# Compressor pressure switch with VFD ...



## FOMOGO (Jul 12, 2019)

Started working on using a VFD on a compressor I picked up. It has a 2hp 3 phase motor on it but I have a 3 hp, 3 phase on the shelf  that i will be using. Will be trying out a 10hp VFD I got on amazon. The 220 single phase in, and the 3 phase out wiring is pretty straight forward, but i need to run the pressure switch function off of one of the low voltage connections. Think I may have it figured out, but wondering if anyone here has done it on a similar setup, and has a method that worked for them. Thanks, Mike


----------



## JimDawson (Jul 12, 2019)

Just connect your pressure switch to GND and FWD on the VFD control wiring.  That's the way mine is set up.  I also have the acceleration set to 10 seconds for a nice soft start, and have it set to Coast to Stop.


----------



## FOMOGO (Jul 12, 2019)

Thanks Jim, I'll give that a try. Cheers, Mike


----------



## mksj (Jul 12, 2019)

You will want to set the acceleration to something like 3 seconds and ideally have it freewheel to a stop, shouldn't have a problem with an oversized VFD. Technically the power input/fusing to the VFD should be something along 125% of the VFD rated current (not the output load), but you should be able to get away with way less fusing/breaker size. On this pump size the minimum pump speed is something like 400 RPM and maximum of 900 RPM, probably would use the pump at a fixed speed as opposed to variable, so as Jim indicated just use the VFD low voltage input 1 across the NO pressure switch.  There are a few VFD parameters that will need changes.


----------



## FOMOGO (Jul 13, 2019)

Thanks for the input Mark. I've done a lot of residential single phase wiring over the years, but never any 3 phase, or much in the way of electronics. I got the 7.5 kw inverter for my lathe which is a little over 4hp, but decided to try it out on compressor first, as I will need temp air in the new shop before I need the lathe. If it works out well, I will put the vfd on the lathe, and a single phase motor on the compressor, as it will eventually be moved up to my old shop (wife's new garage and guest apartment) for tire filling and such. The directions that came with the VFD are pretty good compared to most other stuff I get from China. Have to finish up a three day stint of weed whacking tomorrow, but hope to have this up and running by Monday. If there's something I need to know, this forum seems to always come through. Thanks, Mike


----------



## MikeInOr (Jul 13, 2019)

Hmmm.... I disagree. The way I read that diagram is you push the FWD button and the motor ramps up and runs forward until the Reset button is pressed at which the motor is powered down.  So just wiring the pressure switch to the FWD input won't work.  The motor won't stop when the pressure switch opens.

Looking at the operating panel of the VFD you would press the green button to start the motor.  Then you can press either the green button again or the red button to stop the motor.  I would suspect the logic for the wired buttons would work the same way.

Most of the VFD's I have worked with have a setting you can use to make the FWD input a latching or non-latching input.  Press Fwd and run until a stop command is received or Press FWD and run until the FWD button is released.  I think you will need to find this setting on your VFD before you connect everything up.  After programming the setting simply wiring the pressure switch to the FWD and common terminals should work propery.



But I certainly could be wrong!


----------



## mksj (Jul 13, 2019)

There is what is known as 2 wire control which is almost always the default programming setup for VFDs, this requires a sustained closure of the switch contact to run in the direction of the programmed. You can reprogram the inputs to 3 wire control, where a momentary closure of the run command will give you a continuous run, and hitting the E-Stop or stop command terminates the run command. A separate sustained switch is typically used for the reverse command. The 3 wire control is often used with mills, although 2 wire with a 3 position switch can also be used.

The VFD he purchased is 2 wire control with the default settings. The reset button is used to reset the VFD if it cause into a fault mode say because of an over current or over voltage fault. The reset input is setup for remote operation by default.


----------



## MikeInOr (Jul 13, 2019)

I would have expected a switch symbol "_/_" to denote a non-latching 2 wire control and a button symbol "_-_" to denote a latching 3 wire control... but this is not my area of expertise.


----------



## tq60 (Jul 13, 2019)

Add an additional pressure switch set to lower to be determined pressure.

Program vfd for higher frequency until pressure rise causes increase of motor current above to be determined value.

Then when it opens speed reduces to normal. 

Sent from my SAMSUNG-SM-G930A using Tapatalk


----------

