Two small RPC's or one bigger one ?

I've been in a shop without 3 phase power for the first time for about the last 3 years or so. I have needed a new compressor since my larger one, an Ing. Rand T-30, got damaged in the last shop move, and finally ordered a Kellogg 452 pump and two Baldor motors - one a 10 hp the other a 7 1/2.

I am currently operating with a 3 hp RPC a buddy gave me which starts a 7 1/2 HP idler that easily runs a 3 hp compressor and either of two B&S screw machines or one 20 spindle drilling and tapping machine or a horizontal milling machine along with the compressor. But I need more air as I also run in about 25 to 30 K screws a year with an air driver.

One of the B&S machines has an air drill on the front slide and the drill/tap machine has an air over hydraulic clamp. The drill press has a air power feed and is used in production work as well.

The drilling/tapping machine has an air over hydraulic clamping fixture and an air nozzle is used continually to blow chips out of the fixture which slides along rails back and forth between drilling and tapping heads - air powered of course. And the heads themselves power down to the parts before the lead screw kicks in, by air of course. I need lots of air.

I have never attempted balancing out the voltages with capacitors, and it seems that the more motors are run in the circuit, the less effective the balancing becomes. I checked the voltage on the B&S that runs the motor starters hot and found that the generated leg was 277 volts as opposed to 220 and 220 on the power company legs. That was taken with a Fluke model 322 class III tester - whatever that means.

The current was also all over the place.
However, that machine's spindle motor starter ran hot on good power-company 3 phase when I bought it used. Someone had added a box with capacitors in it, and there was a pile of motor starter heater insert things in the machine electric box.

So I just run it for half an hour and let it cool down before cranking it up again. It sounds a lot like the problem Coountry Boy 19's father has here http://www.practicalmachinist.com/v...ning-multiple-motors-rotary-converter-343135/

What I've done before with 2 compressors (and regular 3 ph. power) was to start both compressors off the same pressure switch, but use an on-delay timer for the control voltage to start the second, and use a separate circuit from the main panel for the high voltage to run that second compressor. Then I didn't have to screw around with trying to pull heavy wire through the conduit nor have such a huge inrush at startup.

So I don't know what the best way to go is. One RPC started with a "3 hp" rpc consisting of two 7 1/2 HP idlers and one panel?

Or should I split them up into two circuits in different panels with the same 3 hp RPC starting a 7 1/2 hp idler starting only a 10 HP compressor while the rest of the machines run off the 3 ph rpc starting a different 7 1/2 hp in a different 3 phase panel? It seems that that way would make it easier to balance the voltages - which I've never attempted before. I'm guessing that the 3 hp plus the 7 1/2 hp would start the compressor 10 hp motor. I plan to let it run down to 90 psi before it trips the pressure switch.

I could also get head unloaders for the compressor so it would become a continuous run motor. The guy I talked to that seemed to definitely know his stuff really didn't like that plan (and he was the one who'd sell me the head unloader kit) - saying that continuous run sucks oil out of the reservoir really fast and can cause big problems. So I can relate to that guy, especially since he will be losing money by talking me out of it. But I do the same thing with my product because I don't want them to be coming back because somebody is using them for an application they weren't designed for.

I've only had one continuous run compressor, a 5hp Emglow single phase wheelbarrow for construction work, and I burned out the motor and screwed the pump by spraying continuously for too long a period of time with the compressor outside where I couldn't hear it or the rain.

Ox, my brain is getting as squirrely as my squirrel mail. I finally checked my old messages and saw that you'd already recommended the lube for the screw machines running only brass that I posted another question about. It was so long ago (at least 3 months) I'd forgotten about it. Anyhow, I just got in 60 gallons of new hydraulic oil for the screw machines and another 15 for various other things. So I thank you for your generosity and advice based on actual experience. This place is a great resource.

So I got my Christmas presents sittin in the shop, with more comin.

