Equivalent Circuit of Induction Motor Referred to Stator Side

Transcription

1 Electrical Machine IV Code: CECE Equivalent Circuit of Induction Motor Referred to Stator Side Fig. 3: eqt. circuit of induction motor per phae at any lip referred to tator ide (Fig. 3 i the ame a Fig. ) Note that: the element RL i the equivalent electrical reitance related to the mechanical load on the motor. he following point may be noted from the equivalent circuit of the induction motor: At no-load (where N N), the lip i practically zero and the load R L i infinite. hi condition look like a tranformer whoe econdary winding i open-circuited. At tandtill (N=0), the lip i unity and the load RL i zero. hi condition look like a tranformer whoe econdary winding i hort-circuited. When the motor i running under load, the value of R L will depend upon the value of the lip. hi condition reemble that in a tranformer whoe econdary i upplying variable and purely reitive load. R L depend on lip. If the lip increae, the load R L decreae and the rotor current increae and motor will develop more mechanical power. hi i expected becaue the lip of the motor increae with the increae of load. 79

2 Electrical Machine IV Code: CECE Power Relation from Equivalent Circuit of Induction Motor (i) Stator input power : P in 3V 1I1 co 1 (ii) Stator Cu loe : P cu ( tator ) 3I1 R1 (iii) Rotor input power 3( I ) R : P rotor (iv) Rotor Cu loe : P cu ( rotor ) 3( I ) R or Pcu ( rotor ) Protor (v) otal mechanical power developed by rotor 1 : P mech 3( I ) R ( 1) 3 ( I ) R 1 Or Pmech Protor Pcu( rotor) 3( I ) R 3( I ) R ( 1) (vi) otal or gro torque developed by rotor : gro P rotor 3( I ) N 60 R 3( I ) 9.55 N R 9.55 N i the ynchronou peed in rpm Note: the haft torque h le than gro by friction and windage loe. P rotor N 80

3 Electrical Machine IV Code: CECE Approximate Equivalent Circuit of Induction Motor A in cae of a tranformer, the approximate equivalent circuit of an induction motor i obtained by hifting the hunt branch (RcjXm) to the input terminal a hown in Figure (Fig.: approximate eqt. circuit of IM per phae referred to tator ide). hi tep ha been taken on the aumption that voltage drop in R1 and X1 i mall and the terminal voltage V1 doe not appreciably differ from the induced voltage E1. From the above approximate circuit, we note that: (i) Unlike a power tranformer, the magnetic circuit of induction motor ha an air-gap. herefore, the exciting current of induction motor (30 to 40% of full-load current) i much higher than that of the tranformer. Conequently, the exact equivalent circuit mut be ued for accurate reult. (ii) he value of X 1 and X in an induction motor are larger than the correponding one to be found in the tranformer. hi fact doe not jutify the ue of approximate equivalent circuit. (iii) In a tranformer, the winding are concentrated wherea in an induction motor, the winding are ditributed. hi affect the tranformation ratio. (iv) Depite the above drawback of approximate equivalent circuit, thi approximate circuit i atifactory for large motor but not preferred for mall motor. 81

4 Electrical Machine IV Code: CECE Example on Equivalent Circuit of Induction Motor [An: I A, pf co 1 co lag ] [Note: -circuit mean approximate circuit, [An: (a) (e) 1 Y G jb & Z ] Y A (b) A (c) lag (d) Pgro 3I RL 3067Watt Pgro 4.9Nm (f) P in 3473Watt (g) % P % ] in 8

5 Electrical Machine IV Code: CECE

6 Electrical Machine IV Code: CECE Method of Starting 3-Phae Induction Motor he method to be employed in tarting a given induction motor depend upon the ize of the motor and the type of the motor. he common method ued to tart induction motor are: 1- Direct-on-line tarting, - Stator reitance tarting, 3- Autotranformer tarting, 4- Star-delta tarting, 5- Rotor reitance tarting. Method (1) to (4) are applicable to all induction motor type. Method (5) i applicable only to lip ring (wound rotor) motor. In practice, any one of the firt four method i ued for tarting quirrel cage motor, depending upon, the ize of the motor. But lip ring motor are invariably tarted by rotor reitance tarting. 84

7 Electrical Machine IV Code: CECE Method of Starting 3-Phae Squirrel-cage Motor Except direct-on-line tarting, all other method of tarting quirrel-cage motor employ reduced voltage acro motor terminal at tarting. 1- Direct-on-line tarting, - Stator reitance tarting, 3- Autotranformer tarting, 4- Star-delta tarting, (in all above method, we control the applied voltage acro the tator) 85

