Motors & Drives Information

I have a 60 HP compressor, I don’t need such a large compressor, will an ac drive reduce the HP? Can I fit a ac drive to this thing?

Discussion Group Answers

• An AC drive will reduce motor speed. By reducing the speed, you could adjust the compressor to the desired flow, and the power requirement will be reduced proportionally. Check the motor input current, which approximately follows the power consumption.
• I assume the compressor is a reciprocating (positive displacement) type. If it a screw operated in parallel with other machines, you may need to look more closely.
• It is a screw type compressor. Does that make a difference for the ac drive add-on?
• AC drives on centrifugal compressors for chillers are routinely used…not sure of what happens when speed is reduced for a screw type. It will reduce HP, but I’m not sure what happens to the compression.
• Reducing the HP will reduce CFM of the compressor. In reality, it’s the reduction in the amount of work required (cfm, pressure, etc.) that results in the reduced horsepower.
• That is what ac drives are for! I use them for my compressors and no more blown fuses, tripped breakers, etc. If properly programmed, you cannot kill your motor no matter what happens to your power or compressor. Your motor will run much cooler and use less power even at 60 HZ. The compressor will have to be positively unloaded on start-up with solenoid valves on both stages. They can be controlled with any ac drive.
• I would prefer ac drives for screws rather than reciprocating types. I have been discussing with manufacturers about variable speed reciprocating types but no concrete decision so far. Some drawbacks I presume are 1) Ineffective lubrication, if the oil pump is connected to crankshaft; and 2) Increased vibration due to imbalance at other speeds than designed.
  I saw and used good many refrigeration chillers (screw and centrifugals) with variable speed application. I have reservations for using ac drives with screw air compressors. Proper receiver sizing, minimization of leakage and correct usage pressure with compressor on/off (or load/unload at least) will give better results and at reduced investment. Power consumption reduces if we reduce mass flow rate through a compressor.
• Another thought is that screw compressors do not load up until at least 1/3 speed, so the area that you can control speed may be greatly reduced.
• There are some additional considerations about lubrication. I have found several screw compressors where the mechanically coupled lube pump performance dropped off at a faster rate than the equivalent speed reduction in the compressor, resulting in rapid compressor failure running at less than 70% speed. That reduced the effective speed control range even further, and thus, the energy savings argument. In some cases, we added positive pressure lube systems, but the added cost affected the payback period adversely.

# posted by Medical Information : 2:59 AM