Merry Christmas all. Any thoughts on the above would be greatly appreciated.

Cheers,
Jim

sounds like a 20 hp phase perfect will solve a lot of problems.


aside from that, you can use a 10kw 3 phase generator head as a rotary phase converter for your needs in my opinion. you will need to start it with a pony motor. to bring it online you need some resistors, on the order of 10 ohms. 4500 watt water heater or 2500 watt oven heating elements will work. so that means two sets of contactors. one to engage the resistors which will drag it into synchronous speed with the grid, the other to directly connect it.

given the generator head is synchronous, it will generate the proper voltage you need. the third leg will drop with current but it will be nearly a dozen times stiffer than an induction motor.

in my experience a 2 hp synchronous motor is better than a 5 hp induction motor when judged by how much starting torque it can deliver into a 1/2 hp motor.

I will warn you though, when i tried to start my 5 hp induction motor from my 2 hp synchronous motor, the load was sufficient to pull the 2 hp synchronous motor out of synchronization. maybe a bigger flywheel would prevent this, i don't know.


anyhow you can also deal with the unloaded voltage of the rpc being too high.. as you mention 277 volts, by connecting the ballencing capacitors at each load, according to that load.. not globally at the rpc. the problem with this is: when you throw the switch to turn on the load, you've got short circuit at the contacts due to the capacitive load. NTC resistors are one method to get rid of this problem.

johansen said:

anyhow you can also deal with the unloaded voltage of the rpc being too high.. as you mention 277 volts, by connecting the ballencing capacitors at each load, according to that load.. not globally at the rpc.

Click to expand...

Thanks Johansen. Sorry it took me so long to get back to you.

So I take that to mean that I do need more than one RPC to do it that way. In other words, I need a separate idler and a separate buss bar that has no other machines connected to it. Is that correct?

johansen said:

The problem with this is: when you throw the switch to turn on the load, you've got short circuit at the contacts due to the capacitive load. NTC resistors are one method to get rid of this problem.

Click to expand...

Sorry, I don't understand the part about NTC resistors, or what happens if I don't have them.

The B&S machine in question had an add-on box with big capacitors and resistors or something attached to the top of the machine. I just figured the previous owner did not have 3 phase power and was using them to run off an RPC. So I disconnected it since I had 3 phase from the power company. However, I saved it and will try to get some pics tomorrow. I will not be able to figure out what they were connected to in any event.

Regards,
Jim

what i was talking about is removing the balancing capacitors from the RPC and placing them at each load.

your 277 volts is because you have too much capacitance on the third leg of the rpc. but you need that capacitance when there is a high load on the rpc to get the third leg up from 200-210 volts to the minimum of 230 or so that it needs to be for it to do any good.

so the solution (one of them) is to remove the capacitors from the rpc and wire them behind the contactors that drive the loads.

so they are only connected to the rpc when the loads are energized. the problem with this is capacitors draw a huge starting current when they are connected to the line, high enough to destroy relay contacts. an NTC is a negative temperature coefficient resistor. they start out at say a dozen ohms and when the current passes they heat up and the ohms drops to a rather low value. there is one in every single computer power supply for exactly this problem. (a good place to find one of about the right size for a 30-60uF capacitor on a 240v circuit.)


so presuming you need say 5uF for each hp of motor load, each motor will have about that much capacitance permanently connected to it. in this way you can keep the voltage approximately right regardless how many loads you have connected.

Thanks, again, Johansen.

I was under the impression that, since the more three phase motors that are running on a circuit, the more motors one can run, that some of the generated leg went back into the buss bar on that circuit, making the generated leg better for extra machines added to that buss bar.

So assuming that, then I assumed that trying to balance the voltages at one machine would be effective only when that one machine was running off that buss bar in the panel. And that as soon as you cranked up more machines - the balancing would be off again.

I will try to take a picture of those capacitors in the box that was on top of the machine when I bought it. There are a bunch of resistors of some sort between the capacitors.

