A single phase SRM driven by commercial AC power supply

The 2014 International Power Electronics Conference

A Single Phase SRM Driven by Commercial

AC Power Supply

Kohei Aiso Noriya Nakao Kan Akatsu Shibaura Institute of Technology

Department of Electrical

Engineering and Computer Science

3-7-5, Toyosu, Koto-ku, Tokyo,

Japan

ma 1300 I@shibaura-it.ac.jp Shibaura Institute of Technology Department of Electrical Engineering and Computer Science 3-7-5, Toyosu, Koto-ku, Tokyo, Japan

nb 121 05@shibaura-it.ac.jp Shibaura Institute of Technology Department of Electrical Engineering 3-7-5, Toyosu, Koto-ku, Tokyo, Japan akatsu@shibaura-it.ac.jp

Abstract-This paper presents a novel single phase switched

reluctance motor (SRM) which can be directly driven by

commercial AC power supply without any switching circuit

and sensor. This proposed SRM is optimized based on a

simple design technique. The optimized machine has aimed

to be higher motor efficiency than conventional induction

motors (IMs). In addition, the novel SRM can achieve the

start-up with directly connecting commercial AC power

supply. In this paper, the performance of the prototype

single phase SRM is verified by performing simulations and

experiments.

Keywords-commercial power AC supply, single phase

SRM, SRM designe method, start up charactersitics,

I. INTRODUCTION

Single phase induction motors (IMs) have been widely

used in home appliances such as fans, refrigerators,

washing machines, hoods cooker, pumps and so on

because direct drive by connecting commercial AC

power supply is possible [1]. The manufacturing cost of

their drive system is low because no circuit and sensor is

required. However the motor efficiency is much lower

than other types of electric motors. To reduce the electric

power consumption by electric machines developing high

efficiency single phase motors without any drive circuits

and sensors is an essential requirement in the industrial

field.

From the view point of the efficiency, Permanent

magnet (PM) machines are expected as a candidate of

IMs because they have high motor efficiency. However,

PM motors are more expensive than IMs because of

using rare earth magnets. Furthermore, PM motor drives

generally require the rectifier and switching circuits and

some sensors [2].

In the previous researches, some single phase switched

reluctance motors (SRMs) for industrial applications

have been presented. They have advantages such as low

manufacturing cost and simple structure. In addition,

these machines can achieve higher motor efficiency than

IMs [3]. In conventional single phase SRMs, the numbers

of stator and rotor poles are equal (For example, 2/2, 4/4,

6/6, and 8/8). The driving methodology of the single phase SRMs is that all stator coils are simultaneously excited to generate positive a torque when the stator and the rotor poles are in unaligned position, the excitation current is immediately turned off when the rotor poles are aligned with the stator poles, and the rotor keeps moving due to stored kinetic energy [4]. Consequently, the torque is discontinuous in the conventional SR motor drive. Because of this torque production mechanism, the start­up of single phase SRMs is difficult when rotor and stator poles are completely aligned or the rotor is in a position where negative torques are generated by armature excitation currents. Furthermore, their drives also require rectifier and switching circuits and sensors to implement the current control. To overcome the start-up problem of conventional single phase SRMs, some researches which improve the motor structure have been reported [5]-[7]. In [5], an advanced design method of single phase SRMs has been introduced. This technique gives the geometry parameter calculation process for the optimal machine design to achieve the smooth torque production. In [6], a novel skewed motor structure overcomes the self-starting problem. The arc of rotor pole is changed to produce starting torque. In [7], a saturation-based starting method is used to start up. In the method, the saturable areas such as a simple hole are added on the rotor reduce the flux, it is possible to produce continuous torque at every rotor position and consequently to start by shifting the aligned position with varied current. Meanwhile it has reported that the smooth start-up is possible with a permanent magnet on the stator in [8]. The stator magnet pulls the rotor away from the alignment at stand-still (i.e. the magnet position is adjusted to achieve the maximum torque production at starting). In [9], magnetically insulated attraction elements which are called a shorting ring form a shade pole on the rotor to achieve start-up. If the ring is aligned with the stator pole, a change in stator coil excitation will produce a current in the ring which makes a magnetic flux of producing torque. However, although the above techniques have been discussed on SRM drive system including switching 978-1-4799-2705-0/14/$31.00 ©2014 IEEE 1141

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Drive circuit

Single phase SRM r---------,

A phase

I

c

c

I I L _________ I

" !

