Current Control Technique for Voltage-Source PWM Converter
Applied for a Slip Energy Recovery Drive
Satean Tunyasrirut1 and Vijit Kinnares2
12

Faculty of Engineering, Pathumwan Institute of Technology, Thailand
Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Thailand

1. Introduction
Induction motor drives with full power control on the statorside are commonly used in industrial applications. Althougheither a cage-type or wound rotor induction machine can be usedin the drive, the former is always preferred in terms of low
weight, low cost, low rotor inertia, speed limitation,maintenance and reliability. One feature of the latter is that theslip power becomes easily available from the slip rings, which
can be either mechanically or electronically controlled for motor
speed adjustment [1]. However, for limited range speed control
applications, where the slip power is only a fraction of the total
power rating of machine, power rating of the converter isreduced, hence reduction in cost. The wound rotor induction
motor takes this advantage. Slip power for adjusting the motor
speed can be recovered through static converters instead ofuseless dissipation on resistors. Slip power recovery drives havebeen used in some applications such as large-capacity pumps
and fan drives, variable-speed wind energy systems, shipboard
variable-speed/constant-frequency systems,
variable-speedhydro pumps/generators and utility system flywheel energy
storage systems. The slip energy recovery drives (SERD) known
as Scherbius system [2-3], offer low cost, simple control
circuitry and high efficiency even at a low speed range. TheSERD consists of a wound-rotor induction machine, a diodebridge rectifier, a large link inductance, a thyristor-bridge
inverter and an optional 3 phase transformer. A SERD transfers
power that is normally wasted in the rotor of an induction
machine back to the ac mains supply to improve overall drive
efficiency. In addition, by inserting a step-down transformer
between the ac supply and the inverter module, the voltage
ratings of the inverter and rectifier devices may be madesignificantly smaller than the rated machine stator voltages.
Thus, in contrast with a stator-voltage-controlled induction
machine drive, the relative power electronic units of the SERD
may be designed to be smaller, lighter and less expensive. The
choice of the turns-ratio of the optional transformer is based on acompromise between achieving the desired speed control range
and optimizing the drive power factor [1]. For smaller values of
inverter firing angle, the rotor speed is higher. Then inverter
draws substantial reactive power and the inverter returns lessreal power to the mains. Thus the drive power factor is reduced.
The drive power factor is found to be optimized for applications
requiring a narrow range of speed control and a large load
torque, where significant real power is returned via the inverter.
A problem of SERD is having lower power factor in general 0.4
to 0.6 more or less.
Drawbacks of SERD such as, 1) low power factor, 2) highcurrent harmonic, and 3) a poor capability of the reactive poweron the ac side, can be avoided by using a pulse widthmodulation (PWM) voltage source inverter. The drive system
can control the active and reactive power on the ac line by acurrent-controlled technique in conjunction with the first andsecond a boost chopper for DC link voltage. The voltage source
inverter is used to increase power factor of the system and to
reduce low order harmonics of the input line current.
This paper presents a simple configuration that cancompletely solve the first and second problems and partiallysolve the fourth problem. Although there are a number ofarticles which analyze the SERD with dc voltage intermediate
circuit. The system proposed in this paper differs frompreviously discussed approaches in the following ways. 1) Thereis a boost chopper on the intermediate circuit providing controlof the machine. 2) A decoupled control system is achieved.
Effectively, the machine speed is controlled by the boostchopper and the inverter controls the reactive power on the acside. 3) The current harmonics on the mains are reduced by aVSI based on PWM [11].
The aim of this paper is two-fold. The first is that theharmonics of line current waveforms are reduced by using theVSI. The second aim is to improve the power factor of the driveby the IGBT type boost chopper applied across the DC terminal.
The boost DC voltage is connected to the voltage source inverter
and the reactor circuit. This scheme leads to be able to adjust
the speed of the motor by varying the duty cycle of the boostchopper operating in a PWM mode.

Fig. 1. Schematic of the proposed slip energy recovery drivewith current control voltage source inverter.
2. Current Control Techniques for Voltage-Source PWM
Converter
The proposed system configuration is illustrated in Fig. 2.
It’s consists of an IGBT boost chopper, a diode D, a dc link
capacitors and a PWM type VSI. The voltage, E is controlled
at a constant value by the IGBT boost chopper. Consequently
variation of torque and speed is achieved by operation of theboost chopper.[4]S1

S3S5S4S6S2VLia i b ic

vα , vβi a , ib , ic
ia* , ib* , ic*
P*va vb vc

vα , vβ
Q*Fig. 2. Current Control Techniques for PWM Converter.

Using the diode bridge connecting the rotor of the machine tothe electronic circuit allows a simplified control system. The
active and reactive power can be controlled by controlling thevoltage and phase angle between the voltage source inverter sideand ac line side connected with the reactor circuits. Aninductance Ld can be added on the dc side to decrease theswitching frequency or the dc current ripple[2].
From fig. 2 the grid voltage can be written to equation 1v grid = vconv + v L(1)

The active and reactive power flow between grid side andconverter side can be written to equation 2 and 3 [4-8]
2 Vgrid Vconvsin δ 
P=ωL  VgridFig. 3. Simulation of current control technique for voltagesource PWM converter.
10(2)0-5-102 Vgrid
1 − Vconv cos δ Q=ωL  Vgrid(3)

Where apparent power is0(4)The instantaneous real and imaginary power are part to theinstantaneous complex power defined equation 5
S = vα iα + vβ iβ + j vβ iα + vα iβ(5)) ()From the transformation matrixes, the Clark transformation andits inverse transformation become1vα  2 1 − 2 = 3v β  3  021  v a2  v  b 3−v 2   c −(6)The instantaneous power of the p − q theory P   vα
Q  = − v   βvβ  iα vα  iβ (7)In the following explanation, the αβ currents will be set asfunctions of voltage and the real and imaginary power P andQ . This is very suitable for better explaining the physicalmeaning of the powers defined in the p − q theory, it ispossible to write

iα vα1i  = 22 v
 β v +v  βαβvβ   P  − vα  Q (8)The inverse Clark transformation can be decomposed into thesum of two terms, as follows

 1ia*  * 2  1
ib  = −i *  3  2
 1
 c 
− 2

0 
3  iα  2  iβ 3−2 (9)

3. Simulation Result

Current control technique for voltage-source PWM
converter applied for a slip energy recovery drive wassimulated in the Matlab/Simulink software. The wound rotor
induction motor is rated at 1 kW, 4 pole with a 380 V, 50Hz as
shown in figure 3.

0.05

0.1

0.15

0.20.25

10005000-500-1000-1500-2000

S = vi* = P + jQ

(iavan5Grid → Loadinverter → gridFig. 4. Voltage and current waveforms, with the grid sideconverter.
4. Conclusion
This paper has proposed a control strategy of current control
technique for voltage-source PWM converter applied for a slip
energy recovery drive. The objective of grid side converter
control is to maintain the dc-link voltage and independent active
and reactive power flow. The performance simulations showthat this strategy is able to a good dynamic responses and highaccuracy to the active and reactive power control.
Acknowledgements
The work presented in this paper has been supported
financially by the Thailand Research Fund, Office ofthe Higher Education Commission and Pathumwan
Institute of Technology, Bangkok, Thailand.
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