Soft Starter vs VFD

What was the duty cycle on the SS stopping and starting the fan?

Somewhere in the tech manual for them gives the sizes you should use, my theory is the SS was undersized.

It's something I feel Siemens should make more of a point of, but if the duty cycle is quite high, or if it's a large inertia load. They often recommend a SS a couple of sizes bigger than you would expect.

I would guess that the fan is energized between 15-20 times per hour. I would also say that unless a fault condition were triggered for a long period of time specifically related to that station the fans inertia would keep it from ever stopping. It would decelerate yes, but it wont ever be at a standstill.

Probably the biggest reason Softstarters are becoming a bit more common these days is the huge across-the-line inrush currents experienced with Premium Efficient Design A NEMA motors. A Code R motor (around 14/1 FLA starting inrush) may be unmanageable even in rather small horsepowers (for example, 25hp 460V 31.5FLA has an inrush of over 430amps typically)

dwoodlock said:

I would guess that the fan is energized between 15-20 times per hour. I would also say that unless a fault condition were triggered for a long period of time specifically related to that station the fans inertia would keep it from ever stopping. It would decelerate yes, but it wont ever be at a standstill.

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Way too high of a duty cycle, IMO. Sounds like a VFD application.

As far as I know, the motor starts/hour rating does not change with a SS, but is infinite with a VFD, assuming you limit VFD accel current to 100% (something that can't be done with an SS if you want the motor to start).

Use VSDs all the time now. No more soft starters. Soft starters are usually mounted in the panel creating heat - even with bypass contactors. The VSDs I use are rated IP66 - mount on a stick alongside the motor - very short run of very expensive cable. No extra heat in the swithchboard either. pF is improved also - energy saving. Mainly swimming pools hence IP66. For swimming pools in Australia all items must be protected by a 30ma RCD as well. The drives I use run on 30ma RCDs - have been designed for it. ABB for example one has to disable a lot of the filtering to run on an RCD - I have our locally made VSDs running with full filters in place on 30ma RCDs up to 75 kW motors. Impressive.

BobB said:

. pF is improved also - energy saving..

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Displacement power factor is improved yes, but True Power Factor suffers unless you deal with harmonics.

Although at this point most utilities don't measure this so you get away with it unless you cause problems in their network or with other customers.

Saffa said:

Displacement power factor is improved yes, but True Power Factor suffers unless you deal with harmonics.

Although at this point most utilities don't measure this so you get away with it unless you cause problems in their network or with other customers.

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Or if the transformer is owned by the company you're installing all the VFDs for.

In the UK, I've seen a number of transformers had to be upgraded to what we call "K type" transformers.

These have a 200% rated star point connection to deal with the harmonics and heating they cause.