• Welcome to the new SAOCA website. Already a member? Simply click Log In/Sign Up up and to the right and use your same username and password from the old site. If you've forgotten your password, please send an email to membership@sunbeamalpine.org for assistance.

    If you're new here, click Log In/Sign Up and enter your information. We'll approve your account as quickly as possible, typically in about 24 hours. If it takes longer, you were probably caught in our spam/scam filter.

    Enjoy.

Controlling Variable Frequency Drives

Bill Blue

Platinum Level Sponsor
The 3 hp 220v motor on my air compressor has died. The compressor is an old industrial unit and I have the 5hp, 3 ph motor that was on it. It is in good shape and I would like to use it with a single phase to three phase VFD. My problem is I don't know how to wire the VFD so the compressor will cycle automatically. It is my understanding the VFD will fry itself if the output current is switched. If the current to the VFD is interrupted, the motor will shut down but will not restart when input current is restored. I can think of two solutions:
1. Restart the compressor manually by pressing the "Start" button on the VFD.
2. Us an unloader valve. The compressor, once started, would run until shut down manually.

Any better ideas?

Bill
 

RootesRacer

Donation Time
Modern VFDs have inputs for doing just that. Program an input connected to a pressure switch that pulls a 10k resistor to ground when you hit pressure (or the opposite). Connect VFD 5v supply to the 10k, the other end which gets grounded at setpoint goes to the input.
 

Bill Blue

Platinum Level Sponsor
Thanks RootesRacer. Let me see if I understand how this works.

The VFD has a 5v output. As long as the input is receiving this 5v after being filtered through a 10k resister, the VFD will output three phase. Grounding the resister stops the filtered 5v input and the VFD stops the three phase output. No harm done, awaiting filtered 5v input to resume operation.

Bill
 

RootesRacer

Donation Time
Thanks RootesRacer. Let me see if I understand how this works.

The VFD has a 5v output. As long as the input is receiving this 5v after being filtered through a 10k resister, the VFD will output three phase. Grounding the resister stops the filtered 5v input and the VFD stops the three phase output. No harm done, awaiting filtered 5v input to resume operation.

Bill
Here is a "schem"

VFDSupply+--------[10k]-----/--------[PressureSw_Closed above SP]---------VFDSupplyGround
VFDEnable--------------------/

You will need to program the VFD to run when the VDFEnable signal is "true"
You can put a switch across the pressure switch to shut the system down before it hits setpoint.

VFDEnable will go to VDFSupply+ through the 10K pullup.
VFDEnable will short to ground when the pressure hits setpoint.
 

RootesRacer

Donation Time
Here is a "schem"

VFDSupply+--------[10k]-----/--------[PressureSw_Closed above SP]---------VFDSupplyGround
VFDEnable--------------------/

You will need to program the VFD to run when the VDFEnable signal is "true"
You can put a switch across the pressure switch to shut the system down before it hits setpoint.

VFDEnable will go to VDFSupply+ through the 10K pullup.
VFDEnable will short to ground when the pressure hits setpoint.

You can also program a frequency ramp to let the motor soft start when the drive is enabled.
 

Bill Blue

Platinum Level Sponsor
Why not just run the 5v + 10k resister through the pressure switch? The compressor is setup with a nice magnetic switch. In operation, I would energize the switch (juice to the VFD) when I want compressed air, let the pressure switch do its thing with 5v, then de-energize when through.

Bill
 

RootesRacer

Donation Time
Why not just run the 5v + 10k resister through the pressure switch? The compressor is setup with a nice magnetic switch. In operation, I would energize the switch (juice to the VFD) when I want compressed air, let the pressure switch do its thing with 5v, then de-energize when through.

Bill
You wouldnt even need a resistor then, but then there is no relatively low impedance signal that clearly says ON versus OFF since in the open switch state, the VFDEnable input will float (high impedance) and the VFD will have a slow or ambiguous response to that state.

In short, doing it the way I suggest is a bit more robust and the common way to use VFDs in a variety of equipment.
 

Bill Blue

Platinum Level Sponsor
I chickened out and bought a 5 hp Baldor. It boils down to my inabilities. I could purchase a VFD and hopefully muddle my way through the electrical hookup, a language in which I am not proficient. Or for $130 more, I could order a bolt in Baldor (not some suspicious, no name) and get it work in a few minutes. Peace of mind, and maybe common sense, won the day.

Bill
 

RootesRacer

Donation Time
I chickened out and bought a 5 hp Baldor. It boils down to my inabilities. I could purchase a VFD and hopefully muddle my way through the electrical hookup, a language in which I am not proficient. Or for $130 more, I could order a bolt in Baldor (not some suspicious, no name) and get it work in a few minutes. Peace of mind, and maybe common sense, won the day.

Bill
Sounds like a safe bet
 

Bill Blue

Platinum Level Sponsor
All is good. Bought the 5 hp Baldor from ironwing on Ebay, a 1725 rpm motor. Delivered Friday afternoon, installed and operational by noon, Saturday. Total cost: $337, includes two bucks for gigantic wiring nuts need for the Baldor motor wires. Reused the motor pulley and drive belts that were on the compressor when its power came from the 5 hp, 3 phase motor. Need to reset the pressure switch, bump it up about 20 psi from where it set while using the 3 hp motor.

Bill
 
Top