LESSONS WE NEED TO LEARN
SIZING TRANSFORMERS WITH LARGE MOTOR LOADS (PART II)
By Doods A. Amora, PEE (Decemer, !""#$
(Las% o& T'o Par%s$ #." )OLTAGE SAGS IN LARGE MOTOR STARTING
The voltage dip at the transforme t ransformerr terminals is proportional to the motor load required in start-up. As discussed earlier, earlier, the voltage drop can be expressed as a percentage of the inrush motor load compared to the maximum capability of the transformer. transformer. * )o+%ae Dro- (Mo%or S%ar%/0 1)A$ 2 3"" 4(Mo%or S%ar%/0 1)A 5 S6or% 7/rc8/% 1)A$
EXAM!E" #o$, supposing a lone %,&&& hp, '.() *+, ode !etter motor /tarting *+A 0 1.) times its rated hp2 is connected to the 1,3&& *+A transformer described above, $ould it not be that the transformer is too large for the motor load4 robably yes, probably not5 !et us see $hy. A$ IF THE MOTOR 7ONTROLLER IS A7ROSS9THE9LINE (F:LL9)OLTAGE 7ONTROLLER$
/tarting *+A of %,&&& hp Motor /*+A 0 1.)2" /tarting *+A of %,&&& hp motor 0 %,&&& *+A x 1.) or (3,%&& *+A Three-hase /hort ircuit apacity of the Transformer" Transformer" 67 / *+A of 1,3&& *+A Trafo 0 1,3&& *+A8&.&9 or :6,13& *+A #ote" ;n this case, the 1.3 M+A transformer has a maximum of :6.13 M+A short circuit capability. The voltage drop on motor inrush $ill be" +< at Trafo Terminals 0 (3,%&&8(3,%&& = :6,13&2 0 &.(6:3 or, (6.:3> +< at Trafo Terminals 0 39& v Trafo Terminal +olts or less. The next higher standard si@e transformer at (&,&&& *+A 9.&> impedance $ould have a short circuit output capability of (%3,&&& *+A $hich is still not sufficient. r a (%.3 M+A transformer could be the choice5 ;n this particular application, the ratio of the selected standard si@e transformer *+A to motor *+A is (%,3&& *+A8%,&&& *+A 0 ).%3. Thus the transformer rating is )%3> larger than the rating of the motor5 Bantastic5
#ote that the voltage sag in reality $ill even be bigger actual voltage smaller2 because the source can never be infinite in fault duty plus in addition, the voltage drop at the distribution cables from the transformer to the motor end. The total voltage sag is the sum of the sag in the secondary unit substation transformer and the secondary circuit. ;n the case of very large motors of several hundred to a fe$ thousand horsepo$er, the impedance of the supply system or the Cavailable fault dutyD2 should be considered.
B$ IF THE MOTOR 7ONTROLLER IS AN A:TOTRANSFORMER RED:7ED )OLTAGE STARTER
#o$ therefore, here comes the $isdom in using reduced voltage starting scheme and the choice of ode !etter specification of a motor. #ormally in practice, large motors need to be equipped $ith AutoTransformer +/ controllers $hile the motor ode !etter specification can be chosen. !et us no$ see the effect $ith a %,&&& hp ode !etter E motor /tarting *+A 0 '.9 times rating2 is controlled by an Auto-transformer +/. Establish further that the Auto-transformer +/ shall be at )3> tap. Brom the table belo$, the line current at )3> tap dra$s ')> of the full voltage starting current.
Table %" MT /TAT;#F METG>* o& L/0e S%ar%/0 78rre0% or 1)A o& 3""* o& F8++ )o+%ae S%ar%/0
#TE" ;n addition to methods listed above, users should consider solid-state soft-start motor controllers and8or adIustable speed drives.
;n this example" Motor /tarting *+A 0 ')> of %,&&& *+A x '.9 or ','() *+A Motor /tarting *+A can therefore be placed at ','() *+A. The voltage drop at transformer terminals on motor inrush $ill be" +< at Trafo Terminals 0 ','()8','() = :6,13&2 0 &.&'3 or, '.3> +< at Trafo Terminals 0 (91.% v Trafo Terminal +olts tap, the voltage drop as seen by the motor $ould still satisfy the (&> +< requirements.
