Seamless Star-Delta Winding Changeover Circuit
for AC Generators
Jun-ichi Itoh, Kazuma Tanimukai
Nagaoka University of Technology
Nagaoka, Japan
itoh@vos.nagaokaut.ac.jp, kazuma_tanimukai@stn.nagaokaut.ac.jp

Abstract— This paper proposes a circuit, which can change thewinding connection of a generator from the star connection modeto the delta connection mode by changing the circuit operation.
In the conventional method, the outage time exits during thedead-time of two magnet contactors. Therefore, the generator
current cannot maintain its continuity. Moreover, the surge
voltage occurs in the terminal of the generator. In order toachieve the seamless winding changeover, this paper proposes a
new circuit, which consists of a PWM rectifier, a diode bridge
rectifier and three additional switches. From the simulation
results, the transition from the star connection mode to the deltaconnection mode, and the continuous generator current during
the transition time are confirmed. In addition, it is confirmedthat the current THD can reduce by supplying the reactive
current with the generator of the delta connection mode. Whenthe generator has percent reactance of 30%, the current THD is
improved from 19% to 3%.
Keywords-component; Permanent magnet synchronous
generator (PMSG); Winding changeover ;Hybrid bridge rectifier
circuitI.

INTRODUCTION

In recent years, the permanent magnet synchronousmachines have attracted many attentions in the engine
generator, the wind turbine system, or the micro-hydro power
plants and more applications due to its high efficiency. In theseapplications, the permanent magnet machines are driven atwide speed range [1]. If the back electromotive force (EMF) is
designed to be higher in order to achieve high efficiency at low
speed range, it is necessary to apply field weakening control to
the PWM rectifier at high speed range. It causes the increasesof the copper loss because the d-axis current increases at highspeed range. In contrast, if the back EMF is designed so thatfield weakening control is not required at high speed range,
efficiency at low speed range becomes low. Therefore, it isdifficult to achieve high efficiency at wide speed range.
Stator winding changeover methods have been proposed inorder to solve this problem [2-5]. In past works, the star-delta
winding changeover methods using magnetic contactors havebeen widely studied and used [6-7]. However, the dead-time
between two magnetic contactors is necessary in order toprevent the short circuit of the generator during the transition.

Moreover, dead-time causes the outage time and an
instantaneous voltage drop of the DC output voltage. Therefore,a large capacity electrolytic capacitor is necessary. It is difficult
to reduce the total volume because magnetic contactors are also
large.
On the other hand, a seamless changeover method whichchanges the windings connection from the star connection to
the delta connection by the mode transition without the outage
time is presented [8-9]. However, this circuit consists of thetwo PWM rectifiers and two additional switches, which uses
for mode transition, i.e. many of switching devices are required[10]. Therefore, the system becomes expensive.
This paper proposes a winding changeover circuit, which is
specialized to the generator operation. The number of theswitches in the proposed circuit is reduced by applying theopen-winding half controlled converter topology [11-12]. This
paper is organized as follows; first, the fundamental operationof the proposed circuit is described; second, the comparison
between the proposed circuit and the conventional method
which uses two magnetic contactors are conducted; finally, theincrease of the generator current corresponding to thesynchronous reactance is evaluated by the simulation.
II.

OPERATION MODE OF PROPOSE CIRCUIT

A. Conventional method
Fig. 1 shows the conventional star-delta changeover circuit.
The method as shown in Fig. 1(a) changes the stator winding
connection of the generator from the star connection to thedelta connection using two magnetic contactors. This method
has been widely studied and used due to its simplicity.
However, in order to prevent the short circuit of the generator,
the outage time is necessary during the mode transition. It
causes an instantaneous voltage drop of the DC output voltage.
On the other hand, a generator inductance can be used as boost
inductor of the PWM rectifier. The star-delta winding
changeover circuit by changing the operation mode as shownin Fig. 1(b) achieves a seamless transient. The seamless
transient leads to the star-delta changeover which maintains thecontinuity of the generator current without the outage time.
However, this circuit consists of many switches.

