A squirrel-cage rotor is the rotating part 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.
Structure The motor rotor shape is a cylinder
mounted on a shaft. Internally it contains longitudinal conductive bars
set into grooves and connected at both ends by shorting rings forming a
cage-like shape. The name is derived from the similarity between this
rings-and-bars winding and a squirrel cage.
The solid core of the rotor is built with stacks of electrical steel
laminations. Figure 3 shows one of many laminations used. The rotor has a
smaller number of slots than the stator and must be a non-integer multiple of
stator slots so as to prevent magnetic interlocking of rotor and stator teeth
at the starting instant. The rotor bars may be made either of
copper or aluminum. A very common structure uses die cast aluminum poured
into the rotor after the laminations are stacked. Some larger motors have
aluminum or copper bars which are welded or brazed to end-rings. Since the
voltage developed in the squirrel cage winding is very low, no intentional
insulation layer is present between the bars and the rotor steel.
Theory The field windings in the stator of an
induction motor set up a rotating magnetic field through the rotor. The
relative motion between this field and the rotor induces electric current in
the conductive bars. In turn these currents lengthwise in the conductors
react with the magnetic field of the motor to produce force acting at a
tangent orthogonal to the rotor, resulting in torque to turn the shaft.
In effect the rotor is carried around with the magnetic field but at a
slightly slower rate of rotation. The difference in speed is called slip and
increases with load. The conductors are often skewed slightly
along the length of the rotor to reduce noise and smooth out torque fluctuations
that might result at some speeds due to interactions with the pole pieces of the
stator. The number of bars on the squirrel cage determines to what extent
the induced currents are fed back to the stator coils and hence the current
through them. The constructions that offer the least feedback employ prime
numbers of bars. The iron core serves to carry the
magnetic field through the rotor conductors. Because the magnetic field
in the rotor is alternating with time, the core uses construction similar to a
transformer core to reduce core energy losses. It is made of thin laminations,
separated by varnish insulation, to reduce eddy currents circulating in the
core. The material is a low carbon but high silicon iron with several times the
resistivity of pure iron, further reducing eddy-current loss, and low
coercivity to reduce hysteresis loss. The same basic design is used for both
single-phase and three-phase motors over a wide range of sizes. Rotors for
three-phase will have variations in the depth and shape of bars to suit the
design classification. Generally, thick bars have good torque and are efficient
at low slip, since they present lower resistance to the EMF. As the slip
increases, skin effect starts to reduce the effective depth and increases the
resistance, resulting in reduced efficiency but still maintaining torque.
The shape and depth of the rotor bars can be used to vary the speed-torque
characteristics of the induction motor. At standstill, the revolving magnetic
field passes the rotor bars at a high rate, inducing line-frequency current
into the rotor bars. Due to the skin effect, the induced current tends to
flow at the outer edge of the winding. As the motor accelerates, the slip
frequency decreases and induced current flows at greater depths in the winding.
By tapering the profile of the rotor bars to vary their resistance at
different depths, or by constructing a double squirrel cage, the motor can be
arranged to produce more or less torque at standstill and near its synchronous
speed. Practical demonstration
To demonstrate how the cage rotor works, the stator of a single-phase motor and a
copper pipe may be used. If adequate AC power is applied to the stator, an
alternating magnetic field will revolve around within the stator. If the copper
pipe is inserted inside the stator, there will be an induced current in the
pipe, and this current will produce a magnetic field in the pipe. The
interaction between the stator's revolving magnetic field and the
copper-pipe-rotor's induced magnetic field produces a torque and thus
rotation. Use in synchronous motors
A synchronous motor may have a squirrel-cage winding embedded in its
rotor, used to increase the motor starting torque and so decrease the time
to accelerate to synchronous speed. The squirrel cage winding of a synchronous
machine will generally be smaller than for an induction machine of similar
rating. When the rotor is turning at the same speed as the stator's revolving
magnetic field, no current is induced into the squirrel-cage windings and the
windings will have no further effect on the operation of the synchronous motor
at steady-state. The squirrel cage winding in some
machines provides a damping effect for load or system disturbances, and in this
role may be designated as an "amortisseur" windings. Large machines
may only have amortisseur bars in the individual pole faces, not
interconnected between poles. Because the squirrel cage winding is not large
enough to dissipate the heat of continuous operation, large synchronous
machines often have protective relays to detect when the machine has fallen out
of synchronization with the supply voltage.
Induction generators Three phase squirrel cage induction
motors can also be used as generators. For this to work the motor must either
be connected to a grid supply or an arrangement of capacitors to provide
excitation current. For the motor to work as a generator instead of a motor
the rotor must be spun just faster than its nameplate speed, this will cause the
motor to generate power after building up its residual magnetism.
References ^ theory and performance of electrical
machines, J.B.Gupta ^ a b Gordon R. Slemon, Magnetoelectric
devices, John Wiley and Sons 1966 pp. 384-389
^ Garr M. Jones, Pumping Station Design Revised 3rd Edition Elsevier, 2008 ISBN
978-1-85617-513-5, pg. 13-4 See also
AC motor Induction motor
Copper in energy efficient motors Mikhail Dolivo-Dobrovolsky