What's new
What's new

Soft Start tutorial ; Cliff Notes

Cyclotronguy

Stainless
Joined
Sep 21, 2005
Location
Northern California
Can someone point me to a resource where I can digest the basics and pitfalls.

Home shop has a universal mill with two spindle drives. Additionally separate motors for each of the rapids, also polyphase. All start across the line, no clutches. HP varies depending upon which motor is running.

All run off of a single idler based phase converter. I'd like to add a soft start for at least the spindles. One a 7-1/2 HP, the other a 3 HP.

I'm guessing each motor needs it own soft start unit?

Does it make more sense to simply add a 3-phase inductor to take a load off of the rotary converter and not rewire all the controls on the mill?
 
What are you expecting from the soft start?

Many regular soft starts are really "reduced line current" start, which is good for the powerco, but has a limited effect on your operation. Less light dimming, and so forth, but not always much easier on the equipment.

If you want a "slow start", that's different from a straight "soft start".
 
Mill is in a residential garage shop, we're current limited. With across the line start of the main spindle there are some spindle speeds that simply overtax the 240 / 60 / 1 mains and trip the service entrance breakers. Ideally more power, but not possible at this location. And I'd rather not change the motor.

The goal is simply to damp the inrush current slightly
 
I switched to a VFD for just this reason. Getting one with multiple programed configurations allows matching to the specs of differing motors.

Sufficiently oversized, (20 hp drive for a 7.5 hp motor in my case, bought surplus) The VFD doesn't even notice the feed motors. But I keep the freq. at 60 hz, +/- 15.

The initial cost, and wiring is a but of a task, but the benefits are lasting!
 
I wonder if you could get a main breaker with a higher trip class, and if that would be enough with as close to okay as you seem to be. Not sure that kind of thing is available for a home box main breakers. Might be a simple and comparatively inexpensive fix.

Not sure if your rotary phase convertor has a bank of starting caps that are switched out after startup. I use to have a big ole shaper that wouldn't start without those capacitors left in circuit. It made a big difference. The convertor people said it was totally fine to leave them in circuit. Wonder if that extra quick shot of juice would be enough to get you going.

I'm a little jaded about soft starters right now. Just removed a nice one from a 10hp 3ph air compressor, because after a month or two it started false alarming and shutting off right after start up. The error code was lost phase slash open load. Neither of which were true. The hardest thing about soft starters is getting used to the sound of it. It's not pleasant. And I had it set at the fastest, (2 sec) highest current setting (65%) available in the unit. It wouldn't start or start without sounding like you were killing it if it wasn't set that way. I'm now back to across the line and loving it. Different story I know, but if you're so close, I'd consider other options if there turn up to be any.
 
Mill is in a residential garage shop, we're current limited. With across the line start of the main spindle there are some spindle speeds that simply overtax the 240 / 60 / 1 mains and trip the service entrance breakers. Ideally more power, but not possible at this location. And I'd rather not change the motor.

The goal is simply to damp the inrush current slightly

Similar challenge, here. Save that I can have all 200A if I really need it.

But.. since Dominion Power went-over to a "demand" component, even on residential billing? I do not want to "really need it" even once!

"Patience" is cheap in Virginia. So.. my solution is a multi-idler RPC.

Start each/any/all of 3, 7.5, 5-to-7.5 (weird motor) and 10 HP idlers, staggered over time. Once enough electrical and kinetic inertia is motating as the pilot and supplementary idlers? Starting any of my 1.5 HP to ~ 8 HP loads has a far less dramatic effect on the single-phase feed.

From THEIR seats? Bang! Up and away, same as utility mains 3-Phase. They have no need of "soft start" .. nor much of a klew there is any limitation at all. Because they are calling on the resources of a fairly well "funded" mentor, upstream.

"BFBI" method, save that the heaviest idler I'd ever start in a demand time-period is the 10 HP one.. and mostly I don't start that one at ALL, what with a pair of 7.5 HP. Stagger-start of a 3 HP and a 7.5 HP, or two 7.5 HP, spaced in time realm, and I'm good to go. Once the heavier load-motor is up and running? I may have the option of dropping one of the idlers off the line.

