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Old 01-11-17, 02:07 PM   #1
oil pan 4
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Default Eliminating start up inrush current

I may try closed circuit soft starts but the way capacitor start motors work this may only be marginally effective. I have a feeling if I really want to do this its going to involve a 3 phase VFD.

I have found that 240v single phase input, 3 phase 240v out put variable frequency drives have been getting a lot cheaper.
I built all my 240v air compressors out of junk I had laying around, the only thing I really had to buy for all of them was pulleys and the shut off pressure switch.
The main problem with them is the single phase capacitor start, capacitor run motors have really bad inrush current.
It also just so happens all my compressors are in the size range for the most common VFDs.
Don't worry about getting 3 phase motors. Small single digit horsepower, delta wound, 3 phase motors that can be wired for 240 or 480v are everywhere because most people cant use them and they are super common in industrial environments. I would prefer a 4 wire wye motor but I don't think its going to happen unless I want to cough up a lot of $.
Reason why is I would like to take steps to move off grid, connecting big power hungry air compressors to an expensive split phase power inverter, seems like not a good idea.

Right now the only way I have to eliminate compressor inrush current is use my 5hp gasoline powered air compressor which can more than match the output of any one of my 240v powered air compressors, but makes a lot of noise and uses some gas. Or use a 12v compressors my Gast air compressor uses a ton of power and doesn't produce a lot of air and my ARB compressor is just tiny. Other way to protect an expensive inverter from compressor inrush is I could connect the air compressor to a huge generator like my 17.5kw generac or my 7kw troybuilt those do not care too much about high inrush, across the line starting but those suck down a lot of gas and make a lot of noise and don't want to run them unless I really need a lot of air.

I would like to greatly reduce the inrush current on one of my compressors with a VFD. I figure I can have the motor kick on at 20 or 25Hz ramp up to the desired speed, throttle down to 20Hz then use the pressure switch to release pressure in the line and hopefully signal a turn off signal to power down second or 2 later.
Depending on the features and inputs I may be able to have it run at 70Hz part of the time when its building up from 0 to 70psi, 60hz from say 70 to 90psi then 50hz tapering down to 20Hz from 90psi to shutoff pressure.

I would need a 3 or 4hp, 4 pole motor that could be wired for 240. These are real common.

Some terms to clear up.
Unloaded motor = no belt, motor not driving anything.
Unloaded compressor = starting with 0psi in the tank or using a dump valve to release pressure while the compressor is turning.
Soft start = any mechanical or electrical means to reduce starting inrush current and/or mechanical stress at startup.


Last edited by oil pan 4; 02-13-17 at 08:01 PM..
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240v, air compressor, compressor, inverter, vfd

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