Single phase semi and full converter DC separately excited motor

SINGLE PHASE SEMI CONDUCTOR DRIVE

Semi convertors are one quadrant converters; they have one polarity of voltage and current at the DC terminals. The motor current can’t be reversed due to thyristors in the converters.

* When AC supply is given the thyristors and diodes combined together and convert AC signal to DC signal.

* The basic circuit diagram of single phase semi conductor drive feeding a separately excited DC motor is given below:

* The power flow through the circuit is positive half cycle is

(+)e – T1 – Df – Ra – La – D2 – e(-)

* The negative half cycle power flow is given as

(-)e – T2 – Df – Ra – La – D1 – e(+)

We are supplying AC supply to the circuit which is time variant (changes with time). The armature voltage and current is depends upon the operation conditions

Continuous armature current operation:

The below waveforms shows the voltage and current limits of semiconductor drive.

* From wave forms the thyristors T1 is triggered at angle α and T2 is triggered at an angle π+α with respect to voltage.

* For period α < ωt < π , the motor voltage is equal to supply voltage.

* For period π< ωt < α , the motor terminals are shorted through Df making ea Zero.

* Beyond period π, the armature emf reverses the input voltage by changing the polarity, this leads the freewheeling diode Df conducts.

* The armature current is in continuous mode due to the presence of freewheeling diode

* During π to π+α period the freewheeling diode recovers the energy from inductance ( storage element) and is supplied to load in the form of kinetic energy.

* By using Df the torque developed in the motor is said to be continuous.
* The torque and speed characteristics are given below:

SINGLE PHASE FULL CONVERTER DRIVE

Single phase full converter is a two quadrant converter. The output current is not reversed due to presence of thyristors (Unidirectional in nature)

* This full converter is limited to application up to 20HP

* The basic circuit diagram of single phase full converter drive is given below:

Current mode operation:

*The circuit is activated by giving AC supply. The thyristors which are connected in bridge connection are unidirectional in nature, due to this property the output current does not take back path, only voltage may reverse.

* For positive half cycle of AC supply the current travelling direction in the circuit is given by (+)e - T1 - Ra - La - eb - T2 - e(-).

* For negative half cycle of AC supply the travelling path of current is given as (-)e - T4 - Ra - La - eb - T3 - e(+).

* In full converter T1 and T3 are Triggered at firing angle α and thyrisytor T2 and T4 are triggered at π+α

* Thyristor T1 and T3 conducts during the interval α < ωt < π+α

* Thyristor T2 an T4 conducts during the interval π+α < ωt < α

* Natural commutation is taken into account here it means after conduction of T2 and T4 the supply voltage appears across T1 and T3 has a reverse biased voltage and turns them OFF automatically. This process is named as Natural commutation.

* The voltage and current waveforms for firing angle greater than 90 degress.

Inversion mode operation:

The average voltage Ea is negative at motor terminal. If motor back emf eb is reversed it will behave as a DC generator and will fed the power back to the AC supply. This operation is known as inversion operation of the converter and this modes of operation is used in the regenerative breaking of the motor. The wave forms for Inversion mode is given below: