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2013 International Conference on Power, Energy and Control (ICPEC)

978-1-4673-6030-2/13/$31.00 ©2013 IEEE 296

Harmonics and Torque Ripple Reduction of Brushless DC Motor (BLDCM)

using Cascaded H-Bridge Multilevel Inverter

M.ArunNoyal Doss1, E.Premkumar

2,G.Ranjith Kumar

3,Jahir Hussain

2

1Department of EEE, SRM University, Chennai.

2PG Scholar, SRM University, Chennai.3Department of EEE, Warangal, A.P.

Abstract

Brushless DC motors are widely used in variable speed

drive application. The torque produced by the Brushless

DC (BLDC) motors with trapezoidal back Electromotive

force (BEMF) is constant under ideal condition. However

in practice the torque ripple appears on the delivered

output torque. Smoothness of variable speed drive

operation is critical due to this torque ripple. The

brushless DC motors uses power electronic switches for

the commutation purpose, it creates the Harmonics in

armature current. This paper proposes Cascaded H-

Bridge Multilevel Inverter with current controller used to

reduce the torque ripple and the harmonics. This has

been simulated with the help of PSIM simulation software

and the amount of torque ripple and total harmonicdistortion (THD) are calculated.

Index Terms: Brushless DC motor (BLDC), Multilevel

Inverter (MLI), PSIM, Harmonics, Stator current, torque

ripple, Total Harmonic Distortion (THD).

I.I NTRODUCTION

Brushless DC motors with trapezoidal Back-EMFhave several inherent advantages. Most prominent among

them are high efficiency and high power density due to theabsence of field winding, in addition the absence of brushesleads to high reliability, low maintenance and highcapability. However in a practical BLDC drive, significant

torque pulsations may arise due to the back emf waveformdeparting from the ideal.as well as commutation torqueripple, pulse width modulation (PWM) switching. Torqueripple due to the current commutation is caused by the

mismatches between the applied electromotive force andthe phase currents with the motor electrical dynamics. It isone of the main drawbacks of BLDC drives. These torque

ripples produces noise and degrade speed-controlcharacteristics especially at low speed. Due the power

electronic commutation, the usage of high frequencyswitching of power devices, Imperfections in the stator andthe associated control system. The input supply voltage to

the motor contains various harmonics components. Duringits operation, high frequency component present in theinput voltage will cause Electromagnetic Interference(EMI) problem.

Nowadays researchers are trying to reduce the torque rippleand harmonic component in the BLDC motor. An activetopology to reduce the torque ripple is synchronous motor

presented in [1]. This paper discusses the hysteresis voltage

control method. The torque ripple is minimized usingPWM switching is presented in paper [2], this scheme has

been implemented using a PIC microcontroller to generate

modified pulse width modulation (PWM) signals fordriving power inverter bridge. In paper [3] the currentcontroller method is used for reducing the torque ripple andharmonics. This method is based upon the generation of the

quasi-square wave armature current. To reduce torqueripple the indirect position detection is used in [4], it is

based on the detection of the zero crossing points of theline voltage measured at the terminal of the motor. The

harmonics and torque ripple reduced by using the L-C filter proposed in [5]. The proposed method based upon the

current controlled technique. The armature current ismeasured and compared with the reference value to

produce the gate pulses for the multilevel inverter.

II.FUNCTIONAL UNITS

The BLDC Motor requires a power

electronic drive circuit and a commutation system for

its operation. The Fig.1 describes the functional units

present in the drive circuit and the associated

commutation controller for the BLDC Motor. A 4

pole BLDC motor is driven by the inverter for 120

degree commutation. The rotor position can besensed by a hall-effect sensor, providing three square

wave signals with phase shift of 120o. These signals

are decoded by a combinational logic to provide the

firing signals for 120o conduction on each of the three

phases. The operation of the system is as follows: as

the motor is of the brushless dc type, the waveforms

of the armature currents are quasi square. These

currents are sensed through current sensors, and

converted to voltage signals. These signals are then

rectified, and a dc component, with the value of the

ceiling of the currents, Imax, is obtained as shown in

Fig.1 This dc signal is compared with a desired

reference Iref, and from this comparison, and errorsignal Ierr is obtained. This error is then passed

through a PI control to generate the PWM as shown

in Fig.2 for all the switches of the multi-level inverter

which are sequentially activated by the shaft position

sensor.




