Page 344 - Compression Machinery for Oil and Gas

Drivers Chapter 7 329

but not by power. The heating effect in the motor is primarily governed by the
motor amps so that 110% amps are likely to cause excessive motor winding
temperatures. In this case, the current pulsation is (140–80)/100¼60%. The
NEMA MG1 limits the current pulsation to 66% API 618 limits the current pul-
sation to 40%. It is recommended that the API 618 limits be applied to induction
motors because the NEMA pulsation limits are not adequate to protect against
overloading the motor.
API 618 requires that the motor nameplate power be 110% of the maximum
operating condition break power. In addition, it is a 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 efficiency over loads of 50%–90% of nameplate power, the effi-
ciency 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.
Reciprocating compressors have large flywheels that require sustained high
motor torque during starting to accelerate to full speed. Because of the high rota-
tional moments of inertia resulting in high starting loads, motor selection must
take this into account.
Motor torques recommended by the NEMA, for reciprocating compressors
starting unloaded are: starting torque equal to or >40% of the full load torque,
pull-up torque equal to or >30% of the full load torque, and breakdown torque
equal to or >150% of the full load torque (NEMA MG-1 Reference from above).
With induction motors, the torque requirements for reciprocating compressors
are achieved with the NEMA class C motors. Synchronous motors or variable
speed drives can also be used.
As an example, suppose a synchronous motor with fairly typical 40/40/150
torques. The voltage dip is 10% per the NEMA and 20% is commonly specified
by clients. The motor manufacturer typically requires 10% free for acceleration.
2
Therefore, torque limit in the unloaded compressor is 40 0.8 –10¼15.6% at
breakaway and pull in. An induction motor has higher torques and so is easier to
start (does not have a pull in issue).
Now, a reciprocating compressor is a constant torque with speed machine.
So, it has to be unloaded for start. There are three methods:
1. Depressurize.
2. At normal inlet pressure but using inlet valve unloaders, there is a check
valve between the compressor discharge and the header and a small bypass
is used to depressurize the final stage back to the inlet suction pressure.
Resulting breakaway torque is usually 5%–15% coming from the static fric-
tion of the bearing and pistons plus for case of two- and four-throw (but not