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VFD Start/Stop Considerations (Limitations?) for a Duplex/Triplex Air Compressor

sae8425

Aluminum
Joined
Mar 30, 2011
Location
western pa
I am in the process of designing a MUCH better compressed air system that will work within the following constraints –

WIDELY varying air consumption requirements: for weeks on end 5 to 15 cfm (typical 'light' shop uses) would
more than suffice; then for a day or two I would use 40 to 50 cfm (blast cleaning) for hours at a stretch.

Powered on for at least 16 hours a day (often 7 days a week) with auto On/Off cycling (NO unloaded running)

200 Amp single phase supply (which the utility is willing to upgrade to 400/320 Amps for free) –

I believe a duplex/triplex system with 2/3 x 7.5hp compressors would provide more flexibility than a single 15/20hp compressor.

I have found an alternator/sequencer that will control up to four air compressors.

ALL the 'turn key' solutions that provide enough CFM incorporate VFD's fed by single phase power and 3 phase compressor motors.

So; being somewhat of an old dog, I'm now having to learn a new trick: VFD technology. And my grey cell is starting to throb!

The more I read the more I realize just why I've shied away from VFD's: sorting the wheat from the chaff is highly problematic.

Document, after document, after document from VFD manufacturers state that VFD's have NO hourly start/stop limitation! NONE!

However, in numerous forums (PM included) a VERY contrary view has been expressed by numerous thread respondents.

These commentators mostly say VFD on/off restrictions are necessary to limit inrush current to the charged DC bus capacitors.

Additionally, some say that using digital inputs, as opposed to the auto-start function, makes a difference in Start/Stop cycles. Eh?

I can read ad nauseum about VFD harmonics, overloading, and sizing considerations; but NOTHING about Start/Stop limitations!

Why can't I unearth a SINGLE authoritative VFD industry document that states a VFD has ANY hourly start/stop limitation?

I've had numerous detailed and lengthy conversations with VFD suppliers, and this constraint has not been mentioned even once!

Why not? EVERY VFD supplier I've talked to was told EXACTLY what I wanted to achieve. Is there some great marketing conspiracy?

The internet is replete with a plethora of seemingly successful applications of VFD's to air compressors of ALL capacities and types.

So, I have a hard time accepting that VFD's are inherently incompatible with air compressors and shouldn't be used with them.

What air compressor issues do I need to consider/resolve to ensure that an appropriate VFD lives a long and happy life?

Staying below the NEMA or compressor manufacturer motor starting limits will likely require some SERIOUS air storage capacity!

I'm not really keen on rotary screw compressors at all: I would MUCH rather continue to use rotary vane compressors.

If I HAD to use piston compressors, they would be pressure lubricated Quincy QR390's running at 400rpm with 7.5hp motors.

Would using a 20hp VFD be better than using a 15hp VFD (both running on 1Ø power) when driving the same 7.5hp 3Ø motor?

Does any forum member have a good insight as to who's right, who's wrong, and/or who's on first?

Many thanks to any forum member who takes the time to respond.

P.S. I'm NOT going to get a 100-150 cfm portable diesel compressor to cater for my infrequent periods of higher air utilization.
 

jwmelvin

Aluminum
Joined
Sep 5, 2018
Location
northern Virginia
It sounds like you aren’t distinguishing between VFD power on/off cycles (which stress the capacitors) and motor run/stop cycles (which should have no limitation). Power up your VFD and keep it powered up, then use control signals (low voltage) for motor run/stop.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
It is not start/stop cycles for the VFD, it is power on/off cycles for a VFD. Bottom line is you leave the VFD on all the time unless you are doing maintenance or gone from the shop. Otherwise you control the VFD on/off run cycles via a programmed input. Issue gets a bit more complex with multiple motors/compressors, and for the most part for a 7.5 Hp 3 phase motor, there are very few native single phase input VFDs. So you need to use a 3 phase input VFD in derated mode if providing single phase input, you need a minimum upsizing of a factor of 2, and sometimes more depending on the manufacturer. I do not see the advantage of using a VFD for variable speed, air is air, either fill up the tank quickly or slowly on a VFD over a longer time.

Have you considered a single phase duplex 7.5 Hp, such as the Quincy Duplex Air Compressor — 7.5 HP, 230 Volt, 1 Phase, 120 Gallon Horizontal, Model# 271CC12DC? Primary down side of single phase motors is start/stop cycles per hour and the failure of the start capacitors. Alternative is a single phase rotary screw, they have 10 Hp variable speed with refrigerated dryer rated at 45 CFM at 100 PSI. They will be much quieter and do well running continuously, maintenance is more costly. EMAX 10-HP 120-Gallon Rotary Screw Air Compressor (208/230V 1-Phase) Model: ERS0101201

Alternative might be to equip the shop with a Phase perfect three phase converter to run the compressor and any other 3 phase machines, and just get a 3 phase 7.5Hp duplex compressor or rotary screw.

A similar discussion.
VFD for duplex compressor
 








 
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