A squirrel-cage rotor is the rotating part of the common squirrel-cage

induction motor An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor ...

. It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface. In operation, the non-rotating

stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric ...

winding is connected to an

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...

power source; the alternating current in the stator produces a

rotating magnetic field A rotating magnetic field is the resultant magnetic field produced by a system of coils symmetrically placed and supplied with polyphase currents. A rotating magnetic field can be produced by a poly-phase (two or more phases) current or by a singl ...

. The rotor winding has current induced in it by the stator field, like a

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

except that the current in the rotor is varying at the stator field rotation rate minus the physical rotation rate. The interaction of the magnetic fields of currents in the stator and rotor produce a torque on the rotor. By adjusting the shape of the bars in the rotor, the speed-torque characteristics of the motor can be changed, to minimize starting current or to maximize low-speed torque, for example. Squirrel-cage induction motors are very prevalent in industry, in sizes from below up to tens of megawatts (tens-of-thousand horsepower). They are simple, rugged, and self-starting, and maintain a reasonably constant speed from light load to full load, set by the frequency of the power supply and the number of poles of the stator winding. Commonly used motors in industry are usually

IEC The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and ...

NEMA The National Electrical Manufacturers Association (NEMA) is the largest trade association of electrical equipment manufacturers in the United States. Founded in 1926, it advocates for the industry, and publishes standards for electrical product ...

standard frame sizes, which are interchangeable between manufacturers. This simplifies application and replacement of these motors.

History

Galileo Ferraris Galileo Ferraris (31 October 1847 – 7 February 1897) was an Italian university professor, physicist and electrical engineer, one of the pioneers of AC power system and inventor of the induction motor although he never patented his work. Many ...

described an induction machine with a two-phase stator winding and a solid copper cylindrical armature in 1885. In 1888,

Nikola Tesla Nikola Tesla ( ; ,"Tesla"
'' Mikhail Dolivo-Dobrovolsky Mikhail Osipovich Dolivo-Dobrovolsky (russian: Михаи́л О́сипович Доли́во-Доброво́льский; german: Michail von Dolivo-Dobrowolsky or ''Michail Ossipowitsch Doliwo-Dobrowolski''; – ) was a Russian Empire ...

developed a wound-rotor induction motor, and shortly afterwards a cage-type rotor winding. By the end of the 19th century induction motors were widely applied on the growing alternating-current electrical distributions systems.

Structure

The motor rotor shape is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars (usually made of aluminium or copper) 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 larger number of slots than the

and should be a non-integer multiple of the number of stator slots so as to prevent magnetic interlocking of rotor and stator teeth at the starting instant. The rotor bars may be made of either copper or aluminium. A very common structure for smaller motors uses

die cast Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly ...

aluminium poured into the rotor after the laminations are stacked. Larger motors have aluminium or copper bars which are welded or brazed to end-rings. Since the voltage developed in the squirrel cage winding is very low, and the current very high, no intentional insulation layer is present between the bars and the rotor steel. Gordon R. Slemon, ''Magnetoelectric devices'', John Wiley and Sons 1966 pp. 384-389

Theory

The field windings in the stator of an induction motor set up a

through the

rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...

. The relative motion between this field and the rotor induces

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movin ...

in the conductive bars. In turn these currents lengthwise in the conductors react with the magnetic field of the motor to produce

force A force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. The concept of force makes the everyday notion of pushing or pulling mathematically precise. Because the Magnitude ...

acting at a

tangent In geometry, the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve. Mo ...

orthogonal In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

to the rotor, resulting in

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...

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.

Skewing

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, by ensuring that at any time the same fraction of a rotor bar is under each stator slot. If rotor bars were parallel to the stator poles, the motor would experience a drop and then recovery in torque as each bar passes the gap in the stator. The laminations shown in the photo have 36 bars in the stator and 40 bars in the rotor. The

greatest common divisor In mathematics, the greatest common divisor (GCD) of two or more integers, which are not all zero, is the largest positive integer that divides each of the integers. For two integers ''x'', ''y'', the greatest common divisor of ''x'' and ''y'' i ...

of 36 and 40 is 4, with the result that no more than 4 bars of the stator and rotor can be aligned at any one time, which also reduces torque fluctuations. The number of bars in the rotor 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 use

prime number A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only way ...

s of rotor bars.

Laminations

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

core to reduce core energy losses. It is made of thin laminations, separated by varnish insulation, to reduce

eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnet ...

s circulating in the core. The material is a low carbon but high-

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

iron with several times the

resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

of pure iron, further reducing eddy-current loss, and low

coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...

to reduce

hysteresis loss Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component o ...

Rotor bars

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, the

skin effect Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the cond ...

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 Slip ratio is a means of calculating and expressing the slipping behavior of the wheel of an automobile. It is of fundamental importance in the field of vehicle dynamics, as it allows to understand the relationship between the deformation of the t ...

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, with a combination of high and low impedance rotor in parallel 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 (as rotor) 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 of its own 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

synchronous motor A synchronous electric motor is an AC electric 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. Sync ...

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 relay In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detec ...

s to detect when the machine has fallen out of synchronization with the supply voltage.Garr M. Jones (ed.), ''Pumping Station Design Revised 3rd Edition'' Elsevier, 2008 , pg. 13-4

Induction generators

Three phase squirrel cage induction motors can also be used as generators. For this to work the motor must see a reactive load, and 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 faster than its stator's synchronous speed. This will cause the motor to generate power after building up its residual magnetism.

References

Induction motors