An AC motor is an electric motor driven by an alternating current (AC).
It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating
current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a
torque by the rotating field.
There are two main types of AC motors, depending on the type of rotor used. The first type is
the induction motor or asynchronous motor; this type relies on a small difference in speed between the
rotating magnetic field and the rotor to induce rotor current. The second type is the synchronous motor,
which does not rely on induction and as a result, can rotate exactly at the supply frequency or a submultiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered
through slip rings or by a permanent magnet. Other types of motors include eddy current motors, and also
AC/DC mechanically commutated machines in which speed is dependent on voltage and winding
Alternating current technology was rooted in Michael Faraday’s and Joseph Henry’s 1830-31 discovery
that a changing magnetic field can induce an electric current in a circuit. Faraday is usually given credit
for this discovery since he published his findings first.
An induction or asynchronous motor is an AC electric motor in which the electric current in
the rotor needed to produce torque is induced byelectromagnetic induction from the magnetic field of
the stator winding. An induction motor therefore does not require mechanical commutation, separateexcitation or self-excitation for all or part of the energy transferred from stator to rotor, as
in universal, DC and synchronous motors. An induction motor's rotor can be either wound type or squirrelcage type.
#A wound-rotor motor is a type of induction motor where the rotor windings are connected through slip
rings to external resistances. Adjusting the resistance allows control of the speed/torque characteristic of
the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the
rotor circuit; as the motor accelerates, the resistance can be decreased.
Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced
voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-up a typical
rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable power resistor.
When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the
resistors reduce the field strength in the stator. As a result the inrush current is reduced. Another
important advantage over squirrel-cage motors is higher start-up torque.
A wound-rotor motor can be used in several forms of adjustable-speed drive. Certain types of variablespeed drives recover slip-frequency power from the rotor circuit and feed it back to the supply, allowing
wide speed range with high energy efficiency. Doubly fed electric machines use the slip rings to supply
external power to the rotor circuit, allowing wide-range speed control. Today speed control by use of slip
ring motor is mostly superseded by induction motors with variable-frequency drives.
#A squirrel-cage rotor is the rotating part (rotor) used in the most common form of AC induction motor. It
consists of a cylinder of steel with aluminum or copper conductors embedded in its surface. An electric
motor with a squirrel-cage rotor is termed a squirrel-cage motor.
Three-phase squirrel-cage induction motors are widely used in industrial drives because they are rugged,
reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as
household appliances like fans. Although traditionally used in fixed-speed service, induction motors are
increasingly being used with variable-frequency drives (VFDs) in variable-speed service. VFDs offer
especially important energy savings opportunities for existing and prospective induction motors in
variable-torque centrifugal fan, pump and compressor load applications. Squirrel cage induction motors
are very widely used in both fixed-speed and VFD applications
Principle of operation
A three-phase power supply provides a rotating magnetic field in an induction motor.
In both induction and synchronous motors, the AC power supplied to the motor's stator creates
a magnetic field that rotates in time with the AC oscillations. Whereas a synchronous motor's rotor turns
at the same rate as the stator field, an induction motor's rotor rotates at a slower speed than the stator
field. The induction motor stator's magnetic field is therefore changing or rotating relative to the rotor. This
induces an opposing current in the induction motor's rotor, in effect the motor's secondary winding, when
the latter is short-circuited or closed through an external impedance. The rotating magnetic flux induces
currents in the windings of the rotor; in a manner similar to currents induced in a transformer's
secondary winding(s). The currents in the rotor windings in turn create magnetic fields in the rotor that
react against the stator field. Due to Lenz's Law, the direction of the magnetic field created will be such as
to oppose the change in current through the rotor windings. The cause of induced current in the rotor
windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor
will start to rotate in the direction of the rotating stator magnetic field. The rotor accelerates until the
magnitude of induced rotor current and torque balances the applied load. Since rotation at synchronous
speed would result in no induced rotor current, an induction motor always operates slower than
synchronous speed. The difference, or "slip," between actual and synchronous speed varies from about
0.5 to 5% for standard Design B torque curve induction motors. The induction machine's essential
character is that it is created solely by induction instead of being separately excited as in synchronous or
DC machines or being self-magnetized as in permanent magnet motors.
For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator's
rotating magnetic field ( ); otherwise the magnetic field would not be moving relative to the rotor
conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed,
the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and
creating more torque. The ratio between the rotation rate of the magnetic field induced in the rotor and the
rotation rate of the stator's rotating field is called slip. Under load, the speed drops and the slip increases
enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes
referred to as asynchronous motors. An induction motor can be used as an induction generator, or it
can be unrolled to form a linear induction motor which can directly generate linear motion.
