Electric Motors

1. Inform motor manufacturers that reciprocating gas compressor torque varies considerably in one

revolution. An Ariel compressor is not a constant-load (uniform torque) device, even if driven at a

constant speed. In severe service, peak torque may vary ±200% of the mean, repeating as often

as three times per revolution. Torque peaks and torque reversals can cause motor shaft fatigue

failure, especially with a keyway. Ensure motor shaft strength suitability for all operating conditions

of intended service. Larger Ariel compressors require robust motors with large diameter,

keyless shafts for long life and successful performance.

2. When designing the motor rotating assembly, electric motor manufacturers must account for

dynamic (alternating) torques generated by the driven equipment as well as the mean torque.

3. The motor stub shaft must be only as long as needed to fully insert in the appropriate coupling

hub and ensure complete contact.

4. The motor stub shaft and the section thru the drive end bearing should equal or exceed the compressor

drive stub diameter. For keyless compressor drive stubs, an equivalent diameter keyed

motor shaft may not be sufficient.

5. Analyze the drive train to ensure there are no dangerous vibratory stresses and that current pulsation

falls below motor or switch gear limits. It is inadequate to simply specify a past satisfactory

motor frame size; prove rotor inertia and shaft strength and stiffness are equal to a past satisfactory

installation before omitting torsional analysis.