A Line-Fed Permanent Magnet Motor Solution for Drum-Motor and Conveyor-Roller Applications

Transcription

1 A Line-Fed Permanent Magnet Motor Solution for Drum-Motor and Conveyor-Roller Applications M. Popescu D. A. Staton Motor Design Ltd. Ellesmere, U.K. S. Jennings 1 J. Schnuettgen 1 T. Barucki 2 1- Interroll Trommelmotoren GmbH Wassenberg, Germany 2 Adapted Solutions GmbH Chemnitz, Germany Abstract This paper describes a new solution for drum motors and conveyor roller applications. The specially designed 3-phase line-fed permanent magnet (LFPM) motor ensures a self-starting capability is combined with a high efficiency and operation at constant speed when the load varies. I. INTRODUCTION The drum motor concept was first produced specifically for conveyor belt applications [1]. The idea is to produce a compact, totally enclosed single component drive unit with high efficiency and lower frictional losses than a conventional geared motor. The motor is fixed to a stationary shaft at one end of the drum and directly coupled through the motor s rotor pinion to an in-line helical or planetary gearbox which is fixed to the other stationary shaft. The torque is transferred from the motor via the gearbox to the drum shell through a coupling or geared rim attached to the shell or end housing (see Figure 1). The design is quick and easy to install, requires no maintenance and because of its totally enclosed hermetically sealed design is not affected by dust, dirt grease or water. There are many examples of the modern day drum motor in airport checks, in conveyors and security machines, supermarket check-outs, food processing conveyors and weighing equipment. Single or 3-phase induction motors are normally used as drum motors. A recent alternative are brushless synchronous permanent magnet motors. All 3 phase motors can be used together with a variable frequency converter drive. This paper describes a new solution for drum motors, which is a line-fed permanent magnet motor. II. LINE-FED PERMANENT MAGNET MOTOR SOLUTION A. Motivation The drum motors are required to operate continuously whilst the temperature of the housing shell has to be below an acceptable limit according to the application s environment. For example, in a food manufacturing unit the outer shell surface cannot exceed 30 0 C. The main cooling method for drum rollers is filling the space between the electric motor elements and the shell housing with mineral oil. The drum motors are quasi-generally equipped with induction motors. This solution is robust and easy to maintain, but the operation at variable load leads to variable speed and lower efficiency. For reference it is considered the induction motor with parameters given in Table I. TABLE I. Figure 1 A drum-motor for conveyer roller REFERENCE INDUCTION MOTOR, 50HZ, 2-POLES, 400V PARAMETERS Parameter Value Stator OD [p.u] 1 Axial active length [p.u] 1 Output rated power [W] 370 Rated current [Arms] 1.91 Rated speed [rpm] 2826 Rated torque [Nm] 1 Efficiency [%] 62 Power factor [p.u.] 0.79

