A1-A2 is the time for adding liquid iron and charging (low speed)
A3-A4 is the time for supplying electricity and supplying electricity and oxygen to smelt (high speed)
A5-A6 is the time for waiting for sampling, discharging and blocking out (Low speed)
A2 is the speed rising point, at which the electricity supplying and oxygen supplying signal are collected at the production site and begin to increase the speed from the low speed to high speed.
A4 is the speed decreasing point, at which the electricity supplying stopping, and oxygen supplying stopping signal are collected at the production site and begin to decrease the speed from the high speed to low speed.

In order to adapt the production variation, 5 levels of air-flow from the dusting fan are needed at the low-speed section for adapting the smelting technology demands of the electric furnace.

A-B is the first slow speed section of the low speed section (lowest speed), B-P is the fifth low speed section (highest speed), Q-R is the first low speed section of the low speed section (lowest speed).

B-D is the second low speed section, in which B-C is the fan speed rising time;
D-F is the third low speed section, in which D-E is the fan speed rising time
F-H is the forth low speed section, in which F-G is the fan speed rising time
H-J is the fifth low speed section, in which H-I is the fan speed rising time
When the fan speed is rising, the speed may leap over any speed rising point according to requirements.
At point J, the fan begins speed decreasing.
J-L is the fifth low speed section turning to the forth low speed section, in which J-K is the fan speed decreasing time;
L-N is the forth low speed section turning to the third low speed section, in which L-M is the fan speed decreasing time;
N-P is the third low speed section turning to the second low speed section, in which N-O is the fan speed decreasing time;
P-R is the first low speed section, in which P-Q is the fan speed decreasing time;
Q-R is the first low speed section of the low speed section (lowest speed).

HARVEST-A06/220 VFD is applied to the dusting fan of the electric furnace of the Forth Steel Making Plant of Daye Special Steel Co., Ltd. to perform speed regulation. A bypass switching cabinet of industry frequency is added. The VFD is of voltage source type with direct high-high mode. The main specifications are as follows: Capacity of VFD 2,250kVA, normal output current 220A, input frequency 45Hz-55Hz, normal input voltage 6,000V, allowed voltage fluctuation ±10%, input power factor≥0.96 (when greater than 20% load), output frequency ranging 5 speed levels, adjustable in the range of 0hz to 50Hz (i.e. speed adjustment ranging from 0 to 100%), VFD efficiency ≥98% (not including input transformer), output frequency resolution 0.01Hz, stepless speed regulation, overload capability 120% 1 minute, 150% protect instantly, Acceleration time may be set according to technology requirements. Deceleration time may be set according to the technology requirements.

Table for VFD & Interfaces at the Site

The interface of VFD with PLC control system of the Forth Steel Making Plant: The PLC of the Forth Steel Making Plant is Siemens S7-300 and the built-in PLC of the VFD is Siemens S7-200. When communicating, the PLC of the Forth Steel Making Plant is as the master station, the built-in PLC of the VFD as the slave station, to perform the data communication. The information needed to be transmitted from the VFD to the PLC of the Forth Steel Making Plant include: failure alarming (comprehensive), high voltage switch on allowable, high voltage emergency switch off, analog signal: operating frequency, VFD input current (4-20mA, current source, loading capability greater than 500Ω)
The interface of VFD with the site of the Forth Steel Making Plant: The local control box for the VFD and the Forth Steel Making Plant use hard wires for connection. The local control box of the Forth Steel Making Plant should include: buttons: button for starting (with indicator), button for stopping (with indicator); indicator lamps: indicator for normal operation of VFD, indicator for VFD operation with industry frequency; The VFD and the before-furnace control box of the Forth Steel Making Plant uses hard wires for connection. The before-furnace control box of the Forth Steel Making Plant should include: buttons: high speed button (with indicator), low speed 1 (lowest speed) button (with indicator), low speed 2 button (with indicator), low speed 3 button (with indicator), low speed 4 button (with indicator), low speed 5 (highest speed) button (with indicator); The signals provided to the local control box by VFD: status signal, normal operation of VFD, VFD operation with industry frequency; The signals provided to the VFD by the local control box: remote control commands: start and stop; The signals provided to the VFD by the before-furnace control box: working condition speed commands: high speed command, low speed 1 command, low speed 2 command, low speed 3 command, low speed 4 command, low speed 5 command; interlock signals for the VFD and high voltage switching cabinet: high voltage emergency switch off. The output for all the above digital signals adopts passive contacts, and it is defined as effective at the closed contacts, which capacity of all contacts is AC220V/3A.
Selection for remote/close control for VFD: When the selection control is set to the position for remote control, the control authority is handled to the local control box. When the selection control is set to the position for close control, the control authority is handled to the VFD.

Operation & Tests of the Dusting Fans
The operation record of the VFD for various speed points:

Before the VFD applied to the dusting fans, the operating current of the motor is 180A under various working conditions. After the VFD applied to the dusting fans, the electricity saving rate is more than 47.5% averagely at the low speed section.