Regards,
Jim

Hi Johansen,

Thanks again. Since your last post I’ve been doing a lot of reading.

I have read the Fitch design several times and watched a three part video explaining it a couple times and am pretty confident about the connections on the regular type with two terminals on them. Also, I zapped a screwdriver real bad once checking to see if it was still hot. I understand you use a resistor to short between the terminals on a 2 terminal capacitor to drain it.

I’m guessing you use a well insulated pair of pliers, alligator clips and well insulated wire and heat shrink to put them on with from what I’ve seen on the internet. I’m also guessing that you don’t need to do drain each of them one at a time, but just go from A to B for instance, if those are the wires you are adding them to.

Are there current and voltage components to resisters too or just voltage? Should I make several “drain tools” of increasing resistance, and how many ohms do I need? I hate to just blindly guess on any of this.

Below is that picture of that capacitor box I removed from one of my screw machines back when I had 3 phase power from the power company – thinking it was for a phase converter and that I didn’t need it. But I saved it.

http://www.miterclamp.com/Images/Caps.jpg

The resistors are marked green, brown, yellow, gold, and a calculator online says they are 510K +- 5%. I don’t know what kind they are though and have no idea which way to connect them nor why there are 5 terminals on the capacitors, which are labeled 1 KVAR and 1.5 KVAR. I’m just going to get rid of them.

I’d like to get the right resistors you mentioned before cranking this thing up – but don’t have a clue what sizes and so on to get. I don’t know if those in the picture will work or not. Are these essential to have before balancing the voltages or does the balancing come first to determine which ones are needed?

Should the VAC and VBC balancing be done after the motor overload relays or between them and the motor contactors or does it matter ?
I ordered a bunch of capacitors today based on the Fitch design and am pretty sure I got plenty of them.

A friend gave me a huge new Hubbel box that’s 4 times the size I need but will allow plenty of air circulation. There is room for enough capacitors to run the block. All it will have is three motor starters with overload relays, fuses, on-delay timers and capacitors for three motors. The motors will come on sequentially instead of all at once when the pressure gauge calls for air.

I am hoping to get the new compressor pump up and running by the end of next weekend.

Thanks for any assistance or suggestions you or anyone else has.

Cheers,
Jim

Your capacitors in that box are for power factor correction. They are no different in type from others, mostly just labeled differently, and with several in one enclosure.. But a nuisance to "translate", plus they are set up for 3 phase.

The resistors have no function other than to drain off the charge after power is removed. Values of resistor are determined by voltage, how much capacitance you have and how fast the voltage needs to be drained off. The resistors are often put on each capacitor, to assure that they all get drained even if a wire comes off.

clampman said:

Thanks, again, Johansen.

I was under the impression that, since the more three phase motors that are running on a circuit, the more motors one can run, that some of the generated leg went back into the buss bar on that circuit, making the generated leg better for extra machines added to that buss bar.

So assuming that, then I assumed that trying to balance the voltages at one machine would be effective only when that one machine was running off that buss bar in the panel. And that as soon as you cranked up more machines - the balancing would be off again.

I will try to take a picture of those capacitors in the box that was on top of the machine when I bought it. There are a bunch of resistors of some sort between the capacitors.

Regards,
Jim

Click to expand...

more motors running at no load means any additional motor will start faster, but they provide no significant 3 phase power to a motor when its under load. you need the capacitors to offset the idler motor's internal inductance, etc, to get the voltage on the third leg up to where it does any good.

when the voltage on the third leg is higher than the other two, then you run into problems with overheating and wasted power, when its lower, it doesn't do much good.


so by sizing the capacitors at each load separately, and wiring them such that they are only connected when the load is connected and running, then you can keep the voltage in reasonable range. if its too high its causing overheating, too low and its contributing no significant torque to the loaded motor.


anyhow the problem with wiring the capacitors after the relay contacts that turn your load (motor) on, is the inrush current of the capacitor. NTC resistors are a good way to deal with this, but they take a minute to cool off.

clampman said:

I've been in a shop without 3 phase power for the first time for about the last 3 years or so. I have needed a new compressor since my larger one, an Ing. Rand T-30, got damaged in the last shop move, and finally ordered a Kellogg 452 pump and two Baldor motors - one a 10 hp the other a 7 1/2.

