A Line-Fed Permanent Magnet Motor Solution for Drum-Motor and Conveyor-Roller Applications · 2016-12-16 · A Line-Fed Permanent Magnet Motor Solution for Drum-Motor and Conveyor-Roller

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. Jennings1 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 0C. 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. 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

Figure 1 A drum-motor for conveyer roller

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

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

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 (kw1 = 0.9577). 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. 5th

and 7th

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 flux-

density level for the LFPM motor operating at supra-

synchronous 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 flux-

density 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

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 4 Cross-section of the newly developed LFPM motor

Figure 5 Phase winding distribution for the LFPM motor

Figure 6 Space harmonics content for LFPM motor

Figure 7 Flux lines and flux-density distribution for LFPM motor operating at zero speed (transient start-up after 0.02sec)

Figure 8 Flux lines and flux-density distribution for LFPM motor operating at supra-synchronous speed (transient start-up after 0.22 sec)

Figure 9 Flux lines and flux-density distribution for LFPM motor operating at synchronous speed (transient start-up after 0.4sec)

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 200C 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, end-

winding 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 700C. 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.

(a)

(b)

(c)

Figure 14 Measured performances of line-fed permanent magnet motor at 50Hz (a) – power and current; (b) – efficiency and power factor; (c) –

bearings (T1) end-winding (T2,T4) and housing temperature (T3)

(a)

(b)

(c)



(a)

(b)

(c)



(a)

(b)

(c)



(a)

(b)

(c)



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

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 [0C]

Outer drum

surface

temperature [0C]

0 78.5 53.7

0.083 77.1 55.6

0.125 72 54.7

0.166 70 54

0.20 68 53.3

0.25 70 54.3

0.33 78.6 66.3

0.5 78.6 68

0.75 75.7 68

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.

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A Line-Fed Permanent Magnet Motor Solution for Drum-Motor and Conveyor-Roller Applications · 2016-12-16 · A Line-Fed Permanent Magnet Motor Solution for Drum-Motor and Conveyor-Roller