Efficiency Evaluation of a Matrix Converterwith a Boost-up AC Chopper in Adjustable Drive System
Kazuhiro Koiwa*

Jun-ichi Itoh**Nagaoka University of Technology
Abstract:- In this paper, the validity of the proposed circuit which consist of a V-connection AC chopper and a matrix converter
is evaluated in an IPMSM adjustable speed drive system in term of efficiency using loss calculations and experiments. As a result,
it is confirmed that the total efficiency of the proposed system can achieve 79.7%. Furthermore, it is revealed that the proposedsystem is effectiveness when the input line voltage is 93.5% lower than the motor rated voltage.
Keywords : Matrix converter, V-connection AC chopper, Flux-weakening control, IPM motorMatrix converter

1. Introduction
A matrix converter (MC) which can convert an AC power supply
voltage directly into an AC output voltage of variable amplitudeand frequency without the large energy storages, such as
electrolytic capacitors, have been actively studied recently [1-5].
However, one of the disadvantages of the MC is that the voltage
transfer ratio, which defines as the ratio between the output
voltage and the input voltage, is constrained to 0.866. As a result,in the adjustable speed drive system, due to implementation offlux-weakening control the motor loss increases inherently.
The authors previously proposed a circuit topology whichconnects a V-connection AC chopper in the input stage of the MCthat enables boost-up functionality [2-3]. However, additional loss
from the AC chopper occurs in the proposed circuit structure.
Thus, in comparison with the conventional MC, the efficiency ofthe proposed circuit is lower due to the additional chopper loss.
In this paper, the validity of the proposed circuit is evaluated in
the adjustable speed drive system in order to clarify the advantageof the proposed system. At first, the chopper loss in the proposed
circuit and the copper loss of the motor in the conventional MC
are introduced theoretically. Then, the difference between thechopper loss and the copper loss are compared, where
flux-weakening control is implemented in the motor control.
Finally, the operation of the proposed system will be demonstrated
as a 3.7-kW Interior Permanent Magnet motor (IPM motor). Inaddition, the chopper loss and the copper loss are evaluated toclarify the validity of the proposed system.

V-connection
AC chopper
Sc1vsSc2

vt

Sc3vuvv
vw

M

Sc4Li

Cf3vcr vcs vct

iu
Fig. 1. Circuit configuration of proposed circuit.

iw

Therefore, the capacitor value does not dominate by the voltage
control response and the current response from the input side,which is contrast to a BTB system. As a result, the V-connection
AC chopper and its components do not dominate the size and theweight in comparison to the origin structure of a MC. In addition,
the maximum output voltage of the proposed circuit is decided by
the duty ratio of the AC chopper. It should be noted that theswitches in the AC chopper do not operate when the voltage
transfer ratio is lower than 0.866 of the input voltage. That is, inthe range of low output voltage, the V-connection AC chopper inthe proposed circuit does not generate the switching loss in low
output voltage.
However, the conduction loss occurs in the AC chopper evenswitching operation is disabled. Therefore, it is concerned that theconversion efficiency is degraded by adding the AC chopper.
2.1 Conduction loss
In order to consider the influence of the chopper loss for theproposed circuit, the chopper losses such as the conduction loss
and the switching loss are theoretically derived in this section.
At first, the conduction loss of Sc1 is expressed by (2).

2. Circuit topology and Chopper losses
Fig.1 shows the proposed circuit which connects a V-connection
AC chopper in the input stage of the MC. Note that the virtual
indirect control method is applied in the MC [4]. Eight IGBTs areused in the AC chopper. The relationship between the input
voltage vin and the output voltage vout is expressed byvout     mc  vin

vrPconS 1 1v i dt ....................................................(2)
 
0ce r

where, vce is the on-state voltage of Sc1 which is expressed by (3)and ir is the input current.

.......................................................... (1),vce  k con1ir  k con2 .........................................................(3)where, mc is the modulation index of the MC,  is the boost-up
ratio of the chopper and vin is the input voltage. Basically, thefunction of the AC chopper is to boost-up the input voltage.
Feedback control is not required for the filter capacitor since boththe input and output sides of the AC chopper are AC voltage.

Note that kcon1 and kcon2 are defined from the on-state voltage
characteristics in the datasheet of the switching device. In thispaper, SK80GM063 which is in SEMIKRON is used as thechopper device. The rated voltage and the rated current of this1IGBT are 600 V and 81 A, respectively. From these equation, theconduction loss of Sc1 is introduced by (4).

Pcon _ S1 k con1

I in 
22 2 k con2

80
Conduction loss of Sc1(Simulation)
60

I in ................................. (4)40where, Iin is the effective value of the input current. In similarly,
the conduction loss of Sc2 is expressed by (5).

