When is it worth the Cost to use a Soft Starter?

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460ThreePhase

New User
Location
Texas
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Beginner Electrical Engineering Tech
Greetings,

I was wondering about when the cost would be worth it to use a soft starter for 460V three phase motors when the energy plan does not charge for momentary spikes (charges for a 15 min sustained period instead)? I've read a number of articles that have spoken of the benefits of using one--which were: energy savings and lower maintenance costs (belts, motor less chance of being destroyed).

I have read a plethora of different responses as to when it is beneficial to use one. Some people saying you should use one all the way down to a 5hp motor !?!
Others have said only after 100hp. This degree of discrepancy in judgment is alarming.

Personally, I'm wondering if it's even worth it with the 100hp motors if they haven't been damaged over the years without one. I'm also wondering why they cost so much if they don't easily pay for themselves in energy costs and they are not necessary.

The energy plan that my company is on does not charge for momentary peaks but rather for an average 15 minute peak. I do see the benefit if your energy plan does charge for the momentary peaks though!

Thank you for suffering my question.
 
IDK how they would save energy. Once running, they might as well be across the line. Some do have a bypass contactor once they are up to speed.

Wear and tear on equipment plus less impact on distribution would be the biggest plus. POCOs in this area are starting to require SS or VFD on larger motors. Last I knew 100 Hp, but it may be less now.
 
Another item to remember here is the electrical environment. In a heavy industry environment on a stiff or dedicated utility (think a mine), starting a 50 - 75 HP motor directly across the line is likely done without a second thought.

On the other hand, in a light industry application, surrounded by residences, located a ways away from the utility source, starting even a 15 - 20 HP motor directly may get a visit from the POCO if the customers notice some lights blinking etc. The utility may even have a rule prohibiting it.
 
Have a client with a steam boiler. Combustion air blower is powered by a 15 HP motor. Is rather heavy blower and takes time to accelerate. If belts are too loose it will slip and wears out belts in relatively short time. They would tighten belts and have less slip but then they started wearing out bearings at a faster rate. Cost of the bearings is one thing but takes most of an afternoon to change them so you have to factor that in as well as boiler down time.

Put a soft starter on that motor and now the belts and bearings last much longer.

Did the same thing on another blower in that facility. Another high speed blower that takes some time to accelerate. That one has a cogged style belt but would slip some while starting. They were going through a lot of belts on that before we put soft starter on it.
 
Biggest fan of VFD'S & soft starters. One place that I worked at had a 250 HP fire pump with a soft starter. That eliminated a timer and a Y Delta starter. Years ago we had very heavy explosion proof motors bolted to a Reeves speed drive and gear box in the bottom center of machines. The Reeves drives never lasted more then a year and took two guys 8 hours to replace the entire unit. We did away with the Reeves belt speed adjusters with an explosion proof motor bolted to a gear box. Set the ramp up speed to 5 seconds. With the slow ramp up speed the steel drive product chain did not receive sudden shock on start up and that & bearings lasted a lot longer and was still running after five years. We had a lot if 1960 era AHCU fans from 40 to 125 HP that we changed over to VFD'S. Seldom ran over 90% during weekdays and still ran supply two fan systems with both fans running at nights & weekends as low as 50% of full speed . Return fans were all on VFD'S. We had a great engineer who told us they paid for them selves in no time. Even the older VFD'S have a total hours & power ( KW hours ) that you could figure out the average power each motor consumed.
 
One thing to remember, a SS will not solve a PP design.

We replaced a part winding start 300 HP with a SS. It took forever to start and tripped out on overload often enough. I gained experience with centrifugal fans because of it. They were turning the fan well past what it was designed for. Surprised it even held together. We got them to change pulley ratio but it was still overdriven.
 
ptonsparky said:
One thing to remember, a SS will not solve a PP design.

