Squirrel-cage rotor

Last updated March 08, 2024

A squirrel-cage rotor is the rotating part of the common squirrel-cage 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 winding is connected to an alternating current power source; the alternating current in the stator produces a rotating magnetic field. The rotor winding has current induced in it by the stator field, like a 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 in the stator and the currents in the rotor produce a torque on the rotor.

Squirrel-cage induction motors are very prevalent in industry, in sizes from below 1 kilowatt (1.3 hp) 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 or NEMA standard frame sizes, which are interchangeable between manufacturers. This simplifies application and replacement of these motors.

History

Galileo Ferraris described an induction machine with a two-phase stator winding and a solid copper cylindrical armature in 1885. In 1888, Nikola Tesla 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 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.^[1]

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. The figure shows one of many lamination sets used. The rotor lamination has a larger number of slots than its corresponding stator 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.^[2]

The rotor bars may be made of either copper or aluminium. A very common structure for smaller motors uses die cast 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.^[3]

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.

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 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 numbers 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 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.

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 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.^[3]

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

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.^[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.

Related Research Articles

An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates in reverse, converting mechanical energy into electrical energy.

<span class="mw-page-title-main">Stepper motor</span> Electric motor for discrete partial rotations

A stepper motor, also known as step motor or stepping motor, is an electrical motor that rotates in a series of small angular steps, instead of continuously. Stepper motors are a type of digital actuator. Like other electromagnetic actuators, they convert electric energy into mechanical energy to perform work.

A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings on the armature are connected to the commutator segments.

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore needs no electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type.

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. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.

A brushless DC electric motor (BLDC), also known as an electronically commutated motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

A DC motor is an electrical motor that uses direct current (DC) to produce mechanical force. The most common types rely on magnetic forces produced by currents in the coils. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.

The shaded-pole motor is the original type of AC single-phase motor, dating back to at least as early as 1890. A shaded-pole motor is a small motor with either two or four poles, in which the auxiliary winding is composed of a copper ring or bar surrounding a portion of each pole to produce a weakly rotating magnetic field. When single phase AC supply is applied to the stator winding, due to shading provided to the poles, a rotating magnetic field is generated. This auxiliary single-turn winding is called a shading coil. Currents induced in this coil by the magnetic field create a second electrical phase by delaying the phase of magnetic flux change for that pole enough to provide a 2-phase rotating magnetic field. The direction of rotation is from the unshaded side to the shaded (ring) side of the pole. Since the phase angle between the shaded and unshaded sections is small, shaded-pole motors produce only a small starting torque relative to torque at full speed. Shaded-pole motors of the asymmetrical type shown are only reversible by disassembly and flipping over the stator, though some similar looking motors have small, switch-shortable auxiliary windings of thin wire instead of thick copper bars and can reverse electrically. Another method of electrical reversing involves four coils.

The universal motor is a type of electric motor that can operate on either AC or DC power and uses an electromagnet as its stator to create its magnetic field. It is a commutated series-wound motor where the stator's field coils are connected in series with the rotor windings through a commutator. It is often referred to as an AC series motor. The universal motor is very similar to a DC series motor in construction, but is modified slightly to allow the motor to operate properly on AC power. This type of electric motor can operate well on AC because the current in both the field coils and the armature will alternate synchronously with the supply. Hence the resulting mechanical force will occur in a consistent direction of rotation, independent of the direction of applied voltage, but determined by the commutator and polarity of the field coils.

A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.

A repulsion motor is a type of electric motor which runs on alternating current (AC). It was formerly used as a traction motor for electric trains but has been superseded by other types of motors. Repulsion motors are classified as single phase motors.

A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance.

<span class="mw-page-title-main">AC motor</span> Electric motor driven by an AC electrical input

An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.

An induction generator or asynchronous generator is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor usually can be used as a generator, without any internal modifications. Because they can recover energy with relatively simple controls, induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure.

The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis.

In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating or linear. While transformers are occasionally called "static electric machines", since they do not have moving parts, generally they are considered not as "machines", but as electrical devices "closely related" to the electrical machines.

<span class="mw-page-title-main">Wound rotor motor</span>

A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. 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.

The switched reluctance motor (SRM) is an electric motor that runs by reluctance torque and thus is a subgroup in reluctance motors. Unlike common brushed DC motor types, power is delivered to windings in the stator (case) rather than the rotor. This greatly simplifies mechanical design as power does not have to be delivered to a moving part which eliminates the need for a commutator, but it complicates the electrical design as some sort of switching system needs to be used to deliver power to the different windings. Electronic devices can precisely time the switching of currents, facilitating SRM configurations. Its main drawback is torque ripple. Controller technology that limits torque ripple at low speeds has been demonstrated. Sources disagree on whether it is a type of stepper motor.

An induction regulator is an alternating current electrical machine, somewhat similar to an induction motor, which can provide a continuously variable output voltage. The induction regulator was an early device used to control the voltage of electric networks. Since the 1930s it has been replaced in distribution network applications by the tap transformer. Its usage is now mostly confined to electrical laboratories, electrochemical processes and arc welding. With minor variations, its setup can be used as a phase-shifting power transformer.

Electromagnetically induced acoustic noise (and vibration), electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.

References

↑ Ion Boldea, Syed A. Nasar, The Induction Machine Handbook, CRC Press 2010 ISBN 1420042653, pages 2-3
↑ theory and performance of electrical machines, J.B.Gupta
1 2 Gordon R. Slemon, Magnetoelectric devices, John Wiley and Sons 1966 pp. 384-389
↑ Garr M. Jones (ed.), Pumping Station Design Revised 3rd Edition Elsevier, 2008 ISBN 978-1-85617-513-5, pg. 13-4

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[1] Ion Boldea, Syed A. Nasar, The Induction Machine Handbook, CRC Press 2010 ISBN 1420042653, pages 2-3

[2] theory and performance of electrical machines, J.B.Gupta

[GRS66-3] 1 2 Gordon R. Slemon, Magnetoelectric devices, John Wiley and Sons 1966 pp. 384-389

[4] Garr M. Jones (ed.), Pumping Station Design Revised 3rd Edition Elsevier, 2008 ISBN 978-1-85617-513-5, pg. 13-4

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