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Induction motor is the most sought after motor in the industry for excellent performance characteristics and robustness. Developments in the Power Electronic circuitry have revolutionised the induction motor industry leading to the developments in various control strategies and circuits for motor control. Direct Torque Control (DTC) is one of the excellent control strategies preferred by industries for controlling the torque and flux in an induction machine. The main drawback of DTC is the presence of torque ripple which is slightly more than the acceptable limit. There are various parameters that introduce ripples in the electromagnetic torque, one of them being the type of inverter circuit. There are various types of inverter circuits available and the effect of each of them in the production of torque ripple is different. This work is an attempt to identify the influence of various multilevel inverter circuits on the torque ripple level and to propose the best inverter circuit. The influence of multilevel diode clamped inverter and cascaded H bridge inverter circuits on torque ripple minimization, is analysed using simulation studies for identifying the most suitable multilevel inverter circuit which gives minimum torque ripple. The results obtained from the simulation studies are validated by hardware implementation on 0.75 kW induction motor.

Direct torque control is one of the most widely adapted schemes of control of induction motors. It is one of the simplest methods to control the stator flux and electromagnetic torque simultaneously. A detailed study on DTC, dealing with the historical and recent developments and major milestones [

The main application of direct torque control of induction motor drive is to control the flux linkage and electromagnetic torque directly by selecting proper inverter switching state with the help of a predefined lookup table. Conventional DTC uses two level and three level hysteresis controllers for flux and torque controls respectively. Even though it has many advantages like no feedback control, no traditional PWM algorithm, no vector transformation, it has some drawbacks like variable switching frequency, inherent steady state torque and flux ripples. Due to hysteresis band controller, steady state torque and flux ripples are high in the direct torque control of induction motor which is undesirable.

This paper deals with a novel inverter circuit along with the appropriate controller for reducing the torque ripple in a DTC scheme as applied to an Induction motor. Study is initially carried out in open loop mode using hysteresis comparator. The main draw back with hysteresis comparator is the introduction of torque ripple. Then, the study is repeated in closed loop mode with PI controller replacing the hysteresis comparator. Space vector modulation technique is adapted along with PWM technique for a three level inverter. The work carried out is presented as given below.

Section 2 deals with the mathematical modelling of induction motor. Section 3 deals with the effect of various multilevel circuits in reducing the torque ripple. Three level neutral point clamped SVM circuit based DTC and the design of PI controllers are described in Section 4. Section 5 discusses about the simulation results of the proposed scheme and the proposed fuzzy based SVM-DTC is discussed in Section 6. Section 7 deals with the hardware implementation and the conclusions are discussed in Section 8.

The circuit model of the induction machine with d-axis fixed along the stator flux axis is shown in

where

Since d-axis is fixed along the stator flux

where, _{s} is the stator flux. ω_{m} is the angular

motor speed. ω_{s} is the stator flux speed. P denotes

The elementary concept of a multilevel inverter to achieve high power by a series of power semiconductor switches with several low voltage dc sources is applied for power conversion to generate staircase voltage waveforms. Capacitors, batteries, and renewable energy voltage sources can be used as the multiple dc voltage sources. Different multilevel circuits used in the proposed study are Cascaded H bridge multilevel inverter (CHBMLI) and Diode Clamped Multilevel Inverter (DCMLI) which are much superior to Flying Capacitor Multilevel Inverter. An induction motor drive with five level CHBMLI and seven level CHBMLI are considered for simulation study using PSIM. The simulation circuit of DTC with a five level CHBMLI in open loop mode is shown in

During simulation, the motor is run on no load and then load is increased gradually up to the rated load of 19Nm. The same procedure is repeated with a seven level CHBMLI. The study is limited to inverter level up to seven as the cost and complexity may increase with levels higher than seven. The variation of the total harmonic distortion for different load torques is shown in

The CHBMLI is then replaced by a DCMLI of three, five and seven levels and simulation study of DTC is repeated on the motor of same rating. A three phase three-level inverter requires twelve switching devices and six clamping diodes per leg as shown in

The comparison of the performance of the motor in terms of variation of THD with load torque for all the three types of inverters is shown in

It is very clear from the simulation analysis that the THD decreases as the inverter level increases and hence it can be concluded that the performance of the induction motor fed by SLDCI is better when compared with the performance of FLDCI or TLDCI fed induction motor. Further, detailed analysis and hardware implementation for validating the results is carried out in closed loop form with a motor using TLDCI with SVM-DTC to reduce

the investment cost.

