DOC PREVIEW
CU-Boulder ECEN 4517 - The Polyphase Induction Motor

This preview shows page 1-2-3 out of 10 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

ECEN 4517 1Experiment No. 4The Polyphase Induction MotorThe polyphase induction motor is the most commonly used industrial motor, findingapplication in many situations where speed regulation is not essential. It is simple andrelatively inexpensive, and the absence of sliding contacts in the squirrel-cage machinereduces maintenance to a minimum. There are two general types of polyphase inductionmotors: the squirrel-cage type and the wound-rotor machine. Both motors have anarmature or stator structure similar to that of the alternating current generator, consistingof a hollow cylinder of laminated sheet steel in which are punched longitudinal slots. Asymmetrical polyphase winding is laid in these slots which, when connected to a suitablevoltage source, produces a travelling MMF wave in the air gap, rotating at a synchronousspeed equal to: RPMsync= 120fp(1)where f is the frequency and p the number of poles for which the stator is wound.The squirrel-cage type of rotor is made up of sheet steel laminations keyed to theshaft and having slots punched in the periphery. The number of slots in the rotor is nevera multiple of the number in the stator, thereby preventing rotor locking under light loadconditions. The rotor conductors in most machines are made of aluminum alloy eithermolded or extruded in place in the slots, with end rings being cast as an integral part ofthe structure and connecting all bars at both ends. The air-gap length between rotor andstator is kept as short as manufacturing tolerances will allow in order to minimize themagnetizing current necessary for the production of normal air-gap flux. A simple two-pole, three-phase, squirrel-cage induction motor is diagrammed in Fig. 1.The wound-rotor induction motor has a rotor similar to that of the squirrel-cagemachine except that the short-circuited squirrel-cage winding is replaced by a three-phaseinsulated winding similar to that on the stator. This winding is usually wye-connectedwith the terminals brought out to three slip rings on the shaft. Graphite brushes connectedto the slip rings provide external access to the rotor winding which is connected to arheostatic controller, the purpose of which is to insert additional resistance in each rotorphase to improve the starting characteristics.In practically all induction motors, either the rotor or the stator slots are skewedone slot width as shown in Fig. 1(a). The purpose is to smooth the flux transition from2one slot to the next, thereby reducing harmonics in the torque characteristic andimproving the operation.(a) (b)Fig. 1. Physical construction of the squirrel-cage induction motor: (a) cross sectionshowing stator and rotor, (b) rotor construction.1. Basic operation of the induction motorAs previously shown, the phase displacement between the voltages applied to the statorwindings produces a travelling MMF or rotating magnetic field in the uniform air gap.This field links the short-circuited rotor windings, and the relative motion induces short-circuit currents in them, which move about the rotor in exact synchronism with therotating magnetic field. It is well known that any induced current will react in oppositionto the flux linkages producing it, resulting herein a torque on the rotor in the direction ofthe rotating field. This torque causes the rotor to revolve so as to reduce the rate ofchange of flux linkages reducing the magnitude of the induced current and the rotorfrequency. If the rotor were to revolve at exactly synchronous speed, there would be nochanging flux linkages about the rotor coils and no torque would be produced. However,the practical motor has friction losses requiring some electromagnetic torque, even at no-load, and the system will stabilize with the rotor revolving at slightly less thansynchronous speed. A mechanical shaft load will cause the rotor to decelerate, but thisincreases the rotor current, automatically increasing the torque produced, and stabilizingthe system at a slightly reduced speed.The difference in speed between rotor and rotating magnetic field is termed “slip”which is numerically equal to:3 Slip = s =synchronous speed – rotor speedsynchronous speed(2)This varies from a fraction of one per cent at no-load to a maximum value of three or fourper cent under full load conditions for most properly designed machines. The speedchange between no-load and full-load is so small that the squirrel-cage motor is oftentermed a constant-speed machine.2. Equivalent circuit modelTheoretical analyses of the induction machine consider it to be a transformer with arotating secondary. The stator windings constitute primary windings that induce flux inthe rotor and stator iron. The rotor windings constitute a secondary winding that isshorted. Hence, an equivalentcircuit similar to thatrepresenting the transformer isderived and appears as in Fig. 2.Since the rotor frequency in theactual machine is dependentupon the rotor speed, all rotorquantities must be modified tobe referred to the frequency andvoltage bases of the stator forinclusion in the equivalent circuit. Since the circuit represents just one phase of the actualpolyphase machine, all values are given on a per-phase basis.Once the equivalent circuit constants have been determined, the operatingcharacteristics may be determined directly from it. The variable load resistance RR (1 –s)/s models the conversion of power from electrical to mechanical form. The powerabsorbed by this resistance is equal to the mechanical output power of the machine Po; fora three-phase machine, this power is equal to: Po=3 IR'21–ssRR(3)Similarly, the torque is proportional to the power divided by the speed. Since the speed isproportional to 1 – s, the torque is given by: T =Po1–s ωs=3 IR'2RRsωs(4)Lm+VSper phase–ISIR'RmRsLsRRLR1– ssRR Vsper phase =VLL3Fig. 2. Equivalent circuit model of the induction machine,per phase.4Here, ωs is the synchronous speed, in radians per second. The torque is expressed inNewton-meters. Note that the synchronous speed in rpm is related to the applied statorfrequency f according to Eq. (1). The torque expressed in the English units of foot-poundsis T =3KIR'2RRsfoot-pounds(5)where K = 0.058 p/f.The losses may be evaluated by realizing that Rs and RR represent stator and rotorresistances per phase respectively, and that Rm models the core loss. For the usualconstant speed application, the mechanical windage (i.e., the resistance of air to rotationof the shaft) and


View Full Document

CU-Boulder ECEN 4517 - The Polyphase Induction Motor

Documents in this Course
Lecture 2

Lecture 2

24 pages

Lecture 4

Lecture 4

16 pages

Lecture 2

Lecture 2

48 pages

Lecture 1

Lecture 1

23 pages

Lecture 6

Lecture 6

26 pages

Battery

Battery

27 pages

Lecture 3

Lecture 3

20 pages

Lecture 4

Lecture 4

23 pages

Load more
Download The Polyphase Induction Motor
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view The Polyphase Induction Motor and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view The Polyphase Induction Motor 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?