Beijing Leader & Harvest Electric Technologies Co., Ltd.
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Application of VFD in #4 Duster Fan in Steel Plant
1. Introduction
The steel works of Fujian Shangang Mingguang Co., Ltd. has three 15ton oxygen topblowing converters. During converting, a large quantity of brown-red smoke may exhaust from their mouthes. The high temperature smoke, which contains combustible gas and metal particles, must be cooled and cleaned, and then exhausted to chimneys for spreading or conveyed to gas recovery system for reservation through induced draft fans. Therefore each converter must be provided with a duster system, the model of the fan is AI850 and the power of the motor is 630KW/6KV. Since oxygen blowing of converters is periodical and intermittent, it is required that the fan operates with variable speed in the process of steel making to satisfy the requirements for energy saving and environmental protection.i.e. with oxygen blowing, the fan should operate with high speed otherwise the fan should operate with low speed. Speed adjustment is fulfilled by means of hydraulic couplers. The high speed is 2700r/min (designed speed 2900r/min) and the low speed is 1,200 r/min. After a period of operation, it is discovered that the hydraulic coupler technology has its limitation, mainly as follows:
Speed adjusting ranges from 30% to 90% and the speed is not stable. The low speed of 1,200 r/min is rather high, leading to energy waste. When operating with high speed, the hydraulic coupler loses speed sometimes, and smoke exhausts from the converter mouth.
The bearings of the hydraulic coupler must be replaced frequently, leading to the converter stopping and continuously producing cannot be satisfied.
Motor efficiency is low. Efficiency is extremely low especially operating at low speed.
Adjusting accuracy and linearity are low and react slowly.
Starting current is still rather high, whichaffects electric network stability.
When the hydraulic coupler fails, It is no way to switch to operate with normal power supply by the bypassand it must be stopped for maintenance.
Serious oil leakageresults in great environment pollution and the floor is contaminated badly.
In consideration of the above problems of the hydraulic coupler, it is decided not to use the hydraulic coupler any longer and but to use VFD for the speed adjustment of the fans on the new converter in 2001.

2. Technical Requirements for VFD and Reform Scheme
The duster fan is the hub of the duster cleaning system. Once the duster fan cannot operate normally, not only production will be affected, causing tremendous economic losses, but also may threaten the safety of personnel at the site. In addition the operating environment of the speed adjustment system is rather bad and the converter blowing is periodical and intermittent, therefore, it is required that the VFD for the duster fan has very high reliability. Based the above operating features, the main requirements to VFD are as follows:
The VFD has high reliability and operates for l ng time without failures.
The VFD has bypass function. Once failed, the motor can be switched to operation with the power supply.
The VFD has wide speed adjusting range and high efficiency.
The VFD has logic control capability and may increase or decrease the speed according to the blowing period automatically.
The VFD has switching device for resonance point interrupt, which enables the motor to operate avoiding the resonance point and the fan will not surge.
After investigation and comparison, it is decided to use the VFD, produced by Beijing Leader & Harvest Electric Technologies Co., Ltd. The reform scheme with VFD for the duster fans for #4 converterwas worked out through the cooperation of the technical personnel of both parties. The reform scheme is as follows:

2.1 Equipment configuration

KM:VFD power supplied medium-voltage vacuum breaker
  KG1、KG2、KG3:VFD Built-in vacuum contactor provided by the manufacturer
  DJ:Asynchronous motor of 630KW/6KV
  KM Original medium-voltage vacuum switch
  DJ Reserved original asynchronous motor
   It is required that remote and local control can be performed. When the fan operates with high speed, VFD will stop automatically if serious VFD failure occurs. With remote controlled, the motor is switched to operate with the power supply through the Power/VFD controlon the operating console. When the blowing period finishes, service the VFD. After servicing the VFD, the motor return to operating with the VFD through the Power/VFD control on the operating console. With local control, the motor is switched to operate with the power supply through the Power/VFD control on the operating console. When the blowing period finishes, service the VFD. After servicing the VFD, the motor return to operating with the VFD through the Power/VFD control on the operating console.

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