Ii'

c

4;

"

II

Fig. 1 Drive system of a novel single phase SRM circuits and sensors, they are unsuited for low-price industry field since the system is complex. In this research, a novel single phase SRM has been proposed[10]. Fig. 1 shows a drive system of a novel single phase SRM. The proposed machine can be driven by directly connecting to commercial AC power supply. To generate the continuous torque and to achieve the smooth start-up, stator poles are sequentially energized by exciting two insulated coils in the proposed SRM drive. Its drive circuit consists of only two diodes and it generates the two-phase excitation current fr? m . commercial AC power supply. Therefore, no sWltchmg device is needed in the circuit and the manufacturing-cost is low. In this paper, the optimal design parameter calculation of the proposed single phase SRM is also presented. The perfo

rmance is verified by performing simulations and experiments. The results show the proposed SRM can be a candidate against the commonly­ used IMs. II. PROPOSED SINGLE PHASE

Fig. 2 Proposed motor structure

TABLET DIMENSION OF THE DESIGNED MOTOR OF SINGLE PHASE

SRM

Number of rotor poles Number of stator poles Stator outer diameter: CPs[mm] Rotor radius: r[mm] Stack length:[([mm] Air gap length: g[mm] Shaft diameter: rsh[mm] Stator pole arc angle: flsi[deg] Slot area: Sf[mm2] Number of turn: N[turn/slot]

6 12 80 15 10 0.3.0.6 10 20. 1 84.2 1442

SRM

As described in the above, typical single phase SRMs deliver the discontinuous shaft torque. Due to the torque production mechanism, the start-up is difficult when rotor and stator poles are completely aligned or the rotor is at a position where the negative torque is generated by excitation currents. To overcome the above problem, a novel motor structure and drive circuit are proposed in this research. A.

system is low because the circuit includes no switching device. C.

Principles of torque generation

The output torque of an SRM is expressed as follow: T=- --- l

1 aL(i,e)

.2

2

ae

(1)

Machine Structure

Fig. 2 shows the proposed motor structure and the design parameters are shown in Table I. The proposed machine has 6 and 12 rotor and stator poles respectively. The armature concentrate windings are wound on each stator pole and the two-phase circuit is formed. The stator poles are sequentially energized by exciting two-phase windings. Therefore, the proposed SRM requires separated two-phase unipolar current. B.

Drive Circuit

As shown in Fig. I, the circuit consists of only two diodes and it rectifies the commercial AC power source into separated two-phase unipolar voltage. The SRM deliver a continuous shaft torque by applying the sequential current excitation and the smooth start-up is possible. In addition, the manufacturing cost of the drive

where T, L, B, and i are the output torque, inductance, rotor position, and phase current, respectively. Then, rotor position e is expressed as follow based on electric angle. (2) where P is pairs of poles, em is mechanical angle. As described in (1), the output torque depends on the self­ inductance variation and the phase current. Therefore, the SRM generates positive torque when the inductance increases. Fig. 3 shows the excitation current and inductance waveform for the proposed single phase SRM. In this paper, the unaligned and aligned positions are defined as shown in this figure (unaligned: 0 degrees, aligned: 90 degrees in the electric angle). An SRM delivers the positive torque in each phase when the rotor position is in the range from 0 to 90 degrees. The proposed SRM can achieve the smooth torque production by applying separated two-phase currents which are shifted 90 degrees each other in the electric angle.

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The 2014 International Power Electronics Conference

r : Negative

tocquc --+ Positive +-tocquc

Rotor

SL<lLor@

Fig. 4 Design flow chart

B. Electrical parameter design U Fig. 3 Output current waveform and inductance waveform ofthe proposed single phase SRM

III. DESIGN METHOD OF SINGLE PHASE SRM

In this section, the design method for the proposed single phase SRM is presented. Fig. 4 shows the design flow chart. As shown in Fig. 4, the design flow chart is categorized into three main parts which are structure parameter design, electrical parameter design and performance evaluation. In the sturucture parameter design part, the detail of the motor structure is determined. In the electrical prameter design part, the magnetomotive force which includes number of turns and current is determined. In the performance evaluation part, the mechanical output, motor efficiency, copper loss of the design motor are calculated and evaluated.

