Control options for compressors with single phase & 3 phase AC electric motors.

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PBcatfish

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separate units always, and the bigger the better. As big as you can justify/afford. Will minimize the changes of filter and purge cycles on the coalescer. The cost/filter size is not linear since it's just a longer tube, so I would always recommend to go bigger.
I just got done looking up a few of the different options from LF. I was surprised to learn that a 20% increase in equipment cost got you a 400% increase in cartridge life compared to a P0. That was an eye opener.
 

Curious_George

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I was hoping that I was misunderstanding something about the way that they worked. The two I looked at each had 2 unloader valves. One dumped the little cup under the second stage & the other dumped from the fitting at the bottom of the P0 tower. Does that not dump all the pressure out of the filter tower?

I was looking at a Bauer Jr II & a Bauer Utilus 10.

Hey PB. The P0 filter is a combination stack. It combines a third stage coalescer as well as the filter stack in a tube-within-a-tube layout. And so an auto unloader needs to drain both the P0 and the second stage coalescer. The bleeder closest to the PMV drains from inside the inner tube where the filter is located and will rarely or never have water. It is part of the design to always have water inside the outer tube of the P0 which is one of the reasons I’m gathering parts to dump the one on my U-10.

There is not a check valve between the third stage and the P0 on the basic version Junior or Utilus 10. My third stage piston isn’t bubble tight so the P0 leaks down to zero after 2-3 days - back through the crankcase. I’m guessing this is what’s happening since I can’t find any other leaks.

I usually use used ones removed from compressors not worth repairing.
Tracy, I’ve heard about these mystery coalescers. When you going to find one for me? :) One thing I like about your above link to the new LF setup is the diagram that shows where all the parts go (check valve, OPV, PMV, etc.). I found this one, but not sure it’s appropriate.

A shameless plug to anyone who’s interested (sorry): I’m going to have a P0 for sale soon and looking for a coalescer and some compressor small plumbing parts in another thread.
 
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Hey PB. The P0 filter is a combination stack. It combines a third stage coalescer as well as the filter stack in a tube-within-a-tube layout. And so an auto unloader needs to drain both the P0 and the second stage coalescer. The bleeder closest to the PMV drains from inside the inner tube where the filter is located and will rarely or never have water. It is part of the design to always have water inside the outer tube of the P0 which is one of the reasons I’m gathering parts to dump the one on my U-10.
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Thank you for confirming the way I thought the P0 was supposed to work. In the manual drain version of the U10, I believe that it works as you describe & if cylinder valves do not leak, then the stack holds pressure.

In the auto drain version of the U10, that I am now looking at, the second dump valve drains the bleeder in the P0 that is furthest from the PMV.

My issue, is that when the compressor shuts down, both dump valves open as soon as power is lost & they stay open. All pressure is lost from the P0 stack, as there appears to be no check valve between the two chambers inside it. When the coalescer in the P0 is drained, the pressure in the center of the stack & the PMV seems to back flow through the filter cartridge and disappear out the dump valve.

It's possible that I am wrong in my assumption about there being no check valve inside the P0. That possibility has me wanting to ask you for a small favor. If you could please drain the left petcock in your P0 the next time you shut down your compressor & drain it all the way down to zero pressure, then open the right petcock, that would test my theory. If you get no pressure from the right petcock, then my assumption would seem to be validated. If you do get substantial pressure, then the unit I am looking at is either different, or defective.
 

Curious_George

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If you get no pressure from the right petcock, then my assumption would seem to be validated.
Both the petcocks open into the same airspace. There is no check valve separating them. My added red lines below show where they vent from. The "tube within a tube" as I called it earlier is open at the top so nothing to keep either one from emptying the pressure from the entire P0 canister.

The path of the air in the P0 enters from the silver tube #2 then goes between the inner aluminum tube #6 and the filter body before entering the filter through the holes in the bottom. It then exits from the bottom center fitting in the filter into the P0 base and then to the PMV.

edited to correct the number of the silver tube where the air enters into the P0.
70730A66-EB9E-4105-BDDB-CF1A5251D0F6.jpeg
 

BRT

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I have one of those if anybody wants to buy it.
 
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I had an opportunity to hang an ammeter on a 3hp Jr today that was running off of 220vac single phase. Up to 2kpsi it drew about 7.5 amps. When it hit 3kpsi, it was drawing 8.25amps. When the unloader valves dumped the entire air pressure load out of the filter canister, the amps dropped to 4.3.

Considering that even a crummy little 1.0sf single phase motor of that size usually has a nameplate rating around 12-13 amps, and higher SF motors often carry ratings more like 17 amps, the compressor would seem to be rigged with a conservative pulley size. Actual compressor shaft speed measured 2040 RPM with a hand held tachometer. Rated RPM is 2100 with this motor and 2300 with the gas or diesel motor.
 
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Magnetic Motor Starters

The next step up from a manual motor starter, is a magnetic motor starter. These are not as simple to wire nor inexpensive as a manual starter, but they have some advantages, especially if you want to add some control features, like automatic shut off at a preset pressure.

A magnetic starter is basically a large mechanical relay, called a contactor, with a motor circuit protector attached to it. It frequently also has auxiliary contacts for operating other "pilot" devices. Most contactors allow you to add auxiliary contacts if you want to. The aux contacts can be normally open or normally closed. They are generally "pilot duty" & can handle no more than 10 amps.

