Soft Start--vs---VFD

[Jraef]

I have many times heard tell of (but never personally seen) “ratcheting instantaneous demand meters”. Most of the time it has been brought up by people trying to sell Soft Starters by claiming they were going to save on the demand charges. I have never seen that actually work in person, although I have seen indirect effects. In the best case of that, I put in soft starts on about 2 dozen large machines in a lumber mill, only to find out afterward that the reason they had me do that was because a salesman had told them it would cut their demand charges. I told them the truth, they were not happy, but after a couple months they told me it had worked. I took a look and their entire energy use went down. A little sleuthing and I discovered that because of having the soft starters, operators were no longer leaving machines running idle during breaks and shift changes because they had been told it was cheaper to run for an hour rather than restart, but the soft starters made them comfortable with shutting down now. That meant there were about 3 hours per day of no machines running, which reduced their overall energy use, so the demand charges were lower too because they are a percentage of the base charges. The actual peaks were the same.

Personally, I think the ratcheting instant demand charges is a myth created from people seeing an old analog demand meter and not knowing that the red needle showing the peak demand does not move instantly, that it took 15-30 minutes to register that peak.

 

[Timpi Electric Solutions]

I'm surprised by your comment because I had been getting the impression that utility demand metering was typically done on a 5 or 15 minute interval basis, and I wouldn't expect in-rush currents to have a meaningful effect on average power over that kind of timescale. Are you referring to a typical utility demand charge, or a different type of metering for inrush specifically?

While I don't have first-hand experience with commercial utility billing, it's what I've been informed of by customers. My locality is somewhat extraordinary, though. My county is very manufacturing-heavy. We have a labor force of 118,400 but have 145,100 jobs (many people commute from adjacent counties). 74,600 of those jobs - or 51.4 percent - are in manufacturing. Nationally, there are 12,974,000 manufacturing jobs in the US out of 160,315,000 jobs - or only 8.1 percent - according to the Bureau of Labor Statistics.

You can't throw a rock around here without hitting a factory, so I imagine that there is an extremely more substantial demand on the grid during peak hours here than in other regions. The city where my primary customer is located consistently has power quality issues that have very likely caused many equipment failures due to surges and/or brownouts. I imagine this is the reasoning for the extreme utility rates for in-rush currents that they complain about.

 

[Besoeker3]

My field is/was mostly high power electronics in the industrial field. Now and again did some some soft starts. The last was three phase soft starts but a little bit different. Two of the phases were soft starts, the third was fixed speed. It was a bit less expensive and it worked just fine.

 

[Open Neutral]

It's been a few decades since my two senior quarters of Machines, but I recall something about soft starters being harder on the motor than across the line starting; SS was done mostly to keep the PoCo happy. Could Jraef/Besoeker comment on motor lifetime effects? I agree re: demand meters/15 minutes.

 

[Besoeker3]

It's been a few decades since my two senior quarters of Machines, but I recall something about soft starters being harder on the motor than across the line starting; SS was done mostly to keep the PoCo happy. Could Jraef/Besoeker comment on motor lifetime effects? I agree re: demand meters/15 minutes.

I would have expected the soft starters to be gentler than direct in line. The direct line starter is typically 6x full load current whereas the soft starters I have known have been 3x or less.

 

[Jraef]

It's been a few decades since my two senior quarters of Machines, but I recall something about soft starters being harder on the motor than across the line starting; SS was done mostly to keep the PoCo happy. Could Jraef/Besoeker comment on motor lifetime effects? I agree re: demand meters/15 minutes.

Depends on the way the Soft Starter was made, there are several iterations and some are not good for the motor. Good designs are fine and actually relive mechanical stress, but cheap ones being sold now cut corners by only ramping 2 phases instead of all three, which causes excess motor heating. There were really old designs that used one SCR and one Diode per phase, which was hard on the motor too. Luckily that company is now long gone.

 

[zooby]

Depends on the way the Soft Starter was made, there are several iterations and some are not good for the motor. Good designs are fine and actually relive mechanical stress, but cheap ones being sold now cut corners by only ramping 2 phases instead of all three, which causes excess motor heating. There were really old designs that used one SCR and one Diode per phase, which was hard on the motor too. Luckily that company is now long gone.

by cheap....certain manufactures?