In the single phase SRM driven by the voltage source, the mechanical output and motor efficiency depend on number of turns because output current depends on the inductance distribution and coils resistance which are determined by number of turns. Therefore, the electric parameter design is mainly presented in this section. In the electric parameter design, the keys to establish the efficient machine design are the inductance modeling, and performance calculation.

A. Structure parameter design

In the design of the single phase SRM, there are more than ten design parameters of motor structure such as stator teeth width: Wst, stator yoke length: Y,h, rotor teeth width: Wrt, and so on as shown in Fig. 1. However, their simultaneous design is difficult. To simplify the complexity, the design parameters are considered as a function of two variable parameters: rotor radius: r, stator pole arc angle: {JSI. In the proposed SRM design, these two parameters are simultaneously determined to maximize the motor efficiency. The detail design of structure parameter such as rotor radius, stator pole arc angle is presented in [10]. In the electrical parameter design, the number of turns which achieves maximum motor efficiency is determined. The motor efficiency is calculated by the effective power and mechanical output. Then, inductance modeling is important because the inductance is used to calculate the output current. Therefore, their performance significantly depends on the magnetic permeability distribution in the motor. Generally, the self-inductance depends on number of turns, the position and current (i.e. the inductance is effected by the spatial harmonics and magnetic saturation) and its spatial distribution includes the components of AC and DC as follow: (3) where LTJC and LAC are DC component and AC component. The accurate inductance distribution is obtained by the finite element analysis (FEA). However, the inductance must be calculated in each machine design. Therefore, it takes a long time to implement the design process as shown in Fig. 4 if the inductance calculation is implemented by the FEA. In this paper, since to calculate the current from the inputted voltage is the first objective, it is assumed that the inductance spatial distribution is negligible only to obtain the current. Adding that, the proposed machine has a lot of coils wound on the stator teeth to reduce the current, the inductance variation including spatial harmonics is negligible because the DC component of inductance is much larger than AC component of that (LTJc»LACJ Therefore, inductance is expressed as follow: (4) Also, as shown in Fig. 4, the maximum flux density is limited to a value to ignore the magnetic saturation effect. Therefore, the inductance is expressed as a function of only number of turns.

(5) L=kLn",(Stllm)N2

where SI is cross-section area of stator teeth, 1m is flux 1143

The 2014 International Power Electronics Conference

35 30

25 4.5 20 4

25 20

15 ..8 iil§: 8 800 1000 1200 1400 1600

0 01

3.5

'G u.J

g 15

;>,

3 '5 '5 2.5 2 1.5

~ 800 1000 1200 1400 1600 Number oftums[turn/slot]

10 5 o -- ---- --

if: 10 5 0 NlUnber oftlmls[ltmlis1ot]

. 800 1000 1200 1400 1600

"" u"

0.5 0

Number ofturns[tunlis1ot]

Fig. 5 Motor efficiency for number of turns

Fig.6 Copper loss for number of turns

Fig.7 Mechanical output for number of turns

path length, N is number of turns, krrms is a constant of proportion which are based on the FEA result[10]. C. Perf ormance evaluation

where Te is average torque, Irms is effective current, kTe is a coefficient which is based on the FEA result[10]. Therefore, the copper loss is expressed as follow: (9) The mechanical power is expressed as follow: (10) where P", is the mechanical output, OJ", is mechanical angular velocity. The effective power is expressed as follow: 1

[n the performance evaluation part, the copper loss, mechanical output and motor efficiency are ca[culated and evaluated to determine the optimum number of turns which achieves high motor efficiency. First, the output current and torque are calcu[ated based on inductance expressed as (5) to calculate the mechanical output, copper loss and motor efficiency. Here, the output current is derived from the voltage equation. The voltage equation is expressed as follow when commercial AC power supply is input. V=Ri+L di dt

