Three phase dual converter for four quadrant operation of dc motor

dual converter for four quadrant operation of dc motor By: Nosheen Memon Electrical engineer QUCEST (B.E)MUET SZAB ( M.E)

Dual converter is the combination of two antiparallel full converters with same load.Each converter has six thyristor (SCR).Dual converter can control separately excited dc shunt motor in either direction and mode i.e. generating and motoring.So the motor can operate in all four quadrant in forward motoring forward generating reverse motoring and reverse generating.The power to dc motor is supplied through dual converter.Motor can deliver maximum torque below its base speed and maximum power above its base speed.To control dc motor speed armature voltage is varied with field voltage.If the armature voltage is more than field voltage speed will be more and if the armature voltage is less than the field voltage speed will be less.The inductor is use in series with motor to reduce impact of notches on line voltage.

Dual converter consist of two converter one is positive converter and another is negative converter .When the positive converter conducts motor operate in forward direction i.e. forward motoring (1st quadrant) and when the negative converter conducts motor operate in reverse direction i.e. reverse motoring (3rd quadrant).

Single-Phase Dual-Converter

(a) circuit (b) waveforms (c) quadrant

If 1 and 2 are the delay angles of converter 1 & 2, respectively, the corresponding average output voltages are Vdc1 & Vdc2.The delay angles are controlled such that one converter operates as a rectifier & other operate as an inverter; but both converters produce same average output voltage.The average output voltages of both the converters are: Vdc1=2 Vm/ cos1 Vdc2=2 Vm/ cos2Since, one converter is rectifying & the other is inverting . Therefore, Vdc1 = -Vdc2 or, cos1 = -cos2 = cos ( - 1)Therefore, 2 = - 1

ModesThe dual converter have one is circulating current mode and other is without circulating current mode.The dual converters can be operated with or without circulating current.

Circulating current mode..Since, the instantaneous output voltages of two converters are out of phase there will be an instantaneous voltage difference & this will result in circulating current between the two converters.If both converter conducts at the same time there is a circulating current mode.This circulating current will not flow through the load & is normally limited by a circulating current reactor Lr . The circulating current depends upon the delay angle.

Without circulating currentWithout circulating current, only one converter operates at a time & carries the load current; and the other converter is completely blocked.

We can use one converter at any instant but there is a small delay when switching from one converter to another.

Disadvantages of circulating current modeThe operation with circulating current though it has disadvantages such as: 1. A reactor is required to limit the circulating current. The size & cost of the reactor may be significant at high power levels. 2. Circulating current gives rise to more losses in converters & hence efficiency & pf is low. 3. As the converters have to handle load as well circulating current, the thyristors of two converters are rated for higher currents.

inspite of the above drawbacks, a dual converter with circulating current mode is preferred as: 1. The circulating current maintains continuous conduction of both converters over the whole control range, independent of the load. 2. Since one converter always operates as a rectifier & the other converter operate as an inverter, the power flow in either direction at any time is possible. 3. Since both converters are in continuous conduction, the time response for changing from one quadrant operation to another is faster.