electric motor, any
of a class of devices that convert electrical energy to mechanical
energy, usually by employing electromagnetic phenomena.
Most electric motors develop their mechanical torque by the interaction of conductors
carrying current in a direction at right angles to a magnetic field. The
various types of electric motor differ in the ways in which the conductors and
the field are arranged and also in the control that can be exercised over
mechanical output torque,
speed, and position.
Induction
motors
Permanent-magnet motors
Reluctance motors
Induction
motors
The simplest type of induction
motor is shown
in cross section in the figure. A three-phase set ofstator
windings is
inserted in slots in the stator iron. These windings may be connected either in
a wye configuration, normally without external connection to the neutral point,
or in a delta configuration. The rotor consists of a cylindrical iron core
with conductors placed in slots around the surface. In the most usual form,
these rotor conductors are connected together at each end of the rotor by a
conducting end ring.
The basis of operation of the induction motor may be developed
by first assuming that the stator windings are connected to a three-phase electric supply and that a
set of three sinusoidal currents of the form shown in the figure flow
in the stator windings. This figure shows
the effect of these currents in producing a magnetic field across the air gap
of the machine for six instants in a cycle. For simplicity, only the central
conductor loop for each phase winding is shown. At the instant t1 in the figure, the current in phase a is
maximum positive, while that in phases b and c is half that value negative. The
result is a magnetic field with an approximately sinusoidal distribution around
the air gap with a maximum outward value at the top and a maximum inward value
at the bottom. At timet2 in the figure (i.e.,
one-sixth of a cycle later), the current in phase c is
maximum negative, while that in both phase b and phase a is
half value positive. The result, as shown for t2 in the figure, is again a sinusoidally distributed
magnetic field but rotated 60° counterclockwise. Examination of the current
distribution for t3, t4, t5, and t6 shows that the magnetic field continues to
rotate as time progresses. The field completes one revolution in one cycle of
the stator currents. Thus, the combined effect of three equal sinusoidal
currents, uniformly displaced in time and flowing in three stator windings
uniformly displaced in angular position, is to produce a rotating magnetic field with a constant
magnitude and a mechanical angular velocity that depends on the frequency of
the electric supply.
The rotational motion of the magnetic field with respect to the
rotor conductors causes a voltage to be induced in each, proportional to the
magnitude and the velocity of the field relative to the conductors. Since the
rotor conductors are short-circuited together at each end, the effect will be
to cause currents to flow in these conductors. In the simplest mode of
operation, these currents will be about equal to the induced voltage divided by
the conductor resistance. The pattern of rotor currents for the instant t1 of the figure is
shown in this figure. The currents are seen to be
approximately sinusoidally distributed around the rotor periphery and to be
located so as to produce a counterclockwise torque on the rotor (i.e., a torque
in the same direction as the field rotation). This torque acts to accelerate
the rotor and to rotate the mechanical load. As the rotational speed of the
rotor increases, its speed relative to that of the rotating field decreases.
Thus, the induced voltage is reduced, leading to a proportional reduction in
rotor conductor current and in torque. The rotor speed reaches a steady value
when the torque produced by the rotor currents equals the torque required at
that speed by the load with no excess torque available for accelerating the
combined inertia of the load and the motor.
The mechanical output power must be provided by an electrical
input power. The original stator currents shown in the figure are
just sufficient to produce the rotating magnetic field. To maintain this
rotating field in the presence of the rotor currents of the figure, it is necessary that the stator
windings carry an additional component of sinusoidal current of such a
magnitude and phase as to cancel the effect of the magnetic field that would
otherwise be produced by the rotor currents in thefigure. The total stator current in each phase
winding is then the sum of a sinusoidal component to produce the magnetic field
and another sinusoid, leading the first by one-quarter of a cycle, or 90°, to
provide the required electrical power. The second, or power, component of the
current is in phase with the voltage applied to the stator, while the first, or
magnetizing, component lags the applied voltage by a quarter cycle, or 90°. At
rated load, this magnetizing component is usually in the range of 0.4 to 0.6 of
the magnitude of power component.
A
majority of three-phase induction motors operate with their stator windings
connected directly to a three-phase electric supply of constant voltage and
constant frequency. Typical supply voltages range from 230 volts line-to-line
for motors of relatively low power (e.g., 0.5 to 50 kilowatts) to about 15
kilovolts line-to-line for high-power motors up to about 10 megawatts.
