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THREE PHASE A.C. COMMUTATOR MOTORS

THREE PHASE A.C. COMMUTATOR MOTORS. Developed to provide Better Speed Control Good p.f. Control Both Speed and p.f. Control. 2 Classes Those having Series Characteristics Those having Shunt Characteristics Most of the machines have Shunt Speed Characteristics.

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THREE PHASE A.C. COMMUTATOR MOTORS

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  1. THREE PHASE A.C. COMMUTATOR MOTORS • Developed to provide • Better Speed Control • Good p.f. Control • 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. DEE_JRN

  2. THREE PHASE A.C. SERIES COMMUTATOR MOTORS Motor having Series Characteristics. i.e. Speed falls with increase in load. 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. The Stator and Rotor are connected in Series . Flux will be dependent on the current, resulting in “Series” Speed Characteristics. (Variable Speed) DEE_JRN

  3. 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. DEE_JRN

  4. 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. DEE_JRN

  5. 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. • 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. DEE_JRN

  6. SPEED CONTROL • By Brush Shifting – most commonly used. • By varying the applied Voltage. • 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. DEE_JRN

  7. 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. DEE_JRN

  8. 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. When = 0, the brushes in effect are Short circuited - the m/c now operates as an Induction Motor. DEE_JRN

  9. 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. DEE_JRN

  10. 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. DEE_JRN

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