Direct flux and torque control with space vector modulation (DTC-SVM) schemes are proposed in order to improve the performance of classical DTC of Induction motor. The DTC-SVM strategy, as shown in

Three level neutral point diode clamped inverter employed, in the proposed DTC scheme, is shown in

Each leg has four active switches S1_{ }to S4_{ }with parallel diodes D1 to D4. The capacitors at the DC side are used to split the DC voltage to provide a neutral point Z. The clamping diodes Dz1 and Dz2 are connected to the neutral point. When switches S2 and S3 are closed, the output terminal A can be taken to the neutral through one of the clamping diodes. The voltage of each DC capacitor is E, which is equal to V_{d}/2. A three-level inverter is characterized by 27 switching states as indicated in

・ Zero vector (V_{0}), represents three switching states [1 1 1], [0 0 0] and [−1 −1 −1]. The magnitude of V_{0 }is Zero.

・ Small vectors (V_{1} to V_{6}), have a magnitude of V_{d}/3. Each small sector has two switching states 1 and −1 and they are classified as P or N type small vector.

・ Medium vectors (V_{7} to V_{12}), have the magnitude of

・ Large vectors (V_{13} to V_{18}), have the magnitude of

Switching state | Device switching status (Phase A) | Inverter terminal voltage V_{AZ} | |||
---|---|---|---|---|---|

S1 | S2 | S3 | S4 | ||

1 | On | On | Off | Off | E |

0 | Off | On | On | Off | 0 |

−1 | Off | Off | On | On | −E |

In Space Vector PWM (SVPWM), the voltage vector is approximately calculated by using three adjacent vectors in the given region.

In a three-level inverter, similar to a two-level inverter, each space vector diagram is divided into six sectors. The switching pattern explained for Sector I is same for all other sectors. Sector I is divided into four regions as shown in _{ds} and V_{q}_{s} and then the sector in which the command vector V_{ref} is located is determined. If θ is between

・ 0˚ ≤ θ < 60˚, Sector 1,

・ 60˚ ≤ θ < 120˚, Sector 2,

・ 120˚ ≤ θ < 180˚, Sector 3

・ 180˚ ≤ θ < 240˚, Sector 4,

・ 240˚ ≤ θ < 300˚, Sector 5,

・ 300˚ ≤ θ < 360˚, Sector 6.

However, the working region is identified as given below, using two additional vectors

・ If _{d}, then

・ If _{d} and (_{d}, and then

・ If _{d}, then

・ If _{d}, then

The principle of SVPWM method is based on the command voltage vector which is approximately calculated by using three adjacent voltage vectors. The duration of each voltage vectors obtained by using voltage time equation of vector

where V_{1}, V_{2} and V_{0} are vectors that define the triangle region in which _{a}, T_{b} and T_{c} are the corresponding vector durations and T_{s} is the sampling time. T_{a}, T_{b} and T_{c} give the switching time for sector I as given in _{ref} in sector I region 1 is shown in

PI controller is the one which controls the dynamic performance of the machine. The gains of PI controllers for the torque and flux loops have to be tuned properly to minimize the torque ripple. In the proposed work, separate PI controllers are used for torque, flux and speed control of induction motor.

1) Torque PI controller

REGION 1 | REGION 2 | |
---|---|---|

T_{a} | ||

T_{b} | ||

T_{c} | ||

REGION 3 | REGION 4 | |

T_{a} | ||

T_{b} | ||

T_{c} |

Intervals | Region 1 | Region2 | Region3 | Region4 |
---|---|---|---|---|

1 | T_{b}/8 | T_{a}/6 | T_{a}/4 | T_{c}/4 |

2 | T_{a}/4 | T_{b}/3 | T_{c}/2 | T_{b}/2 |

3 | T_{c}/4 | T_{c}/3 | T_{b}/2 | T_{a}/2 |

4 | T_{b}/4 | T_{a}/3 | T_{a}/2 | T_{c}/2 |

5 | T_{a}/4 | T_{c}/3 | T_{b}/2 | T_{a}/2 |

6 | T_{c}/4 | T_{a}/3 | T_{c}/2 | T_{b}/2 |

7 | T_{b}/4 | T_{b}/2 | T_{a}/4 | T_{c}/4 |

8 | T_{c}/4 | T_{c}/3 | ||

9 | T_{a}/4 | T_{a}/6 | ||

10 | T_{b}/4 | |||

11 | T_{c}/4 | |||

12 | T_{a}/4 | |||

13 | T_{b}/8 |

Motor Equation from (1) to (8) can be modified and written as given in Equations (15) to (17).

