Page 163 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 61143
The use of PLCs is essential in the control of motors The scheme will also facilitate conservation of
to closely monitor the operating parameters of the process regenerative energy if there are more of such drives
line on which the motors are connected. The inverter without any additional cost.
unit controlling the motors then conducts the required 5 Shear no. I is used to shear the edges of the sheet of
correction in each motor speed through its switching pay-off reel no. I at the beginning as well as at the
logistics, which may be activated by the motor side V,f, end of the coil before it enters the welder to smooth
I,,,, I;, or p etc.. depending upon the inverter logistics this for a correct welding with the outgoing edge.
being used. All such parameters are predefined for a At the beginning of the coil this edge is welded with
particular process and are preset. the tail end of the previous coil and at the end it is
welded with the edge of the fresh coil at the beginning
A brief description of the process and the use of from pay-off reel no. 2. Pay-off reel no. 2, driven by
static drives motor M1, is arranged parallel to pay-off reel no. I
to provide a second feed route for an uninterrupted
We have divided the total process line into three sections: and continuous process flow.
6 The deflector roll guides the sheet to another pinch
Uncoiler section roll no. 4 to carry out precise alignment of the edges
before they enter the welder.
7 Pinch roll no. 4 aligns the edge of the sheet through
I Pay-off reel no. 1 feeds the raw MS sheet to the a feedback control.
process section via feed pinch rolls nos I and 2 8 For further alignment of the edge width just before
which straighten the sheet before it enters the welder.
2 To maintain continuity and achieve an uninterrupted entering the welder the sheet is guided again through
process line a second parallel feed route is provided an entry-side guide.
through a second pay-off reel no. 2. 9 With the help of bridle no. 1 driven by motors Mi
3 These rolls are driven by motors MI and MZ whose and M4, the uncoiler section speed is controlled by
speed is controlled through the tension of the monitoring the tension of the travelling sheet and
hence maintaining constant speed of the sheet in the
travelling sheet. The tension of the sheet is adjusted
by monitoring the diminishing diameter of the pay- uncoiler section. The tensile difference of T, and T2
off roll and the thickness of the sheet. determines the speed of the uncoiler. Speed and
3 The pay-off roll is unwound by the tension of the tension of the sheet must remain constant for absolute
sheet, caused by the speed of the recoiler at the synchronization between the uncoiler process and
finishing line and the bridles positioned at different the recoiler sections.
locations. The pay-off roll motors therefore operate 10 To allow for welding time, a buffer of a certain
sheet length, in the form of entry accumulator, is
in a regenerative mode and can feed-back the energy maintained, generally in a vertical formation, to save
thus saved to the source of supply, if desired. This
can be done by using a full-wave synchronous space. This feeds the line ahead until the welding
inverter, as shown in Figures 6.3 1 or 6.33. operation is completed and the second route is
installed to feed the process.
However, this is a more expensive arrangement.
A more economical method is to use a full-wave, 11 The time gap in carrying out the welding 1s . com-
diode bridge converter and an IGBT inverter unit pensated by raising the speed of the second route
combination as shown in Figure 6.50 in place of' an now introduced until the predefined buffer of an
additional thyristor or IGBT feedback circuit. The excess length of sheet is produced with the help of
d.c. link bus is now made a common bus for all the accumulator drive motor M5.
drives operating on the process. During a regenerative
mode, such as during uncoiling the pay-off reels, Process section
the voltage of the d.c. bus will rise and will be utilized 12 The tension and speed in the process section is
to feed the other drives. This process will draw less maintained again with the help of bridle no. 2. driven
power from the source. The regenerative energy is by motors M, and M,.
now utilized in feeding the process system itself 13 The sheet is now fed through a pair of guide rollers
rather than feeding back to the source of supply. to a furnace section through a degreasing tank, where
There is now only one converter of a higher rating, it is preheated for drying and raising the temperature
reducing the cost of all converters for individual of the sheet up to a required level (40046SoC typical)
drives and conserving regenerative energy again at before it enters the hot galvanizing pot for the desired
a much lower cost. thickness of zinc coating. The movement of the sheet
There is no need to introduce a resistance for the through the furnace is helped by motors M8 and My.
purpose of dynamic braking for the individual drives, 14 The hot-treated sheet is cooled to the required level
but a large resistance will be necessary on the d.c. by fans, driven by motors M,o and MI, before it
bus to absorb the heat energy during shutdown enters the molten zinc pot.
(braking) of the process. The useful energy during 15 The required thickness of zinc coating is achieved
shutdown cannot be fed back to the source due to by dipping it in the molten zinc pot for a preset
the configuration of the converter-inverter combi- time. The thickness of the coating is monitored by
nation. This arrangement can feed the regenerative two rollers through which the coated sheet is passed.
energy to its own process only. The time of welding. degreasing. heating and hot