Magnetic
contuctor

Load

1/2VdcPWM Rectifier 1/2Vdc

Diode bridge rectifier

LoadGenerator
D1

D3

D5

+

+

+

+
D2

Generator

D6-1/2Vdc-1/2Vdc(a) Conventional method using two magnetic contactors.
Diode bridge rectifier

D4

S1 PWM Rectifier 1/2Vdc

Load(a) Star-connection mode which generator has a neutral
point.
PWM Rectifier 1/2Vdc

Diode bridge rectifier

Generator

Generator
D1

+

D3

D5

+

+

+
D2

S2

S1

D4

D6

-1/2Vdc-1/2Vdc(b) Conventional method by changing operation mode [8].
Fig.1 Conventional star-delta connection winding changeover
circuits, which use generator reactance as boost inductor.
Diode bridge rectifier

PWM Rectifier 1/2Vdc(b) Delta-connection mode which consists of three hybrid
bridge rectifier circuit.
Fig.3. Equivalent circuits of proposed circuit configuration.
i*
0 d (star)

Load

x
Vdc*
+ － AVR

Generator
D1

D3

D5

+

D2

vu*(star)+－id*(delta)
+
iq* －

Vdc

+

S3
D4

id

D6

 

a    2Laiq
2
a

S2

Load

-1/2Vdc

Fig.2. Proposed star-delta connection winding changeover
circuit, which uses generator reactance as boost inductor.
B. Proposed circuit
Fig.2 shows the proposed generator circuit which consistsof a PWM rectifier, a diode bridge rectifier and three additionalswitches. The additional switches are used to change theoperation mode. This circuit also achieves the seamless
winding changeover. Furthermore, this circuit reduces thenumber of the switches compared to conventional circuit
shown in Fig.1 (b). The operation of proposed circuit isdescribed as bellow.
Fig. 3 (a) shows the equivalent circuit of the starconnection mode. In the star connection mode, switch S3 areturned on and creates a neutral point in conjunction with dioderectifier. The DC output voltage is obtained by the operation of

2La

2

iq

ACR

dq

ACR

uvw

3

3

vv*(star)vw*(star)+
3 + +vu*(delta)+ + vv*(delta)+ vw*(delta)
vD2
vD4
vD6
uvw
iu
iv
iw
dq

q

Fig.4. Control block diagram of proposed circuit.
the boost type PWM rectifier. Note that, switches S1 and S2
should be turned off to avoid the short circuit between theneutral point and the DC bus. The output voltage of thegenerator is the line-line voltage of the back EMF, when thestar connection mode.
Fig. 3 (b) shows the equivalent circuit of the deltaconnection mode. The DC bus of the diode bridge rectifier and
the PWM rectifier are connected by turning on the switches S1and S2. Note that, switch S3 has to be turned off. In case of thedelta connection mode, the circuit is equivalent to three single
phase hybrid bridge rectifiers in parallel as shown in Fig.3 (b).
Thus, output voltage of generator is the phase voltage of back
EMF.

III.

