Starting Current wth Soft Starter 8

[jwilson3]

I'm in a disagreement with an Engineer with a major electrical equipment manufacturer over the basics of a thyristor controlled soft starter. I am trying to calculate the voltage drop profile on the supply system during starting.

The Engineer's analysis shows the initial starting voltage at 30% and the motor current at 330A with 20%PF and ramps up to 598A as the voltage is raised. That's the range of current values I say needs to be used in the voltage drop calc. The Engineer says the current to use is the current on the source side of the starter, which would be 30% of the current on the motor side at start and would rise to equal the motor side current as the voltage is ramped up.

I say he's wrong, that the current is virtually the same on either side of the starter.

 

[ScottyUK]

jraef,

'Forst' is a word where I come from: forst, sicond, thord, forth...

This has been an interesting thread - thanks to all for making me get my notes out and look some stuff up. Sometimes I forget howe much I used to know!


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[cbarn24050]

Well done Marke! a correct explaination.

 

[stevenal]

We can avoid the whole distortion power question by simply saying that active power is always conserved, while non-active power in general, is not.

Still hanging tough davidbeach? Kind of alone out there.

 

[davidbeach]

I don't know. If all the soft starter can do is switch the current, then current in and current out have to be the same thing; I think I was trying to use input voltage as the reference and carrying the same phase angle over to voltage out, but looking at the various plots above, it becomes clear (to me anyway) that the voltage phase angle has to change across the starter, therefore the phase angle between current and voltage changes between the two sides. What ever the details, I will certainly stand behind my first statement that power doesn't change.

 

[Marke]

Hello davidbeach

The soft starter is literally three solid state AC switches in series with the three phases to the motor.
The switches (SCRs) can only be turned ON or OFF and not change to the phase angle occurs when current is flowing.
The voltage patterns get confusing because of the voltage generated by the motor which fills in some of the OFF time.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jwilson3]

To davidbeach,
You are correct that power doesn't change as you cross thru the starter. However, you can't use full voltage on the source side in the calculation. The "effective" value of that portion of the voltage waveform that's being "utilized" by the motor while the solid state devices are "firing" is what's used in calculating power in. And it's the same as power out. Current in is the same as current out, and voltage in is the same as voltage out.