APPLICATIONS OF LINE COMMUTATED RECTIFIERS IN MACHINE DRIVES

APPLICATIONS OF LINE COMMUTATED RECTIFIERS IN MACHINE DRIVES

Important applications for line commutated three-phase controlled rectifiers, are found in machine drives. Figure shows a dc machine control implemented with a six-pulse rectifier.

 
Torque and speed are controlled through the armature current ID , and excitation current Iexc. The current ID is adjusted with VD , which is controlled by the firing angle α through equation below,

 
This dc drive can operate in two quadrants: positive and negative dc voltage. This two quadrant operation allows regenerative braking when α>90°,

and Iexc <0.

The converter shown can also be used to control synchronous machines, as shown in figure below. In this case, a second converter working in the inverting mode operates the machine as self-controlled synchronous motor. With this second converter, the synchronous motor behaves like a dc motor but has none of the disadvantages of mechanical commutation. This converter is not line-commutated, but machine-commutated.

 The nominal synchronous speed of the motor on a 50 or 60 Hz ac supply is now meaningless, and the upper speed limit is determined by the mechanical limitations of the rotor construction.
There is the disadvantage that the rotational emfs required for load commutation of the machine side converter are not available at standstill and low speeds. In such a case, auxiliary force commutated circuits must be used.

 The line-commutated rectifier through α controls the torque of the machine. This approach gives direct torque control of the commutator less motor and is analogous to the use of armature current control shown in previous figure for the converter-fed dc motor drive.  

The line-commutated rectifiers are also used for speed control of wound-rotor induction motors. Sub synchronous and super synchronous static converter cascades using a naturally commutated dc link converter can be implemented. The figure below shows a super synchronous cascade for a wound rotor induction motor, using a naturally commutated dc link converter.

 In the super synchronous cascade shown in figure, the right hand bridge operates at slip frequency as a rectifier or inverter, while the other operates at network frequency as an inverter or rectifier. Difficulty control is experienced near synchronism when the slip frequency emfs are insufficient for natural commutation, and special circuit configuration employing forced commutation or devices with a self-turn-off capability are necessary for the passage through
synchronism. This kind of super synchronous cascade works better with cycloconverters.