CONTROL METHOD OF PROPOSED CIRCUIT

Load

Fig. 4 shows the control block diagram of the proposed
circuit. A control system consists of the automatic current
regulator (ACR) and automatic voltage regulator (AVR) basedon a rotational d-q frame. The ACR and AVR are implemented
to control the generator current and the DC output voltage,
respectively. A field-oriented control assigns the magnet fluxof the generator, i.e. back EMF is assigned into the q-axis.
Therefore, the d-axis current means the reactive current and theq-axis current means the active current respectively. The AVR
decides the q-axis current shown in Fig.4 In the star connectionmode, id* should be zero to increase the power factor. However,in the delta connection mode, the d-axis current command id*
should not be zero since the current of the generator have somuch harmonic distortion that the circuit cannot drive. The daxis current with generator is required.
Fig. 5 and 6 show the circuit configuration of the single
phase hybrid bridge rectifier and the terminal voltages of thesingle phase hybrid bridge rectifier, respectively. If the backEMF is the sinusoidal waveform, a phase voltage of the dioderectifier vd is the reversed rectangular waveform. Then, thephase voltage of the PWM rectifier vu is the sum of theterminal voltage of the phase voltage of the diode rectifier vd,
the back EMF of the generator vs, and the inductor voltage asshown in 5 and 6. It is understood that the phase voltage of the
PWM rectifier is non-linear. However, the voltage command of
the PWM rectifier vu* is the sinusoidal waveform. Although thevoltage command of PWM rectifier has to be compensated by
the phase voltage of the diode rectifier vd as shown in Fig. 4.
Fig.7 shows the vector diagram of the general hybrid bridge
rectifier circuit and the delta connection mode of the proposed
circuit. The input current of the hybrid bridge rectifier circuit ig
is generally controlled to obtain unity power factor from theview point of the input voltage of the hybrid bridge rectifier
circuit vs. However, the input current ig has to be controlled toobtain unity power factor from the view point of the terminal
voltage of the generator vg due to a large generator reactance. A
phase difference exists between the terminal voltage of thegenerator and back EMF of the generator owing to the voltage
drop of the synchronous reactance as shown in Fig. 7. The daxis current id when the delta connection mode is have to fulfill
id/iq=vd/vq. Although, the d-axis current command id* has to beprovided as following [11].

 id   a   a  2 La iq
2

*



2

2 La





where, a is the flux linkage produced by permanent magnet.
From (1) and q-axis current iq, the generator current on thedelta connection mode can be calculated. It is similar to d-axis
current with field weakening control, the command id* shownin (2).

id   a  VOM   La iq
2

*

La



2





Inductor voltage+

vu
Back EMF ofgenerator, vs

ig

+

vg
vd

Fig. 5 Single phase hybrid bridge rectifier circuit(i) Back EMF of generator, vs(ii) A Phase voltage of diode rectifier, vd

1/2Edc(iii) Phase voltage of PWM rectifier, vu-1/2Edc

Fig. 6 Voltages of single phase hybrid bridge rectifier
circuit.
qq

eLaigvg

eLaig

vs
ig

d’

vs

vg
d

ig

d

iq
d

id

(a) General hybrid bridge(b) Proposed circuitrectifier circuit
Fig.7. Comparison of vector diagrams of hybrid bridge
rectifier circuit.
where,  and VOM are electric angler frequency and the rated
value of terminal voltage of the rectifier, respectively.
IV.

