© ApICS ® LLC 2000

Investment in the present installed base of applications employing MG-Sets is adequately significant to justify upgrading the existing control systems rather than scrapping the MG-Set generators and installing new DC static drive systems. Upgrades of existing systems may become necessary from market pressures to improve product quality, or it may result from a need to realize an improvement in system reliability and consequently, productivity. Typically these upgrades are performed in the field during several short shut-downs. The 5 stand cold rolling mill at Inland Steels' Indiana Harbor works underwent such a field modernization. The objectives of this project were 1) to improve product quality with an AGC upgrade, 2) to reduce scrap with improved speed loop tracking during accelerations and decelerations and 3) reduce down time for control system maintenance.

Before describing the current minor loop (CML) implemented at Inland Steel, a brief synopsis of control strategies employed in MG set applications commissioned circa. 1950 to 1975 is presented. While this brief overview may be unnecessary for the "old hands" of the industry it will be useful for younger engineers who have not been exposed to MG-Sets in their academic curricula.

Control topologies in MG_Sets applications can be condensed to three basic design strategies. The first strategy employed mag-amps (an "amplidyne" configuration for low power applications is presented in Fig. 1). The output voltage of the "amplidyne" is controlled by the field current through the control winding. By supplying current to the field winding, generator action causes a voltage to develop across both pairs of commutator brushes. One pair of brushes is shorted so that a large current occurs in the windings connected to these brushes. The other pair of brushes is connected to the load through a compensating winding. The windings are arranged so that the strong magnetic field developed by the shorted pair of brushes reinforces the control field excitation. The power delivered to the load is therefore, an amplified function of the signal applied to the control winding. Mill stand MG-Sets applied the mag-amp configuration for the control of generator field current which in turn controlled generator voltage.

Figure 1. A Low Power "Amplidyne" Controlled MG-Set

The second design strategy centers around armature voltage regulation (Fig. 2). Armature voltage is a function of the generator shunt field flux which is itself a function of the shunt field current. Armature voltage regulation may be achieved by controlling the firing angle of the generator shunt field power module (and consequently the generator shunt field current) with the output of an armature voltage regulator. Change in shunt field current results in change in shunt field flux and consequently armature voltage.

Figure 2. Armature Voltage Controlled MG-Set

The third design strategy employed armature current regulators Armature current regulators were implemented in much the same way same as the armature voltage regulator in Fig. 2 but with the substitution of armature current feedback for voltage feedback. In both the second and third control strategies the power module providing generator field current is effectively run open loop. Differences in linearization techniques are the most noticeable contrasts in the control schemes of drive vendors who applied the above three strategies.

During the 1960's and 70's speed regulation became more common in cold rolling mills as the quality of interstand tension measurement devices improved and AGC algorithms were developed that controlled interstand tension. These algorithms required individual mill stand speed regulation. In the early speed regulation schemes the speed error was used to provide the generator field power module firing angle in much the same way as the voltage error in design 2 above. Later approaches employed a current minor loop (CML) as described in design 3 above, with a cascaded speed loop, in a manner similar to modern static drives. Despite claims to the contrary the speed loop bandwidths of these systems were typically around 5 [rad/sec]. Performance is severely limited by the electro-magnetic characteristics of the generator field and armature. Drive trains with low torsional natural frequencies and significant backlash imposed additional constraints on the speed loop bandwidth.

More recently, digital and/or analog static DC drives have been designed with self-tuning CML's. The CML is readily modeled as a lag from the current reference to motor torque with a fixed gain and a corner frequency approximately equal to the CML bandwidth. CML bandwidths for these drives can vary from a low of about 100 [rad/sec] to a high of several thousand [rad/sec]. The high bandwidth of the CML ensures that the factors limiting speed loop bandwidths in static drives are attributable either to the mechanical configuration and/or the speed loop control strategy employed. In static drives the bandwidth of the inner loop rarely imposes limitations on the performance of the speed loop.

The armature CML presented in this paper differs from older designs in two ways: 1) The use of a generator field current regulator 2) The implementation of a stiffer CML control algorithm. It will be shown that the small signal response of the rotating CML described in this paper (Fig. 3.) is very similar to that of a static CML with a bandwidth of 50 - 100 [rad/sec]. Because of this, DC static drive speed loop design strategies may be utilized including backlash compensation, inertia compensation, and frictional loss compensation. The net result is a significant improvement in speed loop bandwidth and tracking. In the case of the 5 stand cold rolling mill at Inland Steel the higher bandwidth of the CML and the resulting improvement in speed loop response (around 12 - 15 [rad/sec]) combined to produce a significant reduction in scrap along with improved AGC regulation.

The MG-Set CML regulation scheme in Fig. 3 and its associated tuning algorithm are described in section 2. In the application at Inland Steel an additional complication includes a 12 pulse DC bridge that is wired in parallel with each MG-Set armature and supplies additional armature current during mill acceleration and rolling. Because the bridge is non-regenerative it does not contribute to armature current during deceleration. As a consequence of these differing operating conditions, a scheme was implemented to ensure a stable sharing and transferring of load between the MG-Set and the 12 pulse bridge as a function of the magnitude and sign of the current reference. This scheme is described in section 3. In section 4 a simulation of one stand from the cold rolling mill at Inland Steel is compared to results obtained on-site. Section 5 closes with some observations and conclusions.

Figure 3. Armature Current Controlled MG - Set

2.1 BACKGROUND:

Recently, mill-model based closed loop AGC control algorithms have been installed in rolling mills. These algorithms require speed loop bandwidths in the range of 10 to 25 [rad/sec] to realize significant improvements in gauge control. Speed reference tracking of each stand in the mill must be approximately equal to minimize disturbances to the AGC during line accelerations and decelerations. With these requirements in mind the following control algorithm was developed for MG-Set applications.

Fig. 16 presents simulated 100 - 125 [rpm] speed step responses of mill stand No. 4. The Speed loop was tuned for a 12 [rad/sec] response. For both the locked rotor CML step responses and the speed step responses there was good agreement between the responses from the model and the responses obtained on-site.

Figure 16. Simulated 25 [rpm] Speed Step Response

A Significant improvement in speed loop response in solid frame MG set mill stands can be achieved by implementing a high bandwidth armature current loop. The CML implementation differs from previous designs in that; 1) A loop is closed around generator shunt field current, 2) A combination of feedback forcing and lead pre-filtering is used to provide robustness in the MG-Set CML. The effectiveness of the design is contingent on the generator FML providing some form of compensation for eddy current effects in the generator.