THREE PHASE A.C. COMMUTATOR MOTORS

Developed to provide

(i)Better Speed Control

(ii)Good p.f. Control

(iii) Both Speed and p.f. Control

2 Classes

Those having Series Characteristics

Those having Shunt Characteristics

Most of the machines have Shunt Speed Characteristics.

Speed Control is obtained by injecting a Variable voltage at correct

Frequency to the Secondary winding of the motor via its Commutator.

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THREE PHASE A.C. SERIES COMMUTATOR MOTORS

Motor having Series Characteristics. i.e. Speed falls with increase in load.

Constructional DetailsStator: Similar to that of 3 Phase Induction Motor.Rotor: Similar to that of a D.C. armature with commutator segments 3 Brush sets per pair of poles spaced 120 0 E apart.

The Stator and Rotor are connected in Series .Flux will be dependent on the current, resulting in “Series” Speed Characteristics. (Variable Speed)

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Speed Control usually done by SIMULTANEOUS movement of all the

Brushes round the Commutator using Brush Gear.

• To avoid commutational difficulties, a transformer is usually

employed between Stator & Rotor – thus reducing the voltage

applied to the Commutator Wdg.

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STARTING: No special Starting equipment is required. Effected with the Brushes in the minimum speed position. i.e. from the short circuit position. normally between 100 to 400 . The Stator is energized and the rotor rotates.

(180 )

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CHARACTERISTICS FEATURES

Starting Torque and Starting Current – 50% of the FL Values.

Reducing , enables to obtain TSt upto 200% FL &

ISt upto150% FL

Efficiency HIGH for the whole operating range.

p.f. HIGH approaching unity for Speeds near NS .

Speed range available to about 3:1.

As the brush gear is moved against the direction of rotation of rotor, the torque increases the motor can be bought up to the desired speed. For any given position of the BRUSHES, the Speed falls with increase in Load, as in the case of SERIES MOTORS.

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SPEED CONTROL

(i)By Brush Shifting – most commonly used.

(ii)By varying the applied Voltage.

(iii) Using a Phase Shifting Transformer.

APPLICATIONS

Used for driving loads where torque increases with speed.

eg. Compressors, Centrifugal Pumps

Used when large Starting Torque is required.

eg. Transporters and Turntables.

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THREE PHASE A.C. SHUNT COMMUTATOR MOTORS

Also known as Doubly Fed Motor.

Constructional Details

Stator: Similar to that of 3 Phase Induction Motor.

Rotor: Similar to that of a D.C. armature with commutator segments

3 Brush sets per pair of poles spaced 120 0 E apart.

Both Stator & Rotor are connected to the same supply source.

Stator is directly fed; but the rotor through a variable ratio transformer.

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Purpose of variable Voltage Transformer

(i) To provide low operating voltage to the commutator Wdg.

(ii) For regulating the injected voltage in the rotor circuit for

SPPED and p.f. Control.

BRUSHES are fixed in position.

SPEED CONTROL

Speed Control is obtained by injecting a Variable voltage

at correct Frequency to the Rotor winding of the motor

via its Commutator.

1.When = 0, the brushes in effect are Short circuited

- the m/c now operates as an Induction Motor.

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2. When opposes , Speed is below Ns .

Slip power returned to Supply.

3. When aids , Speed is above Ns .

Slip power delivered to Supply.

Since slip power is either returned to, or taken from the supply,

Doubly fed motor results in a CONSTANT – TORQUE drive system.

More or less Constant speed at all loads for a given tapping.

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APPLICATIONS

Used for fans, pumps, lifts, cranes and haulage equipment,

machine tools, printing machinery, chemical plant

& wire manufacturing machinery

Power and voltage ratings between 2kW at 600V & 2MW at 11kV.

Speed range is usually about 3:1 to 5:1.