Starting Current wth Soft Starter
Starting Current wth Soft Starter
(OP)
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.
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.





RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
Think of the reduced voltage starter as a varibale voltage transformer. Input V*I = Output V*I
The source side voltage is not changing only output(motor) side is, so obviously the current will be different on the two sides when voltages are different to deliver a given power.
RE: Starting Current wth Soft Starter
There is NO transformer action on a solid state starter! Maybe you were thinking of an Auto-Transformer starter. In a solid state starter, voltage is not transformed, it is reduced by using partial waveforms. So really it can be considered line potential for smaller time slices, hence a lower RMS voltage, which in combination with the motor impendence, reduces the current consumption.
Line current and motor current are the SAME in a solid state soft starter!
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
It's easy to show how a conversion device like a transformer changes voltage and current on either side of the device and that Input V*I = Output V*I, but a solid state soft starter is not a conversion device, so I'd like Mr davidbeach, or Mr rbulsara to explain to me how a soft starter changes the current as suggested in their comments.
RE: Starting Current wth Soft Starter
Assume for the moment that the starter is lossless. One method that can be used is to turn on the thyristors for a portion of each half cycle. V in is a sinusoid, V out is non-sinusoidal and has a much lower rms value. Power out equals power in, therefore since V in and V out are not equal, it is not possible for I in and I out to be equal. It is entirely possible for I (scalar quantity, magnitude only) in to equal I out.
RE: Starting Current wth Soft Starter
I would be interested to know if the voltage does not change how does the motor see "reduced" voltage? and if does, then why V.I in will not be V.I out? Less the losses in the electronics?
You possibly can't have 480V in and 144V (30%) out and not have different currents. If so then the solid state starter will have losses equal to a primary resistor or reactor starter. Is that true?
RE: Starting Current wth Soft Starter
But of course, since right at start the motor is not being sent full voltage. The motor side will have the power consumption defined by that (partial) starting voltage and the current. The same amount of 'power' will be drawn by the starter but at Vsource. The result is you will need less current on the start side than the current of the motor side. 30% in this case.
The key is "constant power" as there is no energy "storage" or consumption in the starter. (except losses)
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
And for the moment, trust me when I say that if you put a CT on the input side or the output side of an RVSS starter, the current measures almost exactly the same (1.5W or heat per amp per phase is the only power loss).
Stop thinking of power "conversion", be it by rectification, as in a VFD, or by induction via transformation or by restriction via resistance/impedance. An RVSS is "restricting", but not with resistance/impedance. It plays with time, not magnetism or chemical interaction, to NOT ALLOW full power through the devices. To understand what I mean, go back to basics for a moment. What is a Volt? The potential that makes 1 Amp flow through a conductor and dissipate 1 watt. What is an Amp? 1 coulomb of charge per second through a conductor. What an RVSS is doing is altering the time domain of that basic equation. By only allowing power flow for a restricted time, the current, and by definition the voltage, is lowered. So even though you can measure the RMS voltage on the load side as being lower than the line side, it is different by virtue of a reduced net amount of current flowing through the circuit. But from an Amperage standpoint, the current is the current, regardless of which side of the SCR you measure it on.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Starting Current wth Soft Starter
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Starting Current wth Soft Starter
The current "in" must be the same as the current "out". . However the incoming current is zero for an extended time until the SCRS fire on. On the output the current is the same but the voltage is chopped (not reduced as for a transformer). The product “VxI” is the same on both sides.
See figure below.
RE: Starting Current wth Soft Starter
The hardest thing to overcome is the concept that full voltage on the line side times the current passing thru to the motor side doesn't equal the motor side voltage times the current. If you consider that the voltage on the line side is only "driving" current during the part of the cycle that the thyristors are firing, and you disregard the voltage of the non-conducting part of the cycle, then the applicable line side and the motor side RMS voltages are the same. Then the fundamental equations that we grew up on remain valid. Everyone can then be happy.
RE: Starting Current wth Soft Starter
Good picture. OK. I buy that. Learn a thing everyday.
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
Here's the pics from Siemens "starters" page.
These seem to be showing that they're switching at nonzero voltage points - on AND off. ???
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Starting Current wth Soft Starter
The input voltage waveform during the time that the SCRs are conducting, will be the same as the output voltage wave form. When the switch is closed, the voltage is the same on both sides of it.
If the effective value of this waveform is used instead of the effective value of the input voltage sin wave, the equations will balance.
Think of a flashing light that is on 50% of the time. The output voltage equals the input voltage 50% of the time.
Similarly, the soft starter input voltage equals the output voltage only during the time that the SCRs are conducting.
respectfully
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
Can I blame this temporary blockage of my understanding on my personal termoils going on in my house?
RE: Starting Current wth Soft Starter
I made some recordings. This is what it looked like when we finally got it working:
Voltage in upper trace and current in lower trace. We are at about 50 percent of end speed. Current scale is 3000 A/division.
It may be interesting to see what happened when we tried to start with the fault still in the controller (one firing pulse missing). It looked like this:
The scale factor is now 6000 A/division. As you can see, the missing firing pulse produces a DC component, which not only drives motor current high - it also saturated the transformer feeding the system so that it had a very high overcurrent which made a protective relay fail. It did that after about ten minutes. There were probably other damages to the system as well. But I left when the soft starter was OK. So I do not know what else had happened.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Starting Current wth Soft Starter
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Starting Current wth Soft Starter
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
Distortion power was used as a way of explaining what harmonics do to the apparent power. Remember the 3D Pythagoras rule used by some educators and salespeople that actually had no idea what they were talking about? Nor had their listeners, I think. They just got confused. That's why you do not see this concept any more.