COMPARISON RESULT OF PROPOSED SYTEM AND
CONVENTIONAL SYTEM

A. Simulation condition
Table 1 shows the simulation parameters. The carrier
frequency of the PWM rectifier is 15 kHz and the naturalangular frequency of the ACR and AVR are set to 4000 rad/s,
400 rad/s respectively. The dead-time between the magnetic
contactors is 2 milliseconds and the dead time between the
additional switches of the delta connection mode (S1, S2) andthe additional switch of the star connection mode (S3) is 2 s. It
is considered that the cylindrical machine is applied withproposed circuit. A synchronous inductance is 1 mH (6.28% on
the percent reactance). The comparison between theconventional circuit and the proposed one is described as
bellow.
B. Steady state characteristics
Fig 8 and 9 show the simulation results of the conventional
circuit and the proposed circuit respectively. The simulation
results are divided into following sectors (i) is the starconnection mode (ii) is the delta connection mode. The stator
windings are changed from the star connection mode to thedelta connection mode at 0.6 s. As results, the output voltage ofthe generator is changed from 173 V (line-line voltage) to 100
V (phase voltage) with the stator winding configuration asshown in Fig 8 and 9. Nevertheless the input voltage ischanged, the output voltage remains 350 V i.e. it indicates thatthe output AVR works correctly. The operation of the winding
changeover action is confirmed. In the proposed circuit, theline current is equals to the phase current in the bothconnection modes since the delta connection mode differs fromgeneral delta connection. Hence, in this paper the phase current
is discussed in spite of the delta connection mode. The electric
power and the amplitude of the generator current remain atconstant in spite of the changing of the back EMF. Moreover,the generator current is controlled to achieve unity power factor
from the view point of the back EMF on the conventional
circuit. On the other hand, the generator current is controlled toachieve unity power factor in phase from the view point of theterminal voltage of the generator in the proposed circuit.
C. Transient chractoristics
In the conventional circuit, the outage time exits during thedead time of magnetic contactors. Therefore, the DC output
voltage is dropped by 18 V (5%) as shown in Fig 7. At the endof the outage time, the generator current exceeds 12 A owing to
the voltage drop. The outage time is caused by thedisappearance of the current paths while the transient.
Therefore, in order to reduce the influence of the voltage drop,a large capacity electrolytic capacitor is required. In contrast,
the propose circuit can achieve seamless transient as shown in
Fig. 8. During the transient state, the generator current is
supplied by the freewheeling diodes of S1 and S2. However, thegenerator current increases the reactive current as shown in (2).
D. Comparison results with field weakening control
Fig. 10 shows the comparison results of d-axis current of
the generator. With proposed circuit, the field weakening
control is not required due to the terminal voltage becomes
0.577 times by mode transition. The d-axis current of proposed
circuit, which uses to reduce the current THD is calculated by(1). On the other hand, with field weakening control, theterminal voltage becomes constant by supplying the negative
the d-axis current calculated by (2). In the condition, q-axis
current decrease with increasing of the rotational speed due tothe output voltage of the circuit is set to constant value (350 V).
At the field weakening region, the d-axis current of field
weakening control increases against the increasing of therotational speed. In contrast, with proposed circuit, d-axis
current is decrease with the rotational speed. However, less

Table 1. Simulation parameters.
Inductance of generator, La

1 mH

Synchronous Reactance, %Xa

6.28%

Back EMF of Generator,Vac

100 Vpeak

Frequency of Back EMF, femf

100 HzCarrier Frequency, fsw

15 kHzOutput Voltage,Vout

350 V

Response angler frequency of ACR

4000 rad/sec

Response angler frequency of AVR

400 rad/sec

Output voltage of the generator (LPF: fc=1 kHz) [V]
173 V
100 V

(i) Star connection(ii) Delta connection

Back EMF of the generator [V]Current of the generator [A]12 A

Output Voltage of the rectifier [V]Dead-time of magnetic
contactors (2 millisecond)

Voltage drop

[10 milliseconds/div]

Fig.8. Simulation result of winding changeover using
magnetic contactors.
Output voltage of the generator (LPF: fc=1 kHz) [V]
173 V
100 V

(i) Star connection(ii) Delta connection
Back EMF of the generator [V]Current of the generator [A]Output Voltage of the rectifier [V][10 milliseconds/div]

Fig.9. Simulation result of winding changeover using
proposed circuit.

Fig. 12 shows the comparison results about the RMS valueof the generator current. The generator current RMS value isalso compared when id = 0 and when supplying the reactive
current with the high percent reactance %Xa characteristics. The
RMS value of generator current is measured at two conditions;(i) rated torque, (ii) 50% torque. The torque is proportionalwith the value of the active current iq. Note that, the rotational
speed and the active power are also constant. Thus, when id = 0,
the generator current is constant for %Xa. In the generator has
30% of the reactance, the generator current on the deltaconnection mode increases 5% compared with that of the starconnection mode as shown in Fig. 12. The simulation results
are in good agreement with the calculated values in the
low %Xa area. On the other hand, in the high %Xa area, the error
becomes bigger. The error is caused by the increases of the
distortion components.
V.

