Equivalent Circuit of a Single Phase Induction Motor The double revolving field theory can be effectively used to obtain the equivalent circuit of a single phase induction motor. Imagine the single phase induction motor is made up of one stator winding and two imaginary rotor windings. One rotor is rotating in forward direction i.e. in the direction of rotating magnetic field with slip s while other is rotating in backward direction i.e. in direction of oppositely directed rotating magnetic field with slip 2 - s. 1. Without core loss

2. With core loss

Hysteresis motor A hysteresis motor is a synchronous motor without salient (or projected) poles and without dc excitation which starts by virtue of the hysteresis losses induced in its hardened steel secondary member by the revolving filed of the primary and operates normally at synchronous speed and runs on hysteresis torque It is a single-phase motor whose operation depends upon the hysteresis effect i.e., magnetization produced in a ferromagnetic material lags behind the magnetizing force.

Hysteresis Motors Stator Rotor same as for induction motor Smooth cylinder

Principle of Operation Stator Flux establishes these magnetic poles Rotor poles “induced” by Stator Flux

Rotor poles follow the rotating flux, but lag behind by angle δh Stator poles moving CCW

Ts α φsφrsinα Where, φs = stator flux φr = rotor flux

Hysteresis Power Loss, Ph where fr = frequency of flux reversal in the rotor (Hz) Bmax = maximum value of flux density in the air gap (T) Ph = heat-power loss due to hysteresis (W) kh = constant

Hysteresis Motor at Synchronous Speed No load and negligible rotational losses Induced rotor magnets remain locked with the rotating poles produced by the stator

Hysteresis Motor at Synchronous Speed The rotor slows down and the induced rotor magnets lag the rotating poles of the stator by an angle δmag . The rotor returns to synchronous speed at the new torque angle. Apply a step increase in shaft load.

Hysteresis Motor at Synchronous Speed If shaft load causes δmag>90°, the rotor pulls out if synchronism, the magnet torque drops to zero, and the machine develops hysteresis torque. This torque is not sufficient to carry the load.

Torque-Speed Characteristic Constant Hysteresis Torque allows the motor to synchronize any load it can accelerate “Normal” Operating Range

Advantages Rotor as no teeth and no windings No mechanical vibration Quiet and Noiseless operation Suitability to accelerate High inertia load Possibility of multispeed operation Application Sound recoding Instrument Sound producing equipment Tape recorder Electric Clock Tele-printers Timing Devices High quality recorder player

Reluctance motor Stator Rotor

Operating principle When power is applied to the stator windings, the rotor's magnetic reluctance creates a force that attempts to align the rotor pole with the nearest stator pole.

Reluctance Motor There are various types of reluctance motors:- Synchronous reluctance Variable reluctance Switched reluctance Variable reluctance stepping.

SWITCHED RELUCTANCE MOTOR The stator and rotor of a Switched Reluctance motor have salient poles This doubly-salient arrangement is very effective for electromagnetic energy conversion. The stator part carries coils on each pole, the coils on opposite poles being connected in series. The rotor does not have magnets or coils attached with the rotor slots. It is a solid salient-pole rotor made of soft magnetic material with a laminated-steel.

The Switched Reluctance Motor The Stator contains 3 electro-magnets powered by DC Current.

The Rotor is a Plain Piece of Steel capable of carrying a magnetic flux The rotor is connected To a shaft that is it hoped will turn to make something happen.

Magnetic Reluctance Magnetic flux likes to find an easy flow path A nice piece of steel is a much better flow path than air The magnetic flux will try to get the steel path to line up (considered magnetic reluctance)

Making the Motor Work If I keep turning magnetic fields off And on around the stator I can Have the rotor continuous chasing The magnetic field and thus Turning the shaft – I now have a DC motor with no Rings or brushes. (But one wholly Heck of a lot of switches)

A bunch of extra poles and then let a project logic controller program do the field Switching to smooth out the torque.

Advantages of Switched Reluctance Motor Reluctance motor offers following advantages. These are: Construction of the motor is simple. Brushes, commutators , permanent magnets are absent Starting torque is quite good Accurate speed control is possible Cost effective and easy maintenance. Higher efficiency More power per unit weight and volume Has no windings or slip rings in the rotor Can run at very high speed (upto 30,000 rpm) in hazardous atmospheres. Four-quadrant operation is possible with appropriate drive circuitry.

Disadvantage of Switched Reluctance Motor Noisy in the operation This type of motor not well-suited for smooth torque production. Flux linkage and Non-linear function of stator currents as well as rotor position control of the motors a tough challenge. Applications Washing Machines, Weaving Machinery. Centrifugal Pumps, Compressors, Door Openers Analog Electronic Meters. Control Rod Drive Mechanisms of Nuclear Reactors. Microcontroller Based Operation Control Circuits.