have left that out for simplicity.


Based on your diagram, when you disengage the start capacitor, the RPC is up to speed and the run capacitors are already charged and engaged.

You could eliminate the timers entirely by using a NO/NC relay as part of your starting circuit.
You could set the system up to close the circuit to a magnetic motor starter on air compressor at the same time as you disengaged the start cap.

My limited experience with air compressor circuits is they usual have a pressure activated relay that starts and stops the motor based on tank pressure.
This provides the switch gear large enough to handle the motors starting current.
If you use a set of timers on the legs between the compressor and the switch, they will need to be rated for the full voltage and starting current of the motor.
It may be more cost effective to add a magnetic starter to the circuit and use one smaller timing relay or a NC/NO relay to control it.
You also won’t have the issue of supplying the air compressor motor with single phase power should one timer fail.

My limited experience is big timers are not cheap or all that common, but motor starters are not hard to scrounge up second hand.
I have also noticed that many starters and relays can be disassembled and reconfigured easily from NC to NO.

Just thoughts, not instruction.
 
Better, simpler, more efficient way..

2 VFD's.. Total weight for both ~8 pounds...

No clunky 40 lb 10 hp motor running, no fudged together, non UL approved, wiring and hookup.

Bonus's... Variable speed for Lathe. Ramp start for compressor... Full motor protection, low voltage control circuits...
 
I have a 5 hp 3 phase industrial air compressor with 90 gallons of capacity. I built with Bill Cosby a crude 10 hp 3 phase rotary converter. I want to wire it so that the rotary phase converter will only run when the compressor runs to save power. I also want to have it wired, so that I can run it constantly when I add a lathe to the shop. I've made a basic wiring diagram, but I am not sure if it is correct. Or if there is a better way.

Can someone look over the diagram and give me advice?

-Adam Prince
Crisp Line Metal
View attachment 107170

I was going to stay out of this, but it looks like everyone else had the same impulse. Even if it switches on a peak, the start relay will close again after the start capacitor runs down. The two timers on the compressor motor would have to close at the same instant to prevent single phasing. On a compressor that always follows the same load cycle, I wouldn't bother with an idler, just size the start capacity to get it going and adjust the run capacitor to be right near the maximum pressure. That depends on whether it is a single or two stage compressor. A two stage one will pull the most power at the top pressure but a single stage one will often drop off power consumption due to air that is not exhausted bouncing. You would need to monitor the compressor current through the cycle and size the run capacitor for the peak. I don't know the purpose of the diode in the starting relay circuit. Most people use potential relays but I modify a standard one with a current sensing coil. In any case, it has to be where it will remain actuated after start, in the case of a potential relay it stays pulled in, a current relay releases after the starting surge drops off. To each his own.

Bill
 
Adam,

Is there some reason that you can't just shut the compressor and phase converter off yourself instead of having to do it automatically, like a remote location that is hard to access? I have a 10hp rpc that runs my cnc mill and when I need to start the cnc I start the rpc then flip the switch on the cnc. When I'm done I shut the cnc off and then the rpc. No big deal or lost power.

The rest of my shop is run on a 7.5hp rpc and a 7.5hp static phase converter. When I need 3 phase I turn the needed phase converter on and when I'm done I turn them off. Even when they are idling with no load they don't seem to take a lot of power. That happens when I start applying loads to the machines.

It just looks to me like a whole lot of extra work and possible damage to your compressor motor for something that you probably won't need when you get your lathe anyway.

My 2 cents worth.

Big B
 
-Ahall I like your solution. I have redrawn the diagram to include the magnetic starter switch and got rid of the timers. I still have the check the actual compressor for it's exact wiring. Could you check my plans over?

-Big B. If I cannot find a simple solution I will just go the flick the switch on and off route. I need to check, but I suspect even idling this RPC draws a lot of current. The lathe will only be used occassionally. I do sheet metal work, so I use it for making tooling. The compressor will be used often.

-Abarnsley, unfortunately 10hp name brand phase converting VFDs cost $600-$700 per piece of equipment I want to add. After the RPC is in each additional piece of equipment becomes much much cheaper. I've read a lot of good things about VFD though.

-9100 (Bill), I added a potential relay into the circuit. Did I draw the potential relay in correctly? The compressor is two stage, 175 psi, 5hp. Similar to a champion centurion II. I'd like to use an idler motor to get full power from the compressor. The diode idea comes from this website: http://www.waterfront-woods.com/Articles/phaseconverter.htm
"Without this diode, the system was self-defeating... The bottom line was that the [potential] relay would flicker on and off, but the motor would not start... I can only surmise that it is because I cut the voltage in half by using the diode [solving the starting circuit's flickering problem]."

