Electric Motor Drive

Electric motors, direct connected through a flexible coupling can be utilized to drive Ariel compressors. See the Packager's Standards for application guidelines (button below). Electric motors are available at various speed depending upon the frequency of the power supply and the number of motor poles.

Common motor speeds:

Synchronous rpm = Power Frequency (hz) x 120 / number of motor poles

Induction motor rpm = synchronous rpm x 0.985 (accounts for 1.5% slip)

Induction motors will operate at less than the rated synchronous speed because of �slip�. A typical motor slip is 1.5%. This must be accounted for in the compressor selection.

Number of Motor Poles	Synch	Induction	Synch	Induction
	50 hz	50 hz	60 hz	60 hz
4	1500	1475	1800	1775
6	1000	985	1200	1185
8	750	740	900	885
10	600	590	720	710
12	500	492	600	590
14	428	422	514	506

Electric motor drives will require a review of starting torques to ensure the motor has enough torque to start the rotation of the compressor. This will most often require a full bypass for start-up and may require special motor torques. Variable speed drivers have other details to consider, see Minimum Allowable Rotating Speed topic.

Refer to the Ariel Packager's Standards Section 5 for more details on the drive system.

Common Project information for motor driven applications include:

Quotation:

Start up torque curve (at highest suction pressure) with estimated bypass line pressure losses
Full horsepower torque effort curve
Part load torque effort curves (highest power of each single acting step)
+10% of max horsepower for nameplate power (may be inclusive of service factor)
Shaft diameter to be no less than compressor shaft diameter (keyless shaft)

Purchase (confirming quotation data):

*Start up torque curve (at highest suction pressure) with actual bypass line pressure losses
Full horsepower torque effort curve
Part load torque effort curves (highest power of each single acting step)
+10% of max horsepower for nameplate power (may be inclusive of service factor)
Shaft diameter to be no less than compressor shaft diameter (keyless shaft)

Studies necessary:

Motor start up analysis - Does the motor have enough torque to reach full speed fast (before the high inrush current overheats the motor)
Current pulsation analysis (66% or less for induction motors, per NEMA MG-1)
Torsional analysis
Lateral analysis may be necessary depending upon lengths

Data necessary for Torsional from Motor Supplier:

Mass elastic data (inertia and shaft stiffness)
Shaft detail drawing for vibratory stress calculations
Minimum additional inertia (flywheel) to limit current pulsations (66% or less)

*Final Start Up Torque Curve:

Run start up curve at highest suction pressure (or settle out pressure) and default pressure loss settings of 25 psi times the number of stages
Load cylinders if manual capacity devices are used, and unload cylinders if pneumatic capacity devices are used (suction valve unloaders and pneumatic pockets)
Apply the "flow at startup" value from the start up torque data to size the bypass line
Calculate the bypass line pressure drop at the "flow at startup" (from final stage cylinder discharge flange through vessels, coolers, bypass line, bypass valve, suction scrubber, suction vessel to first stage cylinder suction flange)
Use the final flywheel inertia from the torsional analysis and current pulsation analysis for the final start up torque calculation
Rerun the start up curve with the calculated bypass line losses

Start up torque is used to determine if an engine starter, or electric motor driver has enough torque to start a compressor.

Start up torque is characterized by two main components, break away torque and speed up torque. The break away torque is the static friction and gas load on the piston rod area at zero rpm. The speed up torque is the dynamic friction and the pressure load on the cylinders as the unit speeds up toward full speed.

Ariel provides start up torque data within the Ariel Performance Program. All start up torque calculations provided assume that a bypass or gas recirculation line is installed and open, sized to bypass 100% of the compressor flow.

The break away torque is dependent on the pressure on the unit when the start button is pushed. Higher start up pressures will result in higher break away torque. If the break away torque must be reduced for starting, the pressure on the unit must be reduced. Six throw units are mostly exempt from this as the phasing of the six throws offers a cancellation of the individual throw torques from the pressure on the piston rod.

The speed up torque can be impacted by both the starting pressure, as well as the bypass line pressure loss. Smaller bypass lines will have higher pressure losses, resulting in higher torques as the unit approaches full speed.

Start Up Torque Calculations:

Use the highest suction pressure that the unit will see when the start button is pushed. This may be a normal (max) suction pressure, or a settle out pressure.
Load all manual capacity devices (close VVCP�s, remove spacers, double act cylinders if valves are removed manually for single acting cases).
Unload all panel controlled capacity devices (open FVCP�s, single act if SVU�s are provided)
Consider and calculate the bypass line pressure losses for a more accurate torque curve.
Apply the final flywheel inertia from the torsional analysis and current pulsation analysis for the final start up torque calculation

Bypass Line Pressure Losses:

Ariel provides an entry for the bypass line pressure loss. This is a value multiplied by the number of stages, for a total pressure loss.
The default value is 25 psi times the number of stages. This is only a rough value for a beginning assumption.
The start up torque calculation will include a �flow at Start up�. Apply this value when calculating the bypass line pressure loss. At the lower ratios of start up (bypass line open) the flow can be much higher than the compressor design flow.

Ariel Corporation Application Manual
11 November 2011