2 TABLE II. LINE FED PERMANENT MAGNET MOTOR, 50HZ, 2-POLES, 400V PARAMETERS Parameter Value Stator OD [p.u] 1.05 Axial active length [p.u] 0.54 Output rated power [W] 370 Rated current [Arms] 0.9 Rated speed [rpm] 3000 Rated torque [Nm] 1 Efficiency [%] 84 Power factor [p.u.] 0.82 Figure 2 Stator and rotor assembly for the newly developed line-fed permanent magnet motor Figure 3 Stand test for line-fed permanent magnet motor From Table I, one can notice the low efficiency which is characteristic for drum rollers equipped with induction motor. Line-fed permanent magnet motors represent a solution that would maintain the starting capability and robustness of the induction motors, while a higher efficiency and operation at synchronous speed is achieved [2,3]. B. Line-fed permanent magnet motor (LFPM) design The newly developed LFPM motor is designed to replace the equivalent induction motor described in Table I. A stator lamination with 24 slots available from the standard production of equivalent power induction motors is used. The prototype is a 400V, 50Hz, 2-pole motor. The rated torque is 1Nm, while the efficiency target is over 80%. LFPM motor has to start similarly to an induction motor due to the presence of the cage rotor. This is a die-cast aluminum type rotor with 18 non-skewed open bars. The rotor is equipped with embedded NdFeB type magnets placed in a V-shape polar configuration. The pole arc is limited to 120 electrical degrees to minimized higher order MMF harmonics effect. Magnets can be inserted in the rotor body after die-casting or can be magnetized in situ. Stator winding is a single-layer, concentrically equal type with two coils per pole and phase (see Figure 2). Figure 3 illustrates the settings for the stand test. Figure 4 shows the motor radial cross-section. The winding distribution per phase uses two concentric equal coils per pole and phase and is of single layer type (See Figure 5). This ensure a high fundamental winding factor (kw 1 = ). In Figure 6 the MMF harmonics content is presented. Due to the magnet pole arc value, the effect of higher order space harmonics, i.e. 5 th and 7 th is minimized. Total number of turns per phase is optimized considering two criteria: maximum efficiency at synchronous speed operation and maintaining the starting capabilities. The rated performance of the LFPM motor is given in Table II. A finite-element analysis was used to study the effect of saturation over the motor performance at 50Hz, 400Vrms. The V-shape polar arc configuration from Figures 2 and 4, will lead to a low reluctance torque component, i.e. the ratio between d-q axis inductances is low. Thus, the newly designed LFPM motor will get into synchronism mainly due to the excitation torque, given by the interaction between magnet flux and stator currents. Figure 7 shows the flux lines and the flux-density level for the LFPM motor operating at stall conditions. Most of the rotor magnets flux is short-circuited with minimal impact on the starting torque and current value. Figure 8 shows the flux lines and fluxdensity level for the LFPM motor operating at suprasynchronous speed, i.e. 3500rpm during start-up period. Most of the rotor magnets flux in linking with the stator winding. Similarly, in Figure 9, the flux lines and the fluxdensity levels are plotted for synchronous speed operation under load (1Nm). Figures 10 and 11 show the estimated rotor speed variation during start-up and torque variation during start-up respectively. Back EMF

3 Figure 13 show the phase current and efficiency variation with torque considering steady-state conditions. Estimated pull-out torque is approximately 2.4Nm. Corresponding phase current for pull-out torque is 1.91Arms. Figure 7 Flux lines and flux-density distribution for LFPM motor operating at zero speed (transient start-up after 0.02sec) Figure 4 Cross-section of the newly developed LFPM motor Figure 8 Flux lines and flux-density distribution for LFPM motor operating at supra-synchronous speed (transient start-up after 0.22 sec) Figure 5 Phase winding distribution for the LFPM motor Figure 6 Space harmonics content for LFPM motor Figure 9 Flux lines and flux-density distribution for LFPM motor operating at synchronous speed (transient start-up after 0.4sec)

