Linear loads
 It is a load in which impedance remains constant. The current input is sinusoidal if the applied voltage is sinusoidal.
Non linear loads
 It is a load in which impedance varies for a given time period. The current input is non sinusoidal if the applied voltage is sinusoidal.
Sources of harmonics
( a ) Six phase bridge rectifier
 The current pulse is rectangular in shape which produces harmonic in the order of
n = 6k ± 1 (
Where k = 0, 1, 2, …. )
Amplitude of
current pulse
I_{n} =
I / n
Where n =
harmonic
I = Fundamental
current
I_{n} =
nth harmonic current
 The calculation of harmonic current according IEC is calculated as below
I_{n }= I
/ ( n – 5 / n ) ^{1.2} 5 <
n < 11
 The harmonic current as a percentage of fundamental current
Fundamental current

5^{th}
harmonic current

7^{th}
harmonic current

11^{th}
harmonic current

I

I_{5}

I_{7}

I_{9}

100%

18.9%

11%

5.9%

 The inductance of the DC circuit affects the harmonic current. Lower value of inductance means high ripple content.
( b ) Alternator
 The alternator does not provide pure sinusoidal voltage in spite of linear loads. It supplies pre – existing harmonic voltages
( c ) Switch Mode Power Supply ( SMPS )
 The harmonic current induced by the SMPS depends upon type of load.
( d ) Lighting load ( Fluorescent lamp, Discharge lamp )
 The harmonic current supplied by the electronic ballast and fluorescent lamp is given as below.
 The electronic ballast supply lower harmonic content as compared to switch mode power supply.
Fundamental current

3^{rd}
harmonic current

5^{th}
harmonic current

7^{th}
harmonic current

I

I_{3}

I_{5}

I_{7}

100%

35%

27%

10%

( e ) Uninterruptible power supply
 The harmonic current supply by UPS is shown as shown below
Fundamental current

5^{th}
harmonic current

7^{th}
harmonic current

11^{th}
harmonic current

I

I_{5}

I_{7}

I_{9}

100%

33.0%

2.5%

6.1%

( f ) Arc furnace
 The arc furnace is used for production of steel supplied through either AC or DC.
 The arc is unstable, asymmetric and non linear.
 It will produce odd and even harmonics.
 The level of harmonic depends upon type of furnace, its power rating, process time etc.
 When arc furnace is supplied through DC , arc produced being more stable than the AC.
( g ) Magnetic saturation in the transformer
 When the applied voltage to transformer primary is 10% higher than the rated voltage, magnetic circuit will saturate and current waveform distorted resulting generation of odd harmonics.
( h ) Voltage sources
 The generation of harmonic due to voltage sources such as alternator, UPS even though the load is linear.
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