8 Electrical Machine IV Code: CECE Starting Method Direct-on-line (DOL) he motor i tarted by connecting it directly to 3-phae upply at a rated voltage. he impedance of the motor at tandtill i relatively low and when it i directly connected to the upply, the tarting current will be high (4 to 10 time the full-load current). Conequently, thi method of tarting i uitable for relatively mall machine (up to 7.5kW). he high tarting current in DOL method can be controlled by reducing the applied voltage but thi way i not allowable here. Advantage: implet, cheapet and mot common tarting method. lowet temperature rie at tarting compared to other tarting method. Motor that tart and top frequently often have control ytem, which conit of a contactor and overload protection uch a a thermal relay. Diadvantage: Full voltage i witched directly onto the motor terminal. For mall motor, t i (1.5:3 time) of. Alo, It i (3:8 time) of I or even higher. 86

9 Electrical Machine IV Code: CECE Relation between t and for Direct-on-line (DOL) Method Rotor input power : Protor. gro gro cont. 60 gro 1 Rotor Cu loe : P N 3( I ) cu( rotor) 3 ( I) R Protor rotor R P From 1 & : I R 3( ) ( I. gro gro cont ) Alo, gro. I ) I1 ( I1 At tarting : At full-load: From 3 & 4: t I I t I ( 1 at tarting ) t I t I 3 4 t t c I I.at tarting: the tarting current t the hort-circuit (blocked-rotor) current I c. I i 87

10 Electrical Machine IV Code: CECE Relation between t and for Direct-on-line (DOL) Method t I I Let u illutrate the above relation with a numerical example. Suppoe Ic = 5I and the full-load lip =0.04. hen, t I 5I c I I c Note that the tarting current i a large a five time the full-load current but the tarting torque i jut equal to the full-load torque. herefore, the tarting current i very high and the tarting torque i comparatively low. If thi large tarting current ow for a long time, it may overheat the motor and damage the inulation. t 88

11 Electrical Machine IV Code: CECE Stator Reitance Starting Method In thi method, external reitance (primary reitor or rheotat or reactor) are connected in erie with each phae of tator winding during tarting. hi caue voltage drop acro the reitance o that voltage available acro motor terminal i reduced and hence the tarting current. he tarting reitance are gradually cut out in tep (two or more tep) from the tator circuit a the motor pick up peed. When the motor attain rated peed, the reitance are completely cut out and full line voltage i applied to the rotor. here are two drawback. Firt, the reduced voltage applied to the motor during tarting reduce the tarting torque. (becaue the current varie directly with the applied voltage, but the torque varie a quare of the applied voltage). For example, if the applied voltage i reduced by 50%, the tarting current i reduced by 50%, but the torque i reduced by 5% of the full-voltage value. Secondly, a lot of power i wated in the tarting reitance. 89

12 Electrical Machine IV Code: CECE Stator Reitance Starting Method Operation of the ued rheotat i hown in the Figure. he rheotat conit of (R1 + R). Firtly: cloe the witche 1,,3 (i.e., cloe a 3-ph circuit breaker) to make the tator reitance ha a high value of (Rtator +R1 + R). Secondly: cloe the witche 4,5,6 to make the tator reitance ha a le value of (Rtator + R). hirdly: cloe the witche 7,8,9 (by keeping the witche 4,5,6 are cloed) to make the tator reitance ha a lower value of (Rtator). hi mean that the tator current gradually increae (alo the applied voltage i gradually increae) by a fraction x, where (x>1). 90

13 Electrical Machine IV Code: CECE Relation between t and for Stator Reitance Starting Method Let (V) be the rated voltage/phae at the beginning of inerting the rheotat (i.e., intant of motor tarting). After get out the rheotat from the circuit, then the voltage applied to the motor/phae will be (xv). t I t I c It x I x c hen, I I hu, at direct witching of the motor (the rheotat i inerted completely to the tator circuit) the current reduce by a fraction x of the rated-voltage tarting current (Ic), the tarting torque i reduced by a fraction x of that value obtained by the direct witching. So, the reduced voltage applied to the motor during the direct witching period reduce the current but at the ame time increae the accelerating time becaue of the reduced value of the torque. herefore, thi method i ued for mooth tarting of mall motor only. 91

14 Electrical Machine IV Code: CECE