08-17-2017, 12:13 AM
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DOUBLY FED INDUCTION MACHINE
INTRODUCTION
ELECTRICAL MACHINES
Based on the supply, 2 types-AC machine & DC machine
AC machines are of different types
1. Induction machine
2. Synchronous machine etc.
INDUCTION MACHINE
Induction machines are of different types and one of the type in particular are Singly Fed and Doubly Fed.
SINGLY FED INDUCTION MACHINE
1. Only ONE winding active in ENERGY conversion Process
2. Can have speed around Synchronous Speed.
DOUBLY FED INDUCTION MACHINE
TWO windings participate in ENERGY conversion Process.
Can work in DOUBLE synchronous speed for constant torque.
Similar to Synchronous Machine but in synchronous machine only one winding participate in energy conversion
DFIM is to operate in Narrow SPEED ranges
CONSTRUCTION
Two types-
1. Wound Rotor DFIM.
2. BRUSHLESS wound rotor.
a. Brushless DF Induction electric machine.
b. Brushless wound rotor DF electric machine.
WOUND ROTOR DFIM
Uses the TWO windings of same POWER Rating.
One winding on STATOR & other on ROTOR.
Stator supply is Normal 3F supply.
Rotor supply is from power frequency converter.
Slip ring assembly used to transfer Power to rotor winding.
Brushless DFIM
Two windings adjacent to each other on the STATOR .
Windings are excited separately.
Brushless wound rotor DFIM
Similar to wound rotor DFIM but slip ring assembly is not used.
Large efficiency and less cost but instability is more.
WORKING
Field can be from rotor or stator or from both.
Both Active Power (for torque) and Reactive Power (for Flux) has to be fed to ROTOR.
Multi-phase supply with frequency f given to stator.
CONTROL
Frequency converter converts power from supply frequency to slip frequency.
VECTOR CONTROL
ADVANTAGES
Theoretically system cost is half of other machines with same rating.
HIGHER efficiency due to less LOSS.
Rotor core is effectively utilized hence Power density is large.
Active & Reactive Power to GRID can be controlled using Electronic Converters.
DISADVANTAGES
The wound rotor type is not reliable due to SLIP RING assembly.
Brushless DFIM has less efficiency and high cost.
APPLICATIONS
WIND MILL
Used as Wind Generators
DFIG can work in variable speed range around synchronous speed.
DFIG kept synchronized with grid uses wind source more effectively.
High power pumps and fans.
Used in hydro generators.
Used in ships etc
CONCLUSION
Wound rotor DFIM found commercial success in very large applications with limited speed range.
For a low cost ,highly efficient and reliable electronic controlled DFIM is kept under study.