2 IntroductionCycloconverters directly convert ac signals of one frequency (usually line frequency) to ac signals of variable frequency. These variable frequency ac signals can then be used to directly control the speed of ac motors.Thyristor-based cycloconverters are typically used in low speed, high power (multi-MW) applications for driving induction and wound field synchronous motors.
3 Phase-Controlled Cycloconverters The basic principle of cycloconversion is illustrated by the single phase-to-single phase converter shown below.
4 Phase-Controlled Cycloconverters (cont’d) A positive center-tap thyristor converter is connected in anti-parallel with a negative converter of the same type. This allows current/voltage of either polarity to be controlled in the load.The waveforms are shown on the next slide.
6 Phase-Controlled Cycloconverters (cont’d) An integral half-cycle output wave is created which has a fundamental frequency f0=(1/n) fi where n is the number of input half-cycles per half-cycle of the output. The thyristor firing angle can be set to control the fundamental component of the output signal. Step-up frequency conversion can be achieved by alternately switching high frequency switching devices (e.g. IGBTs, instead of thyristors) between positive and negative limits at high frequency to generate carrier-frequency modulated output.
7 Phase-Controlled Cycloconverters (cont’d) 3 to single phase conversion can be achieved using either of the dual converter circuit topologies shown below:
8 Phase-Controlled Cycloconverters (cont’d) A Thevenin equivalent circuit for the dual converter is shown below:
9 Phase-Controlled Cycloconverters (cont’d) The input and output voltages are adjusted to be equal and the load current can flow in either direction. Thus,where Vd0 is the dc output voltage of each converter at zero firing angle and p and N are the input and output firing angles. For a 3 half-wave converter Vd0 =0.675VL and Vd0 = 1.35VL for the bridge converter (VL is the rms line voltage).
10 Phase-Controlled Cycloconverters (cont’d) Voltage-tracking between the input and output voltages is achieved by setting the sum of the firing angles to . Positive or negative voltage polarity can be achieved as shown below:
11 Phase-Controlled Cycloconverters (cont’d) A 3 to 3 cycloconverter can be implemented using 18 thyristors as shown below:
12 Phase-Controlled Cycloconverters (cont’d) Each phase group functions as a dual converter but the firing angle of each group is modulated sinusoidally with 2/3 phase angle shift -> 3 balanced voltage at the motor terminal. An inter-group reactor (IGR) is connected to each phase to restrict circulating current.
13 Phase-Controlled Cycloconverters (cont’d) An output phase wave is achieved by sinusoidal modulation of the thyristor firing angles.