Three Phase Input DC Drive

Controlled bridge rectifiers are not limited to single-phase designs. In most commercial & industrial control systems, AC power is available in three-phase form for maxi mum horsepower & efficiency. Typically six SCRs are connected together, to make a three-phase fully controlled rectifier. This three-phase bridge rectifier circuit has three legs, each phase connected to one of the three phase voltages. It can be seen that the bridge circuit has two halves, the positive half consisting of the SCRs S1, S3, & S5 & the negative half consisting of the SCRs S2, S4, & S6. At any time when there is current flow, one SCR from each half conducts.

The variable DC output voltage from the rectifier sup plies voltage to the motor armature in order to run it at the desired speed. The gate firing angle of the SCRs in the bridge rectifier, along with the maximum positive & negative values of the AC sine wave, determine the value of the motor armature voltage. The motor draws current from the three-phase AC power source in proportion to the amount of mechanical load applied to the motor shaft. Unlike AC drives, bypassing the drive to run the motor is not possible.

Larger-horsepower three-phase drive panels often consist of a power module mounted on a chassis with line fuses & disconnect. This design simplifies mounting & makes connecting power cables easier as well. A three phase input DC drive with the following drive power specifications:

• Nominal line voltage for three-phase-230/460 V AC
• Voltage variation-+15%, -10% of nominal
• Nominal line frequency-50 or 60 cycles per second
• DC voltage rating 230 V AC line: Armature voltage 240 V DC; field voltage 150 V DC
• DC voltage rating 460 V AC line: Armature voltage 500 V DC; field voltage 300 V DC