 

[Jraef]

by cheap....certain manufactures?

Unfortunately, many of the top names are succumbing to the “cheaper is better” pressure and selling these lower cost 4 SCR versions, albeit usually in parallel with the full 6 SCR versions too. They rarely explain the risks, or do so along with a claim that “Ours is a better version (of the wrong way to do it)!” Siemens, Schneider, Eaton, Weg, Benshaw all offer both versions, but in all cases the cheaper ones are the bad ones that shouldn’t be used, yet that’s what people gravitate to. Motortronics (and their brand label Toshiba) and Rockwell are the only ones I know of not offering the 4 SCR option that I am aware of. The dirt cheap no-name ones that you can buy from China are almost all the bad versions now.

 

[zooby]

Unfortunately, many of the top names are succumbing to the “cheaper is better” pressure and selling these lower cost 4 SCR versions, albeit usually in parallel with the full 6 SCR versions too. They rarely explain the risks, or do so along with a claim that “Ours is a better version (of the wrong way to do it)!” Siemens, Schneider, Eaton, Weg, Benshaw all offer both versions, but in all cases the cheaper ones are the bad ones that shouldn’t be used, yet that’s what people gravitate to. Motortronics (and their brand label Toshiba) and Rockwell are the only ones I know of not offering the 4 SCR option that I am aware of. The dirt cheap no-name ones that you can buy from China are almost all the bad versions now.

Me : look boss We are going to tell the bidders that we are not going with the low cost 4 SCR soft starts.
Boss : why is that?
Me : I wont act like I truly understand it but..... Jraef seems to know his stuff and that should be reason enough! Thanks!

 

[Jraef]

Ok, fair enough.

Think about it. If I am using a Soft Start to ramp up the voltage to a 3 phase motor, but only have SCRs on two of the phases, that means one phase is just a bus bar, straight through line to load. That then means that two of the phases will have half of their sine waves uncontrolled, while the other half is at reduced voltage, creating a relatively severe unbalanced voltage in the motor as it accelerates, which significantly increases the heat in the motor. You can hear it in the motor as well, it makes it sound like it has gravel in the bearings. IIF the application is something like a low head pump or fan that will accelerate to full speed in 10 seconds or less, and then only start infrequently, like once per hour, the damage risk is lower. The problem there is, they are not marketed that way.

And for the purists, yes, I know that my description of how it acts on the phase relationships is less than detailed, but the details are pretty boring to most people so I was boiling it down to the important aspects.

 

[zooby]

Ok, fair enough.

Think about it. If I am using a Soft Start to ramp up the voltage to a 3 phase motor, but only have SCRs on two of the phases, that means one phase is just a bus bar, straight through line to load. That then means that two of the phases will have half of their sine waves uncontrolled, while the other half is at reduced voltage, creating a relatively severe unbalanced voltage in the motor as it accelerates, which significantly increases the heat in the motor. You can hear it in the motor as well, it makes it sound like it has gravel in the bearings. IIF the application is something like a low head pump or fan that will accelerate to full speed in 10 seconds or less, and then only start infrequently, like once per hour, the damage risk is lower. The problem there is, they are not marketed that way.

And for the purists, yes, I know that my description of how it acts on the phase relationships is less than detailed, but the details are pretty boring to most people so I was boiling it down to the important aspects.

Hey Thank You so much. I was just joking a bit. The time and effort some of you take to explain things is admirable!

 

[Open Neutral]

I'm trying to recall what was the reason given in class, and can't say I've succeeded. [1] But what glimmer I have relates to soft starters & undervoltage. Is it accurate to think of SS's are starting the motor at lower voltage? Alternately, the SS prolongs the time spent in the region where slip is higher, and counter-EMP is lower, so more heating?



1] And several years ago I'd sought out that prof for another issue. Alas, given I'm old, he was OLD, and it was soon clear that while he remembered me, he was no longer able to grasp the question, sigh.