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978-1-4244-8542-0/10/$26.00 ©2010 IEEE

Abstract—The direct torque control (DTC) speed control

scheme of an induction motor surpasses the vector control in the sense that it controls the torque directly and does not use transformations and current modulator. The only snag in DTC scheme is the presence of high content of torque ripples in the output torque. This paper presents a novel approach towards the minimization of torque ripples by modifying the conventional three-level hysteresis torque controller used in DTC. A unique switching strategy has been developed to generate desired voltage vector. The comparison of the simulated results based on % ripple content shows that the proposed method has successfully reduced ripples in the torque under the various operating conditions.

Index Terms—Direct torque control, three-level hysteresis torque controller, switching strategy, torque ripple.

I. INTRODUCTION irect torque control (DTC) is a new control strategy of the induction motor (IM) drives, suggested by Takahasi et al [1]. Depenbrock et al [2] gave it a

name direct self control. This method presents an expedient solution to conform to the hardware of the V/f-based induction motor drives and in unison, to obtain the performance of the vector controlled induction motor drives. This scheme as such doesn’t require a mechanical speed sensor and hence, can be called a sensor-less drive. Despite, giving fast torque control, dearth of current modulator and transformations, this scheme faces certain snags. The torque and flux control at very low and zero speed is cumbersome, high ripples in torque and current, continuously varying inverter switching frequency and high level of noise [3] at low speed are a few shortcomings of this method discussed by Casadei et al in [4]. It has been observed by Casadei et al [5] that the drive behavior, in terms of torque and flux, is based on the selection strategy of voltage vector An appropriate selection of this strategy may improve the dynamical response of the drive along with the production of minimal torque and flux ripple. The success of DTC scheme has been implemented not only in induction motor drives but also in other AC motor drives for e.g. brushless dc motors etc. [6].

Arunima Dey and Bharti Dwivedi are with the Electrical Engineering Department, Institute of Engineering and Technology, Sitapur Road, Lucknow, India (corresponding author phone: 919415018627; off.: 91-522-4048747; e-mail: arunimadey@gmail.com, bharti_dwivedi@yahoo.com).

Bhim Singh is with the Electrical Engineering Department, Indian Institute of Technology, Delhi, India. (e-mail: bsingh@ee.iitd.ac.in).

Dinesh Chandra is with the Electrical Engineering Department, MNNIT Allahabad, India (email: dinuchandra@rediffmail.com)

Most of the papers surveyed have analyzed DTC using conventional three-level torque controller (TC) and all have presented a high degree of torque ripple in the results under dynamic situations. This has a repercussion in speed ripple also. Various methods have been reported with regards to minimization of torque and speed ripple using various PWM strategies but the modification in the three-level TC has been reported by Lai et al [7].

The minimization of torque ripple requires improvement in switching strategy of a three-level hysteresis torque controller which is most commonly utilized in DTC scheme. The paper aims at developing a new torque controller with a novel switching strategy. The three-level torque controller has been replaced by a five-level torque controller (TC). To generate an appropriate voltage vector for this five-level TC a unique set of switching pattern is devised. The proposed controller has been implemented in the DTC induction motor drive model. The PI speed controller used in this model is tuned using one of the most widely used tuning method i.e. Zeigler-Nichols tuning method. The model is simulated and the results are compared with the conventional three-level TC based DTC drive. The basis of comparison is the percentage of ripple content present in the torque.

II. DIRECT TORQUE CONTROL The generalized schematic block diagram of a DTC IM

drive is shown in fig.1.

Fig. 1 Schematic block diagram of DTC IM drive.