The soft-starter just lets a smaller part of the sine voltage through. The smaller the part, the smaller the RMS voltage - and hence the current. It is no more complicated than that.
jwilson3 is right. And so are the guys supporting his view. I jumped in only because I thought that my recordings are illustrative. The soft-starter has a very limited ability to manipulate the voltage (Can only fire thyristors, has to take into consideration that the phase/phases returning the current must be "on" when thyristor is fired. It must also take the inductive nature of the load into account. As you can see, the thyristor conducts long after zero voltage crossing) All these circumstances complicate things considerably. But it does not change the fact that Current In = Current Out.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Starting Current wth Soft Starter
You are batting 1000 buddy!
That 2nd waveform posting is not a wave form for a true 3 phase controller. Siemens (and a few others) have a "low cost" version of their soft starters that only control 2 of the 3 phases in an attempt to reduce the component count. One phase is just a piece of bus bar. While this works OK on low inertia loads, it is somewhat controversial and IMHO should not be used where long acceleration times are expected. That waveform shown is for their 3RW - 2 phase controller only, and even then it is a little "simplistic" to say the least. They publish this sort of stuff because people who don't understand it get excited from comments from competitors about negative sequence currents and DC components imposed on the sine wave which can theoretically damage the motor windings. They are trying to simplify the concept for the masses.
Most of the "waveforms" published for soft starter outputs are simplified graphical representations of one phase, partly because of exactly what Gunnar's post shows; the complexity of a scope readout. Remember, a soft starter is never controlling just one phase at a time, it is controlling 6 SCRs which are almost carrying current from more than one line phase 2/3 of the time. The effect on scope traces of the output voltage waveform in any one phase becomes very complex to interpret.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Starting Current wth Soft Starter
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Starting Current wth Soft Starter
Skog's scope pictures are quite a bit better. Nasty complex especially in the one phase of three aspect.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Starting Current wth Soft Starter
With SCR's the switching is done at zero current, not zero Voltage, and from the manufacturers diagram of the current waveform, you can see that indeed it is switching at zero AMps.IF you look at the opeation of a SCR Rectifyer operating in the regeneration region, the switching is always done at quite high voltages, both on and off.
Like Gunnar, I spend enormous amounts of my customers money investigating problems with these things. Have literally CD's full of high speed recordings of all sorts of things, except, sadly, Thyristor starters.
Stars to jreff and others for some very good points
RE: Starting Current wth Soft Starter
I have a lot of respect for the knowledge and experience of both of you. Your remarks are more flattering than quite a few LPSs.
Thanks
Respectfully
RE: Starting Current wth Soft Starter
When the SCR is conducting, we have the same voltage input and output minus tha voltage drop across the SCRs which is typically around 1 - 1.2 volts per SCR. The current is the same, so at each instant that the SCR is ON, the VA input equals the VA output minus the conduction losses of the SCR. For each instant that the SCR is OFF, the VA input equals the VA output, so the laws of conservation etc are fully met.
The difficulty comes when considering discontinuous waveforms. At that time, you can not take the average voltage times the average current to get the average power. This is a stunt commonly pulled by the promoters of energy savers!!
Unfortunately, much of the technical literature about SCR control of induction motos shows waveforms that are true for an SCR based controller with a resistive load. This shows the current waverform to be very different from reality and the commutation occuring at 0 and 180 degrees.
The reality is that the current is smoothly rounded and takes on the double hump, and it commutates at an angle which reflects the power factor of the motor, hence the commutation angle changes with slip.
During the non conduction period, the voltage on the motor terminals does not fall to zero as logic would suggest. The spinning motor acts as a generator and creates it's own voltage profile to fill in the gap. The actual voltage seen is dependant on the voltage generated, but can make the waveform "applied" to the motor seem prety wicked at times. The rality is that the waveform applied to the motor is only the portion where the SCRs are conducting.
So in calculating power and VA, we need to take the integral of the products of the instanteous values to get meaningful results.
Returning to the OP, for voltage drop calculations, is is usually sufficient to consider the average start current to get reasonable results. The maximum start current flows around the voltage zero crossing at zero shaft speed and increases higher up the sinewave as the motor reaches full speed. This is the same for a standard induction motor starter. The major difference is that there is a non conduction period before the voltage crest that can at times include the voltage crest.
So treat the soft starter like a full voltage starter with a reduced average start current. - somewhat like a primary resistance starter with a very small power loss.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Starting Current wth Soft Starter
I have learned a truck-load on this one.
Looking at a truly 3ph system by looking at only one of the ph's really adds to the confusion.
Rather like looking at an elephant thru a soda straw at night.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Starting Current wth Soft Starter
I think that MarkE's comment is the final word. He is THE Soft-Starter Authority. He, and Jeff, I meant to say.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Starting Current wth Soft Starter
Mark, you need to get the Google tool bar spell checker!
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Starting Current wth Soft Starter
'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!
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Sometimes I only open my mouth to swap feet...
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
Still hanging tough davidbeach? Kind of alone out there.
RE: Starting Current wth Soft Starter
RE: Starting Current wth Soft Starter
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
RE: Starting Current wth Soft Starter
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.