4 Figure 10 Calculated synchronization performance of the LFPM motor for start-up under load (1Nm) Figure 11 Calculated instantaneous torque performance of the LFPM motor for start-up under load (1Nm) Figure 12 Calculated line-line back EMF at 3000rpm and 20 0 C for the LFPM motor C. Experimental results A special test stand (Figure 3) was developed for the measurement of the performance for a drum roller equipped with LFPM motor. It was investigated the electromechanic and thermal behavior of the LFPM motor for variable loads (0 to 1.4Nm) and variable frequency values (50Hz to 100Hz). Results are presented in Figures 14 to 18. It is interesting to observe that at 50Hz (Figure 14), the phase current experiences a relatively minor variation at lower load levels. Figure 13 Calculated phase current and efficiency at synchronous speed (50Hz) for newly developed LFPM motor The torque increase is due to the reluctance torque component. At higher load level, i.e. over 0.5Nm, the saturation will lead to decreased inductance levels and thus the output torque will increase due to higher absorbed current. The newly designed 2-poles LFPM motor has a tendency to behave like a DC motor at higher load and frequency levels, i.e. torque varies linearly with current. When frequency is increased to 60Hz, the back EMF and d-q axis reactances increase with the same ratio. Hence, the reluctance torque component contribution to the total output torque is diminished. Output torque continues to vary approximately linearly with current starting for loads over 0.5Nm. Further increase of the frequency (over 60Hz) leads to further minimization of the variable reluctance effect and the torque will vary linearly with current for the whole range of the load variation. One should note that the maximum phase current level is constant for all frequency levels, i.e. 1Arms at 1.4Nm load. Efficiency is maximized for the rated load (1Nm) when frequency is close to 60Hz. This corresponds to a ratio between the back EMF and phase voltage of 0.80 when the absorbed current required for the rated torque has a minimum value. For higher frequencies the efficiency decreases and this is due to the increased iron losses. The power factor is increasing when frequency is increasing Thermal experimental results are given for bearings, endwinding and housing temperatures. From Figures 14 to 18, one should note that all measured temperatures: bearings (T1), end-winding (T2 and T4) and housing (T3) are increasing when frequency is increasing. However, in all cases temperatures are not exceeding 70 0 C. The thermal effect of the oil s presence in the cavity between the stator housing and drum shell is given in Table III. The cooling effect of the mixture air-oil has an optimum value for which the temperature of the stator winding reaches a minimum, while the temperature of the outer surface of the drum shell is maintained constant. With reference to the tested drum roller, the optimum quantity of oil is 20% of the drum s inner volume. Higher oil quantity will lead through friction losses and increased conductivity to higher temperatures of the stator winding and drum shell.

5 (a) (a) Figure 14 Measured performances of line-fed permanent magnet motor at 50Hz (a) power and current; efficiency and power factor; Figure 15 Measured performances of line-fed permanent magnet motor at 60Hz (a) power and current; efficiency and power factor;

6 (a) (a) Figure 16 Measured performances of line-fed permanent magnet motor at 70Hz (a) power and current; efficiency and power factor; Figure 17 Measured performances of line-fed permanent magnet motor at 80Hz (a) power and current; efficiency and power factor;

7 (a) Figure 19 Thermal image of a drum roller with 75% volume filled with oil Figure 20 Thermal image of a drum roller with 7.5% volume filled with oil Figure 18 Measured performances of line-fed permanent magnet motor at 100Hz (a) power and current; efficiency and power factor; TABLE III. THERMAL MEASURED RESULTS FOR DRUM ROLLER EQUIPPED WITH LFPM MOTOR, 0.37KW, 50HZ, 2-POLES, 400V, 0.9ARMS Quantity of oil inside the drum roller [p.u. volume] Winding temperature [ 0 C] Outer drum surface temperature [ 0 C]

8 Figures 19 and 20 show thermal images of two different drum rollers with different level of oil inside their inner cavities, 75% filled with oil and 7.5% filled with oil respectively. III. CONCLUSIONS A new solution for drum motors and conveyor rollers is proposed by using a newly designed line-fed permanent magnet motor. This solution leads to a significantly higher efficiency compared to an equivalent induction motor and material savings. The motor performance is investigated for variable frequency values. Thermal behaviour of the drum roller is analysed. REFERENCES [1] E. Drexler, Conveyer Roller, US Patent # 1,889,173, 1932 [2] Miller TJE, Popescu M, Cossar C, McGilp MI, Strappazzon G, Trivillin N, Santarossa R. Line Start Permanent Magnet Motor: Single-Phase Steady-State Performance Analysis IEEE Trans. on Ind. Appl. Vol. 40, No. 2, March/April 2004, pp [3] Honsinger, V.B. Performance of polyphase permanent magnet machines, IEEE Trans. Power Appl. Syst., vol. 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