Introduction to Induction motors

The induction motors are probably the simplest and most rugged of all electric motors. They
consist of two basic electrical assemblies: the wound stator and the rotor assembly.

The rotor consists of laminated, cylindrical iron cores with slots for receiving the conductors.
The rotor assembly resembles a squirrel cage when viewed from the end, hence the name squirrel
cage motor is used to refer to induction motors. In modern induction motors, the most common
type of rotor has cast aluminum conductors and short circuiting end rings. The rotor turns when
the moving magnetic field induces a current in the shorted conductors. The speed at which the
magnetic field rotates is the synchronous speed of the motor and is determined by the number of
poles in the stator and the frequency of the power supply.

Ns=120f/P
where Ns = synchronous speed
f = frequency
P = number of poles

Synchronous speed is the absolute upper limit of motor speed. At synchronous speed, there is no
difference between rotor speed and rotating field speed, so no voltage is induced in the rotor bars
and hence no torque is developed. Therefore, when running, the rotor must rotate slower than the
magnetic field. The rotor speed is just slow enough to cause the proper amount of rotor current to
flow, so that the resulting torque is sufficient to overcome windage and friction losses and drive
the load. This speed difference between the rotor and magnetic field, called slip, is normally
referred to as a percentage of synchronous speed:

S=100(Ns-Na)/Ns

where: s = slip
Ns = synchronous speed
Na = actual speed
Polyphase motors:
NEMA classifies polyphase induction motors according to locked rotor torque and current,
breakdown torque, pull up torque and percent slip.
 Locked rotor torque is the minimum torque that the motor develops at rest for all angular
positions of the rotor at rated voltage and frequency.
 Locked rotor current is the steady state current from the line at rated voltage and
frequency with the rotor locked.

 Breakdown torque is the maximum torque that the motor develops at rated voltage and
frequency, without an abrupt drop in speed.

 Pull up torque is the minimum torque developed during the period of acceleration from
rest to the speed that breakdown torque occurs. The below figure illustrates typical speed-
torque curves for NEMA Design A, B, C and D motors.

 Design A motors have a higher breakdown torque than Design B motors and are usually
designed for a specific use. Slip is 5%, or less.

 Design B motors account for most of the induction motors sold. Often referred to as
general purpose motors, slip is 5% or less.

 Design C motors have high starting torque with normal starting current and low slip. This
design is normally used where breakaway loads are high at starting, but normally run at
rated full load, and are not subject to high overload demands after running speed has been
reached. Slip is 5% or less.

 Design D motors exhibit high slip (5 to 13%), very high starting torque, low starting
current, and low full load speed. Because of high slip, speed can drop when fluctuating
loads are encountered. This design is subdivided into several groups that vary according
to slip or the shape of the speed-torque curve. These motors are usually available only on
a special order basis.
Wound-rotor motors
Although the squirrel cage induction motor is relatively inflexible with regard to speed and
torque characteristics, a special wound-rotor version has controllable speed and torque.
Application of wound-rotor motors is markedly different from squirrel-cage motors because of
the accessibility of the rotor circuit. Various performance characteristics can be obtained by
inserting different values of resistance in the rotor circuit.

Wound rotor motors are generally started with secondary resistance in the rotor circuit. This
resistance is sequentially reduced to permit the motor to come up to speed. Thus the motor can
develop substantial torque while limiting locked rotor current. The secondary resistance can be
designed for continuous service to dissipate heat produced by continuous operation at reduced
speed, frequent acceleration or acceleration with a large inertia load. External resistance gives
the motor a characteristic that results in a large drop in rpm for a fairly small change in load.
Reduced speed is provided down to about 50%, rated speed, but efficiency is low.

Single-phase motors
These motors are commonly fractional horsepower types, though integral sizes are generally
available to 10 hp. The most common single phase motor types are shaded pole, split phase,
capacitor start and permanent split capacitor.
More about Induction motors
An induction motor (or asynchronous motor or squirrel-cage motor) is a type of alternating
current motor where power is supplied to the rotor by means of electromagnetic induction.

An electric motor converts electrical power to mechanical power in its rotor (rotating part).
There are several ways to supply power to the rotor. In a DC motor, this power is supplied to the
armature directly from a DC source, while in an induction motor, this power is induced in the
rotating device. An induction motor is sometimes called a rotating transformer because the stator
(stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is
the secondary side.

Unlike the normal transformer, which changes the current by using time varying flux, induction
motor uses rotating magnetic field to transform the voltage. The primary side's current evokes a
magnetic field which interacts with the secondary side's emf to produce a resultant torque, hence
serving the purpose of producing mechanical energy. Induction motors are widely used,
especially polyphase induction motors, which are frequently used in industrial drives.

Induction motors are now the preferred choice for industrial motors due to their rugged
construction, absence of brushes (which are required in most DC motors) and the ability to
control the speed of the motor.

The basic difference between an induction motor and a synchronous AC motor is that in the
latter, a current is supplied into the rotor (usually a DC current) which in turn creates a (circular
uniform) magnetic field around the rotor. The rotating magnetic field of the stator will impose an
electromagnetic torque on the still magnetic field of the rotor causing it to move (about a shaft)
and rotation of the rotor is produced. It is called synchronous because at steady state, the speed
of the rotor is the same as the speed of the rotating magnetic field in the stator.

On the other hand, the induction motor does not have any direct supply onto the rotor, instead, a
secondary current is induced in the rotor. To achieve this, stator windings are arranged around
the rotor so that when energized with a polyphase supply, they create a rotating magnetic field
pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the
rotor conductors. These currents interact with the rotating magnetic field, created by the stator,
and in effect cause a rotational motion on the rotor.

However, for these currents to be induced, the speed of the physical rotor must be less than the
speed of the rotating magnetic field in the stator, or else the magnetic field will not be moving
relative to the rotor conductors and no currents will be induced. If by some chance this happens,
the rotor typically slows slightly until a current is induced again and then the rotor continues as
before. This difference between the speed of the rotor and speed of the rotating magnetic field in
the stator is called slip. It is a unit less quantity and is the ratio between the relative speed of the
magnetic field as seen by the rotor (the slip speed) to the speed of the rotating stator field. Due to
this, an induction motor is sometimes referred to as an asynchronous machine.
Speed control
The synchronous rotational speed of the rotor (i.e. the theoretical unloaded speed with no slip) is
controlled by the number of pole pairs (number of windings in the stator) and by the frequency
of the supply voltage. Before the development of low cost power electronics, it was difficult to
vary the frequency to the motor and therefore the uses of the induction motor were limited. As an
induction motor has no brushes and is easy to control, many older DC motors are being replaced
with induction motors and accompanying inverters in industrial applications.

The induction motor runs on induced current. Speed of induction motor varies according to the
load supplied to the induction motor. As the load on the induction motor is increased, the speed
of the motor gets decreased and vice versa.