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Home » Soft Starters Knowledge » Does soft starter causes current harmonics?
Does soft starter causes current harmonics?
A soft starter does not create harmonics during Run but only during the soft start process. Typically this is a short time process (about 1-15 seconds typically).
The current harmonics created depends strongly on the ratio (r) between the actual (soft) starting current and motor DOL (Direct on Line) current.
During most of start process, current THDi is 8 – 20% but there are cases (r=0.8-0.9) where it is much lower 4 – 8%
Strongest Harmonics is 5th. 7th is maybe 50% of 5th and rest are much lower.
THDV is mostly determined by 5th harmonics.
Let us now have a case study as follows:
• Transformer kVA is 3 times more than motor's kVA.
• Uk% of the transformer is 6%.
• Assume that the motor will start at 350% of its rated current.
• Assume DOL current is 450% of motor rated current.
Since the transformer power are 3 times bigger than motor, normalized to motor Uk%' = 6*1/3=2%.
With 5th harmonic of 10% (practical value for starting at 3.5In for motor with DOL current of 4.5In), we get, at 3.5 times rated current:
5th harmonic voltage drop can be calculated as:
5(harmonic causes the impedance to increase 5 times)*0.1(5th current harmonic)*0.02(transformer impedance per unit motor power)*3.5(times rated current)*100(%)=3.5% (5th voltage harmonic).
7th harmonic voltage drop can be calculated as:
7th Current Harmonic is 5%.
7*0.05*0.02*3.5*100=2.45% (7th voltage harmonic).
Other Harmonics are negligible.
Total expected THDV is therefore: sqrt(3.5^2+2.45^2)=4.272%
Note that this value only applicable in short time during the starting process.
All other running machines in the plant practically reduce the system impedance so THDV may be even lower.
One thing to remember – Install the soft starter's mains input line and output line separated from other lines (especially from any control lines) so that during starting conditions, you avoid any disturbances.
See also a typical harmonic level of a 140A 6.6kV soft starter with a motor of 105A. You can see that the THDi is around 38% only at the beginning of the starting process and then it decays to much lower values.
Based on the above and on our rich experience on industry, we believe it should cause no disturbance to other systems.
The current harmonics created depends strongly on the ratio (r) between the actual (soft) starting current and motor DOL (Direct on Line) current.
During most of start process, current THDi is 8 – 20% but there are cases (r=0.8-0.9) where it is much lower 4 – 8%
Strongest Harmonics is 5th. 7th is maybe 50% of 5th and rest are much lower.
THDV is mostly determined by 5th harmonics.
Let us now have a case study as follows:
• Transformer kVA is 3 times more than motor's kVA.
• Uk% of the transformer is 6%.
• Assume that the motor will start at 350% of its rated current.
• Assume DOL current is 450% of motor rated current.
Since the transformer power are 3 times bigger than motor, normalized to motor Uk%' = 6*1/3=2%.
With 5th harmonic of 10% (practical value for starting at 3.5In for motor with DOL current of 4.5In), we get, at 3.5 times rated current:
5th harmonic voltage drop can be calculated as:
5(harmonic causes the impedance to increase 5 times)*0.1(5th current harmonic)*0.02(transformer impedance per unit motor power)*3.5(times rated current)*100(%)=3.5% (5th voltage harmonic).
7th harmonic voltage drop can be calculated as:
7th Current Harmonic is 5%.
7*0.05*0.02*3.5*100=2.45% (7th voltage harmonic).
Other Harmonics are negligible.
Total expected THDV is therefore: sqrt(3.5^2+2.45^2)=4.272%
Note that this value only applicable in short time during the starting process.
All other running machines in the plant practically reduce the system impedance so THDV may be even lower.
One thing to remember – Install the soft starter's mains input line and output line separated from other lines (especially from any control lines) so that during starting conditions, you avoid any disturbances.
See also a typical harmonic level of a 140A 6.6kV soft starter with a motor of 105A. You can see that the THDi is around 38% only at the beginning of the starting process and then it decays to much lower values.
Based on the above and on our rich experience on industry, we believe it should cause no disturbance to other systems.
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