Pcon _ S 2 (   1)k con1

I in 
22 2 (   1)k con 2

Conduction loss of Sc1(Calculation)
Switching loss of Sc2(Calculation)

20

I in ........... (5)(Simulation)

02.2 Switching loss
The switching loss of the AC chopper is obtained by the input
line voltage vrs and the input current iin. Thus, the switching loss
Pton_s1 and Pton_s2 are expressed by (6) and (7), respectively.
v
1 
Pton _ S 1  5 E on f s rs dt 6
Vs

0

10

2030
40
Input current Iin (A)5060Fig. 2. Chopper loss by simulation result.

q axisLqiqv

eq

2Vin f s(   1)[( 6  6)k ton1 I in  12( 3  2)k ton 2 ]24Vsiqi....................................................................................... (6)v
1 Pton _ S 2   E on f s rs dt 6VsLdid+Lqiq)v

2 Vin f s[(5 6  3 2 )k ton1 I in  12(1  3 )k ton 2 ]24Vs

id....................................................................................... (7)where, Eon is the instant switching loss which is shown by (8).

d axisFig. 3. Phasor vector diagram on flux-weakening control.

Eon  k ton1iin  k ton2 ........................................................ (8)is expressed by (9).

Note that kton1 and kton2 are defined from the switching loss
characteristics in the datasheet of the switching device.
Fig. 2 shows the conduction and switching losses of the ACchopper by the theoretical calculation and the simulation. Notethat the chopper loss of the calculation results is the sum of theequations of (4), (5), (6) and (7). Additionally, kcon1 and kcon2 are0.0182 and 0.9773, respectively. On the other hand, kton1 and kton2
are 0.00005 and 0.0, respectively. These parameters were obtained
from the datasheet of the switching devices. As the result, thecalculation results are corresponding to the simulation results.
Thus, the validity of the derived equations of the chopper loss isconfirmed.

Vom  v d  v q
22

 (Lq i q ) 2  (Ld i d  e q ) 2........................................(9)where, Vom is the limitation of the output phase voltage that theMC can output. Therefore, the d-axis current in theflux-weakening control is calculated by (10).
 eq  Vom  (Lq i q ) 2
2id  Ld.....................................(10)The motor current is increased due to the flux-weakening control.
As a result, the motor loss and the converter loss will be increased.
In order to validate the AC chopper, the copper loss which isincreased by the flux-weakening control is introduced. In otherwords, the equation of the copper loss that subjects to the input
line voltage is calculated.
At first, the torque of an IPM motor is expressed by

3. Relationship flux-weakening control to copperloss
In order to drive an IPM motor in high rotating speed area within
the output voltage limitation of the MC, the flux-weakening
control is necessary to apply in the MC control. This is because
the back electromotive force becomes higher with increasing therotating speed. The output voltage which is necessary to drive themotor can be degraded by the flux-weakening control. There, thed-axis current id for the flux-weakening control is introduced.
Fig. 3 shows the phasor vector diagram of the flux-weakening
control where eq is the back electromotive force, v and v’ are theterminal voltage in the IPM motor without the field-weakening
control and with the field-weakening control, respectively.
Field-weakening control in the IPM motor can equally weaken themagnetic flux in the permanent magnet from the d axis armaturemagnetic flux. As a result, the rotation speed area can be extended
by implementing the field-weakening control. From the Fig. 3, itT  Pn [eqiq  ( Ld  Lq )id iq ] ...................................... (11),where, Pn is the number of pole.
Thus, Vom is expressed by (12) from (10) and (11).

Vom  [LdT Lq1(eq iq ) ] 2  (Lq iq ) 2( Ld  Lq ) Pn Ldiq....(12)Note that the q-axis current should be obtained to derive thecopper loss. However, Eq. (12) cannot obtain the q-axis current.
For this reason, the q-axis current is calculated by numericalanalysis. It should be noted that the q-axis current is limited by(13).
2

Loss (W)Fig. 4. Comparison between experiment and calculation.

Fig. 5 Acceleration test.

Table 1. Motor parameters.

Table 2 Experimental conditions.
Fig. 6. Experimental results in motor-load.