In this type of speed control scheme employed in IM drive, the flux linkage and electromagnetic torque are controlled directly and independently by the selection of optimum inverter voltage vectors. The selection is made to restrict the flux linkage and electromagnetic torque errors within the respective flux and torque hysteresis bands, in order to obtain the fastest torque response for low inverter switching frequency. The induction motor is supplied by a

A Novel Approach to Minimize Torque Ripples in DTC Induction Motor Drive

Arunima Dey, Student Member IEEE, Bhim Singh, Fellow IEEE, Dinesh Chandra, Member IEEE and Bharti Dwivedi, Member IEEE

D

IM

Flux & Torque estimator

( )s V R i d t= −∫Φ ( )d q q dT = -i i

3 P2 λ λ

Switching Table Inverter

Vdc

Ia

Ib

s∠Φ

¢*s

T*e

-

-

2 voltage source inverter; the outputs of two-level flux hysteresis comparator, three-level torque hysteresis comparator and the stator flux position are used to get the optimum stator voltage vector through the look-up table.

III. CONVENTIONAL THREE-LEVEL TORQUE CONTROLLER AND TWO-LEVEL FLUX CONTROLLER

The DTC speed control scheme consists of three level torque controller and two level flux controller as shown in fig. 2 below:

Fig. 2 (a) Two Level Hysteresis Flux Controller (b) Three Level Hysteresis Torque Controller.

The reference stator flux *sψ and torque *

eT magnitudes are compared with the respective estimated values, and the errors are processed through hysteresis band controllers. The flux controller has two levels of digital output which follows the relations given by equation 1.

1

1

f fs

f fs

for HbH

for HbH

ψ

ψ

= Δ > +

= − Δ < − (1)

Where, 2 fHb = total hysteresis bandwidth of the flux

controller. The actual stator flux sψ is restricted within the hysteresis band and tracks the reference flux in a crisscross path. The torque control loop has three levels of digital output having the following relations in equation 2.

1

0

1

t e t

t et t

t e t

for HbH T

for Hb HbH T

for HbH T

< <

= Δ > +

= − Δ +

= − Δ < −

(2)

The stator voltage impresses directly the stator flux in accordance with the following equation:

tV ssψΔ = Δ (3)

Decoupled control of the stator flux and torque is achieved by acting on the radial and tangential components respectively of the stator flux-linkage space vector in its locus. This gives the optimum selection of the switching vectors for all the possible stator flux-linkage space vector positions i.e. six positions, corresponding to the six sectors and two zero vectors as shown in fig. 3.

Fig. 3 Definition of Switching States and Sectors for Conventional Three- Level Torque Controller in DTC.

The feedback flux and torque are calculated from the machine terminal voltages and currents. The sector number Sec (k) where k varies from 1-6 in which the flux vector ψs lies. There are six sectors each 600 apart as shown in Figure 3. To generate appropriate control voltage vector, i.e. switching states, for the inverter, the input signals required are fH , tH and Sec (k). The switching states are described in Table 1 using these input signals.

Table 1 SWITCHING TABLE OF INVERTER VOLTAGE VECTORS FOR THREE-LEVEL

TORQUE CONTROLLER

Table 1 applies the selected voltage vector, which necessarily changes both the flux and torque concurrently.

IV. DEVELOPMENT OF FIVE-LEVEL TORQUE CONTROLLER Since a conventional three level torque controller DTC

drive selects the inverter switching states using a switching table; it does not require current controller or PWM modulator, resulting in a fast torque response. However, such a DTC scheme has some demerits because the sampling frequency for the calculations of torque and flux ought to be very high in order to achieve gratified tracking performance and to restrict the errors of torque and flux within the specified hysteresis bands [8]. Consequences of not meeting this requirement by the switching table based DTC scheme are the occurrence of ripples in torque and speed [9].

To overcome the above drawback, the paper presents the design of a five level torque controller and its switching strategy to generate required voltage vector for electromagnetic torque and stator flux vector control. However, the two-level flux controller is retained. The technique drastically minimizes the torque and speed ripples without introducing increment in sampling frequency or using SVM modulator. Further, the tuning of PI speed

Hf Ht S(1) S(2) S(3) S(4) S(5) S(6) 1

1 V4 V5 V1 V3 V2 V6 0 V0 V7 V0 V7 V0 V7 -1 V2 V6 V4 V5 V1 V3

0

1 V5 V1 V3 V2 V6 V4 0 V7 V0 V7 V0 V7 V0 -1 V3 V2 V6 V4 V5 V1

0

1

-1

2Hbf

0

1

-1

2Hbt

Hf Ht

3 controller aids in minimizing the speed ripple.

