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

induction motor An induction motor or asynchronous motor is an AC motor, AC electric motor in which the electric current in the rotor (electric), rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding ...

. It consists of a cylinder of steel laminations, with aluminum or copper conductors

cast Cast may refer to: Music * Cast (band), an English alternative rock band * Cast (Mexican band), a progressive Mexican rock band * The Cast, a Scottish musical duo: Mairi Campbell and Dave Francis * ''Cast'', a 2012 album by Trespassers William ...

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 (such as bacterial flagella or ATP synthase). Energy flows through a stator to or from the rotat ...

winding is connected to an

alternating current Alternating current (AC) is an electric current that 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 w ...

power source; the alternating current in the stator produces a

rotating magnetic field A rotating magnetic field (RMF) is the resultant magnetic field produced by a system of Electromagnetic coil, coils symmetrically placed and supplied with Polyphase system, polyphase currents. A rotating magnetic field can be produced by a poly-phas ...

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

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

, 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 in the stator and the currents in the 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; ) is an international standards organization that prepares and publishes international standards for all electrical, electronic and related technologies. IEC standards cover a vast range of ...

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 ne ...

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"
. ''Random House Webster's Unabridged Dictionary''. ; 10 July 1856 – 7 ...

received a patent on a two-phase induction motor with a short-circuited copper rotor winding and a two-phase stator winding. Developments of this design became commercially important. In 1889,

Mikhail Dolivo-Dobrovolsky Mikhail Osipovich Dolivo-Dobrovolsky (; or ''Michail Ossipowitsch Doliwo-Dobrowolski''; – ) was a Russian-born engineer, electrician, and inventor of Polish-Russian origins, active in the German Empire and also in Switzerland. After study ...

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 Squirrel-cage may refer to: *a squirrel-cage rotor *a squirrel-cage fan, another name for a centrifugal fan *a hamster wheel A hamster wheel or running wheel is an exercise device used primarily by hamsters and other rodents, but also by othe ...

. The solid core of the rotor is built with stacks of electrical steel laminations. The figure shows three of many lamination sets used. The rotor lamination has a larger number of slots than its corresponding

lamination, and the number of rotor slots should be a non-integer multiple of the number of stator slots 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 to ...

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 was an elaborate air defence radar system built by the British Government in the early 1950s to counter possible attack by Soviet bombers. To get it operational as quickly as possible, it was initially made up primarily of WWII-era syst ...

. The relative motion between this field and the rotor induces

electric current An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...

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

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

acting at a

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

orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...

to the rotor, resulting in

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

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), also known as greatest common factor (GCF), of two or more integers, which are not all zero, is the largest positive integer that divides each of the integers. For two integers , , the greatest co ...

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 (mathematics), 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 ...

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

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

iron with several times the

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

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 Magnetization, demagnetized. Coercivity is usual ...

to reduce hysteresis loss.

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 In electromagnetism, skin effect is the tendency of an alternating current, alternating electric current (AC) to become distributed within a Conductor (material), conductor such that the current density is largest near the surface of the conduc ...

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, 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 integer 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 Electrical fault, fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts ...

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