Upload
hari-reddy
View
218
Download
0
Embed Size (px)
Citation preview
8/13/2019 Ac+Commutator+Motors
1/10
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
8/13/2019 Ac+Commutator+Motors
2/10
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 segments3 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
8/13/2019 Ac+Commutator+Motors
3/10
Speed Control usually done by SIMULTANEOUS movement of all the
Brushes round the Commutator using Brush Gear.
To avoid commutational difficulties, a transformer is usuallyemployed between Stator & Rotor thus reducing the voltage
applied to the Commutator Wdg.
3DEE_JRN
8/13/2019 Ac+Commutator+Motors
4/10
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 10 0 to 40 0 . The Stator is energized and the rotor rotates.
(180 )
8/13/2019 Ac+Commutator+Motors
5/10
DEE_JRN 5
CHARACTERISTICS FEATURES
Starting Torque and Starting Current 50% of the FL Values.
Reducing , enables to obtain T St upto 200% FL &
ISt 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 directionof rotation of rotor, the torque increases the motorcan be bought up to the desired speed. For any given position of the BRUSHES, the Speedfalls with increase in Load, as in the case of SERIES MOTORS.
8/13/2019 Ac+Commutator+Motors
6/10
DEE_JRN 6
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.
8/13/2019 Ac+Commutator+Motors
7/10DEE_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.
8/13/2019 Ac+Commutator+Motors
8/10DEE_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
1. When = 0, the brushes in effect are Short circuited
- the m/c now operates as an Induction Motor.
j E
j E
8/13/2019 Ac+Commutator+Motors
9/10DEE_JRN 9
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.
j E 2 E
j E 2 E
8/13/2019 Ac+Commutator+Motors
10/10DEE_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.