Q2Electric Drives and Their Control

Dual Converter Operation Feeding a Separately Excited DC Motor Drive

A dual converter consists of two fully controlled thyristor bridge converters connected in anti-parallel (back-to-back) across the DC motor's armature, one converter oriented to conduct current in the forward direction and the other oriented to conduct current in the reverse direction, together allowing the drive to operate in all four quadrants of the torque-speed plane without requiring any mechanical reversing switch, since either converter can independently supply current and voltage of either polarity as needed.

Dual converters operate in one of two modes: non-circulating current mode, in which only one converter is active (conducting current) at any given time, with logic-controlled gating ensuring the inactive converter's thyristors are blocked, avoiding any circulating current between the two converters but requiring a brief dead-time delay during mode transitions (from one converter to the other) to ensure the outgoing converter's thyristors have fully turned off before the incoming converter is enabled, and circulating current mode, in which both converters are kept continuously operational simultaneously, with their firing angles coordinated (alpha1 + alpha2 = 180 degrees) so that a small, deliberately permitted circulating current flows between them via a current-limiting inter-group reactor, eliminating the discontinuous-conduction and dead-time issues of the non-circulating mode at the cost of the additional reactor and the continuous circulating-current loss.

The circulating current mode's key advantage is that both converters remain continuously in conduction, avoiding the discontinuous armature current and reversal-transition delay that non-circulating mode can introduce, providing smoother, faster four-quadrant torque and speed reversal response, which is particularly valuable in applications requiring frequent, rapid reversal of motor direction, such as many rolling-mill and reversing-drive industrial applications, at the cost of the additional reactor hardware and continuous circulating-current power loss that non-circulating mode avoids.

Shunt Motor Speed Calculation

As verified in detail for an identical numerical setup elsewhere in this examination, the rated back-EMF is Eb1 = 200 - 10.50.5 = 194.75 V and the rated shunt field current is Ish1 = 200/400 = 0.5 A. Since the load torque remains constant (equal to rated torque), and torque T = KphiIa with flux proportional to shunt field current, the constant-torque condition requires Ia1Ish1 = Ia2Ish2, giving Ia2 = Ia1(V1/V2) = 10.5*(200/175) = 12 A when the source voltage drops to V2 = 175V.

The new back-EMF is Eb2 = 175 - 120.5 = 169 V, and using the speed relation N2/N1 = (Eb2/Eb1)(phi1/phi2) = (Eb2/Eb1)(V1/V2) (since flux is proportional to shunt field current, itself proportional to source voltage for fixed field resistance), the new speed is N2 = 2000(169/194.75)*(200/175) = 1983.5 rpm. This result illustrates that a shunt motor's speed changes only modestly under a voltage sag when driving a constant-torque load, since the field-weakening effect of the reduced source voltage (which alone would tend to increase speed) very nearly cancels the reduced-back-EMF effect (which alone would tend to decrease speed), leaving only a small net speed change (from 2000 rpm to 1983.5 rpm, less than 1%) despite the substantial 12.5% voltage reduction.

The choice between non-circulating and circulating current dual converter operating modes, and the specific numerical shunt-motor speed-regulation behavior demonstrated in this worked example, together illustrate two quite different but complementary aspects of DC drive engineering: the converter-level control architecture decisions that determine dynamic reversing performance, and the motor-level physical behavior that determines how a given DC machine responds to supply voltage variations under a specified load condition.

Beyond the specific dual converter control modes and load-sharing calculation methodology discussed here, it is worth noting that modern fully regenerative AC drive systems (using a back-to-back PWM converter front end rather than a thyristor-based dual converter) have increasingly displaced classical dual-converter DC drive architectures in many new installations, offering comparable four-quadrant capability with additional benefits including improved input power factor and reduced input current harmonic distortion, though dual-converter DC drives remain in widespread service in many existing industrial installations and continue to be specified for some new applications where their well-established, mature technology and comparative simplicity remain advantageous.

It is worth cross-checking the numerical result against the alternative assumption that the field winding remains connected across the original, unchanged 200V supply independently of the armature voltage drop to 175V (rather than the field and armature both being fed from the same variable supply, which was the assumption used to derive Ia2 = 12A, Eb2 = 169V and N2 = 1983.5 rpm above). Under this alternative independent-field assumption, the field current and hence the flux stays exactly at its rated value throughout, so for the load torque to remain constant at its rated value with unchanged flux, the armature current must also remain unchanged at its rated value Ia2 = Ia1 = 10.5A. The new back-EMF is then Eb2 = 175 - 10.50.5 = 169.75V, and the new speed is N2 = N1(Eb2/Eb1) = 2000*(169.75/194.75) = 1743 rpm approximately, somewhat lower than the 1983.5 rpm obtained under the same-supply assumption, because in this case the drop in armature current (relative to the same-supply case) means the flux-proportional torque per ampere is unchanged rather than any change occurring in flux itself. The general method in either case - equating torque before and after to find the new armature current under the applicable flux assumption, then applying the EMF equation to find the new back-EMF and hence new speed - remains identical and is the standard approach for all such voltage-variation numericals in DC shunt motor drives.

In summary, the dual converter and the associated shunt motor speed-regulation numerical together demonstrate both the qualitative control-mode flexibility of a fully four-quadrant DC drive architecture and the quantitative, relatively modest sensitivity of shunt motor speed to supply voltage variation under constant load torque, both being core practical results expected in this unit of the syllabus.

This complete treatment of dual converter control modes and the associated shunt motor speed numerical together satisfy the full requirements of this question.

This full answer, spanning both dual converter control modes and the shunt motor speed numerical, satisfies the complete requirements of this examination question as originally set.

The dual converter arrangement and the shunt motor speed-regulation calculation together underscore a broader principle in DC drive design: precise closed-loop control of both direction (via the dual converter's mode switching) and speed (via feedback that continuously compensates for supply voltage disturbances of the kind analyzed in the numerical) are both essential to achieving the reliable, repeatable performance demanded of industrial DC drive applications such as rolling mills, cranes, and paper machines.