In a five level hysteresis controller shown in fig. 4 below, the torque vector is restricted within the bands at two levels each for positive and negative limits besides zero value i.e. the torque vector changes twice its value for exceeding its upper and lower values.

Fig. 4 Five-Level Hysteresis Torque Controller.

According to fig. 4, the controller requires modification in the switching table to obtain the right amount of voltage vector for the same stator flux vector. Thus, a novel switching strategy is designed to accomplish the desired voltage vector. In a five-level torque controller, the switching space vector consists of 12 non zero or active vectors and two zero vectors. The switching states are decided on the basis of the position of the stator flux vector moving at synchronous speed and the outcome of the two-level flux and five-level torque controllers. The switching space vectors and sectors are shown in fig. 5

Fig. 5 Definition of Switching States and sectors for proposed Five-level Torque Controller in DTC scheme.

From fig. 5 it is clear that to obtain the exact voltage vector, the 12 non zero switching vectors are V1 to V6 (i.e. the usual six vectors being applied for three-level) and V10 to V60, i.e. all six active vectors V1 to V6 synthesized by two zero vectors either V0 or V7. These added vectors are applied for 50% duty. The switching sequence for sector 1 and for all sectors employed in five-level torque controller is given in tables 2 and 3 respectively.

Table 2 SWITCHING SEQUENCE APPLIED IN SECTOR 1OF FIVE-LEVEL TC

Flux Controller

O/P

Torque Controller

O/P

Switching States/Duty (%)

for Sector 1 (-π/6~π/6)

1

2 V4 /100 1 V4 /50 and V0/50 0 V0 /100 -1 V2 /50 and V0/50 -2 V2 /100

0

2 V5 /100 1 V5 /50 and V7/50 0 V7 /100 -1 V3 /50 and V7/50 -2 V3 /100

Table 3

SWITCHING PATTERN APPLIED TO FIVE-LEVEL TORQUE CONTROLLER

Sector No. Flux Error

Torque Error

Sec 1

Sec 2

Sec 3

Sec 4

Sec 5

Sec 6

1

2 V4 V5 V1 V3 V2 V6 1 V40 V50 V10 V30 V20 V60 0 V0 V7 V0 V7 V0 V7 -1 V20 V60 V40 V50 V10 V30 -2 V2 V6 V4 V5 V1 V3

0

2 V5 V1 V3 V2 V6 V4 1 V50 V10 V30 V20 V60 V40 0 V7 V0 V7 V0 V7 V0 -1 V30 V20 V60 V40 V50 V10 -2 V3 V2 V6 V4 V5 V1

Table 3 changes the bang-bang control of conventional

switching technique applied in three-level torque controller DTC scheme. The inverter switching frequency is boosted when the torque error lies within 2nd tHb band defined in fig. 5 by invoking zero vectors. The introduction of zero vectors diminishes speed ripple unlike in conventional DTC where they do not play a role in determining switching states. Further, the fast response during dynamic situations is maintained by making torque error value greater than 1st

tHb band.

V. RESULTS AND DISCUSSIONS The modeling and simulation of both the three-level and

five-level TC based DTC drives are successfully carried out subjecting the drive to different operating conditions namely starting, speed reversal, load application and load removal. The simulation is carried out using the conventional tuning method i.e. Z-N method in PI speed controller. The details of the motor and inverter data are given in appendix. The results obtained are thus using optimized gain parameters in conventional PI controller.

