The Knowledge Bank at The Ohio State University

Ohio State Engineer

Title: Testing Variable Speed Induction Motor

Creators: Spohn, G. A.

Issue Date: Nov-1927

Publisher: Ohio State University, College of Engineering

Citation: Ohio State Engineer, vol. 11, no. 2 (November, 1927), 5-6, 8, 28.

URI: http://hdl.handle.net/1811/34270

Appears in Collections: Ohio State Engineer: Volume 11, no. 2 (November, 1927)

THE OHIO STATE ENGINEER

Testing Variable Speed Induction MotorBy G. A. SPOHN, '25

UNNING tests in the electrical laboratoryat school was very interesting, but it isa far call from these to tests conductedduring the past summer on electric drive

equipment for the rolling mill of a large steelmanufacturer, in the Schenectady factory of theGeneral Electric Company.

There were five men conducting the test, rep-resenting the following school: Oklahoma A. &M.; Carnegie Tech., Univ. of California, MichiganState and Ohio State.

Electric power in steel mills has been found toperform the manufacturing operations in a bet-ter, more efficient, and more reliable manner thanotherwise possible, and many of these operationsand processes would be practically impossiblewith any other form of power. Hardly any newmills are being equipped with anything but elec-tric motors and older steam driven mills are beinggradually electri-fied for purelyeconomic reasons.

The equipmentwe tested w a sb u i l t for theSparrows Point( M a r y l a n d )plant of the Beth-lehem Steel Com-pany and willdrive a continu-ous rolling mill.

Following is adescription of themotors, genera-tors and controlapparatus:

An adjustablespeed inductionmotor with itsScherbius equip-ment consists, es-sentially of twounits, the maininduction motordirect connected ohmic drop exciter; and the twoScherbius machines with direct connected syn-chronous exciter, driven by a 6100 k. va. syn-chronous motor which also drives a 2300 kw. di-rect current generator for a separate direct cur-rent supply.

THE SYNCHRONOUS EXCITERThe synchronous exciter has three field wind-

ings, the first is in series with a rheostat and isexcited by means of 250 volts direct current, thesecond is across the armature of a small directcurrent generator driven by an induction motorwhich operates at slip frequency, and the thirdfield winding is in series with the field windingof the above generator. The armature of thesynchronous exciter is connected to the slip ringsof the ohmic drop exciter. Thus the synchronousexciter supplies a variable voltage, constant fre-quency excitation to the ohmic drop exciter.

NOVEMBER, 1927

Testing the Electrical equipment for the continuous rolling mill of theSparrows Point plant of the Bethlehem Steel Co.

is shown at the extreme left

THE OHMIC DROP EXCITERThe ohmic drop exciter is in reality a rotary

converter supplying a three phase voltage at slipfrequency. The value of voltage obtained at thecommutator of this machine is not dependentupon the slip of the induction motor as long asthe voltage applied to its collector rings remainsconstant; however, this voltage will have a fre-quency the same as the slip frequency of themain induction motor because it has the samenumber of poles and is driven at the speed of theinduction motor which is the synchronous speedplus or minus the slip.

The commutator voltage of the ohmic drop ex-citer is used to supply a part of the excitation tothe field windings of the Echerdius machines.The rest of the excitation current for the Scher-bius machines is supplied by means of an auto-transformer which is connected in Y across theslip-ring circuit of the induction motor.

The Scherbiusm a c h i n e is athree-phase com-mutating motoror generator onwhose s t a t o rthere are threeshunt-field wind-ings spaced 120electrical degreesa p a r t . Thebrushes in thecommutator o fthe Scherbius ma-chine are con-nected to therotor slip-ringsof the main in-duction m o t o r .The two Scher-b i u s machinesoperate of seriesto a b s o r b ordeliver power tothe rotor of themain induction

motor depending upon whether it is operating be-low or above its synchronous speed. There areother windings on the stator of this machine toimprove the commutation.