We replaced a part winding start 300 HP with a SS. It took forever to start and tripped out on overload often enough. I gained experience with centrifugal fans because of it. They were turning the fan well past what it was designed for. Surprised it even held together. We got them to change pulley ratio but it was still overdriven.
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Sounds similar to where I retired from. They had 4 roof top cooling towers with 75 HP motors. They changed a pulley on one tower to have it run at least 25% faster. Destroyed the gear box .
 
ptonsparky said:
One thing to remember, a SS will not solve a PP design.

We replaced a part winding start 300 HP with a SS. It took forever to start and tripped out on overload often enough. I gained experience with centrifugal fans because of it. They were turning the fan well past what it was designed for. Surprised it even held together. We got them to change pulley ratio but it was still overdriven.
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Never had the tripping problem with long ramp up times on VFD'S. We had a lot of 1960 AHCU units from 30 to 75 HP that had a 10 minute ramp up time for supply fans during the winter to not trip out the three to four freeze stats on units.Once units started they would run 24/7.
 
I spent a lot of years doing a lot of soft starters and anyone trying to sell you on “energy savings” is scamming you. There WAS, for a brief time when they first came out in the late 70s, early 80s, a big push to sell them for energy savings by using what was called a “Nola circuit”, invented by a NASA engineer named Frank Nola to save energy on unloaded AC induction motors used in a proposed space station (at the time). It detected an UNLOADED motor and phased back the voltage to reduce the magnetic losses in the windings. It worked, but the reality is, if a motor is UNLOADED, why not just turn it off? That reality, coupled with the fact that it cannot work if the soft starter is bypassed when at full speed, killed the energy savings concept. Yet the IDEA of it persists in the industry because it was a useful tool to sell more product needlessly.

The TANGIBLE benefits of a soft starter are mechanical and regulatory. A manufacturer I used to work for had a slogan that was “Electrical solutions for mechanical problems”, and that was dead on accurate. It reduces mechanical stresses on the motor, couplings, gears, belts etc. These are very real issues and represent real costs to machinery owners. I highly recommend them for this purpose.

The regulatory issue however is variable. Some utilities impose draconian limits to what size of motor you are allowed to start across the line. Most are at 100HP and up, some are 50HP, I have been in places (Seattle City Light) where it’s as low as 15HP! They dictate that you must use “reduced voltage starting”, but they do not care how you attain that. You can used Y-Delta, Autotransformer, Primary Reactor, Primary Resistor, Part Winding or Solid State. The advantage to solid state is that it does not require a special motor, it is smaller and lighter than an Autotransformer, and is infinitely adjustable to the load /task, not just two hard steps. Many also now come with a lot of extra niceties such as high end motor protection, communications and lots of flexibility to changing conditions. I love them for applications that don’t need to change speed, but there is no energy savings.

A lot of marketing types also glom onto the idea that they eliminate “peak demand charges” from the utility. That’s also GROSSLY exaggerated. Most utilities used a “demand window” of 15 to 30 minutes for this calculation, so the act of starting a large motor has no tangible effect on that, it only registers the ACTUAL load on the system that motor represents, after that sliding demand window. In my 30+ years working with soft starters, I encountered exactly ONE small utility, an irrigation district, who had a ratcheting instantaneous demand charge, where reduced voltage starting would make a difference. I’ve heard tell that there are MAYBE a dozen such utilities across the US. But their existence, however rare, makes the exaggerated claims by soft statement markets have a tiny kernel of truth behind it, ie IF you are served by one of those small utilities it MIGHT make a difference. But for 99% of us, it doesn’t.

So use them for the correct benefits, but ignore the energy savings and peak demand charge BS… it’s not real enough to consider.
 
One other reason (but not referenced in the OP). Generators. When you have a site with one or two large motors that are served by a generator, it can make the difference between being able and not being able to start, or more commonly, controls dropping out before motor can get up to speed.
 
Birken Vogt said:
One other reason (but not referenced in the OP). Generators. When you have a site with one or two large motors that are served by a generator, it can make the difference between being able and not being able to start, or more commonly, controls dropping out before motor can get up to speed.
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I've had that happen. Had to run the controls via a small portable until we got the motor up to speed.
 
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