?." M:LTIPLE MOTORS IN A TRANSFORMER
Jhat if there are smaller motors plus other loads among a fe$ large ones in a single transformer
source4 EXAM!E" Assume that in the 1.3 M+A /ubstation transformer described above, there are %,(3& *+A smaller ')& v motors in the system through a number of transformers do$nstream the ',()& v bus of the substation. ther load, load gro$th included is 13& *+A. Assume also that t here are 9&& hp '.() *+ ode !etter E motors directly connected at the '.() *v bus. Go$ many 9&& hp motors can be energi@ed into the system complying $ith the (&> voltage sag requirements4 Assume that +/Ds for the 9&& hp motors are at )3> tap and the motors shall not be started at one time. ;f there are several motors on one transformer" TRAFO @)A (Ma2/m8m Dema0d 1)A o& Sma++ Gro8- Mo%ors$ 5 (Ma2/m8m Dema0d 1)A o& O%6er Loads I0c+8d/0 Load Gro'%6$ 5 (1)A Ra%/0s o& a++ Lare Mo%ors$ 5 (Add/%/o0a+ Tra&o 1)A ca-ac/%y 0ecessary %o accommoda%e %6e /0r8s6 c8rre0% o& %6e +ares% mo%or$
Thus" TAB K+A 0 1,3&& *+A Max tap" 9&& hp /tarting *+A 0 &.') x 9&& x '.92 9&& hp /tarting *+A 0 (1)) *+A /olving for the Additional Trafo *+A #eeded for 9&& hp /tart-?p" Trafo / *+A #eeded for one 9&& hp /tart-?p to maintain a (&> +< 0 (,1)) x(& Trafo / *+A for one 9&& hp /tart-?p to maintain a (&> +< 0 (1,))& *+A Additional Trafo *+A #eeded for one 9&& hp /tart-?p 0 / *+A x >;L of Trafo Additional Trafo *+A for one 9&& hp /tart-?p 0 (1,))& x &.&9 0 (,'(6 *+A /olving for the #umber of 9&& hp Motors that can be placed into the system" Reca++/0 TRAFO @)A (Ma2 Dema0d 1)A o& Sma++ Gro8- Mo%ors$ 5 (Ma2 Dema0d 1)A o& O%6er Loads I0c+8d/0 Load Gro'%6$ 5 (1)A Ra%/0s o& a++ Lare Mo%ors$ 5 (Add/%/o0a+ Tra&o 1)A ca-ac/%y 0ecessary %o accommoda%e %6e /0r8s6 c8rre0% o& %6e +ares% mo%or$
1,3&& *+A 0 %,(3& *+A = 13& *+A = K+A!M = (,'(6 *+A K+A !arge Motors 0 1,3&& %,(3& 13& - (,'(6 K+A !arge Motors 0 6,(91 *+A #umber of 9&& hp motors 0 6,(9189&& #umber of 9&& hp motors 0 6.:9', say ' The transformer selected $ill be capable of running and starting all motors provided that only one large motor is started at any one time. Additional capacity $ill be required for motors starting simultaneously.
;." BA7@ TO THE BOARD PROBLEM
A %3& hp motor driving a pump is served by 6 x (&& *+A (6.9*+- '9&v transformer ban*. ther data are as follo$s" Transformer ;mpedance" 6.1> ;L /hort ircuit apacity of /ource" %&& M+A Motor /tarting urrent" 9 times of B! !ength of able from Transformer to Motor ontroller" 3& ft /olve for" a2 /i@e of able N onduit b2 Oranch ircuit Orea*er N ontroller /i@e if Auto-Transformer type +/ c2 . /olution" a2 /olving for the able /i@e N onduit Motor Bull !oad urrent 0 6&% A Brom #E Table2 /i@e of able 0 (.%3 x 6&% or 611 A ?/E" 3&& MM TGJ in 'P7 onduit b2 /olving for the Oranch ircuit Orea*er N +/ ontroller /i@e Oranch ircuit Orea*er 0 %.& x 6&% A or )&' A ?/E" )&&AT8)&&AB, 6, )&&+ MO ?/E" Auto-Transformer +/ #EMA /i@e 3 c2 /olving for the +oltage ;L8(&& x *+A Oase 8Trafo *+A X T 0 6.18(&& x 6&&86&& X T 0 &.&61 pu X EH 0 &.&&(3& = &.&61 X EH 0 &.&693 pu ; / 0 ;OA/E 8 X EH ; / 0 6)( A 8 &.&693 ; / 0 :,611 A Three-hase /hort ircuit apacity of the Transformer" K+A / 0 R6 x &.'9 x :,611 K+A / 0 1,1:) *+A
Motor /tarting *+A" Motor /tarting *+A 0 %3& hp x 9 Motor /tarting *+A 0 %,&&& *+A The voltage drop on motor inrush at the transformer terminals $ill be" +< at Trafo Terminals 0 %,&&&8%,&&& = 1,1:)2 0 &.%&'% or, %&.'%> +< at Trafo Terminals 0 :9 v Trafo Terminal +olts ;L can barely satisfy the requirement5 There could be t$o remedies for the situation. ne is to change t he motor controller from full voltage to auto-transformer +/ type. The other alternative is to change the Transformer from 6&& *+A 6 x (&& *+A2 to (,&&& *+A 6 x 666 *+A2. d2 /olving for the +oltage " Motor /tarting *+A 0 &.') x %3& hp x 9 Motor /tarting *+A 0 :%& *+A The voltage drop on motor inrush at the transformer terminals $ill be" +< at Trafo Terminals 0 :%&8:%& = 1,1:)2 0 &.(&33 or, (&.33> +< at Trafo Terminals 0 3&.)' v Trafo Terminal +olts
+oltage at Motor ontroller !ine /ide Terminals during /tart-?p 0 '%6.9%) v +oltage equirements during /tart ?p 0 '(' v for ')& v motors Therefore, the 6 x (&& *+A transformer need not be replaced if its Auto-Transformer +/ is tapped at )3>. #ote that if the +/ is tapped at 9&>, the transformer needed must be 13& *+A 6 x %3& *+A25 ." THE REAL ENGINEERING E7ONOMI7S
Jhile first costs are very important to clients N plant o$ners correct engineering practice should not be sidestepped in selecting best systems. More often than not, too much cheap engineering is in fact, costly. Oy ma*ing the first cost not obIectionable, it does not mean that the engineer shall use cheap, undersi@ed and substandard materials or equipment. Applying correct +alue Engineering, the design must fundamentally conform to good engineering practice, codes and standards. This fundamental or minimum requisite must not be compromised under the cloa* of costs. ;t is important for the ;ndustrial o$er /ystems
