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Unformatted text preview: 2. Electronic power supplies for dc motors There are following electronic power supplies used for control of DC motors: 1) Controlled rectifiers, 2) Choppers. 2.1 Controlled rectifiers They are used if there is an AC supply. There are two types of controlled converters:- full converter: all switching devices are controlled devices- semi-converter: if some switching devices are diodes. The output voltage depends on the firing angle . If the output current is continues the average value of the output voltage of controlled rectifiers is calculated from the following formulae:- single-phase input a) full-converter 2 2 cos out p V V = (1) b) semi-converter 2 (1 cos ) out p V V = + (2)- Three-phase input a) full-converter 3 6 cos out p V V = (3) b) semi-converter 3 6 (1 cos ) 2 out p V V = + (4) where V p is the rms of the ac phase voltage. 2.2 Choppers Choppers convert a fixed-voltage dc supply into a variable-voltage dc supply. Output voltage is: on out t V V T = (5) 1 or out V d V = (6) where: V input voltage t on is on-time of the chopper T is the chopping period d - is the duty ratio of the chopper 3. Open loop control The block diagram of open-loop circuit to control the speed of dc motor is shown in Fig.5. The converters used are controlled rectifiers or choppers. Solid-state converter to control the armature voltage DC Motor DC variable voltage Load DC or AC power supply I f Solid-state field controller Fig.5 Block diagram of open loop control of dc motors 3.1 First quadrant control As an example, a separately excited (or permanent magnet) DC motor is considered. 3.1.1 Six-pulse controlled rectifier The armature winding is connected to a 3-phase line via 3-phase, 6-pulse controlled rectifier as shown in Fig.6. The field winding is separately excited by a single-phase half rectifier as shown in Fig....
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- Fall '06