SCHRAGE MOTORS

Designed by Mr. H.K. Schrage - Swedish Engineer in 1911.

It is a ROTOR fed, Shunt type, Brush shifting, 3 Ø Commutator Induction Motor which has built in arrangement both for speed control and p.f. control. Same is an “Inverted Induction Motor” with Slip regulator.

CONSTRUCTIONAL DETAILSMotor has three windings: Two in Rotor and One in Stator.

(i) Primary wdg: Placed on the lower part of the slots of

the Rotor. 3 Ø supply at line frequency is fed through slip rings and brushes.

Generates working flux in the machine.DEE_JRN 1

(ii) Regulating Wdg: Placed on the upper part of the slots of the Rotor.

Connected to commutator segments in a manner similar

to that of d.c. machine.

Also known as tertiary wdg / auxiliary wdg / commutator wdg.

(iii) Secondary winding: Same is Phase wound & located on Stator. Each Wdg connected to a pair of brushes arranged on the commutator. Brushes mounted on brush rockers. Designed to move in opposite directions, relative to the center line of its stator phase.

Brushes A1 , B1 & C1 move together and are 1200 E apart.

Brushes A2 , B2 & C2 also move together and are 1200 E apart. DEE_JRN 2

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WORKING OF SCHRAGE MOTOR

• Primary energized with line frequency voltage.

• Transformer action occurs between primary and regulating wdgs.

• Induction motor action occurs between primary and secondary wdgs.

• Commutator acting as a frequency converter converts line frequency

voltage of reg. wdg to slip frequency voltage and feeds the same to

secondary wdg on the stator.

• Voltage across the brush pairs A1 - A2, B1 - B2 & C1 - C2 increases

as brushes are separated.

• Magnitude of voltage injected into the secondary wdg depends on the angle

of separation ‘θ’ of the brushes A1 & A2, B1 & B2 , C1 & C2.

(‘θ’ – Brush separation angle)DEE_JRN 4

• When primary is energized synchronously rotating field in clockwise

direction is set up in the rotor core.

• Assume that the brushes are shortcircuited through commutator seg.

i.e. the secondary is short circuited.

• Rotor still at rest, the rotating field cuts the stationary secondary wdg,

induces an e.m.f. The stator current produce its own field.

• Stator field reacts with the rotor field thus a clockwise torque

produced in the stator.

• Since the stator cannot rotate, as a reaction, it makes the rotor rotate

in the counter clockwise direction.

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Suppose that the rotor speed is Nr rpm.

Rotor flux is rotating with NS relative to primary & regulating wdg.

Rotor flux will rotate at slip speed (NS - Nr ) relative

to secondary wdg in stator / with reference to space.

SPEED CONTROL

Speeds above and below Synchronous speed can be obtained by

changing the Brush position. i.e. changing “ θ “ .

Fig (a) – Brush pair on the same commutator segment.

i.e. the secondary wdg short circuited. Injected voltage Ej = 0

and the m/c operates as an INVERTED Induction Motor. Nr < NS .

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Fig. (b) – Brushes parted in one direction - produces sub-synchronous speed.

Injected voltage Ej , is obtained from the section of the regulating wdg between them.

If the center line of this group of conductors is coincident with the centre line of the

corresponding secondary phase, then E2 and Ej are in phase opposition.

Neglecting impedance drop, sE2 must be equal and opposite to Ej.

“β” – angle between E2 and Ej. β=1800 & Nr < NS .

Fig. (c) – Brushes parted in opposite direction - produces super-synchro. speed.

Ej is reversed relative to E2. i.e. β=00 & sE2 must also be reversed.

Same occurring only because ‘s’ becoming negative .

i.e. The speed is thus above synchronous speed. Nr > NS .

The commutator provides maximum voltage when the brushes are

separated by One pole pitch. i.e. ‘θ’ = 1800 E.DEE_JRN 8

POWER FACTOR IMPROVEMENT Same obtained changing the phase angle of the injected voltage

into the secondary winding.

In this case one set of brushes is advanced more rapidly

than the other set.

Now the two center lines do not coincide,

have an angle ‘ρ’ between them. (“ρ” – Brush shift angle)

Fig (d) : Brush set is moved against the direction of rotation of rotor.

In this case Speed decreases, the p.f. is also improved.

Fig (e) : Brush set is moved in the same direction of rotation of rotor.

In this case Speed increases, the p.f. is also improved.

Both p.f. and speed can be controlled by varying ‘θ’ & ‘ρ’.

‘Ej Cos ρ’ and ‘Ej Sin ρ’ effect the speed and p.f. respectively.

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Speed Torque Characteristics

Same reveals that the Schrage Motor is almosta constant speed motor. (SHUNT CHARACTERISTICS)

Applications Can be applied to any individual drive requiringVariable speed. Especially in(i) Hoisery knitting & Ring spinning m/cs.(ii) Cranes & Hoists(iii) Fans & Centrifugal Pumps(iv) Printing Machinery(v) Conveyors, Packing machinery & Paper Mills

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ADVANTAGES

(i)Good Speed Regulation.

(ii) High p.f. for high speed setting.

(iii) High efficiency at all speeds except NS.

DRAWBACKS

(i)Operating voltage has to be limited to 700V because

the power is to be supplied through slip rings.

(ii)Low p.f. at low speed settings.

(iii) Poor commutation.

(iv) High Cost.

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Variation of no load speed with Brush Separation

K= Turns Ratio= No: of Secondary turns / No: of tertiary turns