Soft Starter Design

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ejon1801

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Augusta,GA, USA
Good morning everyone:

Here is my situation. I am redesigning (retrofitting) a very old reduced voltage starter, by employing a soft starter. The soft starter will have a small footprint compared to the reduced voltage starter stuff. The application is for a motor(100hp) connected to a large fan. This is my first time utilizing a soft starter in any design. Matter of factly, I never heard of soft starter before working on this design. Just wanted to know if there were any engineers who have designed or implemented a soft starter in any design. If so, here are my questions.

1. This particular soft starter has a bypass, however the SCR will generate harmonics during the ramp up period ,due to switching mechanisms of the thyristors (SCR). Will the soft starter need any harmonic mitigation devices to address the harmonics during startup? Or is the harmonics so negligible it doesn't need any..please explain based on your experience


2. I noticed many soft starters have a "main contactor output".. I am assuming this could be used for some type of isolation relay? I wanted to know what other application can this main contactor output be tied to?

Thanks for any help/advice/suggestions
 
ejon1801 said:
Good morning everyone:

Here is my situation. I am redesigning (retrofitting) a very old reduced voltage starter, by employing a soft starter. The soft starter will have a small footprint compared to the reduced voltage starter stuff. The application is for a motor(100hp) connected to a large fan. This is my first time utilizing a soft starter in any design. Matter of factly, I never heard of soft starter before working on this design. Just wanted to know if there were any engineers who have designed or implemented a soft starter in any design. If so, here are my questions.

1. This particular soft starter has a bypass, however the SCR will generate harmonics during the ramp up period ,due to switching mechanisms of the thyristors (SCR). Will the soft starter need any harmonic mitigation devices to address the harmonics during startup? Or is the harmonics so negligible it doesn't need any..please explain based on your experience


2. I noticed many soft starters have a "main contactor output".. I am assuming this could be used for some type of isolation relay? I wanted to know what other application can this main contactor output be tied to?

Thanks for any help/advice/suggestions
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1) no you do not need to address harmonics. It's only there during ramping, not continuously. No need to mitigate a transitory event like that.

2) no idea what "main contactor output" means without knowing the context, it's likely something specific to that brand. Posting the brand and model will help. It might be related to a special application that has an odd wiring configuration. Some soft starters are set up to be used in what's called an "inside the delta" configuration when used on Wye-Delta wound motors that have 6 leads brought out to the starter. It lets OEMs use a smaller cheaper soft starter for big motors, but is more difficult to troubleshoot and is very risky for the end user, things an OEM rarely cares about. I strongly suggest not doing that, especially if this is your first soft starter.
 
ejon1801 said:
The model, I am considering using, is General Electric ASTAT Xbm soft starter. Points 13 & 14 =Main contactor output, and pin 23 & 24 = output. (See link below) I was inquiring about 13 & 14, and trying to figure out what can it be tied to OR how it can be utilized in my design. Thanks for any suggestions

http://apps.geindustrial.com/publib...llation and Instruction|GEIS-00049-NL|generic
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Sections 3.4.1 and 3.4.2 on page 7 describe what the functions of the two sets of contacts mentioned are for.

13 and 14 is to supply a main contactor, that is closed when there is a running signal. This contactor is not switching the control power to your soft starter just the incoming leads for the motor circuit.

23 and 24 can be programmed to be used for trip or run status signal.
 
LOL, I wrote an entire paragraph on the other possibility of what that meant, then decided not to post it because it was irrelevant without knowing. Now as it turns out, that's exactly what it meant.

In some areas, people like to use an Isolation Contactor (IC) / Main Contactor ahead of a soft starter, usually for one of two reasons:

1) When you measure the voltage across any two phases on the OUTPUT side of a soft starter, you will read line voltage all of the time it is connected. That's because of "leakage" through the SCRs themselves and the "snubber circuit" used to protect them. The total energy is severely limited, in the neighborhood of a few milliamps, but there are some places where if a voltage can be measured, it is considered "unsafe". Ford Motor Co. is one such place. So when you use an IC ahead of the soft starter, when the starter is Off, the IC is open, providing an air gap isolation, so zero voltage reading on the output terminals.

2) If you are in an area where lightning strikes are a constant danger, SCRs can be damaged by the voltage spikes that take place on the line side, even when the unit is off. Nothing can survive a direct hit, but the bigger problem for soft starters is a lightning strike 5-10 miles away. The IC ahead of the soft starter puts that air gap in place that will protect the SCRs from being damaged. I used to work in the Tampa area, the "lightning capital of the world". Every soft starter sold in that area gets an IC ahead of it, or at least everyone that has to REPLACE the first one buys the IC for the 2nd one... Out here where I am in California now, I've seen lightning storms twice in the past 20 years. Not worth it here.

So Terminals 13 and 14 are aux contacts of the RVSS control board that will respond to the "Start" input command to close that IC (Main Contactor) as soon as you give the command, and open it again once it is done. There isn't much other use for it. You could use it for a "On Light", but often on a soft starter, you don't really want to just know if it is On, you want to know if it is At Speed, which is one of the functions of 23 and 24.
 
drcampbell said:
Is a soft starter even required? Fans have very low starting torque demands.
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Some are high/very high inertia and take a long time to run up. The soft starter may be there to ease the requirements on current drawn from the supply during this period.
 