A. Starting Characteristics When the motor is started at no load, the actual speed

gradually picks up the reference value of 1430 rpm. Initially the starting current and the starting torque are high. Once the speed attains its reference value, the current and the torque acquire their rated values as well. The starting dynamics for

-Hbt

1

-1

2Hbt

2

-2

Hbt

Hbt

-Hbt

Ht

V0 (110), V7 (111)

d axisV1 (100)

V2 (010)V3 (110)

V5 (101)

V6 (011)Sec 1(-π/6~ π/6)

Sec 2

Sec 3

Sec 5

Sec 6

V4 (001) q-axis

V50

V60 V10

V20 V30

V40

Sec 4

4 3-level TC and 5-level TC based DTC IM drive are shown in Fig. 6(a) and 6(b) respectively.

Fig. 6(a) Starting characteristics of three-level TC based DTC IM drive.

Fig. 6(b) Starting characteristics of five-level TC based DTC IM drive.

The inset graph of the output electromagnetic torque shows the percentage of torque ripple content during the starting period. The use of three-level torque controller results into a high value of ripple content of 186.22% in the electromagnetic torque. It is observed that the ripples are reduced in case of five-level TC to 110.66 %. Thus, comparing with the results of three-level in Fig. 6(a) it is established that the torque ripples are minimized using five-level. The testing conditions remain the same incorporating Z-N tuning method in PI speed controller in both the cases. The values of peak overshoot and settling time of three-level remain the same in five-level TC also.

B. Speed Reversal Characteristics The dynamics of three-level and five-level based DTC

IM drive when the motor is suddenly made to run in the reversed direction along with the percentage of torque ripple

content are shown in Figure 7(a) and 7(b) respectively. The reference speed is reversed at 2.2secs to a value of 500 rpm; the actual speed follows it and tries to attain the reference value at the minimum possible time. The torque Tem also reduces drastically only to attain its reference value in the minimum time. The stator currents’ frequency and magnitude are decreased during speed reversal, depicting the application of regenerative braking, followed by the reversal in phase sequence enabling the motor to run in the opposite direction.

Fig. 7(a) Speed reversal characteristics of three-level based DTC IM drive.

Fig. 7(b) Speed reversal characteristics of five-level based DTC IM drive.

A three level torque controller based DTC IM drive produces high torque ripples content of 93.44% whereas a five-level TC based DTC IM drive shows the improvement in the torque ripple content with magnitude 67.22%.

C. Load Application and Load Removal The consequences of load application and load removal

on output torque, speed and stator currents are shown in Fig. 8 (a) and 8(b) for three and five level TC based DTC IM drives respectively. The same figures also present the graph

5 of ripple contents in the torque. The sudden application and removal of load is taken simultaneously at t=2 secs and 3secs respectively. It is observed that in DTC IM drive, the current shows the corresponding behavior by increasing its magnitude during the load applied period and immediately drops as the load is removed. With overshoots at the time of application and removal of load in the speed still the speed is maintained constant throughout the load disturbance period.

Fig. 8(a) Three-level based DTC IM drive response for load application and load removal disturbances.

Fig. 8(b) Five-level based DTC IM drive response for load application and load removal disturbances.

The transient specifications remain constant in magnitudes in both the types of controller performance. The load dynamics are very important as the slightest disturbance in load produces increased ripples in the output torque resulting in astringent performance of the motor. The magnitudes of currents are constricted within the limits or rated value which is an important factor in determining the performance of the motor.

The developed five-level torque controller is able to improve the performance of the motor under the load disturbance conditions by reducing the torque ripple content

115.87% unlike three-level one which has higher ripple content in the torque of 210.36% during the disturbances.

D. Field Weakening The dynamics of the DTC IM under field weakening

region is shown in Fig. 9(a) and 9(b) for three and five level TC based DTC IM drives respectively. The speed is increased beyond the rated value of 1430 rpm to 1680 rpm. As soon as the speed rises above 1430 rpm at t = 1.75 secs, the rotor flux reduces showing the effect of field weakening region The torque instantly shoots but finally settles to the reference value as soon as the speed acquires its command value. The current shows the same behavior as that of the torque.

Fig. 9(a) Three-level based DTC IM drive response under field weakening region.

Fig. 9(b) Five-level based DTC IM drive response under field weakening region.

