Rotor (electric)

The rotor is a moving component of an

A salient pole ends in a pole shoe, a high-permeability part with an outer surface shaped as a segment of a cylinder to homogenize the distribution of the magnetic flux to the stator.^[9]

Non-salient rotor

The cylindrical shaped rotor is made of a solid steel shaft with slots running along the outside length of the cylinder for holding the field windings of the rotor which are laminated copper bars inserted into the slots and is secured by wedges.^[10] The slots are insulated from the windings and are held at the end of the rotor by slip rings. An external direct current (DC) source is connected to the concentrically mounted slip rings with brushes running along the rings.^[8] The brushes make electrical contact with the rotating slip rings. DC current is also supplied through brushless excitation from a rectifier mounted on the machine shaft that converts alternating current to direct current.

Operating principle

In a three-phase induction machine, alternating current supplied to the stator windings energizes it to create a rotating magnetic flux.^[11] 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.^[5] The action of the rotating flux and the current produces a force that generates a torque to start the motor.^[11]

An alternator rotor is made up of a wire coil enveloped around an iron core.

Characteristics of rotors

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• 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 1500-3600 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.^[11]

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

where:

${\displaystyle E}$ = induced voltage

${\displaystyle B}$ = magnetic field

${\displaystyle L}$ = conductor length

${\displaystyle V_{syn}}$ = synchronous speed

${\displaystyle V_{m}}$= 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

${\displaystyle F=(BxI)L}$

${\displaystyle T=Fxr}$

where:

${\displaystyle F}$ = force

${\displaystyle T}$ = torque

${\displaystyle r}$ = radius of rotor rings

${\displaystyle I}$ = rotor bar

Induction motor slip

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

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

where:

${\displaystyle f}$ = frequency

${\displaystyle p}$ = number of poles

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

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

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

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

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

Relative speed of slip:

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

Frequency of induced voltages and currents

${\displaystyle f_{r}=sf_{e}}$

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