SPEED REGULATION OF THE INDUCTIONMOTOR

Since the Scherbius machines are capable ofabsorbing or delivering power at slip frequencyto the ring circuit of the induction motor it ispossible to vary the speed of that motor becausethe speed is dependent upon the voltage appliedto the rotor circuit.

In all Scherbius equipment previously built theexcitation voltage of the ohmic drop exciter wasconstant, and therefore the only way to vary thefield strength of the Scherbius machine was bymeans of th© taps on the auto-transformer excited

{Continued on Page 6)

G. A. Spohn

6 THE OHIO STATE ENGINEER

from the clip-ring circuit. Thus the main in-duction motor would have a series of definitespeeds corresponding to the tap setting of theauto-transformer. From the preceding discus-sion of this set it may be seen that by adjustingthe excitation voltage of the ohmic drop exciterit is possible to vary the speed between each ofthe rough points of speed caused by varying thevoltage by tapping off from the auto-transformer.

Thus it is possible to get any speed desiredbetween 250 and 500 r. p. m., and where fine con-trol of the speed is necessary—as in some rollingmills—this feature is very important.

TESTING THE APPARATUSThe set was assembled on the test floor of

Bldg. 18 and the first job of the testmen was towire it up just as it would be wired if it werebeing actually installed for use. This was quitea task for it is no easy matter to drag thousandampere cables around and to make connectionsin places where it is almost impossible to squeezethe human body.

But finally all the connections were made andeverything was ready to put the set into opera-tion. The induction motor was first operated anda great many readings taken at various speedsto be sure that it was working properly. It wasfound that some minor adjustments had to bemade and so all of these readings had to be takenover again. Although I never heard anyone com-plain, I can imagine that many of the men werereading meters in their sleep. At last it wasworking properly, so it was mechanically con-nected to a three-phase alternator, the powerfrom this alternator being pumped into a three-phase water rheostat. Thus by varying the re-sistance of the water rheostat or by varying thearmature voltage of the machine it was possibleto get any load on the induction motor that wasdesired. Again we took innumerable readingsof all the currents and voltages until finally theset was running perfectly, from the top to the

Power LinesStator or Main Motor

Rotor or Main Motor

Starting Control orContractors

Speed AdjustingTransformer

Armatures or RegulatingMachines

Main Fields of RegulatingMachines

Elementary diagram of electrical connections of a Scherbiusadjustable speed drive with two regulating machines

connected in series

6600 Volt - 3 Phas©- 25 Cycl©

250

jjf} 2600 H.P.275/320

R.P.M.

Arrangement of electric drives for a larger continuous mill

lowest speed: and from maximum load to thepoint where there was no load on the inductionmotor at all.

Then came the well known heat run. This wastaken at the lowest speed of 250 r. p. m., for it isat this speed that the most severe operating con-ditions are found. Thermometers were placedon all of the machines and readings of the tem-perature were taken every half hour until allwere constant.

At last all of the operating tests were com-pleted and the set was ready for its real work inthe steel mill. The wiring that we had taken somuch pains with was soon torn out and the ma-chines were given the hi-potential test and theset passed on to the shipping department.

The Sparrows Point mill will have a numberof stands arranged in tandem. The rolling re-quirements being different, the first several standswill be driven by constant speed motors. Thepower supply is 6600-volt, 25-cycle.

The roughing train will take a 4000-h. p., 83.3rev. per min. motor, A, the intermediate train willbe driven by a 6500-h. p., 187.5-rev. per min.motor, B. The following group of stands willbe jointly driven through a train of gears by anadjustable speed equipment, C, developing 6700h. p. at 500 rev. per min. and 3350 h, p. at 250rev. per min.

The last finishing stand will take a separatedirect-connected drive, D, with an output of 2600h. p. at a speed of 275 rev. per min.; constanthorse power output will be maintained for speedsabove 275 rev. per min., and reduced output onconstant torque basis, for speeds below this value.

These drives will never be required to starttheir respective mills with metal in the rolls.Mill friction on a cold winter day, after a pro-longed shut-down, would be the most severe start-ing condition. Several tests have shown that atorque of about 25 or 30 per cent normal willstart a continuous mill under most adverse con-ditions.

