# Variable frequency drive for compressor

If it is a centrifugal compressor with consistent gas density (mole weight), the load is quadratic and you can size the variable frequency drive based on the motor hp (as long as you don't require over-speed, which needs more detailed review).

If the compressor will be operating with a range of gases of different mole weights, then one must look at each operating point to ensure sizing is proper for full envelope of required operation (for example, there may be a high torque requirement at low speed that could be problematic).

Reciprocating compressors are much more difficult to size, due to their low-speed torque requirements and high torque transients (which can cause substantial current transients in the VFD that result in frequent nuisance trips).

Hence, consistent gas profile and centrifugal-type compressor, sizing is easy. Otherwise, more info and more care is required.

The co-efficient of 2 should be safe for reciprocating compressors, unless there is significant continued operation below 25% speed, in which case additional care may be advised. The co-efficient of 2 should provide sufficient margin to avoid the over-current nuisance trip problem, as well as the heating from an essentially constant torque load operated over a wide range of speed. A detailed application review may result in sizing a little bit smaller, but almost certainly above a 1.5 co-efficient.

If the compressor will be operating with a range of gases of different mole weights, then one must look at each operating point to ensure sizing is proper for full envelope of required operation (for example, there may be a high torque requirement at low speed that could be problematic).

Reciprocating compressors are much more difficult to size, due to their low-speed torque requirements and high torque transients (which can cause substantial current transients in the VFD that result in frequent nuisance trips).

Hence, consistent gas profile and centrifugal-type compressor, sizing is easy. Otherwise, more info and more care is required.

The co-efficient of 2 should be safe for reciprocating compressors, unless there is significant continued operation below 25% speed, in which case additional care may be advised. The co-efficient of 2 should provide sufficient margin to avoid the over-current nuisance trip problem, as well as the heating from an essentially constant torque load operated over a wide range of speed. A detailed application review may result in sizing a little bit smaller, but almost certainly above a 1.5 co-efficient.