I am currently operating with a 3 hp RPC a buddy gave me which starts a 7 1/2 HP idler that easily runs a 3 hp compressor and either of two B&S screw machines or one 20 spindle drilling and tapping machine or a horizontal milling machine along with the compressor. But I need more air as I also run in about 25 to 30 K screws a year with an air driver.


The current was also all over the place.
However, that machine's spindle motor starter ran hot on good power-company 3 phase when I bought it used. Someone had added a box with capacitors in it, and there was a pile of motor starter heater insert things in the machine electric box.


What I've done before with 2 compressors (and regular 3 ph. power) was to start both compressors off the same pressure switch, but use an on-delay timer for the control voltage to start the second, and use a separate circuit from the main panel for the high voltage to run that second compressor. Then I didn't have to screw around with trying to pull heavy wire through the conduit nor have such a huge inrush at startup.

So I don't know what the best way to go is. One RPC started with a "3 hp" rpc consisting of two 7 1/2 HP idlers and one panel?

Or should I split them up into two circuits in different panels with the same 3 hp RPC starting a 7 1/2 hp idler starting only a 10 HP compressor while the rest of the machines run off the 3 ph rpc starting a different 7 1/2 hp in a different 3 phase panel? It seems that that way would make it easier to balance the voltages - which I've never attempted before. I'm guessing that the 3 hp plus the 7 1/2 hp would start the compressor 10 hp motor. I plan to let it run down to 90 psi before it trips the pressure switch.

I could also get head unloaders for the compressor so it would become a continuous run motor. The guy I talked to that seemed to definitely know his stuff really didn't like that plan (and he was the one who'd sell me the head unloader kit) - saying that continuous run sucks oil out of the reservoir really fast and can cause big problems. So I can relate to that guy, especially since he will be losing money by talking me out of it. But I do the same thing with my product because I don't want them to be coming back because somebody is using them for an application they weren't designed for.

I've only had one continuous run compressor, a 5hp Emglow single phase wheelbarrow for construction work, and I burned out the motor and screwed the pump by spraying continuously for too long a period of time with the compressor outside where I couldn't hear it or the rain.
Jim

Click to expand...

Jim, a few thoughts and questions come to mind reading the thread.

First is your electric service your working with. Is it marginal in terms of supplying the planned connected load? The existing equipment and the additional 10HP compressor. I suggest putting together a load schedule list with HP's, current and voltage values. Then see what is the size of the utility transformer supplying your building. Does that transformer have other buildings connected to it? What arrangement and size service panels do you have existing to connect to? How big and stiff your service is will be a big determining factor on how successful you are in getting that 10HP pump running along with your RPC setup.

The screw machines look like they came from a plant with a 480V service, judging buy the voltage on the old power factor correction box that you removed. Were those machines motors and controls properly setup for 230V operation? You did the correct thing by removing the PFC cap box. It was used at the old plant to correct the power factor of the machine to reduce utility penalty charges on poor power factor and peak demand.

A motor wired for 460V and supplied by 230V will run OK, but will heat up quickly when run under load, leading to burnout. Also severely reduces output HP. You mentioned it ran hot and had a pile of heater elements in the control cabinet?

Second item is the compressed air system. You said you need lots of air, but then indicate that at one time a 5HP portable Emglow (great machines) on continuous run was enough. Then you got a IR T30 unit, that crashed in the move. How much cycle time did the T30 have with your typical day, did it run hard during the day, or was it more than adequate? What happened to the T30 motor, is it available for use?

