Drivers should be selected to provide enough power to meet the compression requirements. Best Practice and industry standard is to apply a 10% margin over the greatest compressor power demand for the driver selection. For electric motor drivers, this 10% margin can be applied to the motor-rated horsepower or to the Service Factor. If applied to the Service Factor, a specific review is required with the motor and motor controller suppliers to ensure continuous operation within Service Factor can be allowed. End users must also be aware of and accept operations within Service Factor.

This 10% margin is a selection criterion to account for the variables affecting power demand and power supply. This is not meant to limit the use of the available driver power. Once installed, the full power rating may be used.

For proper sizing of a variable frequency drive (VFD), consult motor and motor controller suppliers, especially if considering use above motor nameplate horsepower.

There are two main types of motors: synchronous and induction. To calculate a general maximum speed of a synchronous motor, you can apply the following formula:

Induction, or asynchronous, motors will operate at less than the rated synchronous speed because of slip. A typical motor slip is 1%. This 1% slip is applied by default in the Ariel Performance Software but can be edited. To calculate a general maximum speed of an induction motor, you can apply the following formula:

For motor driven compressors, it can be beneficial to match an electric driver with the compressor frame that will provide a lower piston speed when the selection allows. Slower piston speeds can provide higher efficiencies and a longer time between scheduled maintenance. For example, if a KBC or a KBD frame could be used for an application where 60 Hz power is available, an electric driver could run the KBD at 1200 RPM, or it could run the KBC at 900 RPM for a lower piston speed. The KBC selection would be more efficient and could have longer times between scheduled maintenance due to the slower piston speed. The KBC will have slightly larger cylinders.

To avoid complications in the torsional analysis, Ariel recommends a motor stub shaft of comparable strength and stiffness to the compressor crankshaft. Using a motor shaft with a keyway, low tensile strength material, or small diameter may make the driveline design difficult, resulting in exotic coupling selections, speed restrictions, or requiring a larger motor. Ariel recommends the motor stub shaft and the section thru the drive end bearing equal or exceed the compressor drive stub diameter. For flanged crankshafts (JGE:K:T/6 and KBB:V/4/6), use the main bearing diameter for the compressor shaft size (x). See ER-83 for frame shaft diameters. Coupling hubs should be fully engaged on the motor shaft.

Select motor drivers for severe duty, or reciprocating compressor duty. Reciprocating compressor torques vary considerably within one revolution. Drive end flywheels and dampers are effective to reduce vibratory torque on the motor. Ariel offers custom drive end flywheels to meet driveline requirements. Contact Ariel Applications Engineering for more information.

Startup torque review is required. Consideration for starting pressure inside the cylinders should include a review of highest pressure at start up; this may include a blocked in settle out pressure. A gas recycle line or bypass for startup is required.

Refer to the Ariel Packager Standards Section 5 for more details on the drive system and ER-83 for torsional analysis guidelines and limits.