Basics of Soft start 1

[gokulkrish2]

Hi all.

I am in the process of getting known about soft starters. So, I thought it will be good to get some inputs from the experts.

We have a SMC Flex Soft Starter.

We use current limit Start.

Now i want to know how the soft starter is able to reduce the current.

Is is reducing the voltage. I hope it acts in that way.

Now they claim that the by pass contactor will come in once the motor comes up to speed. They sense the speed by back emf.

How do they sense the back emf. is it the actual voltage minus the drop?

May be its a dumb question but it keep running in my mind. Is there a way to measure the back emf of a motor?

gokul

 

[jraef]

Yes, Current Limit is accomplished by measuring current and adjusting RMS output voltage by way of Phase Angle Firing of the SCRs to maintain that limit setting.

"Sensing the back EMF" is a very short answer they use to describe an issue that is generally too complex for the average user to comprehend, but it sounds like a good explanation and most people don't question it further. What they really do is that in order to accomplish the Phase Angle control they are using, they need to know the Zero-Cross point of the incoming AC sine wave. But you can't sense Zero, so you sense the cross point of two phases and calculate it. Now that you already know this, you can compare this to the phase angle you are firing at to know where the output RMS voltage sine wave going to the motor is at any given point. Current is of course measured as mentioned above, so knowing the voltage phase angle and the current phase angle, they can indirectly determine the Power Factor. When the PF stops increasing, they can assume that the motor has finished accelerating.

Other mfrs do it more simply by just monitoring current and when the current goes above FLA and then drops again, it must mean that the motor has accelerated. Neither method is terribly accurate, but for the purposes it is used for, it's more that adequate.

The third method, and the one that both of the other two seek to avoid, is to just have a timer that closes the bypass at a fixed time value. The problem with this is that if it closes too soon, the current will still spike and if too late, there may end up being too much heat in a sealed enclosure.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[JBinCA]

Jraef,

I'd never considered it before, but the method you describe as "Sensing the back EMF" makes a lot of sense.

I can't begin to understand, however, how it would come to be know as ""Sensing the back EMF".

Anyone have any ideas about this?

JBinCA

 

[Skogsgurra]

There was a discussion on back EMF in thread237-227962
It may straighten a few question marks.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

It's marketing. The unit is not really measuring the back emf.

In the simplest form, the unit will just regulate the current. Eventually, the SCR's will end up fully turned on as the motor accelerates and the current falls from the starting current to the running current. At that point, the motor is very close to being up to speed so the unit may also check for things such as a timer timing out or the current falling below a certain value before energizing the bypass.

 

[jraef]

It's marketing. The unit is not really measuring the back emf.

Yes, that was I was trying to get at. When you try to describe what is really going on to a non-engineer end customer, it gets too heady. So the marketing people spit out "sound bites" such as "sensing the back EMF" that essentially scares most people away from asking for more details. I once heard this term being used by one of the snake-oil "energy saver" sales people at a trade show and I asked him to explain it, knowing he couldn't. Turns out he didn't even know what EMF stood for!


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[Marke]

Not necessarily so, I have done some work many years ago using the voltage generated by the motor during the non conduction period to gain an insight into the operation of the motor.
It is commonly done in SCR based DC controllers and can also be done with soft starters.

Best regards,

Mark Empson
L M Photonics Ltd

 

[jraef]

If anyone in the world had done it Mark, I would have bet it was you...

But as we know, most manufacturers are unwilling to pay for someone of your calibre to work on their product designs.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[LionelHutz]

Interesting Mark, but in theory anyways, the bypass should be energized when there is no non-conduction period, ie when the SCR's are fully on. There would not be a time to measure back emf when the SCR's are fully on.

 

[Marke]

Hello LionelHutz

Correct, this is a means to determine when to close the bypass contactor etc and is reliant on there being a non conduction period for the SCRs.

I used this technique to predict a stall condition back in the Nola energy saver days as once you stalled the motor, the slowing motor exhibited a falling power factor and that drove the voltage down lower etc when once the motor began to stall you needed to drive the voltage up, not down.
Way back then, I had a patent on the technique and it was effective.

Best regards,

Mark Empson
L M Photonics Ltd