Page 183 - Compression Machinery for Oil and Gas

172 SECTION II Types of Equipment

of the stroke, in addition, the gas is compressed discharged and then on the first
part of the reversed stroke is expanded. This implies a severe torque pulsation
which on a two- and four-throw compressor will include a peak torque that will
usually exceed 200% of the mean torque and a torque reversal for part of each
revolution. A six throw is not as extreme but still has a severe pulsating torque.
This torque pulsation is partially absorbed by the inertia of the flywheel (when
present) and/or the motor inertia so as to reduce the torque variation that must be
provided by the motor magnetic field.

Electric Motors
Motor Sizing
API 618 requires that the motor nameplate HP be 110% of the maximum oper-
ating condition BHP. In addition, it is good practice to ensure that the motor
insulation is class F or better and the temperature rise is limited to class B rise
at 1.0 service factor. This will ensure some thermal reserve and ensure long
life of the insulation. It should be noted that unlike other driver types, motors
will provide good efficiency over a wide load range, offering maximum effi-
ciency over loads of 50%–90% of nameplate, the efficiency starts to drop
off over 90% load. So there is no detriment to operating motors at less than
nameplate load. Care needs to be taken to ensure electric motors are operating
at no more than nameplate load, nameplate amps and within the insulation
temperature limits otherwise operating life and reliability will be severely
impacted.

Induction Motors
An induction motor develops torque by inducing current to the rotor, which is
proportional to the differential speed of the rotor and the rotating magnetic
field in the stator. For NEMA design B motors the differential speed (called
slip) is between 1% and 2% at full load. Due to the torque variation at each
revolution the instantaneous speed will vary. For example, if the speed vari-
ation was 0.8% so the slip would vary between 0.6% and 1.4% giving a torque
variation of 60% torque to 140% torque. But the power factor would vary with
load so at 60% power it might be 80% amps and at 140% load 140% amps for
an average of 110% amps. Note how the power is at 100% but the amps are at
110%, that is, in an overload situation by amps but not by power. The heating
effect in the motor is primarily governed by the motor amps so at 110% amps
it is likely that the motor winding temperatures would be excessive. In this
case the current pulsation is (140 80)/100¼60% NEMA MG1 limits the cur-
rent pulsation to 66% API-618 limits the current pulsation to 40%. It is recom-
mended that the API 618 limits be applied to induction motors because the
NEMA pulsation limits are not adequate to protect against overloading
the motor.