Soft Starter / Energy Saving

By the way, soft starters don't save energy in the starting inerval even current limiting feature is used. The required amount of energy to drive the shaft from stand still to full speed is fixed, but it is absorbed gradually from the supply via current limiting feature or other techniques.

Sounds logical so I stand corrected. However, soft-starting can still save energy in other ways.

If it saves the guys from exerting energy to fix structural cracks due to starting stresses then I'd still claim some savings.

We used to have a big soft-starter that didn't limit the starting current. I remember the power plant boys complaining loudly when we started up the motor without informing them. Seems the generators were set up to provide only so much power that when the big motor engaged, the generators tripped. Putting a soft-start with current limit prevented that. The motor took much longer to start but that wasn't so much of an issue.

"... Actually, my knowledge says "YES, Soft Starters now a days DO".

Recently, a new feature had been added to soft starters for induction motors, such that a very efficient energy saving can be performed.

This new feature starts after reaching the maximum speed of the motor, in condition of the motor is not fully loaded which is mostly common for industrial applications."

There is nothing at all new about this feature, in fact this feature was invented in 1969 by Frank Nola of NASA and was the basis for launching the ENTIRE SOFT STARTER INDUSTRY!

However, as an energy savings feature it was a bust. Although it works in theory, in application it requires that the motor be significantly unloaded, i.e. more than 50%, before there is any appreciable savings. The reality is, the vast majority of motors are turned off if unloaded that much for any length of time, so using that feature as an energy saver is somewhat pointless. There are a few applications where it still makes some sense, such as seldom used escalators that must be left running, or some injection molding machines, but most of the applications where it worked in the past have been taken over by VFDs, which are a much more effective means of saving energy.

"Actually, my knowledge says "YES, Soft Starters now a days DO".

"Recently, a new feature had been added to soft starters for induction motors, such that a very efficient energy saving can be performed."

The energy savings algorithm has been available in soft starters since the late 70's and there have been many companies come and go on the strength of saving money on induction motors by reducing the voltage when the load is reduced. This is not new, and new derivatives can not save lots of energy.
The induction motor has iron loss due to the flux in the iron. If the flux is too high, the iron loss will be very high also (bad design). In reality, the iron loss is pretty low. On larger motors, the total losses are less than 5% of the motor rating and the iron los is 30 - 50% of this. If you eliminate the flux, you will elimiate the iron loss and the motor torque and the motor will not work. (OFF - ON switch)

If the motor is operating at reduced efficiency, then you can reduce the voltage applied to the motor and thereby reduce the iron loss, but it is a very small saving relative to the motor rating except for very small single phase motors. Have a good look at motor efficiency figures and you will see how small the iron loss is. NB the iron loss is constant, independent of load, so if the iron loss is 2% of the rated motor load, then at no load, the iron loss (in watts) is the same as at full load.

Have a look at http://www.LMPhotonics.com/energy.htm

You can only save a portion of the energy that is being wasted. How much energy is really wasted by the induction motor?

Best regards,
Mark Empson
http://www.LMPhotonics.com | http://www.LMPForum.com

You can only save a portion of the energy that is being wasted. How much energy is really wasted by the induction motor?

You are right. And how much energy will be comsumed by the control circuit?...

If this refers to the recent posts regarding the lift of a borehole pump to the surface, then to reiterate, the shaft work required is the product of the volumetric flowrate times the rise in pressure. No soft-start can give a justification for investment on the energy savings to be had in that particular case, as the lift and the flowrate are constant-ish, so the power demand while the pump is running will be constant-ish.

Now, slow down the flowrate, and that is another matter. But then, why soft-start? Why not simply alter the on/off time for the pump (rhetorical question)?

It depends also upon the tariff. If electricity is purchased by the peak demand rate instead of by the kWh consumed, then a soft-start may be justifiable if a starting peak bursts the normal peak demand rate; the soft-start is being justified on the basis of clipping the top off the peak load, and not the continuous energy savings.

All that it is possible to say is that, without looking at each individual case, it is not possible to say!

I'd like to thank all of you for your contribution:

Vulcan, JRaef, Marke, Qqberci,and PWSlack.... Really it was great.

Thanks to all of you

Regards... Samak