Air Compressor and a VFD
I am about to aquire a Quincy 7.5 HP two stage compressor. It is 3 phase with a balador motor that says it pulls 20A. This is a new unit. I want to run it with a VFD as I have them on a two mills and everything works OK.
I plan on getting a TECO MA 7200 15HP which according to the supplier will work best with an air compressor application like mine.
My question is what experience does anyone have with a VFD driving a large air compressor?
I have run one compressor-5 hp- on vfd for about 10 years no problem. Do not use the vfd power as part of control circuit. Coils, relays, solenoids will burn up with vfd power. Do not exceed rated compressor rpm. This usually means 60 Hz max frequency to compressor motor.
Do you "Soft Start" your compressor?
I hooked up a 15hp VFD to a 10hp recip compressor just to try it out, it worked fine even down to 20 hz.
I did soft start mine, 2 sec accelleration, the compressor had an unloader on a timer and I did not want the VFD to trip on overcurrent.
What's the best way to power an air compressor,RPC,VFD?
Honestly, an aircompressor doesn't need a VFD.
If I had an aircompressor that had a 3 phase motor on it, and only had single phase power, I'd put a single phase motor on it. I really don't see much point in trying to run a VFD on a compressor, unless you were going to put an air demand based speed control on it to save power, or make more airflow. Or use the VFD to overspeed the motor at low tank pressures if the compressor head is rated for the RPM. For just your standard garage air compressor, I personally think it's better to stick a single phase motor on it. Unless you allready have an RPC that can run your compressor, and whatever other machine at the same time. Then I'd run it off the RPC.
With a VFD you can do some NEAT things with a compressor. You can set it up so the relay inside the VFD switches an air solonoid valve as an unloader so that when the motor hits it's target speed, it closes. This allows you to get rid of the unloader, and have a LOT LESS inrush current that might trip breakers. You could also overspeed the motor IF the compressor head itself can handle it, and have it slow down to normal speed as the pressure builds. Like say set the motor to 80 or 90 hz when it hits the starting pressure, and have it slow down to 60 when it gets near shutoff. You'd have to build your own pressure sensor, but that's not that hard. Some compressor heads come with smaller motors, and spin them at slower speeds on different model compressors. You also have the advantage of having as much time as you need to get the motor up to speed before the unloader closes. In other words, you can get a LOT more airflow out of a smaller motor with a VFD. If I were going to use a VFD on an aircompressor, I'd take FULL advantage of this. You can also program different modes. Say you want to give the compressor less abuse when the airflow isn't needed. You can set it to run slower when you only need minimal airflow. Or you can have a max airflow mode where you overspeed the motor when the pressure drops. You can also put a temperature sensor on it so that if it does get too hot, it slows down, or shuts off. You could also put a knob on it to make the airflow variable. A VFD could make a compressor a LOT more efficient, and effective.
At the very least, you'll probably have to put an electric unloader on it, because I dunno if the motor will be able to acelerate fast enough to not cause an overcurrent trip without it. The VFD has a relay output you can use as an electronic unloader. You'll also have to wire the pressure switch up the the 24v terminals on the VFD. It will work just doing those 2 things. DO NOT use the pressure switch to switch power to the VFD or the motor. You can try it without an electronic unloader first if you like, but I wouldn't count on that working.
thanks for the advice. Junkyard - much of what you wrote is what I was thinking might be possible. According to Quincy I can run the compressor down to 460 RPM from its standard 1042. the 460 is min required for lubrication to work. The motor is a Balador 1725 3ph and according to TECO the 7200 is best suited for hardstarting like an air compressor. What I was told was to just wire up the remote start switch to the pressure switch and leave the acceleration set to factory, and change the amp settings to match the motor. I was not going to try setting anything else up until I got this simple approach working.
The Quincy should be here in about three weeks, when I get it hooked up I will let you know how things worked out.
Frequency drives have been used in industrial compressor control circuits for years ... now even little machines are available with them. On a recip, you have the advantage of soft start & infinite speed control. On a rotary screw (which is only shut down at the end of the shift) the drive will throttle the motor up & down as demand varies.
15HP, derated 50% for single phase input, is the correct choice for a 7.5HP motor.
I am confused: why you use 15hp VFD for a 10hp compressor? That's a waste of money. I think 10hp VFD will be enough for 10hp compressor.
Originally Posted by HelicalCut
Richard, You are correct, but in this instance, the poster is combining single phase to three phase conversion in the VFD selection. In order to avoid problems with the electrical elements inside the VFD, common wisdom is to double the VFD capacity. Phase conversion sends the amps that are supposed to be carried by three components through only two.
Originally Posted by richardhuang