Reciprocating compressors are hard starting loads during cycling under load. And I"m doubtful that your 3HP + 7.5HP RPC combination is up to the task, for a 10HP two stage compressor, especially if your service is minimal.
Typically RPC's need to be sized X2 or X3 for hard starting loads. As in your existing 3HP pump and 3+7.5HP RPC setup.

I can think of a few different things to employ on the air system that you haven't mentioned, but that depend greatly on the electric service and conditions of use of the system.

1Φ compressor motor. Old T30 motor?
3Φ compressor motor, continuous run using the motor as an contributing idler during periods of unloaded time.
Slowing down the pump speed by the sheave ratio, to reduce starting demands, but longer run times. Less dramatic starts, but longer running at reduced load.
A VFD on the 3Φ motor as a phase converter and motor control.
Transformer type static converter for the 10HP 3Φ, well suited to constant fixed loads.

The compressor can be setup with a selectable continuous run/off/start-stop mode by the addition of the unloader kit, selector switch and a solenoid valve. With the continuous run mode it does use more oil, and deposits and burns it on the 2nd stage high pressure outlet check valve, creating increased maintenance/service.

Tell us more about the big picture, for better suggestions.

SAF Ω

Wow! Thanks everybody. Was up late until about 3AM checking out things on the internet, and have to get over to the shop soon.

JST, I downloaded a capacitor discharge calculator and ran a couple of different scenarios and the results were different, so at least I know it is doing something.

Here is one:
Capacitance 250 mf
Initial Charge 351 v
Safety threashold 11 v
Discharge time 5 seconds
Answer: 5.55 K ohms

Does that sound reasonable? I have absolutely zero experience to draw on.

SAF, the Emglow that I fried happened when I was in my real trade doing residential construction and cabinet work. I was spraying conversion varnish on a library and the trim in a house when it went – years before switching over to metal.

I have three idlers – the 3 hp RPC bought to start the 7 ½ hp 3 phase motor (from the dead T-30)than has been used as an idler without capacitors. That’s what I’ve been using for a long time– but it has never been enough air and now is really bad.

So I bought the new pump (Kellogg 452) rated to pair with 6.5 to 11 hp motors. Along with it, I bought two new baldor motors, one 10 hp and one 7 ½ hp, both with the same 213T / 215T frames and same shaft diameters.

So right now I’ve got 18 hp of idlers to run the 10 hp compressor (and the other tools). I could quickly have 20 ½ hp to run a 7 ½ compressor motor by switching the new motors. So, as you say, I have other options as well as reducing the sheave size on the 10 HP (my preference) if necessary.

In fact, the motor pulley I got is a 6.8” vs the 6.2” I wanted because the former was in stock not the latter. The latter would have required about 7.8 hp BHP(about 6.5 KW) whereas the one I got requires about 8.7 hp BHP (about 7.5 kw) according to the Kellogg charts.

Power into the shop is good. Transformer is about 100 feet from my panel and services only 2 other businesses in my compound, one who tig welds (one guy) one who tig and mig welds (one guy usually). However, I run machines primarily on weekends when no one else is working. Neither of the Brownies is doing any heavy cutting. None of the machines are loaded much or straining.

The biggest air hog is the drilling and tapping machine primarily because I have to blow chips out of the fixture because the parts will not fit into it if a chip of 0.005” gets in there. That way, not many parts wind up scrap. The fixture also has an air over hydraulic clamp, an air shuttle to run the fixture and parts back and forth under each head, both of which drop and retract via large air cylinders before the lead screw begins turning the taps and before the hydrocheck kicks in on the drilling head.

The other air hog is running in thousands of screws with a Cleco air driver. I changed the thread pitch from a 32 to a 24 years ago primarily to save air. The bender has an air over hydraulic clamp. The drill press has a Schrader Bellows air feed with a long initial throw and the other brownie has an air drill to chamfer one side of a cross drilled hole.