297

Fig. 1 Current Controller of BLDC Motor with MLI

Fig. 2 Current Controller Block

III .CASCADED H-BRIDGE MULTILEVEL INVERTER

The clamped inverter, also known as a

neutral clamped converter is difficult to be expanded

to multilevel because of the natural problem of the

DC link voltage unbalancing. The flying capacitor

inverter is difficult to be realized because each

capacitor must be charged with different voltages as

the voltage level increases. Moreover, the clamped

inverter, also known as a neutral clamped converter is

difficult to be expanded to multilevel because of the

natural problem of the DC link voltage unbalancing.

It consists of two capacitor voltages in series and uses

the center tap as the neutral. Each phase leg of the

three level converters has two pairs of switching

devices in series. The center of each device pair is

clamped to the neutral through clamping diodes. The

waveform obtained from the three level converters is

a quasi-square wave output. The Switching sequence

of three phase five level MLI is represented in Fig.3

Fig. 3 switching sequence of three phase five level MLI

A simulation analysis is made on this proposed work

which is presented in Fig.4.This circuit has been

simulated using the PSIM simulation software.

Fig. 4 Closed loop PSIM diagram of BLDC Motor with Current

Controller using MLI

IV.SIMULATION RESULTS

Various simulations are done using

simulation software for the converter circuit presented in Fig. 4. The output of the inverter is

given to the BLDC motor. The motor currents are

sensed and it is given to the rectifier and the obtained

value is compared with the reference value and the

error value is processed using PI controller. The

obtained value is compared with the triangular wave

to generate controlled PWM signals. The obtained

pulses are taken from position sensor signals of the

motor to give pulses to the multilevel inverter. Fig. 5




298

shows the phase to phase voltage of the Brushless DC

motor. The harmonics effect is analyzed by taking the

FFT analysis for the voltage and the current

waveforms of the Brushless DC motor.

Fig. 5 phase to phase voltage of BLDC motor

The motor currents are sensed and it is given to the

rectifier and the obtained value is compared with the

reference value and the error value is processed using

PI controller. The obtained value is compared with

the triangular wave to generate controlled PWM

signals. The obtained pulses are taken from position

sensor signals of the motor to give pulses to

multilevel inverter.Fig.6 shows the output signal

from the Hall Effect sensor of the Brushless DC

motors.

Fig .6 output of Hall Effect sensors.

V.TORQUE R IPPLE CALCULATION

It is defined as periodic increase and

decrease in output torque .The formula for finding the

torque ripple the percentage of the difference

between the maximum torque (Tmax) and the

minimum torque (Tmin) compared to the average

torque (Tavg). Torque ripple can be calculated by the following formula

Fig .7 shows the output torque waveform of the BrushlessDC motor.

Fig. 7 Torque waveform of BLDC motor

Let us take the values for the one cycle. The

maximum value of torque ( Tmax) is 2.24,the

minimum value of the torque is (Tmin) 1.82 and theaverage value of this torque is 2.03.from using the

above value the torque ripple is calculated the

obtained torque ripple value is 20.68 %. Which is low

compared to other type of control techniques. The

amount of torque ripple is reduced due to the

smoothness of the current.

VI.THD CALCULATION

The Total Harmonic Distortion (THD)

which tells the amount of harmonics present in the

current or voltage. The Fig.8 which shows the FFT

analysis of the Phase A current of the Brushless DC

motor.it is found that THD is 14.1%.The Fig.9 which

shows the FFT analysis of the Phase-Phase voltage of

the Brushless DC motor. The amount THD alsocalculated for this waveform. The amount of THD is

24.3 %. Reduction in THD results smoothness in

armature current and the voltage waveforms. This

improves the commutation effect of the motor.

Fig. 8 FFT analysis of phase current A.




299

Fig. 9 FFT analysis of phase voltage.

Fig. 10 speed of BLDC motor.

Fig.10 shows the speed waveform of Brushless DC motor.The harmonics and the torque ripple can be reduced bysmoothening the current waveform.

VII.CONCLUSION

This paper has proposed harmonics and

torque ripples have been reduced using multilevel

inverter with the current controlled technique. The

harmonics content of the voltage and Current for a

BLDC motor is analyzed and the amount of torque

ripple and the THD also calculated. From the

simulated results it is evident that the harmonics

Components at the switching frequency and multiples

of switching frequency are reduced also the torque

ripples are reduced. The main advantage of this

method is it uses one current controller for the three

phases.

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