electrical-engineering-portal.com

Three phase squire cage induction motor, fullyenclosed

http://electrical-engineering-portal.com/the-cage-induction-motor-explained-in-details

The Cage Induction Motor Explained In DetailsThis simplest form of ac induction motor orasynchronous motor is the basic, universalworkhorse of industry. Its general construction isshown in Fig. 1. It is usually designed for fixed-speed operation, larger ratings having suchfeatures as deep rotor bars to limit Direct onLine (DOL) starting currents.

Electronic variable speed drive technology isable to provide the necessary variable voltage,current and frequency that the induction motorrequires for efficient, dynamic and stablevariable speed control.

Modern electronic control technology is able notonly to render the ac induction motor satisfactoryfor many modern drive applications but also toextend greatly its application and enable usersto take advantage of its low capital andmaintenance costs.

More striking still, microelectronic developments have made possible the highly dynamic operationof induction motors by the application of flux vector control. The practical effect is that it is nowpossible to drive an ac induction motor in such a way as to obtain a dynamic performance in allrespects better than could be obtained with a phase-controlled dc drive combination.

The stator winding of the standard industrial induction motor in the integral kilowatt range is three-phase and is sinusoidally distributed. With a symmetrical three-phase supply connected to thesewindings, the resulting currents set up, in the air-gap between the stator and the rotor, a travellingwave magnetic field of constant magnitude and moving at synchronous speed. The rotational speedof this field is f/p revolutions per second, where f is the supply frequency (hertz) and p is the numberof pole pairs (a four-pole motor, for instance, having two pole pairs). It is more usual to expressspeed in revolutions per minute, as 60 f/p (rpm).

The emf generated in a rotor conductor is at a maximum in the region of maximum flux density andthe emf generated in each single rotor conductor produces a current, the consequence being aforce exerted on the rotor which tends to turn it in the direction of the flux rotation. The higher thespeed of the rotor, the lower the speed of the rotating stator flux field relative to the rotor winding,and therefore the smaller is the emf and the current generated in the rotor cage or winding.

The speed when the rotor turns at the same rate as that of the rotating field is known as synchronousspeed and the rotor conductors are then stationary in relation to the rotating flux. This produces noemf and no rotor current and therefore no torque on the rotor. Because of friction and windage therotor cannot continue to rotate at synchronous speed; the speed must therefore fall and as it doesso, rotor emf and current, and therefore torque, will increase until it matches that required by thelosses andby any load on the motor shaft. The difference in rotor speed relative to that of the rotating stator fluxis known as the slip.

It is usual to express slip as a percentage of the synchronous speed. Slip is closely proportional to

Fig. 1 - Sectional view of a totally enclosed induction motor

torque from zero to full load.

The

most popular squirrel cage induction motor is of a 4-pole design. Its synchronous speed with a 50Hz supply is therefore 60 f/p, or 1500 rpm. For a full-load operating slip of 3 per cent, the speed willthen be (1 – s)60 f/p, or 1455 rpm.

Torque characteristicsA disadvantage of the squirrel cage machine is its fixed rotor characteristic. The starting torque isdirectly related to the rotor circuit impedance, as is the percentage slip when running at load andspeed. Ideally, a relatively high rotor impedance is required for good starting performance (torqueagainst current) and a low rotor impedance provides low full-load speed slip and high efficiency.

Fig. 2 - Typical rotor bar profiles

Fig. 3 - Typical torque-speed and current-speed curves (a - standardmotor, b - high torque motor (6 per cent slip))

This problem can beovercome to a useful extentfor DOL application bydesigning the rotor bars withspecial cross sections asshown in Fig. 2 so that rotoreddy currents increase theimpedance at starting whenthe rotor flux (slip) frequencyis high.

Alternatively, for special highstarting torque motors, twoor even three concentric setsof rotor bars are used. Relatively costly in construction but capable of a substantial improvement instarting performance, this form of design produces an increase in full load slip. Since machinelosses are closely proportional to working speed slip, increased losses may require such a highstarting torque machine to be derated.

The curves in Fig. 3indicate squirrel cagemotor characteristics. In thegeneral case, the higherthe starting torque thegreater the full load slip.This is one of the importantparameters of squirrelcage design as itinfluences the operatingefficiency.

SOURCE: NewnesElectrical Power EngineersHandbook – Warne