The inset characteristic in Fig. 9(b) shows the % ripple content in torque during the field weakening region. The torque waveform has 69.93 % ripple content much less than what has been produced by the 3-level TC. The % ripple content in the three-level TC DTC drive is very high with a value of 241.97 % during the field weakening zone of operation.

Thus, Five-level TC has been successful in minimizing the ripple in the torque during various operating characteristics to a great extent as compared to the three-level TC DTC IM drive.

VI. CONCLUSIONS The objective of developing the five-level torque

controller without disturbing the two-level flux controller

6 has been achieved in this paper. The results obtained from the five-level based DTC IM drive have established its operational superiority over the conventional three-level based DTC drive. The torque ripples are minimized by producing appropriate and novel switching strategy for the VSI used in the drive. The outcome of the five-level based DTC IM drive is outstanding both during the steady state and dynamic conditions. It has effectively minimized the ripple content in the output torque during the braking and loaded conditions of the drive.

VII . APPENDIX 3-Φ, delta connected, 20 hp (15Kw) squirrel cage IM Motor Parameters

• Fundamental frequency f = 50 Hz, • Rated voltage = 415 V • No. of poles P = 4, Moment of inertia J = 0.063 kg-

m2, coefficient of friction B =0 • Rotor resistance rR = 2.3268 Ω, stator resistance

sR = 1.752 Ω • Rotor inductance rL = 0.2104 H, stator inductance

sL = 0.2104 H, mutual inductance mL = 0.2028 H Inverter Parameters

• DC voltage Vdc = 651.88 V • AC Voltage available Vcm = 380 V • Switching Frequency fs = 20 kHz

VII. REFERENCES [1] Takahasi I and Noguchi T, “Quick torque response control of an

induction motor based on a new concept”, IEEE Tech. Meeting Rotating Mach., Vol. RM 84-86, Sept. 1984, pp. 61-70.

[2] Depenbrock M, “Direct self control (DSC) of inverter fed induction machine,” IEEE Transactions on Power Electronics, Vol. 3, No. 4, Oct. 1988, pp. 420-429.

[3] Vijayraghavan Praveen and Krishnan R, “Noise in Electric Machines: A Review”, IEEE Transactions on Industry Applications, Vol. 35, No. 5, September/October 1999, pp. 1007-1013.

[4] Casadei Domenico, Profumo Francesco, Serra Giovanni and Tani Angelo, “Assessment of direct torque control for induction motor drives”, Bulletin of the Polish Academy of Sciences: technical Sciences, Vol. 54. No. 3, 2006, pp. 237-254.

[5] Casadei Domenico, Serra Giovanni, Tani Angelo, “Analytical Investigation of torque and flux ripple in DTC schemes for induction motors”, IEEE Transactions on Industrial Electronics, Vol. 48, No. 3, Jun. 2001, pp. 552-556.

[6] Liu Yong, Zhu Zi Qiang and Howe David, “Instantaneous torque estimation in sensorless direct-torque-controlled brushless dc motors”, IEEE Transactions on Industry Applications, Vol.42, No. 5, Sept./Oct. 2006, pp. 1275-1283.

[7] Lai Yen-Shin, Wang Wen-ke and Chen Yen-Chang, “Novel Switching technique for Reducing the Speed Ripple of AC Drives with direct torque Control”, IEEE Transactions on Industry Applications, Vol.51, No. 4, August 2004, pp. 768-775.

[8] Casadei Domenico, Serra Giovanni, Tani Angelo, “Steady-State and Transient Performance Evaluation of a DTC Scheme in the Low Speed Range”, IEEE Transactions on Power Electronics, Vol. 16, No. 6, November 2001, pp. 846-851.

[9] Kang Jun-Koo and Sul Seung-Ki, “New Direct Torque Control of Induction Motor for Minimum Torque Ripple and Constant Switching Frequency”, IEEE Transactions On Industry Applications, Vol. 35, No. 5, September/October 1999, pp. 1076-1072.