The last term in the right side of above equation is negligibly small and it can be considered as zero compared to the values of other terms.

Under no load condition the change in motor speed can be expressed as shown in Equation (18).

The current I_{qs} can be expressed as shown in Equation (19).

Based on the equations of the motor, the open loop transfer function can be expressed as

where the constants A, B and C_{ }are defined as given in Equations (22) to (24).

Closed loop block diagram for the torque PI Controller is shown in _{p} and K_{i} of the PI controller are calculated assuming the values of settling time t_{s} and peak overshoot Mp such that t_{s} £0.003 sec and maximum peak overshoot Mp £ 2%. The values of K_{p} and K_{i} are obtained 1.5 and 100 respectively. Similarly, the parameters of PI Controller for flux and speed are calculated. The value of K_{p} for flux and speed controller are 4441 and 187.56. The value of K_{i} for flux and speed controller are 107.38 and 132.38 respectively.

An induction motor of rating 0.75 kW is considered for analysis. The inverter used here is a Three Level Diode Clamped Inverter(TLDCI). The simulation of TLDCI fed IM with DTC-SVM with torque controller is performed using MATLAB software. The motor is started and run on no load at a speed of 1500 rpm and a load of 5 Nm is applied at 0.25 seconds. The speed drops to 1478 rpm and settles at 1500 rpm within 0.005 seconds as shown in

The load on the motor is, then, reduced to 3 Nm at 0.45 sec and the corresponing torque response is shown in

Due to unexpected load changes or environmental factors, the motor shaft vibrates and produces oscillations in the motor speed and torque until it reaches the set speed. The presence of these oscillations reduce the performance of the machine. The PI controller is replaced by fuzzy logic controller with a view to improving the performance

Fuzzy logic control is the process of formulating the mapping from a given input to an output using fuzzy logic. The classical PI controller is replaced by fuzzy logic controller (FLC) to improve the performance. The torque controller shown in

based DTC drive. The FLC is designed with the knowledge of the response of the system obtained with PI controllers. The error and the change in error obtained from the reference torque and simulated torque are scaled and fed to the fuzzy logic controller. The scaling factor for change in error is 1/30 and for the output, it is 17. The FLC controller is designed to give the change in crisp voltage cV_{qs} and it is integrated to get the voltage V_{qs}. By controlling the torque and flux amplitude, a gate signal for inverter is generated. The Equations (25) to (28) are used in the implementation of the FLC scheme.

The inputs e(k) and ce(k) are mapped to the FLC to generate the change in the voltage cV_{qs} as the output. The fuzzy set L for the error and change in error is defined as

The input variables are the error (e) and the change in error (ce) of the torque. The error variable is quantized into seven fuzzy set as Negative Small NS, Negative Medium NM, Negative Big NB, Zero Z, Positive Small PS, Positive Medium PM and Positive Big PB. All the membership functions chosen are of the triangular type. The membership function of the fuzzy controller input variables and output variable are shown in Figures 17-19. The rules of the fuzzy controller are shown in

The simulation of TLDCI fed IM of DTC-SVM with fuzzy controller is performed using MATLAB software. The motor is started at no load and a load torque of 5 Nm is applied at 0.25 sec, the speed oscillates for short duration and settles at set speed of 1500 rpm and the response is shown in

The load on the motor is reduced to 3 Nm at 0.45 sec and the torque response is shown in

E ce | NB | NM | NS | Z | PS | PM | PB |
---|---|---|---|---|---|---|---|

NB | NB | NB | NB | NM | NS | NVS | Z |

NM | NB | NB | NM | NS | NVS | Z | PVS |

NS | NB | NM | NS | NVS | Z | PVS | PS |

Z | NM | NS | NVS | Z | PVS | PS | PM |

PS | NS | NVS | Z | PVS | PS | PM | PB |

PM | NVS | Z | PVS | PS | PM | PB | PB |

PB | Z | PVS | PS | PM | PB | PB | PB |

Machine | Speed (rpm) | Load Torque (Nm) | Torque ripple (%) | |
---|---|---|---|---|