(6)

where V and R are the voltage amplitude and winding resistance. Generally, it is difficult to solve the equation because the inductance has non-linear characteristic such as spatial harmonics and magnetic saturation. However, (6) can be simply solved in the design method because the inductance can be assumed as a constant in (5). Therefore, the output current is expressed as follow when (5) is substituted into (6).+ (7) v: 0 WL wLe 2 2 2 ( 2 (;(/)= 6 R+ WL) R+ WL)

=

T

where Pe is the effective power, v is single phase voltage. Therefore, the motor efficiency is calculated by (10) and ( 1 1). D. Design result

r v(t)i(fylt

)

(11)

[

+sm wI-tan

.{

-1

( )} R

]

where OJ is electric angu

lar velocity. Therefore the effective current is calculated by (7). The torque of SRM is expressed as (1). Generally, although the average torque is obtained by integrating instantaneous values calculated from (1), it is difficult to accurately calculate the instantaneous values of the output torque by (1) because inductance distributions have non-linear characteristic. However, in the design method, the inductance can be assumed constant (5). Therefore, the average torque is expressed as follow by the production of the inductance (5) and the squared effective current. (8)

Fig. 5 shows the calculated motor efficiencies for the each number of turns. As shown in Fig. 5, the highest motor efficiency 32% is achieved when the number of turn is 1442 turn/slot with the 60% space factor. Fig. 6 and Fig. 7 show the calculated copper loss and mechanical output for each number of turns, respectively. As shown in Figs. 5 and 6, the motor efficiency is increased as increasing the number of turns because the copper loss is decreased. Oppositely, as shown in Fig. 7, the mechanical output increases as the number of turn decreases. Therefore, the motor efficiency and mechanical output are relation of a trade-off. In the paper, as shown in Fig. 1, the number of turns is determined to[442 turn/s[ot to achieve high motor efficiency. Fig. 8 shows a comparison result of the designed motor with a general commercial 1M with shading coil as same motor volume. As shown in Fig. 8, the calculated efficiency is much higher than it of IMs with shading coil.

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Optimising model Commercial 1M with shading coil 35 30

.§<E"'

25 20 15 10

G'

Fig. 8 Efficeincy comparison designed motor and commercial 1M with shading coil

Fig. 9 Prototype single phase SRM Drive circuit

IV.

EXPERIMENTAL RESULTS

In this section, the designed motor performances including starting characteristics, output current, static torque and motor efficiency are evaluated by experiment. Fig. 9 shows a picture of a prototype motor. The motor is manufactured based on the design method as described above section. However, the number of turns of the motor is different from that of the calculated one. The number of turns of the prototype motor is 700 turn/slot because the calculated number of turns cannot be wound as 60% winding space factor because of the manufacturing limitation. The winding space factor of the prototype motor is approximately 26%. A.

Single phase Voltage 100V.50Hz

-1! 8 hase

Fig. 10 Experiment setup for measuring starting characteristics 1000

Start timing

Starting characteristics

Starting characteristics which include starting rotation speed and output torque are verified. Fig. 10 shows an experiment setup to measure starting characteristics. As shown in Fig. 10, the rotation speed and output torque are measured by torque meter (TYPE: TMB304, Rated torque: INm) and these measured values are monitored by PC-based

measuring instrument (WE7000 produced by Yokogawa electric corporation) which synchronizes with the time when commercial AC power supply is inputted. Also, the position sensor is used only for measuring the rotation speed, the speed is not applied to the SRM drive. A fan is used as a load of the motor. Fig. 11 shows the starting rotation speed and Fig. 12 shows the starting output torque. Start timing is the time when the commercial AC power is supplied. As shown in Figs. 11 and 12, the prototype motor can start up by supplying commercial AC power supply. Also, the rotation speed is converged to synchronous speed; 500rpm. B.

-0"" 0VJ

E E

500

o o 2 4 6 10 12 14

Time[s]

Fig. 1 1 Starting rotation speed 0.15 Start liming 0.1

E 6 0)

0.05

0

f-- -0.05

Current and voltage of each phase

The current and voltage are measured by PC-based measuring instrument (WE7000). Fig. 13 and 14 show the current waveforms and the voltage waveforms of each phase, respectively. It is confirmed that the experimental output current is rectified in each phase by the additional diode.

-0.1

-0.15 o 2 4 6 10 12 14

Time[s]

Fig. 12 Starting torque

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