Converter fed DC Drives

• Controlled conversion of ac into dc

Operation in the continuous conduction mode

Single Phase Full Converter

cos*2 m

out

VV

When firing angle is 0 , we have uncontrolled converter with output Vdc as

mm

out

VVV 2cos*2

Operation in the discontinuous conduction mode

condition for continuous conduction

Average DC Output Voltage

Thyristor Converter: Discontinuous Mode

• This mode can occur in a dc-drive at light loads

DC Voltage versus Load Current

• Various values of delay angle

Single Phase Full Converter with freewheeling diode

Single Phase Half Controlled Bridge Converter

• In a half controlled converter the output voltage does not become negative and hence the converter cannot operate in the inverter mode. • For the same firing angle and input voltage the half controlled converter in the continuous conduction mode gives higher output voltage compared to a fully controlled converter. • For the same input voltage, firing angle and load parameters the half controlled converter has better output voltage and current form factor compared to a fully controlled converter. • For the same firing angle and load current the half controlled converter in the continuous conduction mode has better input power factor compared to a fully controlled converter. • Half controlled converters are most favored in applications requiring unidirectional output voltage and current.

Three-Phase Thyristor Converter

• Realistic load

Thyristor Converter Waveforms

• Realistic load; continuous-conduction mode

Thyristor Converter Waveforms

• Realistic load; discontinuous-conduction mode

In a fully controlled converter the output voltage can be controlled by controlling the firing delay angle (α) of the thyristors.

Single phase fully controlled bridge converters are extensively used for small dc motor drives.

Depending on the load condition and the firing angle a fully controlled bridge converter can operate either in the continuous conduction mode or in the discontinuous conduction mode. In the continuous conduction mode the load voltage depends only on the firing angle and not on load parameters.

In the discontinuous conduction mode the output voltage decreases with increasing load current. However the output voltage is always greater than that in the continuous conduction mode for the same firing angle.

The fully controlled bridge converter can operate as an inverter provided α › π ⁄ 2

dc power source of suitable polarity exists on the load side.