The contactor itself is rated either in amps, or in HP at a given voltage. The 3 phase HP rating will be much higher than the single phase HP rating at a given voltage. Higher voltages will have higher HP ratings.

The contactor is operated by voltage that is applied to it's coil. They normally have a wide variety of different coil voltage ratings available. You can often change the coil if it is not set up for the voltage that you need to run off of.

The motor circuit protector, often called a thermal overload relay, or simply the overload relay, is a device that trips when too much current is being fed to the motor. It typically has a normally closed contact that is wired in series with the wire to the contactor coil. When that contact opens, the contactor drops out & no more power is sent to the motor.

The magnetic motor starter normally is controlled by two push buttons for start & stop. These are pilot duty pushbuttons. The start is tropically normally open. The stop is typically normally closed.

If you want to add a pressure switch to shut down the compressor at a given setting, you get one with a normally closed contact & wire it in series with the coil wire.

Most modern overload relays will have a dial to let you set the trip current withing a certain range. Most are calibrated for a motor with a 1.15 service factor. If your motor has a 1.0 service factor, you should set the dial to 0.87 x nameplate amps, if you want to properly protect the motor from burning out.
 
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PBcatfish

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This guy highlights some of the reasons why a 3 phase motor & a drive can be advantageous. He manages to run a Bauer Jr with a 2hp motor off a drive that runs off a 3kw inverter that runs off a big battery bank. He takes about 45 minutes to fill a scuba tank, so he is belted for a slow running block, due to the small motor.

I'll do a write up on drives when I find enough time.
 
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PBcatfish

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Variable Frequency Drives (aka "VFD;s" or just "Drives")
These are electronic control boxes that let you run AC motors at variable speeds, not just the nameplate RPM.

They also allow varied acceleration rates, which can reduce start up currents & allow a big motor to run on a small power source. Almost all of them are only compatible with 3 phase motors. Most use 3 phase power as a primary power source. Some can use single phase primary power. A few have been set up to use DC primary power. Most can run motors at lower voltage than the primary power. A few can run motors at higher voltage than the input power. Most will let you run variable torque as well as variable speed, but that is not of particular interest when running a compressor. Some can communicate over a wide variety of comm buses like device net, Profibus, Canbus, Ethernet, Fiberoptix, etc., but the ones I will discuss here are some of the most basic that only need a couple of buttons & maybe a turn dial to control them.

There is a wide variety of drives out there these days that let you also control other aspects of motor response, but those are beyond the scope of running something as simple as a compressor.

When selecting a drive, you need to start with a few basic questions -
Is my motor the proper type for the drive (usually you need a 3 phase motor)
Is my motor the correct voltage for the drive?
Is the drive's capacity big enough to handle my motor? The rating can be in HP or Kw (1hp = 0.746kW)
Is the drive controlled by push buttons & maybe a dial that are on the unit itself, or does it need external controls?
Do I have space to mount the drive somewhere? Preferably mount it where it will not get shaken to death.
Do I have a power source that can run the drive. Some 3 phase output drives can run on single phase input, but the amps in will be much greater than the amps out in this case.
Is my compressor in a wet location? If so, is my drive rated for this? Or do I need an enclosure? Will an enclosure derate it's power capacity?

A few advantages to consider would include -

Drives can be useful when you want to run a 3 phase motor from a single phase power source. This is of interest to anyone with a 3 phase compressor that wants to us it in a home that only has single phase power available.

Drives can start up the motor slowly, which reduces starting current & will likely allow you to run a 13 amp motor on a 15 amp circuit that would have tripped out from a normal motor start up. Locked rotor current at motor start up is typically calculated at 6 times nameplate current. Most supply circuits can take a certain amount of inrush for a short time, but motor start loads can be pretty big and often trip out circuits that are close to nameplate current.

The variable speed feature may be useful in nitrox blending operations, for fine tuning the mix, provided that the compressor is OK to run at the different speeds. I don't currently have information on what range of speeds compressors can run at. I have noticed that a factory original Bauer Jr with an electric motor is set to run a couple of hundred RPM slower than one that runs off of a gas or diesel motor, so there does seem to be at least some range in acceptable speed.

The external control features on most drives make it easy to tie in something like an automatic shut off switch that is controlled by pressure or temperature. Most can be set to monitor things like the current they are consuming, line voltage, heat sink temperature, etc. & many can be set to activate an alarm or shut down the unit when a selected parameter reaches a preset value.

A slow start up may reduce battering of floating pistons, & may reduce other mechanical wear in the compressor.

When running below the nameplate RPM of the motor, the drive is in the constant torque mode & full motor torque is available. When running above nameplate RPM, the drive is in constant horsepower mode, & the torque reduces as speed increases. When running below nameplate RPM, you are running at reduced Horsepower. Because of this, it may be possible to run a 5 hp compressor on a power feed that is only suitable for 3hp, if you run the 5hp motor at sufficiently reduced speed, & therefore sufficiently reduced effective hp.
 
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PBcatfish

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Has anyone found this stuff helpful? Or am I just spinning my wheels?
 
https://www.shearwater.com/products/teric/

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