-Adam Prince
www.crisplinemetal.com
 

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Christopherson's description is just another case of someone who has only a vague idea of what he is doing floundering around and putting his nonsense on the internet. To begin with, his opening statement is wrong. A static converter can develop full power at any one load, but you are not likely to do it by just voltage readings. Phase angle is just as important, but most of these clowns probably don't even know how to measure it. The last time I was involved in one of these converters, two pretty clever people were unable to make it work. Using my Bridgeport clone mill brake to simulate loads and looking at the waveforms on an oscilloscope, I was able to tune it in a few minutes.

All the idler motor does is extend the range of a reasonable manufactured leg. With a two stage compressor, the load increases through the pressure range and the only place you need full power is at the highest pressure at shutoff. You don't need to develop full power at lower pressures, so a properly set up static converter should be all you need.

the problem with the start switch requiring a diode probably is mostly a matter of not having the hysteresis between pull in and release correct. I don't use potential relays. I convert ordinary relays to current sensors and adjust the springs to give the desired action. I had one on a mill with a two speed motor for ten years. When I started it on full speed, the relay would pull in, connecting the starting capacitor, and drop out when the motor got up to speed. On half speed, the motor started fine on the run capacitor and the relay did not pull in on the lower starting load. When running at half speed, i switched to full, the relay would pull in, get the speed up, and drop out. All I had to do was turn the mill on and off. All there was in the box was two capacitors and a relay.

Bill
 
You've obviously put good thought into this. Personally I would choose low voltage (24ac) controlling contactors, and maybe with 2 pressure switches at 10-20 psi apart get the RPC to fire before the compressor wants to kick in and eliminate the timer business?
 
seems like a long way to go for a couple cents worth of power
one burnt motor from single phasing while your in the house taking a dump and your out 25 years worth of savings
 
-Abarnsley, unfortunately 10hp name brand phase converting VFDs cost $600-$700 per piece of equipment I want to add. After the RPC is in each additional piece of equipment becomes much much cheaper. I've read a lot of good things about VFD though.

I've got VFDs on nine different machines, ranging from 3hp to 10hp, and I don't have $700 invested in the whole lot of 'em. They're all Allen-Bradley 1305 and 1336 drives, bought surplus by-the-pallet.

I'll hafta agree with Kevin here- you've got a 5hp compressor, and you're going through 93hp of engineering to develop a 18hp solution to a 2hp problem.

Planting a single-phase motor on that compressor will be a whole lot simpler and safer, take up less space, and provide much more flexibility that trying to robotize it with a rotary converter. The self-starting rotary converter puts down a pretty hard pull when you turn it on... at least as much as an equivalent single-phase scenario, and usually, worse.

For what you're going to have in your future, the best long-run solution, is to make the compressor a single phase stand-alone machine, and then fit your machine tools' spindles with VFDs. If you decide to make a simple RPC for an interim solution (whilst climbing the 'learning curve' of VFD), so be it... keep it simple, and stay near the switch.
 
9100 Bill, I have an older vacuum tube powered oscilloscope that works. What is a phase angle? How is it measured? Can you give me a list of parts to order?

I'm unable to find a potential relay rated for the starting amps for a 10 hp motor. I think I have to use a separate contactor. Are the amp ratings on the contactors rated for starting amps or just continuous amps? To start a 10 hp is around 100 amps?
 
9100 Bill, I have an older vacuum tube powered oscilloscope that works. What is a phase angle? How is it measured? Can you give me a list of parts to order?

I'm unable to find a potential relay rated for the starting amps for a 10 hp motor. I think I have to use a separate contactor. Are the amp ratings on the contactors rated for starting amps or just continuous amps? To start a 10 hp is around 100 amps?

OK, here goes- A three phase source has three voltages that are equal and swinging back and forth from positive to negative and back at the same rate but with different timing. Sort of like musical instruments playing to the same beat but one playing notes between the other's. Imagine a round footrace track that a normal runner takes 1 minute to make a lap. Start one off, wait 20 seconds to start the next, then another 20 seconds for the third. You now have three runners going around at the same speed but 1/3 of a circle (120 degrees) apart. Now imagine a motor field with the magnetic poles rotating in the same manner and pulling a wheel in the center around with them. That is a three phase motor. If a pole gets behind the proper place, it is said to be lagging, conversely one ahead is leading.

A single phase motor field acts like a single runner running back and forth on a straight track. The motor doesn't know which way to turn, so it needs a starting circuit to kick it off. A three phase motor has a rotating field and the armature just follows. If one of the magnetic poles is out of place, it is grabbing the armature in the wrong place and loses power and efficiency.

