Page 334 - Compression Machinery for Oil and Gas

Drivers Chapter 7 319

Electric Motors
The two common electric motor types are the induction (asynchronous) and the
synchronous motors. Both types are made up of stationary (stator) and rotating
(rotor) components, where an alternating current is applied to the stator to create
a rotating magnetic field. The rotor, a permanent magnet or electromagnet,
reacts with the magnetic field to rotate within the stator.

Induction Motor

Basic Principles
The induction motor is the motor in which alternating current is supplied
directly to the stator and current in the rotor is induced from the stator. When
the stator is excited from a balanced three-phase source, it will produce a mag-
netic field in the air gap rotating at synchronous speed n s as determined by the
number of stator poles and the applied stator frequency f e :

120
n s ¼ f e (7.1)
poles
Three-phase current in the stator produces a magnetic flux of constant mag-
nitude rotating at synchronous speed. As per Faraday’s law, rotating flux cuts
the rotor conductors causing EMF (elector-motive force) induced in the rotor
bars. EMF in shorted rotor bars induces rotor current flow. By the Lenz’s
law, the current flow in the rotor conductors creates mechanical force. The
result is the rotor turns in the direction of the magnetic field trying to catch
up. The rotor speed n m is less than the synchronous speed of the stator-induced
magnetic field n s and it is defined by the slip s:
n s n m
s ¼ (7.2)
n s
An induction motor can be described as a rotating transformer. Its input is a
three-phase system of voltages and currents. For an ordinary transformer, the
output is electric power from the secondary windings. The secondary windings
in an induction motor (the rotor) are shorted out, so no electrical output exists
from normal induction motors. Instead, the output is mechanical. The relation-
ship between the input electric power and the output mechanical power of this
motor is illustrated in Fig. 7.9.
The input power to an induction motor P in is the three-phase electric power.
2
The first losses encountered in the machine are I R losses in the stator windings,
that is, the stator copper loss P SCL . Some amount of power is lost as hysteresis
and eddy currents in the stator P core . The power remaining at this point is trans-
ferred to the rotor of the machine across the air gap between the stator and rotor.
This power is called the air gap power P AG of the machine. After the power is
2
transferred to the rotor, some of it is lost as I R losses, that is, the rotor copper