Except
for a small voltage drop in the resistance of the stator winding, the supply
voltage is matched by the time rate of change of the magnetic flux in the
stator of the machine. Thus, with a constant-frequency, constant-voltage
supply, the magnitude of the rotating magnetic field is held constant, and the
torque is roughly proportional to the power component of the supply current.
With
the induction motor shown in the foregoing figures, the magnetic field rotates
through one revolution for each cycle of the supply frequency. For a 60-hertz
supply, the field speed is then 60 revolutions per second, or 3,600 per minute.
The rotor speed is less than the speed of the field by an amount that is just
enough to induce the required voltage in the rotor conductors to produce the
rotor current needed for the load torque. At full load, the speed is typically
0.5 to 5 percent lower than the field speed (often called synchronous speed),
with the higher percentage applying to smaller motors. This difference in speed
is frequently referred to as the slip.
Other synchronous speeds can be obtained with a constant
frequency supply by building a machine with a larger number of pairs of
magnetic poles, as opposed to the two-pole construction of thefigure. The possible values of magnetic-field
speed in revolutions per minute are 120 f/p, where f is the
frequency in hertz (cycles per second) and p is the number of poles (which must be
an even number). A given iron frame can be wound for any one of several
possible numbers of pole pairs by using coils that span an angle of
approximately (360/p)°.
The torque available from the machine frame will remain unchanged, since it is
proportional to the product of the magnetic field and the allowable coil
current. Thus, the power rating for the frame, being the product of torque and
speed, will be roughly inversely proportional to the number of pole pairs. The
most common synchronous speeds for 60-hertz motors are 1,800 and 1,200
revolutions per minute.
Permanent-magnet motors
The magnetic field for a synchronous machine may be provided by
using permanent magnets made of neodymium-boron-iron, samarium-cobalt, or
ferrite on the rotor. In some motors, these magnets are mounted with adhesive
on the surface of the rotor core such that the magnetic field is radially
directed across the air gap. In other designs, the magnets are inset into the
rotor core surface or inserted in slots just below the surface. Another form of permanent-magnet
motor has
circumferentially directed magnets placed in radial slots that provide magnetic
flux to iron poles, which in turn set up a radial field in the air gap.
The
main application for permanent-magnet motors is in variable-speed drives where
the stator is supplied from a variable-frequency, variable-voltage,
electronically controlled source. Such drives are capable of precise speed and
position control. Because of the absence of power losses in the rotor, as
compared with induction motor drives, they are also highly efficient.
Permanent-magnet
motors can be designed to operate at synchronous speed from a supply of
constant voltage and frequency. The magnets are embedded in the rotor iron, and
a damper winding is placed in slots in the rotor surface to provide starting
capability. Such a motor does not, however, have means of controlling the
stator power factor.
Reluctance motors
Reluctance motors operate on the principle that forces are
established that tend to cause iron poles carrying a magnetic flux to align with
each. One form of reluctance
motor is shown
in cross section in the figure. The rotor consists of four iron poles
with no electrical windings. The stator has six poles each with a
current-carrying coil. In the condition represented in the figure, current has
just been passed through coils a and a′, producing a
torque on the rotor aligning two of its poles with those of the a-a′
stator. The current is now switched off in coils a and a′ and switched on to coils band b′. This produces a
counterclockwise torque on the rotor aligning two rotor poles with stator poles b and b′. This process is then repeated with stator coils c and c′ and then with coils a and a′. The torque is dependent on the magnitude of the
coil currents but is independent of its polarity. The direction of rotation can
be changed by changing the order in which the coils are energized. Reluctance
motors can have other pole configurations, such as eight stator poles and six
rotor poles.
The currents in the stator coils are usually controlled by
semiconductor switches connecting the coils to a direct voltage source. A
signal from a position sensor mounted on the motor shaft is used to activate
the switches at the appropriate time instants. Frequently a magnetic sensor
based on theHall
effect is
employed. (The Hall effect involves the development of a transverse electric
field in a semiconductor material when it carries a current and is placed in a
magnetic field perpendicular to the current.) The overall system is known as a self-synchronous
motor drive. It can operate over a wide and controlled speed
range.
No comments:
Post a Comment