A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is
synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral
number of AC cycles. Synchronous motors contain electromagnets on the stator of the motor that create
a magnetic field which rotates in time with the oscillations of the line current. The rotor turns in step with
this field, at the same rate.
The synchronous motor and induction motor are the most widely used types of AC motor. The difference
between the two types is that the synchronous motor rotates in exact synchronism with the line
frequency. In contrast the induction motor requires "slip", the rotor must rotate slightly slower than the AC
current alternations, to develop torque. Therefore small synchronous motors are used in timing
applications such as in synchronous clocks, timersin appliances, tape recorders and
precision servomechanisms in which the motor must operate at a precise speed.
Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower industrial
sizes. In the fractional horsepower range, most synchronous motors are used where precise constant
speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important
functions. First, it is a highly efficient means of converting AC energy to work. Second, it can operate at
leading or unity power factor and thereby provide power-factor correction.
In non-excited motors, the rotor is made of steel. At synchronous speed it rotates in step with the rotating
magnetic field of the stator, so it has an almost-constant magnetic field through it. The external stator field
magnetizes the rotor, inducing the magnetic poles needed to turn it. The rotor is made of a highretentivity steel such as cobalt steel, These are manufactured in
permanentmagnet, reluctance and hysteresis designs:
Reluctance motors[edit source]
These have a rotor consisting of a solid steel casting with projecting (salient) toothed poles, the
same number as the stator poles.
The size of the air gap in the magnetic circuit and thus
the reluctance is minimum when the poles are aligned with the (rotating) magnetic field of the
stator, and increases with the angle between them. This creates a torque pulling the rotor into
alignment with the nearest pole of the stator field. Thus at synchronous speed the rotor is
"locked" to the rotating stator field. This cannot start the motor, so the rotor poles usually
havesquirrel-cage windings embedded in them, to provide torque below synchronous speed. The
machine starts as an induction motor until it approaches synchronous speed, when the rotor
"pulls in" and locks to the rotating stator field.
Reluctance motor designs have ratings that range from fractional horsepower (a few watts) to
about 22 kW. Very small reluctance motors have low torque, and are generally used for
instrumentation applications. Moderate torque, integral horsepower motors use squirrel cage
construction with toothed rotors. When used with an adjustable frequency power supply, all
motors in the drive system can be controlled at exactly the same speed. The power supply
frequency determines motor operating speed.
Hysteresis motors[edit source]
These have a solid smooth cylindrical rotor, cast of a high coercivity magnetically "hard" cobalt
This material has a wide hysteresis loop (high retentivity), meaning once it is
magnetized in a given direction, it requires a large reverse magnetic field to reverse the
magnetization. The rotating stator field causes each small volume of the rotor to experience a
reversing magnetic field. Because of hysteresis the phase of the magnetization lags behind the
phase of the applied field. The result of this is that the axis of the magnetic field induced in the
rotor lags behind the axis of the stator field by a constant angle δ, producing a torque as the rotor
tries to "catch up" with the stator field. As long as the rotor is below synchronous speed, each
particle of the rotor experiences a reversing magnetic field at the "slip" frequency which drives it
around its hysteresis loop, causing the rotor field to lag and create torque. There is a 2-pole low
reluctance bar structure in the rotor.
As the rotor approaches synchronous speed and slip goes
to zero, this magnetizes and aligns with the stator field, causing the rotor to "lock" to the rotating
A major advantage of the hysteresis motor is that since the lag angle δ is independent of speed, it
develops constant torque from startup to synchronous speed. Therefore it is self-starting and
doesn't need an induction winding to start it, although many designs do have a squirrel-cage
conductive winding structure embedded in the rotor to provide extra torque at start-up.
Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as
servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are
used where precise constant speed is required.
Permanent magnet motors[edit source]
These have permanent magnets embedded in the steel rotor to create a constant magnetic field.
At synchronous speed these poles lock to the rotating magnetic field. They are not self-starting.
Because of the constant magnetic field in the rotor these cannot use induction windings for
starting, and must have electronically controlled variable frequency stator drive.
Made in sizes larger than 735 W, these motors require direct current supplied to the rotor for excitation.
This is most straightforwardly supplied throughslip rings, but a brushless AC induction and rectifier
arrangement may also be used. The direct current may be supplied from a separate DC source or from
a DC generator directly connected to the motor shaft.
Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and
move at the same speed.
For industrial and mining applications, 3-phase AC induction motors are the prime movers for
the vast majority of machines. These motors can be operated either directly from the mains or
from adjustable variable frequency drives. In modern industrialized countries, more than half the
total electrical energy used in those countries is converted to mechanical energy through AC
induction motors. The applications for these motors cover almost every stage of manufacturing
Applications also extend to commercial buildings and the domestic environment. They are used
to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools,
cranes, etc, etc.