Besoeker said:
Some are high/very high inertia and take a long time to run up. The soft starter may be there to ease the requirements on current drawn from the supply during this period.
Click to expand...

Exactly, most of the soft starters we have in a plant I work for are on high speed belt driven fans, the sheaves are arranged that the fan spins faster then the motor. Before we put soft starters on them they were started across the line, there was a lot more maintenance expense. A lot of belt slippage when starting, so they would tighten belts to lessen that slippage to extend belt life - that ended up putting more strain on bearings and more frequent bearing failures. We have anywhere from 10 to 100 hp high inertia blowers in this plant and almost all of them are on either soft starters or VFD's now.
 
kwired said:
Exactly, most of the soft starters we have in a plant I work for are on high speed belt driven fans, the sheaves are arranged that the fan spins faster then the motor. Before we put soft starters on them they were started across the line, there was a lot more maintenance expense. A lot of belt slippage when starting, so they would tighten belts to lessen that slippage to extend belt life - that ended up putting more strain on bearings and more frequent bearing failures. We have anywhere from 10 to 100 hp high inertia blowers in this plant and almost all of them are on either soft starters or VFD's now.
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We've put a few big drives for fans in cement works. Big being 700kW to 2,200kW.
Those at 2,200kW are on Kramer drives. At start up with cold air they sit in current limit at about 80% speed.
 
Besoeker said:
We've put a few big drives for fans in cement works. Big being 700kW to 2,200kW.
Those at 2,200kW are on Kramer drives. At start up with cold air they sit in current limit at about 80% speed.
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Kramer drive... had to look that one up. It's a Slip recovery system for a WRIM. New one on me, not the concept, just the name. I like your name better. We just call it a Slip Recovery System for a Wound Rotor Induction Motor, but because I've only seen one manufacturer here that uses it, I have just referred to it as a "Wattmiser"...

They were, at one time, fairly common here on the West Coast for large sewage and irrigation lift pump systems. I've worked on a couple of them, combined with Liquid Rheostat controllers, it's an impressive system. VFDs have all but killed them as far as new installations go, but this company, Flowmatcher, still makes a living servicing the existing units. I'm pretty sure everyone that works there (likely less than 10 left) is over 60 by now however, so it's hard to say if they will survive 10 more years.
 
Jraef said:
Kramer drive... had to look that one up. It's a Slip recovery system for a WRIM. New one on me, not the concept, just the name. I like your name better. We just call it a Slip Recovery System for a Wound Rotor Induction Motor, but because I've only seen one manufacturer here that uses it, I have just referred to it as a "Wattmiser"...

They were, at one time, fairly common here on the West Coast for large sewage and irrigation lift pump systems. I've worked on a couple of them, combined with Liquid Rheostat controllers, it's an impressive system. VFDs have all but killed them as far as new installations go, but this company, Flowmatcher, still makes a living servicing the existing units. I'm pretty sure everyone that works there (likely less than 10 left) is over 60 by now however, so it's hard to say if they will survive 10 more years.
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Ours was a bit different to the conventional system. Efficiency is what sold it.
In the water industry, energy consumption is a major operating cost.
 
Another question related to soft starter

Another question related to soft starter

I also had another question... The reduced voltage starter circuit had a switch for "forward and reverse" for the motor.. I was wondering are you guys familiar with a soft starter allowing forward or reverse direction for the motor. The possible soft starter (ASTAT Xbm) has a dial ((#8) see link) that addresses "Phase sequence protection" that mentions fwd+reverse. Again, I am a new engineer and do not understand the concept of this (fwd reverse) OR how to implement this with the new soft starter. Any explanation/suggestions will be greatly appreciated.

http://apps.geindustrial.com/publib...llation and Instruction|GEIS-00049-NL|generic
 
kwired said:
a plant I work for are on high speed belt driven fans, the sheaves are arranged that the fan spins faster then the motor. Before we put soft starters on them they were started across the line, there was a lot more maintenance expense. A lot of belt slippage when starting,
Click to expand...

Another way to cure that is to use larger pulleys on both the motor and the fan keeping the ratio the same but providing more surface area for the belt to work with

so they would tighten belts to lessen that slippage to extend belt life - that ended up putting more strain on bearings and more frequent bearing failures.
Click to expand...

We had a ride at the amusement park with four 25 HP motors working together to spin a ride. The driven pulley was large, the motor pulley was small and the two pulleys very close together so there was very little contact area of the belts to the motor pulley.

The ride used resistor banks and contactors to have five speeds and when it made the switch from first to second speed the pulleys would often slip.

So we would run them tight, very tight and when you have five V-belts on each motor so tight they don't deflect at all things happen.