I initially was going to get a dual control head unloader, but after talking with a Kellog compressor guy (who recommended trying it without for exactly the reasons you listed) I held off . Once running, the compressor will be filling two 60 gal tanks, so it won’t cycle so quickly and it will shut off only one of the 7 ½ hp idlers. The tank pressure switch will turn on the second idler, and an on-delay timer which will then turn on the compressor motor. Just one 7.5 has been running the other stuff fine.

Johansen said:

more motors running at no load means any additional motor will start faster, but they provide no significant 3 phase power to a motor when its under load. you need the capacitors to offset the idler motor's internal inductance, etc, to get the voltage on the third leg up to where it does any good.

Click to expand...

Now I get it, Johansen! Not what inductance actually is, but that the motors will start great without capacitors until they have to do some work. When that 10 hp ramped up to full RPM in milliseconds I thought “Man, I don’t need no stinking capacitors! This is great. “ I shoulda known it wasn't going to be so easy.

But one way or another, I am sure I can get this done so it’ll work, once I figure out all the stuff about the resistors, how to avoid electrocution and so on. My confidence is growing with every one of your posts. I think the main thing now it to figure out the NTS resistors thing and start wiring.

Thanks again guys.
Cheers,
Jim

The resistor needs to be as high as possible. Since usually, 32V is about the limit for "safe" voltage, you might go to that value. That will raise the required resistor value.

Because the resistor sees approximately line voltage, you can assume 240 V as a first approximation. Power = V^2 / R, so that is 240^2 / 5500 = 10.5 watts. For that dissipation I would use a 20W resistor.

BUT, for start capacitors, they are not connected except a short time. Many (the electrolytic type) will burn up if connected very long, so you can use maybe a 5W resistor or even 3W resistor, IF it is a type that will stand a higher wattage "pulse". Even then, it is more than needed. use a higher discharge voltage and the resistor will be higher resistance, lower power dissipation

For any resistor that is in-circuit continuously, the 20W would be a good bet for the 5500 ohm value you mentioned (which is lower than needed).

As an example, if you could use a 10,000 ohm resistor, the same calculation gives a watt dissipation of about 6W so a 10W resistor would handle continuous operation (a tad small). For 15,000 ohms, 4W dissipation, and again a 10 watt resistor would be fine for continuous operation, this time being a slight overkill. A 5W 22000 ohm resistor would be fine, so there are several sizes for you to consider.

If the parts and wiring are only accessible with use of a tool, (screwdriver), then you need not necessarily use 5 seconds as your discharge time. It may be possible to use a longer time.

Thanks for the resistor information, JST. I will use your values for the ones in continuous operation. I just plugged those figures into that calculator to see if I was doing it right. I read a bunch about Peak vs RMS voltage and didn't know which to use so figured safe to use the higher one.

I stuck in 11 volts to see if the answer changed much from the previous one - and because I'm sure sure 11 volts won't bother me. The 5 second thing was for a discharge tool while balancing the capacitors. They should be here this week.

I got the pump and motor mounted on the tank and #8 wires run to where I will mount the box - maybe tomorrow or maybe this coming weekend.

I couldn't restrain myself, so I turned on the juice to the pump to see what happened. It cranked up to full RPM in about 2 seconds without any capacitors. But there was no pressure in the tank - just a 1/2" open port. There was no "scrubbing" or "groaning" sound from any of the idlers. It looks like a Harley and sounds like one in that tiny little shop. It also pumped a prodigious amount of air out the tank output pipe. I didn't run it but for about 10 minutes and all the motors were still cool or very slightly elevated above room temp.

So I figured I might as well get some current and voltage measurements. Amps at the motor itself; and voltage in the breaker box 20 feet away.

Amps a 25
Amps b 17
Amps T3 9

Voltage vab 239
Voltage vac 211
Voltage vbc 217

The FLA rating on the motor plate is 24.6A at 230vac.

