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Compressor Duty VFD for 3ph

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
I am considering replacing the RPC connection to my 5hp air compressor with a VFD. The compressor seldom runs, as most of my shop needs are satisfied with the little 220 1ph "tank on wheels". But, shop space is getting tight, so I'm thinking to have only the larger unit plumbed to all points, wired in a way that I need not remember to turn on the RPC before energizing the compressor. (Oh to have 3ph off the line ;-)

It seems that compressor duty would fall under the "hard starting' designation, though I have never had any trouble with the 5hp RPC in service now.

I have a NIB Siemans VFD that might suit the task
I am not actually considering using the drive to vary the motor speed, though at times I consider slowing the darn thing down to a more rhythmic pace.

Will anyone care to comment on the application of VFD to compressor service?
TIA

CalG

PS I did a "search" of this forum with the key word "compressor" and got back the entire form list. How to narrow the search?
 
"It seems that compressor duty would fall under the 'hard starting' designation ..."

It does.


"... though I have never had any trouble with the 5hp RPC in service now."

An alternative, as you really don't need the variable speed feature, rather a phase converter feature, is to convert your three-phase motor to 12-wire, and then to use a Stealman Electric Service-type static converter. This will give you at least 90 percent of the motor's three-phase HP rating, while running on single-phase.

The H.A. Steelman patent, and the Steelman Electric Service documents are available on-line.

References have been made to these in the Monarch Forum.

Essentially, the three-phase motor is converted to the equivalent of a capacitor start/capacitor run motor.

As many may know, a single-phase capacitor start/capacitor run motor has a higher starting torque, and a higher running power factor than a three-phase motor (whether run on three-phase power, or on converted power), in most applications, which certainly includes compressor applications.
 
I am considering replacing the RPC connection to my 5hp air compressor with a VFD.

Will anyone care to comment on the application of VFD to compressor service?
TIA

Short answer is "what a waste". A surplus single phase 5hp motor is going to run $50-100.
Many of those large tank compressors are sold with three phase motors. Swap the
three phase motor out and you will not look back.
 
So long as there is a reasonable flywheel effect on the compressor drive pulley and you have proper start-up unloading valves with a decent size outlet manifold between compressor and reservoir there will be no great start-up load and pretty much any decent VFD will handle the job. Typically for a twin cylinder unit the manifold needs to be about 4 times the cylinder volume. Most inexpensive units have either no manifold or one far too small! Provided the manifold bleeds down between restarts and the unloaders stay engaged until the compressor is securely up to speed you will have no problems. Best unloader system is the Atlas Copco one that my KE series compressor had where the valves are direct in the head to destroy both induction and compression. Many of the manifold mount ones are a bit wimpy.

Clive
 
Peter

A web search for H.A. Steelman was unfruitful. Steelman Industries in Tx came up, however I see only power factor correction caps and RPC suited to this topic.

Any ideas on searches?

A static converter at 90% "should" do the job. (Cap start cap run sounds interesting)
I have built and use such devices. But a "twelve lead configuration" might take a complete motor rewind. hmmm.

Regards

Cal
 
Rons

I'll take the first 184t frame unit I can come across for $50. (good condition of course)

Sure they are out there, just not here ...yet;-)

Send me one if you find it! (No add shipping fees allowed ;-)

CalG
 
I'll take the first 184t frame unit I can come across for $50. (good condition of course)

Sure they are out there, just not here ...yet;-)

Send me one if you find it!

Send a few my way as well :D

Large single phase motors bring a premium whenever they are sold. 5 hp on eBay will go for $100 + shipping.

-------------
Barry Milton
 
Rons

I'll take the first 184t frame unit I can come across for $50. (good condition of course)

Sure they are out there, just not here ...yet;-)

Send me one if you find it! (No add shipping fees allowed ;-)

CalG

CalG,
Good condition or not, it does not make much difference. I can make any motor
look like and run like new and so can you. The standard price per hp for a three
phase motor is $10 (surplus). People generally do not want these types, single
phase motors are the ones most are looking for. I witnessed this behavior in
a large surplus electronics house I went to for more than 10 years. The owner
also voiced the same thought. A used single phase motor can be made to look
and run like new for a small "fistful of dollars" more.:D
 
"Found the Steelman page. Thanks"

Good.

Using Steelman's approach, isolated facilities such as petroleum rigs and irrigation pumps have been converted to single-phase, and many of these exceed 10 HP, the usual single-phase maximum at this point in time.

Some of Steelman's products operate on a non-standard, but perhaps "industry standard" voltage of 720, which is ( 3 ) ^ 0.5 * 440. The goal of this would be to transmit more power, while using less copper ... a traditional objective of power transmission designers.
 
VFD with an unloader works. The unloader can be wired to be released when the VFD attains a preset frequency.

Another option is to wire your compressor so that the pressure switch requests the RPC to start and once the RPC gives a power good signal to a timer 0.5 seconds later the compressor will start. If the RPC is already running then the system should ignore that state and start the compressor immediately. A another timer can shut down the RPC in 5 minutes if no more start requests come in, sounds complex? but it is basic industrial control stuff with relays and timers or a PLC if so inclined. VFD's are actually better for compressors running between 50 and 100% output as they can slow the machine down before having to actually stop the compressor I maintain constant pressure in a blower system using a VFD and process controller, the blower changes speed in response to pressure changes.
 








 
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