Presentation on theme: "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."— Presentation transcript:
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. 1DEE_JRN
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) 2DEE_JRN
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. 3DEE_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 10 0 to 40 0. The Stator is energized and the rotor rotates.
DEE_JRN5 CHARACTERISTICS FEATURES Starting Torque and Starting Current – 50% of the FL Values. Reducing, enables to obtain T St upto 200% FL & I St upto150% FL Efficiency HIGH for the whole operating range. p.f. HIGH approaching unity for Speeds near N S. 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.
DEE_JRN6 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.
DEE_JRN7 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_JRN8 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.
DEE_JRN9 2. When opposes, Speed is below N s. Slip power returned to Supply. 3. When aids, Speed is above N s. 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_JRN10 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.