Under circumstances it was decided to buildthe drives A and B as synchronous motors andto take advantage of their leading kilovolt-am-peres for power factor correction of the steelplant.

(Continued on Page 8)NOVEMBER, 1927

THE OHIO STATE ENGINEER

TESTING INDUCTION MOTOR(Continued from Page 6) .

The large adjustable speed drive, C, will con-sist of a 5000-h. p.,, 375-rev. per min. slip-ringinduction motor, the speed of which will be ad-justed up to 33 per cent above, and up to 33 percent below, synchronism (i. e., from 500 rev. permin. to 250 rev. per min.) by means of the twoScherbius regulating machines Rl and R2. Withthis constant torque layout, the capacity of thedrive will be 6700 h. p. at 500 rev. per min. and3300 h. p. at 250 rev. per min. An A. C. drive ofsuch capacity and speed can be built more eco-nomically and with a much higher efficiency thanany combination of D. C. machines. The fact thatthe power supply was 25-cycle gave the Scherbiussystem an advantage over the Kraemer drive.

The last finishing mill drive, D, will have awider speed range, is of smaller capacity andruns at a lower speed than the drive C. Whilea Scherbius equipment for the drive, D, wouldbe fully competitive in first cost, the differencebetween it and that of a D. C. drive was not aswide as in the case of the drive C. For the sakeof greater flexibility of control it was decided tomake the drive D of the D. C. type.

A 500-rev. per min. synchronous motor, S, willdrive a 2300-kw., D. C. generator G (furnishingpower to the motor D) and the two 650-kv-a.Scherbius speed regulating machines Rl and R2used for adjusting the speed of the inductionmotor C. When the motor C runs below its syn-chronous speed, the slip energy flows to the ma-chines Rl and R2; the latter run as motors andassist the synchronous motor S in driving thegenerator G. In other words, the slip energydoes not have to be returned as electric power tothe incoming line; instead of this, it may be usedfor driving wholly or in part, the finishing millD. The flow of power is indicated by arrows.

When the drive C is running above synchron-ism, the slip energy becomes negative and arrowsshown by the dotted line, will be reversed. Themachines Rl and R2 act then as generators, andderive their power from the synchronous motor S.

A direct-connected exciter provides the neces-sary 250-volt excitation to the synchronous mo-tors A, B and S, and to the D. C. machines Gand D.

The use of two regulating machines Rl and R2for controlling the speed of the Motor C presentssome interesting features. The maximum amountof the slip energy to be handled by the speedregulating equipment is 1700-h. p.; it is not prac-ticable to build A. C. commutator machine of suchcapacity and to run at 500 rev. per min.; a lowerspeed like 375 rev. per min. or 300 rev. per min.would be required. With the proposed layoutsuch reduced speed would considerably increasethe cost of the D. C. generator G and of the motorS. It would be still more expensive to providea separate low speed drive for the regulatingmachines Rl and R2, and to drive the generatorG by another 500-rev. per min. motor. It wasquite advantageous, therefore, to split the ca-pacity of the regulating equipment in two unitsand to run them at 500 rev. per min.

(Continued on Page 28)NOVEMBER, 1927

23 THE O H I O . S T A T E E N G I N E E R

TESTING INDUCTION MOTOR(Continued from Page 8)

The connections of the regulating machines tothe secondary winding of the induction motor areshown. The 5000-h. p. motor is equipped withsix slip-rings, with both ends of each phase ofthe rotor brought out. Each set of three slip-rings is connected electrically to the commutatorof the regulating machines Rl and R2, which thusforms the two Y-points of the secondary circuit.In other words, the two machines Rl and R2 actas if they were connected in series with eachother, their e. m. fs. added together. The shuntfields Fl and F2 are adjusted simultaneously bya common speed control apparatus.

By disconnecting one regulating machine andby short-circuiting the corresponding set of slip-rings, it is still possible to operate the drive withthe other regulating machine; full torque of thedrive will be obtainable, but the speed range willbe cut in half; i. e., it will be in this case ap-proximately 312/437 rev. per min.