I don't think I will need any start capacitors other than the one in the 3 hp convertor that starts off the whole shebang - especially after I get the capacitors installed.

If the centrifugal unloader is not a long enough delay, I can just use a solenoid switch operated off a timer and tank pressure air to unload the head long enough for the motor to start. Things like this are way easier for me because they are all in the visible dimension and I can usually figure them out.

Cheers,
Jim

Finally after all these months I've gotten a chance to work on the new compressor.

I decided to add the motor run capacitors, motor contactors/overloads timed relays etc. in a separate box which has "unregulated" or "untuned" 3 phase entering it from 18hp of idlers creating the third phase.

So I just started hanging capacitors like Christmas tree lights off the motor overloads and checked voltages with the compressor motor running the compressor - without start caps and with an open 3/8” air port on the compressor tank

In other words, I have been "tuning" the voltages going to the 10 HP compressor motor after the motor contactors/overloads that start it. T1 to T3 took 143 mf, T2-T3 took 289 mf.

I have not yet hooked up the control voltage going to the various relays, so I just manually shoved the contactor to the compressor motor with my thumb and took voltage readings between T1 and T2, T1 and T3 and T2 and T3 with the 10 hp compressor motor running the compressor.

I have got them all to within a volt of the same figure of 240 VAC with the compressor running but spilling air to atmosphere through the 3/8" unplugged tank port.

I am assuming that is the objective. Is this correct, or will the voltages of the generated leg drop as back pressure on the head increases?

Should the voltages involving T3 be higher to compensate for the back pressure?

The time it takes to get to full motor RPM is about 3 seconds. Though I have not wired it up yet, I have a solenoid valve between the tank check valve and pump with ¾” ports to atmosphere which will operate from an adjustable off delay timer.

So my additional questions are: will additional start capicators add to inrush current and cause more contactor arcing or less arcing? And is a three second startup time for a compressor motor too long?

Thanks for all your help.

Regards,
JimC

Well, the project is virtually done and I have good shop air again off single phase for the 10 hp 3 phase compressor. Except for a couple of momentary buttons on the cabinet door - which would be there but for the red one which arrived being normally open instead of closed. I didn't look at my "order confirmation" email or I'd have caught it before it shipped.

The converter seems to work fine.

Below is a pic of the cabinet grabbed from a gopro - my only camera. I don't own a cell phone. The three motor contactors operate in cascade fashion.

Turning on the main service panel breaker to the compressor energizes the three motor contactor feeds in the box and feeds the pressure switch that controls the operation of two compressors, the old smaller one and the new bigger one. The two air receivers are connected together.

When the pressure drops below 125psi or so, the pressure switch starts another 71/2 hp idler mounted below the box. The auxiliary contact on that motor starter initiates an "on delay" timer which starts the little compressor.

The little compressor motor auxiliary contact goes to another "on delay" timer which starts the big compressor, and simultaneously goes to an "Interval ON timer" which opens a compression relief solenoid valve between the big compressor pump and the tank check valve for 6 compressor revolutions and also initiates a " dual mode delayed interval timer" that kicks in the start capacitors after a delay of about 200 milliseconds through a couple of old salvaged relays I had kicking around, and drops them out after an interval of about 600 milliseconds.

http://www.miterclamp.com/Images/compressor_cabinet.jpg

I don’t know if all of this was necessary or even right. But Johansen’s post about inrush current and it’s effect on motor starters got me thinking that splitting up the inrush into two smaller chunks, the second after the expensive contactors were already closed, might be better than having it all hit at the same time. Anyhow, I am now getting a lot of air without any scary noises, and nothing seems to be straining.

So I wanted to thank you all for your insight and help. I am now getting 120 gallons of air from 125 psi to 170 psi in 40 seconds compared to air at 110 psi to 150 psi in over 11 minutes.

Hopefully, nothing will blow up. I’d never have been able to do it without the knowledge base here.

Regards to all.

Cheers,
Jim