4. Experimental results

iq 

Fig. 5 shows the acceleration tests for the IPM motor which isdriven by the proposed circuit with vector control. Table 2 showsthe experimental parameters. In Fig. 5, the AC chopper starts
operating while the rotating speed command * is over 0.2pu
because the harmonic distortion is decreased by the damping
control. Additionally, it is confirmed that the input and q-axis
current are not drastically changed because of the operation of ACchopper. Therefore, the proposed circuit can improve the transfer
ratio of the MC.
Fig. 6 shows the experimental results at rated rotating speed.
Note that the torque is 85%. In addition, the damping control isapplied in the AC chopper [5]. According to the waveform, it isconfirmed that the total harmonic distortion (THD) of the input
current is 12.2%. In order to reduce the THD, it is necessary tooptimize the damping gain.
Fig. 7 shows the output line voltage characteristic based on themechanical output power. The rated line voltage of the IPM motor
Vn is 180V. Thus, the output line voltage is limited up to 180V by
the flux-weakening control. According to the result, it isconfirmed that the output line voltage is limited up to 180V by theflux-weakening control. Thus, the flux-weakening control isvalidated by experimental results. Moreover, as the mechanicaloutput power is over 1400 W, d-axis current become larger due tothe flux-weakening control. As a result, the copper loss isincreased due to flux-weakening control.
Fig. 8 shows the total efficiency, which is included the converter
efficiency and the motor efficiency, proportional to the mechanicaloutput power. In addition, the motor efficiency is calculated fromthe rotating speed and the estimated torque. As a result, it isVomLq.................................................................... (13)On the other hand, d-axis current can be calculated by (10).
Therefore, the copper loss Pc is expressed by (14).

(id  iq )2Pc  3Rw I o  3  Rw 222................................... (14)where, Rw is the primary wire resistance of the IPM motor. Inaddition, Io is the effective value of the output current.
Fig. 4 shows the calculation result of the chopper loss and thecopper loss. Note that the copper loss was calculated by the motor
parameter which is shown in Table 1. Since, the flux-weakening
control in the IPM motor is not capable of wide range ofapplication. Therefore, in order to extend the applications, theinput voltage of the proposed circuit and the conventional MC aredegraded. Then, high rotating speed of the IPM motor is possible
to be simulated. According to the result, in case that ratio between
motor rated voltage Vn and input voltage Vin (Vn/Vin) is less than
1.01 p.u., the chopper loss is larger than the copper loss. On theother hand, as Vn/Vin is higher than 1.01 p.u., the chopper loss isless than the copper loss. This is because the motor current and thecopper loss are increased with the ratio of Vn/Vin.

3Output line voltage Vuv (V)confirmed that the total efficiency is 79.7% in 2200-W load.
Fig. 9 shows the input current THD characteristic of theconverter. Note that the damping control is applied in the ACchopper in all of area. According to the result, it is confirmed thatthe input current THD is over 8%. In other words, the input filter
resonance cannot be suppressed by the damping control. This isbecause the damping gain is not optimized.
Fig. 10 shows the total loss comparison between the calculation
results and the experimental results. Note that the input voltage Vin
was changed. Additionally, the iron loss and the mechanical loss
were obtained from experiment. Accordingly, the total efficiencyof the conventional MC which applied the flux-weakening control
becomes low as the input line voltage decreased. This is because
the conduction loss of the converter and the primary order copper
loss of the IPM motor are becoming larger due to theflux-weakening control.
On the other hand, the AC chopper in the proposed circuit canimprove the input line voltage which has been decreased. Thus,
the flux-weakening control is not necessary to apply in theproposed circuit. For this reason, high efficiency of the proposed
circuit can be achieved.
In the calculation and experimental results, in case that the ratioof input line voltage and motor rated voltage is over 1.07 p.u. , thetotal loss of the proposed system is less than that of theconventional MC. However, in comparison with Fig. 4, there is
the different the turning point. This is because the iron loss andmechanical loss is not allowed in Fig. 4. The iron loss isdepending on the output line voltage and frequency. Therefore, thevalidity of the proposed system can be revealed as the input line
voltage is 93.5% lower than the motor rated voltage.

Total efficiency (%)

Fig. 7. Output line voltage characteristic.

Input current THD (%)

Fig. 8. Total efficiency characteristic.

5. Conclusion
In this paper, the validity of the proposed circuit is evaluated interm of the efficiency in an IPM motor drive system. Consequently,
the chopper loss and the copper loss are derived theoretical andsimulation. As a result, the effective area that the chopper loss isless than the copper loss is confirmed.
The chopper loss and the copper loss were demonstrated by a
3.7-kW IPM motor. As a result, it was confirmed that the total
efficiency of the proposed system was 79.7% in 2200-W load.
Moreover, it can be accomplished that the validity of the proposedsystem is confirmed as the input voltage is 93.5% lower than themotor rated voltage.
In future works, the damping gain will be optimized. Moreover,the harmonic distortion in the input current will be suppressed.

Fig. 9. Input current THD characteristic.

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[2]

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[4]

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Fig. 10. Total loss comparison between calculation results andexperimental results.
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