To get three phase power from single, that is to generate another pole in the right place, we use a capacitor which produces a leading current. Why is too complicated to go into here, so just accept that. The problem is that the effect changes with the motor load, so it can only be right at one load. The basic approach is to size the capacitor for the maximum load so you get full power there. Efficiency will fall off at lower loads, but you don't need full power then.

Frequently the optimum running capacitor will not be enough to start the motor, which is in an overload state until it picks up some speed. Sometimes a motor will start on just the running capacitor. I had a mill with a two speed motor that started fine on low speed without switching in a starting capacitor, but needed one on high speed.

What make and model is your oscilloscope? If I am familiar with it, I can advise you on hooking it up.

Bill
 
In my prior shop I ran a 3 phase 5HP Quincy 325 from a 30HP phase converter. The shop was small so I dug, formed and poured a vault below the floor for the converter, compressor and 3 phase distribution panel. I just used a timer relay to start the converter. The converter shut off by itself when the compressor stopped if the converter switch was in run. It worked like a champ for 4 years.

At the time I wasn't aware I could drive 20 minutes and buy a 5HP cap start Baldor single phase motor brand new for $375. That's what I would have done had I known they were that affordable.

My current compressor is a 7.5HP single phase Quincy 350. Much simpler.
 
-9100 (Bill) I built a static phase converter to test the compressor motor. 330+ uF start capacitors, 50 uF L2 to L3 and 30 uF L1 to L3. I want to hook it up for a 20 second test of the Baldor 5 hp 184T 3 phase motor to make sure it's not burned out. I rewired the motor from 480 volts to 230 volts as per the plate on the motor. The plug for the compressor runs into an Allen-Bradley 709-CAA automatic starter with a 72A288 coil rated at 480 volt 60 CY and 440 volt 50 CY.

-What is an Automatic Starter?

-How is an Automatic Starter different from a Definite Purpose Contactor?

-Do I have to replace the 480 volt coil with a 230 volt one?

My scope is an EICO 427. It's working, but I'm not sure how to use it.

Here is a photo of the Automatic Starter:
Img01_2014-10-22.jpg

Thank you.
 
-9100 (Bill) I built a static phase converter to test the compressor motor. 330+ uF start capacitors, 50 uF L2 to L3 and 30 uF L1 to L3. I want to hook it up for a 20 second test of the Baldor 5 hp 184T 3 phase motor to make sure it's not burned out. I rewired the motor from 480 volts to 230 volts as per the plate on the motor. The plug for the compressor runs into an Allen-Bradley 709-CAA automatic starter with a 72A288 coil rated at 480 volt 60 CY and 440 volt 50 CY.

-What is an Automatic Starter?

-How is an Automatic Starter different from a Definite Purpose Contactor?

-Do I have to replace the 480 volt coil with a 230 volt one?

My scope is an EICO 427. It's working, but I'm not sure how to use it.

Here is a photo of the Automatic Starter:
View attachment 120057

Thank you.

It looks like a typical starter to me. As to the difference, I don't know.

You can run a 480 V coil on 240 with a capacitor in series. The inductance of a 480 V coil is higher so you cancel part of it with capacitive reactance. I have a 3 hp Roto Clone dust collector that had the same problem, originally run on 480. I have a 2 mfd 1000 volt oil filled capacitor in series with the starter coil which has been running happily for over 20 years. To set it up, you need to measure the current when the coil is supplied the rated voltage and the contactor pulled in, then run it on the lower voltage and adjust the capacitor for the same current. The contactor must be pulled in when you do it because its inductance changes radically between open and closed.

For your test, just close the starter manually, pushing on the armature with an insulating stick.

The Eico instruments were low end kits the buyer assembled. One would probably work well enough for your purpose. I found schematics on line, but I am just starting my day right now and need to get moving. Later.

Bill
 
Did I miss something? Going from 480 to 240 in addition to changing the coil out his heaters will be to small.

Correct, I didn't think of that. Thanks for pointing it out. You need to get heaters rated at twice the current. The originals will probably hold long enough for a test/

Bill
 
Did I miss something? Going from 480 to 240 in addition to changing the coil out his heaters will be to small.

Alright so I need to change the both the coil and the heater. The chart inside the box shows a N34 heater for 15.1 full load amps at 40 C (Motor shows 14.8 amps at 230v). I think the coil I need is a 72A83. Buy 72A83 - Allen Bradley Coils

I will get that pulled off today and order everything tonight.
 








 
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