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.
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.