Air compressor VFD drive conversion - Page 1
Forum section: electronics / projects

author: Veketti
author_rank: Regular Contributor
timestamp: September 09, 2023, 07:56:11 am
content: Hi,I’ve got 5.5kW screw air compressor which was originally fitted with star-delta starter. I didn’t like the blinking of lights (twice) whenever it started so bought second hand Vacon VFD. Rewired everything and it works now as a soft starter. I used the VFD’s programmable relays to control bypass magnet valve and hour meter. No more light blinking so that was success.I’ve been thinking of utilizing the full potential of the VFD. Adding extra pressure sensor, microcontroller to read it and send 0-10V signal to VFD to control the motor speed according to the air demand. Program MCU like following. Full speed to 9 bar, then every second or two, measure pressure and if it is higher than previous value, drop the frequency by 5Hz. Do this until the speed matches the air demand or lowest specified speed. If the pressure still rises with lowest speed run it up to the pressure switch which cuts at 10 bar. Etc…Few doubts that are holding me back. First, the motor is duty cycle S6 75% (Soga MT1 112MB/2). So it is one frame size smaller than S1 duty cycle motors, which are designed for 100% duty cycle. Second is, how do I determine the lowest frequency I’m able to drive the motor without overheating it. Less speed equals less cooling from the fan. I’ve found one article which stated that 1:4 speed is ok for Totally Enclosed Fan Cooled (TEFC) motors. That would be 12.5Hz when we have 50Hz. There is extra fan attached to the belt pulley, so that enhance the cooling quite a bit. So I’m turning to you motor and drive experts that would this end up in fried motor or is it ok to proceed with the variable speed? And what could I use as the lowest Hz value?Thank you in advance

author: wofritz
author_rank: Contributor
timestamp: September 09, 2023, 08:21:16 am
content: We have a 5.5 kW motor on the flue gas fan of our 500 kW wood chip boiler. The frequency range in the VFD (Siemens Sinamics) is set to 5 - 50 Hz, but I have never seen a frequency less than 7.5 Hz during operation.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 09, 2023, 01:11:05 pm
content: Add an external fan and a sensor to monitor the motor temperature.

author: BradC
author_rank: Super Contributor
timestamp: September 09, 2023, 02:49:29 pm
content: Make sure your compressor has a lubrication system that can cope with your proposed speeds.

author: Veketti
author_rank: Regular Contributor
timestamp: September 09, 2023, 05:21:13 pm
content: Quote from: BradC on September 09, 2023, 02:49:29 pmThat is very good point. Lubrication is based on the pressure, but whether flow magnitude has impact, I don't know. For this reason I set the acceleration time to the same 4 seconds that there was between the star-delta timer as I think that there's no lubrication for the screw until the pressure build up. I might test shorter acceleration time and see the minimum without lights blinking to even further reduce wear. It's 2840 rpm motor and the acceleration is basically no load situation due to bypass valve being active so fairly short time should be ok.I'll try to ask Atlas Copco whether it can be ran with lower rpm... However I highly doubt they are willing to give me answer or they answer "absolutely not" as VFD controlled compressors are around double the price and they most likely don't want anyone doing similar thing by their own. One thing could be to check if the same part # screw can be found from VFD versions as well.