No-load And Blocked Rotor Tet On An Induction Machine Aim To etimate magnetization and leakage impedance parameter of induction machine uing no-load and blocked rotor tet Theory An induction machine in it equivalent circuit form i akin to a tranformer with lip a an additional parameter. A a reult, the equivalent circuit parameter etimation of induction machine proceed on imilar line a that of a tranformer. The equivalent circuit for a ingle-cage induction machine i hown in Fig.(1). Here, (R r + R r( 1 )) + jx r1 repreent the rotor circuit impedance a a function of lip. I R X 1 R r 1 Ir V I m Io I c R o V R (1 ) r Figure 1: Per phae equivalent circuit of Induction Motor No-load tet The no-load tet approximate the tator circuit (R and X 1 ) and magnetization branch parameter( ) of an induction machine. The machine i brought to it rated peed by applying rated three phae voltage at the tator (V nl ). Correponding no-load current (I nl ) and no-load real power input(p nl ) are recorded. When no mechanical load i driven by the machine, lip () i a very mall value. A a reult, referring to Fig.(1), the rotor circuit branch reitance quantity, R r( 1 ) carrie a large value. The impedance of rotor circuit branch i thu much higher compared to the magnetization branch impedance and their parallel combination would turn out to be cloe to j (neglecting core conductance). 1
keeping thi in mind, we have, X nl = X 1 + (1) Z nl = V nl I nl () R nl = P nl I nl where, Z nl and R nl are the no-load equivalent impedance and reitance repectively. Next, uing (1-3), we get, X nl = Znl R nl (4) Alo, a no load current i drawn, the loe that occur under thee condition would repreent the only the rotational loe ariing from friction, core lo. Hence, Blocked rotor tet P lo = P nl I nlr (5) The blocked rotor tet i performed to etimate parameter that affect machine performance under load uch a it leakage impedance, imilar to the hort circuit tet done for a tranformer. In blocked rotor tet, the machine haft i locked or i prevented from rotating via external mean. Blocking the haft eentially amount to making the lip equal unity (n r = 0). If E i the voltage appearing acro the rotor circuit, we have (3) = n n r (6) n I r = E R r + jx r1 (7) It can be oberved that, the rotor reitance offered in thi cae i effectively lowered by a fraction of 1 0, where 0 i the lip under rated operation (0.01-0.04). Rated tator current can thu be etablihed for much lower than rated value of tator terminal voltage. With the rated current (I br ) flowing in the tator, we note the tator applied voltage (V br ) and the power input(p br ). It hould be noted that the rotor poition in blocked tate affect the tator voltage (V br ) required for etting up I br. Hence, an average calculated over different rotor poition can be taken. Auming we have the tator circuit parameter R ready, the other machine parameter can be calculated a indicated below. Z br = V br I br (8) R br = P br (9) I br X br = Zbr R br (10) Here, Z br = R br + jx br i the equivalent impedance offered by the machine with rotor blocked. A the real power conumed during blocked rotor tet i almot entirely the real power lo in the machine, we
can ue it to calculate the machine equivalent reitance a in (4). In other word, the applied tator voltage being low, the core lo component i quite a mall fraction of rated core-lo. To approximate X 1, X r1 and R r one need to calculate the Thevenin equivalent impedance looking into the equivalent circuit of the machine from tator terminal. Z br = R br + jx br (11) = R + jx 1 + [(j ) (R r + j1 )] (1) = R + jx 1 + j[r r + jr rx r1 jr r(x r1 + ) + 1 (1 + )] (13) R r + (1 + ) Letting = 1 + (14) = R + jx 1 + j[r r jr r + 1 ] R r + X r Equating the real and imaginary part in (10) (15) X m R br = R + R r R r + Xr X br = X 1 + [R r + 1 ] R r + Xr (16) (17) = X 1 + [ R r + ] (18) R r + However, R r, the term R r in (13) and R r in the denominator of (11) can be neglected. Alo, R br = R + R r( ) (19) R r (R br R )( ) (0) X br = X 1 + 1 (1) = X 1 + 1 1 + 1 () (3) Again, in general, a 1, 1 can be neglected which yield, X br X 1 + 1 (4) For wound-rotor contruction, one can aume that X 1 1 reulting in X 1 = 1 = 0.5 X br. However, for quirrel cage induction machine, the ditribution of X br can be obtained from a look-up table indicating empirical ditribution of leakage reactance for the machine type. If X 1 i known, from the no-load tet data, we have, = X noload X 1 (5) 3
In thi way, the machine parameter namely, X 1,, R r, X r1 can be calculated from the blocked rotor tet uing the above analyi. Finally we note that, the blocked rotor tet can be performed at different value of applied tator voltage frequencie. To obtain the leakage impedance parameter under rated condition (where the rotor frequency i given by F ) with being unity, the tator voltage frequency can be reduced to match the frequency of rotor current during uual operation. Procedure Figure : Circuit Diagram for no-load and blocked-rotor tet on 3 Phae Induction Motor A. Note down the machine rating and calculate rated current of the machine No-load tet : B. Diconnect any mechanical load connected to the induction motor. C. Gradually apply three phae voltage acro motor tator terminal via the autotranformer D. A the tator voltage i increaed, the machine peed up. Make ure the voltage i applied uch that the machine doe not peed up too fat E. Adjut the tator voltage to it rated value F. Note down no-load quantitie uch a V nl, I nl, P nl G. Slowly decreae the tator voltage to zero and diconnect the upply. Blocked rotor tet : B. Faten the machine rotor haft to the fixed dic with the help of crew provided. Make ure the rotor haft i tightly held in poition. C. Slowly increae the three phae tator voltage from zero with the help of autotranformer D. Stop when rated current i etablihed in the machine tator 4
E. Record relevant quantitie from the power analyzer F. Slowly decreae the autotranformer voltage back to zero. G. In cae the blocked rotor tet i to be performed at input voltage frequencie other than the power frequency(50 Hz), ue a Variable Voltage Variable Frequency (V.V.V.F) ource in place of autotranformer. Record No-load tet : Input tator voltage at rated tator current (V nl ), tator current (I nl ), input real power(p nl ) Ue (-4) to calculate Z nl, R nl and X nl Ue (5) to calculate rotational loe Blocked rotor tet: Input tator voltage at rated tator current (V br ), tator current (I br ), input real power(p br ) Ue (4) and (5) to calculate R br and X br Further ue (15),(19) and (0) to calculate machine parameter Ue reult of no-load tet wherever neceary Follow-up Quetion 1. Compare and contrat the no-load and hort circuit tet performed on tranformer to the no-load and blocked rotor tet performed on induction machine. Refer to a table giving ditribution of leakage reactance X br between tator and rotor for different clae of machine 3. How can the blocked-rotor tet be performed to obtain induction motor characteritic near ynchronou peed? (Hint: Think about varying the upply frequency) 4. Calculate the equivalent circuit parameter with the following machine data Machine rating : 8 kw, 400 V, R dc = 0.6 Ω per phae No-load tet: 400 V, 7 A, 50 W Blocked-rotor tet: 90 V, 35 A, 1350 W Reference (1) M.G.Say, Alternating Current Machine, 5th Ed., Ch.7-9 () P.S.Bimbhra, Electrical Machinery, Ch.5 (3) A.E. Fitzgerald, C. Kingley Jr., S.Uman, Electric Machinery, 6th Ed., Ch.6 Write-up prepared by - Aditya D Nadkarni (Reearch Scholar, 014 batch) 5