PI Controller | Fuzzy controller | |||

1000 | 5 | 24.30 | 17.84 | |

0.75 kW | 1200 | 5 | 22.34 | 16.91 |

1500 | 5 | 21.78 | 14.33 |

Hardware implementation of three level diode clamped inverter fed induction motor using DTC-SVM is carried out in the laboratory with a view to validate the simulation results of PI and fuzzy controllers. The block diagram of the experimental set up of 0.75 kW TLDCI fed IM with DTC-SVM using DSP processor as shown in _{d}, I_{a}, I_{b}, I_{c} and one hall effect voltage transducer senses the DC link voltage (V_{d}). The speed sensor is used to measure the speed of the induction motor in terms of frequency of square wave which is fed to a frequency to voltage converter circuit. The signal conditioner circuit is used to convert the current and voltage signals combatable with the protection circuit and the ADC of the DSP processor. The protection circuit is used to provide protection against over voltage, over current and under voltage. The current and voltage obtained from signal conditioners are given to the ADC inputs of DSP. The space vector modulation scheme is implemented in DSP processor. The schematic diagram for firing pulse and protection circuit is shown in

The speed response is shown in

On comparing the simulation results in

The above simulation method is extended to 3 kW machine and the performance analysis is as explained below.

Analysis of 3 kW Induction MotorAn induction motor drive of rating 3 kW is fed with TLDCI fed DTC-SVM with PI controller run at no load at a speed of 1500 RPM and a load torque of 15 Nm is applied at 0.25 seconds. The electromagnetic torque reaches the steady state value of 15 Nm at t = 0.257 s after the decay of transients and then reduced to 10 Nm at 0.45 seconds. The corresponding torque response is shown in

The simulation study is repeated on the same drive for 1000 rpm and 1200 rpm and the torque ripple in all the cases are measured. The PI controller is replaced by a fuzzy controller and the simulation study is carried out. Initially, the motor runs under no load, the electromagnetic torque overshoots to a value of 22 Nm and reaches the steady state value at 0.07 seconds after the decay of transients. A load of 15 Nm is applied at 0.25 seconds, the electromagnetic torque reaches the steady state value of 15 Nm at t = 0.257 s after the decay of transients and then reduced to 10 Nm at 0.45 seconds. The corresponding torque response is shown in

The percentage of torque ripples calculated for different set speeds for a constant load torque of 15 Nm with PI and fuzzy torque controller is tabulated in

Machine Rating | Speed (rpm) | Load Torque (Nm) | Experimental response of Torque ripple (%) | |
---|---|---|---|---|

PI controller | Fuzzy controller | |||

1000 | 5 | 23.38 | 16.90 | |

0.75 kW | 1200 | 5 | 22.12 | 15.25 |

1500 | 5 | 21.42 | 14.28 |

Machine Rating | Speed (rpm) | Load Torque (Nm) | Simulated response of Torque ripple (%) | |
---|---|---|---|---|

PI controler | Fuzzy controler | |||

1000 | 15 | 14.5 | 12.47 | |

3.00 kW | 1200 | 15 | 12.80 | 11.80 |

1500 | 15 | 10.07 | 9.9 |

This paper investigates the influence of multilevel inverter circuits on the production of torque ripple and to identify the most suitable multilevel inverter to reduce the torque ripple when DTC scheme of induction motor is adapted. A detailed analysis of DTC is carried with H Bridge multilevel inverter and diode clampled multilevel inverter and it is concluded that DTC with Diode clamped multilevel inverter gives less ripple in comparison with DTc using H Bridge multilevel inverter. The proposed scheme is implemented and validated using a three level Diode clamped inverter circuit. The proposed scheme uses PI and fuzzy controllers for three-level space vector modulation based neutral point clamped inverter. It is observed that the torque ripples are significantly reduced and better dynamic performance is achieved with fuzzy controller in comparison with conventional PI controller. The simulation study is validated with the help of an experimental set up for 0.75 kW motor. The simulation analysis is repeated with a 3 kW motor and there also, the ripples are less with a fuzzy controller in comparison with those obtained with PI controller. Based on the study carried out, it can be concluded that torque ripples in a DTC scheme as applied to induction motor can be considerably reduced by using a multilevel inverter circuit with space vector modulation technique along with fuzzy controllers.

Manoj Kumar Nadesan,Geetha Ramadas,Chellamuthu Chinnagounder, (2016) A Novel Multilevel Inverter Circuit for the Performance Enhancement of Direct Torque Controlled Induction Motor. Circuits and Systems,07,2771-2794. doi: 10.4236/cs.2016.79237

Specifications of the motor used:

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