User:Durghums/sandbox

A selection of various types of rotors

Rotor from Hoover Dam generator

For other uses, see Rotor.

The rotor is the moving component of the electromagnetic system in an electric motor or generator. Its rotation is due to the interaction between the electric current in the windings on the rotor and the magnetic field generated by the stator, which produces a torque on the rotor around its axis.

Early development

An early example of electromagnetic rotation was the first rotary machine built by Ányos Jedlik with electromagnets and a commutator in 1826-27. Other pioneers include Hippolyte Pixii who built an alternating current generator in 1832, and William Ritchie's construction of an electromagnetic generator with four rotor coils, a commutator and brushes, also in 1832. Development quickly included more useful applications such as Moritz Hermann Jacobi's motor that could develop about 15 watts of mechanical power in 1834. The next year, Francis Watkins described an electrical "toy" he created. He is generally regarded as one of the first to understand the interchangeability of motor and generator.

Type and construction of rotors

AC and DC motors and generators are electromagnetic systems consisting of a stator and a rotor. There are two main categories rotor: squirrel-cage and wound. Wound rotors have coils of insulated wire wrapped onto them, and these windings are usually connected externally. The windings on squirrel-cage rotors are made of solid uninsulated metal, and have no external connections. The magnetic cores of both types are made of stacks of die-cut plates of electrical steel.

Wound rotor

Main article: Wound rotor

The cores of wound rotors have axial slots to hold their windings. Depending on shape and depth of the slots, they are divided into two categories, cylindrical or salient pole. The windings of both types are most commonly electrically connected to fixed terminals on the stator housing through commutators, slip rings, or, rarely, rotary transformers.

Salient pole rotor

Salient pole rotor with slip ring winding connections, and externally fed with direct current

The core of a salient pole rotor has large sections removed lengthwise, leaving straight fins pointing out like the spokes of a wheel. Each fin is wound with wire to form an electromagnet. One pole of each electromagnet is at the root where it joins the rest of the core. All of these inner poles are magnetically bonded together into the common central body of the rotor. When referring to the number of poles on a given rotor, it is only the outward-facing ends of the coils that are counted. They are the poles which interact with the poles of the stator. Each rotor pole ends in a rounded cap, which exerts a centripetal force on the windings, without which they would tend to fly off the spinning rotor. The caps also serve to spread out the magnetic field of the pole, which helps to reduce harmonics. The name "salient pole" refers to the fact that the pole tips stand out farther than any other part of the rotor. For AC operation, there must be at least one pole pair for each input phase to the rotor.

The diagram at right shows a simplified version of a salient pole rotor with only two poles.

Cylindrical rotor

Cylindrical rotor with slip ring winding connections, and externally fed with direct current

Like salient pole rotors, cylindrical rotors have laminated steel cores with lengthwise slots on the outside to hold the rotor windings. The difference is the size and shape of the slots. The slots in cylindrical wound rotors are often elliptical, and are connected to the outer surface of the core by a thin slot. The slots are insulated from the windings using sheets of plastic,

Squirrel-cage rotor

Main article: Squirrel-cage rotor

Squirrel-cage rotor construction, showing only the center three core laminations

A squirrel-cage rotor consists of a cylindrical core made from laminated steel, with evenly spaced bars of copper or aluminum placed axially around the periphery, permanently shorted at the ends by the end rings. This simple and rugged construction led to its near-universal adoption in induction motors. The assembly is slightly twisted: the axial bars are skewed to reduce magnetic hum and slot harmonics. Some rotors have extension(s) on one end for speed sensors or other electronic controls. The name "squirrel-cage" refers to the way the shape of the bars and end rings resembles the spinning exercise wheels found on antique pet squirrel cages.

The magnetic field of squirrel-cage rotors is generated by currents induced by the relative rotation between the movement of the rotor and the rotating magnetic field generated by the stator windings. Because of this, squirrel-cage rotors can only be used in AC induction motors. Induction motors are by nature asynchronous, which means they cannot be used to produce fixed-speed rotational output from a fixed-frequency electrical input. Machines with squirrel-cage rotors are poorly suited to electricity generating applications.

Operating principle

In a three-phase induction machine, alternating current supplied to the stator windings energizes it to create a rotating magnetic flux. The flux generates a magnetic field in the air gap between the stator and the rotor and induces a voltage which produces current through the rotor bars. The rotor circuit is shorted and current flows in the rotor conductors. The action of the rotating flux and the current produces a force that generates a torque to start the motor.

An alternator rotor is made up of a wire coil enveloped around an iron core. The magnetic component of the rotor is made from steel laminations to aid stamping conductor slots to specific shapes and sizes. As currents travel through the wire coil a magnetic field is created around the core, which is referred to as field current. The field current strength controls the power level of the magnetic field. Direct current (DC) drives the field current in one direction, and is delivered to the wire coil by a set of brushes and slip rings. Like any magnet, the magnetic field produced has a north and a south pole. The normal clockwise direction of the motor that the rotor is powering can be manipulated by using the magnets and magnetic fields installed in the design of the rotor, allowing the motor to run in reverse or counterclockwise.

Characteristics of rotors

• Squirrel-cage rotor

This rotor rotates at a speed less than the stator rotating magnetic field or synchronous speed.

Rotor slip provides necessary induction of rotor currents for motor torque, which is in proportion to slip.

When rotor speed increases, the slip decreases.

Increasing the slip increases induced motor current, which in turn increases rotor current, resulting in a higher torque for increase load demands.

• Wound rotor

This rotor operates at constant speed and has lower starting current

External resistance added to rotor circuit, increases starting torque

Motor running efficiency improves as external resistance is reduced when motor speed up.

Higher torque and speed control

• Salient pole rotor

This rotor operates at a speed below 1500 rpm (revolutions per minute) and 40% of its rated torque without excitation

It has a large diameter and short axial length

Air gap is non uniform

Rotor has low mechanical strength

• Cylindrical rotor

The rotor operates at speed between 1500-3000 rpm

It has strong mechanical strength

Air gap is uniform

Its diameter is small and has a large axial length and requires a higher torque than salient pole rotor

Rotor equations

Rotor bar voltage

The rotating magnetic field induces a voltage in the rotor bars as it passes over them. This equation applies to induced voltage in the rotor bars.

$E=BL(V_{syn}-V_{m})$

where:

$E$= induced voltage

$B$= magnetic field

$L$=conductor length

$V_{syn}$=synchronous speed

$V_{m}$= conductor speed

Torque in rotor

A torque is produced by the force produced through the interactions of the magnetic field and current as expressed by the given: Ibid

$F=(BxI)L$

$T=Fxr$

where:

$F$=force

$T$=torque

$r$=radius of rotor rings

$I$=rotor bar

Induction motor slip

A stator magnetic field rotates at synchronous speed, $n_{s}$ Ibid

$n_{s}={\frac {120f}{p}}$

where:

$f$= frequency

$p$= number of poles

If $n_{m}$= rotor speed, the slip, S for an induction motor is expressed as:

$s={\frac {n_{s}-n_{m}}{n_{s}}}\times 100\%$

mechanical speed of rotor, in terms of slip and synchronous speed:

$n_{m}=(1-s)n_{s}$

$\omega _{m}=(1-s)\omega _{s}$

Relative speed of slip:

$n_{slip}=n_{s}-n_{m}$

Frequency of induced voltages and currents

$f_{r}=sf_{e}$

See also

• Armature (electrical engineering) - any "rotor" that carries some form of alternating current
• Balancing machine
• Commutator (electric)
• Electric motor
• Field coil
• Rotordynamics
• Stator

References

Category:Electric motors