Tian-ye Yu Zhi-hua Zhang

(1.Tianjin Emaging Technology Co.,Ltd,2.Esurging Tianjin Technology Co.,Ltd)

Abstract：Due to advantages of zero friction and none lubrication of magnetic bearing,it becomes one of the best choice for turbo blower.Professional standards and industrial demand of magnetic levitation blower highly request the magnetic bearing designs.According to the blower’s characteristics,a 5 degrees of freedom active control magnetic bearing is designed,dynamics of rotor is analyzed through rotor dynamics calculation,accurate systems model is obtained through system recognition,and a good control is achieved through active control.The test shows that highspeed turbo blower with magnetic bearing could meet completely industrial demand.

Keywords：AMB,Magnetic Bearing,Magnetic Levitation,Turbo-Blower

Nomenclature

M distributed mass matrix chord

G gyroscopic matrix

K stress-displacement factor matrix

ZB=[x1,x2,x3,x4,x5]Tdisplacement vector

UC=[Ic1,Ic2,Ic3,Ic4,Ic5]Tcontrol current vector

R unit matrix

0 Introduction

As general machinery,blower is widely applied in sewage treatment,steel smelting,petrochemical engineering,food fermentations,etc.Traditional blowers include roots blower,multistage centrifugal blower,single stage gear speed increasing centrifugal blower,etc.For magnetic levitation turbo blower,gear-box and lubrication system have been removed,instead high-speed motor drives directly centrifugal impeller,by this way transmission efficiency is high and loss is low,so it is recognized as the most advanced blower in the world,whose efficiency is 30%higher than roots blower,20%higher than multistage centrifugal blower[1],and 10%higher than single stage gear speeding increasing blower.Gearbox speed increasing centrifugal blower is widely used in big sewage treatment factories,the power is usually more than 710kW,and leading examples are HV-TURBO and Howden.At present,the biggest commercialized magnetic levitation blower is from ABS of Finland,its power is 400kW,and is still far from the single stage gearbox increasing blower’s power level,so it can’t replace completely those traditional high energy consumption blowers.Therefor it is quite necessary to develop 700kW high power magnetic levitation turbo blower to replace gearbox speed increasing centrifugal blower.

1 System Demand of High Power Magnetic Levitation Turbo Blower

Magnetic levitation turbo blower consists of high-speed permanent magnetic motor,magnetic bearing,turbo blower,inverter,etc.Surface-mounted structure is applied in rotor of high-speed permanent magnetic motor,and carbon fiber is used for sleeves.In compare with non-magnetic material,carbon fiber has higher strength,which could reach more than 1 600 MPa,meanwhile it could effectively reduce rotor’s eddy-current loss,inhibit overheating,and avoid demagnetization of permanent magnet material,and hence it is an ideal sleeve material for permanent magnetic motor’s rotor[2].Together with inductive sensor,active electromagnetic structure is applied in magnetic bearing;unshrouded titanium alloy impeller is used in turbo blower,and connects directly with motor rotor.Development of the current product is oriented the design parameters are:flow rate:270 m3/min;pressure ratio 2.4.According to the aerodynamic parameters,blower power is calculated,which is 700kW,highest rotational speed is 18 000rpm,axial force of magnetic bearing is more than 4653N.

2 Structure Design and Rotor Dynamics Calculation

Figure 1 is the main structure of 700kW magnetic levitation turbo blower,clearly visible,and there is no component like scroll showed in this figure.Inside it is a five-axis magnetic levitation system that includes high-speed motor.Impeller is installed directly at the end of the rotor.To increase the strength,and guarantee the security under high-speed rotation,titanium alloy and monolithic machining is applied for impeller.

Figure 2 is the structure of rotor,which includes the main magnetic bearing and the connected impeller.

Fig.1 Magnetic levitation turbo blower structure

Fig.2 Components of rotor

Fig.3 Rotor dynamics result

In practical modeling calculation,rotor model is simplified:blower impeller and motor rotor’s magnetic steel is simplified to lumped mass,and magnetic bearing supporting is simplified to spring-damper system.When the bearing stiffness is set up,only radial stiffness is considered,axial stiffness is ignored.[3]According to experience,supporting stiffness is set up as 1×106N/m.

After rotor dynamic calculation,the first bend mode is 416Hz,which means this rotor is a rigid rotor,and over-critical complicated calculation could be avoid[4].Meanwhile isolation margin could reach 39%and meet API617 standard requirement.

3 Magnetic Levitation Controller Design and System Identification

According to the system requirement,firstly select matrixQwith relatively low rigidity,when it is 300Hz,feedback matrix is obtained,and then preliminary static suspension could be achieved,further based on this,system could be identified,figure 3 is the identification result.From the figure,it could be found that the first bend mode frequency of this static rotor is 428Hz,the error is only 2.8%in compare with the theoretically calculated characteristic frequency.

Fig.4 Radial open-loop identification bode figure(impeller included)

Fig.5 Blower test rig

Fig.6 Blower performance curve

4 Magnetic Levitation Turbo Blower Test Result

There is several evaluation indexes to turbo blower’s characteristics in international standard ASME PTC 10-1997,in which flow rate,pressure ratio,efficiency,etc.are the most important indexes.After 2 hours continuous operation and reaching stable working condition,blower is tested according to standard,figure 6 is flow rate-pressure curve,both flow rate and pressure ratio have reached the designed parameters,beside polytrophic efficiency of nominal working condition is 86.7%.

5 Conclusions

Design of the 700kW magnetic levitation turbo blower mentioned in the present article is based on blower’s aerodynamic requirement,a magnetic levitation high-speed permanent magnetic motor system is specially applied.In this system,the rotor is suspended by the magnetic bearing,and impeller is connected directly with the rotor.According to rotor dynamic calculation and system identification,rotor of this blower is a rigid rotor under working condition,and the error is only 2.8%.The mentioned 700kW magnetic levitation high-speed turbo blower could replace traditional gearbox speed increasing blower,simplify system structure and increase blower’s entire efficiency.Higher power of this kind of blower is reached.According to test,the blower meets all respective standard,and could satisfy air flow requirement of those big sewage treatment factories.