EXPERIMENTAL RESULTThis chapter evaluates the fundamental operation of the
proposed circuit when the star connection mode by experiment.
Fig. 13 shows the experimental result of the proposed
circuit when the star connection mode. In this experiment, thegrid voltage and inductors, which have a percent reactance of
8.97% (in case of rated frequency is 50 Hz) are used instead of
the generator. Similarly, the DC output is connected to thepower supply, which maintains an output voltage of 350 V. In
this experiment, the ACR based on d-q flame is only
implemented. The electrical angle q, which is used for the d-qtransformation is calculated by grid voltage. As shown in Fig.
13, the input current and the phase voltage of the PWM
rectifier are sinusoidal. The proposed system achieves unity

D-axis current, id [A]With proposed
circuit-5
With field
weakening control-10field weakening
region

-15
2000

3000

4000

5000

6000

Rotational speed, N [rpm]Fig.10. the comparison results of d-axis current of thegenerator with the proposed circuit and the field weakening
control.
30
Generator current THD [%]E. Effect of the synchronous reactanceFig.11 shows the comparison results of Total Harmonic
Distortion (THD) in the generator current. The generator
current THD is compared when id = 0 and when supplying thereactive current. The generator, which has the percent
reactance of the synchronous reactance %Xa, is used. In thiscondition, %Xa was changed from 3% to 40%. The generator
current THD is yielded on simulation and it contains 40th orderharmonics of the electric frequency of 100 Hz. When id = 0,
THD of the generator current increases in proportion tosquared %Xa as shown in Fig. 11. Thus, when the generator has
30% of the percent reactance, THD of the generator current
reach 19%. Owing to the increase in THD of the generator
current, increase of generator loss, noise and torque ripple isunavoidable. It is possible to reduce THD of the generator
current by supplying the reactive current to the generator. In
the generator has 30% reactance, THD of the generator current
are reduced from 19% to 3%.

020

When id=010
When supplying the reactive
current (id≠ 0)

0

10

20

30

40

%Reactance [%], %Xa

Fig.11. Generator current THD versus synchronous
reactance of the generator changes from 3% to 40% of thepercent
reactance.
12
Generator current [A], ig

than field weakening region, d-axis current of proposed circuit
exceeds which of field weakening control. Although withproposed circuit, the star connection mode is used, less thanfield weakening region. It can become id = 0. Above those, it ispossible to reduce the d-axis current in whole operating region
by winding changeover, except for around transition point. It
shows that proposed circuit has an advantage in a view point ofcupper loss of the generator at high speed range as shown in
Fig. 10. The rated voltage is set to 160 Vpeak per phase.

11

Rated torque

When supplying the reactive
current (id≠ 0)
5.7%

10When id=098When supplying the reactive
current (id≠ 0)

7
50% torque

6
5

0

10

When id=0

20

30

40

%Reactance [%], %Xa

Fig.12. Comparison results of RMS value of generator
current characteristics when the synchronous reactance ofthe generator changes from 3% to 40%.
power factor. As a result, fundamental operation of the starconnection mode is confirmed.

VI.

CONCLUSIONREFERENCES

This paper presented a seamless winding changeover circuitfrom the star connection mode to the delta connection mode ofgenerator stator windings. In this circuit, the output voltage ofthe generator changes in the case of each mode; (i) in the starconnection mode, it becomes the output of which is defined asline-line voltage, (ii) in the delta connection mode, the output
of which is defined as phase voltage. From the simulation
results, the output voltage of the generator is changed from 173
V to 100 V with the stator windings configuration. Furthermore,
the changeover with the seamless generator current is also
confirmed. From the comparison with the field weakening
control, it is possible to reduce the d-axis current by winding
changeover nonetheless d-axis current required to reduce thecurrent THD. The current THD is reduced from 19% to 3% by
supplying the reactive current with the generator of deltaconnection mode when the generator with 30% of the percent
reactance. In spite of the increase of the reactive current, thegenerator current increase to 5.7% in comparison with the starconnection mode. In future works, (i) the study of the proposed
circuit when it is applied to salient-pole generator, (ii) proposed
circuit will be tested by experiments.

0Phase voltage of grid [250 V/div]0Phase voltage of rectifier(LPF) [250 V/div]0
Input current [10 A/div]
DC output current [5 A/div]0

5 millisec/div

Fig.13. Experimental result of the proposed circuit when
the star connection mode.

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