Page 341 - Compression Machinery for Oil and Gas

326 SECTION II Types of Equipment

Motor classes A and D are obtained by designing the rotor resistance to be
either low (class A) or high (class D). Classes B and C are obtained by exploit-
ing the skin effect to obtain a variable resistance rotor circuit, as the frequency
of the current induced in the rotor changes during motor acceleration. The rotor
frequency may be expressed as
(7.6)
f 2 ¼ s f e
where f 2 is the rotor frequency, f e is the stator frequency, and s is the slip that
changes during motor acceleration from 1 in the locked rotor condition to some
final value at the full speed. Typically, slip s at the full speed is <0.1 for NEMA
classes A, B, and C motors, and <0.5 for NEMA class D motors.
The skin effect is the tendency of an alternating electric current to become
distributed within a conductor such that the current density is largest near the
surface of the conductor, and decreases toward the center of the conductor.
The current is evenly distributed across the conductor cross section for the direct
current, so there is no skin effect when the frequency is equal to 0. The skin
effect becomes more and more prominent as the frequency increases. The elec-
tric current flows mainly at the skin of the conductor, between the outer surface
and a level called the skin depth. The skin effect causes the effective resistance
of the conductor to increase at higher frequencies where the skin depth is smal-
ler, thus reducing the effective cross section of the conductor. For the NEMA
design motors, the rotor bar width may vary in order to enhance the skin effect
and its influence on the rotor resistance and consequently its influence on the
torque. Class C rotors are typically fabricated with two separate cages. Only
the outer cage will conduct at starting when current frequency in the rotor bars
is the highest. With the design class C, there is air between the cages and at the
top of the slot to reduce leakage flux. Class D has a small conductor, giving a
high resistance at all slips.
A summary of the performance of the NEMA motor classes is provided in
Table 7.1 (https://people.ucalgary.ca/ aknigh/electrical_machines/induction/
i_standard.html).

Compressor Load
Gas compressors may be reciprocating, rotary, or centrifugal, which present dif-
ferent loads for the electric motor.

Reciprocating Compressors
Reciprocating compressors produce an oscillating load on the motor. As each
volume of gas is compressed, the torque required from the motor increase to a
maximum and then decreases. Often, motors are sized for average torque. That
means the motor will be running over its design load during the peak of the com-
pression cycle. Due to their design, reciprocating compressors can have very
large variations in torque over one revolution.