We did not have bearing issues, we had a motor shaft break between the pulley and the bearing. This would be bad enough but the motors spin with the ride so a 6" long section of maybe 1.25" shaft with a 5 belt pulley still attached on it came spitting out of the inside of the ride and flew past riders and watchers. Pure luck it was not imbedded in someone's head.
 
ejon1801 said:
I also had another question... The reduced voltage starter circuit had a switch for "forward and reverse" for the motor.. I was wondering are you guys familiar with a soft starter allowing forward or reverse direction for the motor. The possible soft starter (ASTAT Xbm) has a dial ((#8) see link) that addresses "Phase sequence protection" that mentions fwd+reverse. Again, I am a new engineer and do not understand the concept of this (fwd reverse) OR how to implement this with the new soft starter. Any explanation/suggestions will be greatly appreciated.

http://apps.geindustrial.com/publib...llation and Instruction|GEIS-00049-NL|generic
Click to expand...


I did not look at the link, but "phase sequence protection" sounds like an internal phase monitor of some sort, if input rotation is wrong it would likely lock out and not run, likely also locks out on a phase loss.
 
kwired said:
I did not look at the link, but "phase sequence protection" sounds like an internal phase monitor of some sort, if input rotation is wrong it would likely lock out and not run, likely also locks out on a phase loss.
Click to expand...
Yes, that's correct.

In a soft starter, you have to determine WHEN to fire the SCRs in order to affect the Phase Angle Firing that reduces the RMS voltage getting to the motor. That "when" is based on accurately knowing the "zero cross" point of the sine wave of each individual phase, because each SCR gate signal is timed off of that zero cross point. But you cannot measure a value of zero reliably, so what they do is measure the CROSSOVER point of at least two phases, which they know happens at a specific moment, then work backward from there to determine when it crossed zero, then base the firing of the NEXT sine wave in that phase from that. It's called a "Phase Locked Loop" scheme. Some low cost soft starter designs (and I can go into a long historical dissertation on this, but I'll spare you all) use one PLL firing control loop, then assume the other two phases will be timed accordingly every 120 degrees. That works most of the time but has trouble when the frequency drifts, such as when running from a backup generator. That's where the stories come from where people say they don't trust soft starters when run on generators.

In that scheme using a single PLL, that critical timing must look at the crossover of two phases, then assumes the zero cross of all three. But with only two phases, there is no way of knowing the rotation of the three, so you must ASSume that as well. Therefore, that type of design is inherently phase rotation sensitive. They give you a switch that lets you change what it looks for, essentially just changing the direction of the timing sequence.

There are other soft starters out there that use three separate PLL loops, so they are not phase rotation sensitive; they pass through whatever hits the line terminals. They are also not affected by frequency drift.
 
Last question

Last question

This is my final question concerning soft starters....for now..LOL. Are there any soft starters that are compatible with grounded delta power systems. Did not realize until recently that the power was "corner-grounded delta":?. This changes a lot since many soft starters are not compatible. From what I research this is different from "inside delta" configuration .Additionally, Ive noticed there are a lot of post concerning these corner-grounded deltas.I would like to know if anyone has any experience implementing a soft starter with this type of power??What type of starter is compatible?
 
ejon1801 said:
This is my final question concerning soft starters....for now..LOL. Are there any soft starters that are compatible with grounded delta power systems. Did not realize until recently that the power was "corner-grounded delta":?. This changes a lot since many soft starters are not compatible. From what I research this is different from "inside delta" configuration .Additionally, Ive noticed there are a lot of post concerning these corner-grounded deltas.I would like to know if anyone has any experience implementing a soft starter with this type of power??What type of starter is compatible?
Click to expand...
As a gross general rule, soft starters made for "world markets" by companies who consider North America a secondary market are going to have that issue, companies that make products that are primarily for North America and consider the rest of the world a secondary market will not. That's because delta power distribution systems don't exist outside of North America (grounded or not), they are always Wye. That then means the components they use are never required to handle a voltage reference to ground that is more than 58% of the line to line voltage: translate; less expensive. So if your products are designed with that in mind, they may not survive in a delta system.

It's not however as easy as saying "American vs Foreign" suppliers because there is a lot of brand labeling and most of the "old guard" American mfrs are now owned by EU companies and when the EU owners bought out NA companies, they killed many of the products that already existed in favor of their EU counterparts. Siemens is a classic example; when they bought Furnas, that came with Nordic, one of the original soft starters on the market and built here to handle anything we could throw at them. Within a few years. Siemens killed Nordic and shut everything down, bringing in their German versions. The German versions could not be connected to delta power systems. I worked at Siemens at the time and pointed out the "flaw" as we saw it, basically the Germans said they didn't care, because NA was less than 2% of their world wide market.

I can't say exactly which ones have this problem, but I know who does not (last I checked). Allen Bradley, Mototronics and Benshaw are the only ones left that I know for sure do not care if the voltage is Wye or Delta. Maybe Eaton, but I'm not a fan of their soft starter for lots of other reasons. Other old NA brands that are now EU owned or use imported units; Sq. D (Schneider), GE (brand labeled Agut from Spain) and of course Siemens. Flat out EU or IEC designs; ABB, WEG, Solcon, Aucom /Danfoss. But without looking at manuals, I can't say for sure if they can or can't handle delta power systems. I know the founder of Aucom personally, I doubt he overlooked that issue, but it's never come up in a conversation...
 
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