Application of
Magnetic Energy Recovery Switch (MERS)
for Power Factor Correction
IPEC-Niigata 2005
April 4-8, 2005
Toki Messe
Niigata, Japan

Jun Narushima, Kouta Inoue, Taku Takaku,
Takanori Isobe, Tadayuki Kitahara, Ryuichi Shimada
(Tokyo Institute of Technology)


Table of contents
1 Operation principles of MERS.
2 Power factor correction of singlephase induction motor.
3 PWM control of MERS.
4 Energy saving by MERS.


1

Magnetic Energy Recovery Switch
S2

MERS

S1

(Magnetic Energy Recovery Switch)

L

S3
Power
Source

S4
R
Load

Connected in series.
Simple circuit, simple
control.
Bi-directional current
switch.
Can shut off current
in any phase.

MERS has a fucnction of automatic power factor correction
regardless of the load inpedance.


2

Operational mode (a)(b)
i

S1
ON


i

S4

S1

L

OFF

C

L

C

+ ON

S2

S4

R

S3
v

(a) On condition
When the capacitor is
not charged, Current flows

in parallel.

OFF

S1 S3
turn off

S2

R

S3
v

(b) Magnetic energy is absorbed
The current charges the capacitor
through diodes.
Current decreases rapidly.


3

Operational Mode (c)(d)
i

i

S1
OFF


S4

S1

L
ON

C

L

C

+ -

+ OFF

S2

S4

R

S3
v

(c) Off condition
The magnetic energy is
completely converted to
capacitor energy.


ON

S1 S3
turn on

S2

R

S3
v

(d) Capacitor energy is discharged
The capacitor discharges the
energy to load.
Current incleases rapidly.


4

Reactance voltage compensation
S2

S1

S1 S3
is on.

Vc


Vc

L

Vc

L

L

S3

VL

R

S4
R

S2 S4
is on.

VC is generated by LC resonance.
VL is compensated by switching
S1~S4.

The power factor becomes 1.

R


VL


5

Computed waveforms (X C > X L )
S1,S3
ON
Voltages (V)

200

δ

vc

vc

v

id
vd

0

v

-200
0

40
Current (A)

S2,S4
ON

vd
20

40

id

0
-40
0

20
time (ms)

40

Discharging time of C is
shorter than the switching
cycle.
When vc is the maximum
value, i d becomes zero.
A gate signal shifted by δ
is required for power factor
correction.

δ depends on the load
impedance ratio XC / XL .


6

Computed waveforms (X C < X L )
S1,S3
ON
Voltages (V)

200

vc

δ

id
vd

v
0

vd

-200
0
40

Current (A)


vc

S2,S4
ON

v
20

40

id

δ should be fixed at π/2
regardless of XC , X L and
frequency.

0
-40
0

Discharging time of C is
much longer than the
switching cycle.
v c is almost constant.

20
time (ms)

40


Power factor automatically
becomes 1.


7

Gate angle for power factor 1
3
90

2

1

XC / XL < 1

δ = 90(deg)
(constant)

δ (deg)

Normalized VCmax

δ

60

VCmax


Gate angle δ should be
always fixed at 90(deg).
VC ≒ 0.7~1 p.u.

30

XC / XL > 1

δ < 90(deg)
0
0.1

To maintain the power
factor at 1...

1

XC / XL

10

0

Gate angle δ should be
fixed at an appropriate
value within 0~90(deg).
VC > 1 p.u.

Automatic power factor correction is realized under the
conditionof XC / XL < 1.



8

Single-phase induction motor
Single-phase induction motors
are widely used.
(home, agriculture, factory...)

Low cost.
Easy to use.
Bad power factor.

The power factor is
0.3~0.7 (at the best).

It requires large power capacity
and causes an increase in cost.

Costly power converter
is not suitable.

Capacitor start type single-phase
induction motor (400W).

MERS is suitable because of
its simple circuit and
simple control.



9

Experimental setup
Phase shifting control

Controller

IGBTs of MERS are controlled
synchronusly with the AC system.
Gate signals advances in phase
by π/2.

Load machine

V1 is controlled to maintain V2 at 100 V.
Tested machine

Vmers
I1

I2
V1

V0
100V
(rms)
50Hz

Single-phase induction motor
400 W / 100 V / 9.0 A


VC

V2
Gate Control
Circuit

IM

DC

Load machine DC motor


10

Voltage (V)

Experimental wafeforms
VC

100

I2
VC

V1
0

-100


V1

20
0

20

40

I2

The current waveforms
are distorted.
The power factor of the
fundamental component
of V1 and I1 is nearly 1.

I1

-20
0

V2

IM

V2

0

Current (A)

I1

20
time (ms)

40

The power factor of input
is improved.


11

Power factor
1

Power factor

with MERS(820µF)
0.8
without MERS

0.6

0.4
0

200


400

The power factors
without MERS are
varying between
0.8~0.4.
The power factors
with MERS are
maintained within
0.9~1.0.

600

Load (W)

MERS can automatically correct the power factor regardless
of load conditon.


12

Input current

Input current

I1 (A)

15


10

without MERS

5

0
0

The power factor
correction decreases
input current.

with MERS(820µF)

200

400
Load (W)

600

Improves utilization of
electrical equipment.
(decreases required
power capacity.)
Decreases transmission losses.


13


Efficiency
Efficiency =
Efficiency (%)

60

Mechanical output
Input power

without MERS

Includes losses of
motor and MERS.

40

20

with MERS(820µF)

0

200

400
Load (W)

600


Efficiecy does not
decreases, although
the current waveform
is distorted.

MERS improve the power factor while it does not affect the
load side.


14

Experiments by PWM control
The current waveforms by phase shifting control have large
distotion and harmonic components.
PWM control can be used for applications which require clean
sinusoidal waveforms.
PWM control
A reference current i* is
determined by input voltage
to maintain the power factor
at 1.
MERS is controlled to make
a difference (i * - i ) to be
zero by PI control and
PWM control.

VC

i* i
S1,S3


S2,S4

K

2 kHz


15

Voltage (V)

Experimental waveforms
VC

100

V2 includes high frequency
components.

V1

0

-100

Voltage (V)

0


20

100

40

V2

I 2 is almost sinusoidal without
distortion.
The phase of I 2 is almost the
same as that of V1 .

0

-100

Currnet (A)

0

10

I1

20

40

I2


MERS with PWM control also
achieves the power factor
correction.

0
-10
0

20

time (ms)

40


16

Power factor

Power factor

1
0.8

with MERS(PWM)

The power factor by
phase shifting control
decreases to 0.9 at low

load.
(Higher harmonic components cause insufficient
power factor correction.)

with MERS
(Phase shifting)

0.6
without MERS

0.4
0

200

400
Load (W)

600

The power factor by
PWM control is always
maintained to be 1.

The sinusoidal current generated by the PWM control
improves the power factor more efficiently.


17


Efficiency
Efficiecy of PWM
control is almost the
same as that of phase
shifting.

without MERS
Efficiency (%)

60
with MERS(PWM)

Although the sinusoidal
current waveform
decreases losses of
motor, switching losses
are increased by PWM.

40
with MERS(Phase shifting)
20
0

200

400
Load (W)

600


The PWM control of MERS does not have significant effect
in efficiency.


18

Energy saving by MERS
The experiment to demonstrate energy saving by MERS
were carried out.
Refrigerator(100W)
Electric power
company side

Custumer
side

I

Wattmeter

Using single-phase
induction motor.
The power factor is 0.72.

MERS

Long wire
(100m 5.6 Ω)

The voltage applied to the

refrigerator is fixed at 100V.

V