The 3 HP, BTW, is all I need for a couple of sub 2 HP 3-Phase loads, but does double-duty as a "trimmer", for better balance of the heavier "nameplate" loads , which may or may not be loafing or working hard, any given tasking.

The whole rig takes a LOT more SPACE than a VFD. Or even SEVERAL VFD.

Especially as there is a 27 KVA Delta-Wye 1:1 Drive Isolation transformer and a transfer switch to run off a 10 HP Phase-Perfect instead.

Some make chips. I make tests. I didn't want to re-configure this a third time. 28 HP max idler when all are singing and dancing? That will handle anything I am likely to drag home.

There was a chunk of freight involved. I did say HEAVY idlers? - but the net cost isn't all that different from multiple VFD'ing.

Rugged as it all be, it won't mind living sorta "outdoors" in a half-vast shelter with the Diesel gen set, either. Arse-end of the carport for now, so "BFD". Noise is exiled outdoors with it.

Not that a deaf-ass Old Fart much notices "noise". More a "high value space" vs low-value thing.

:)

Only the 3-P panel, breakers, and controls take up any shop space at all, and even then "not really" - where they go weren't "premium" space.

VFD? They work. Kinda neat. Lotta nice features.

That I don't need, give every one of my loads has either a Reeves or cousin vari-drive, the odd-man-out a PIV-Werner-Reimers metallic equivalent.

And ... with 3 of the 4 idlers massive Reliance Duty-Masters and ALL of them new or NOS?

The rig is likely to outlive ME ...even if I clock a hundred years of age.

VFD's aren't near as good at that part! Nor do they "share" as well as an "RPC Array"

4 idlers worth...

Your shop?

You could be as well-off as you NEED to be with only TWO idlers - primary and supplementary?

Give it a thunk.

Dave Kamp has posted a schematic as to how dead SIMPLE that is. I mean. .... any SIMPLER ....it would have to be a blank sheet of paper!

:D
 
Most powerco rules make you put a soft start of the type you want, on any motor of at least 5 HP.

Makes sense.

So what you want is less line current at start. that can be done. Is it a hard start?


One type uses an auto-transformer to cut voltage. That usually is the type that has the least line current at start, and generally the most torque, Transformer is cut out after the start, usually on a timer.

Other types start through resistors, or even inductors, to cut the current. Those reduce the line current less, but often have lower start torque. The more they cut the start current, the lower the torque. There are SCR-based versions of this, usually for single-phase. They vary in performance.

The last type that is reasonably common is a wye-delta for 3 phase. The wye connection is for higher voltage than the delta, by 1.73x. So if started on wye, and then switched to run on delta, the initial line current is reduced, by almost as much as by the transformer method. Start torque varies.

Wye-delta starting line current is higher than, but start torque is often similar or less than, the transformer method, depending on the ratio used in the transformer. The transformer method often uses a ratio close to the same as the wye-delta, leading to a similar current to the motor, but less from the line.
 
Most powerco rules make you put a soft start of the type you want, on any motor of at least 5 HP.
I submit you meant they ony pay attention at ALL when in excess of 5 HP?

And few "make you" do anything ... until there is actually some form of chronic deleterious effect on a last few yards, multiple subscriber shared distribution serious enough THEY see it...

And/or wherein another subscriber, same pole or vault "pig", has lodged a complaint.

YAh, an induction motor has a nasty inrush, but still.

My KITCHEN, where more than one "machine tool" is commonly in use at the same time, draws more than my shop!

And then there is summer HVAC, winter all-electric heat, year-round 90 gallon water heater, and GE's largest electric dryer?

Nothing special. Joe Average 200A split-phase 240+ VAC service all-electric home of 40 years ago "best practice of the day".

5 HP motor isn't even a blip on the radar.
 
The push over the edge for my application was a fenzied month of "got to get this done" one brutally cold winter. (I had a salamander in the shop for heat that frosted up all the machine tools about an inch thick, Not doing that again? )

Any way, the shared transformer neighbor approached me one day and asked if I new anything about refrigerators. It seemed that his was making strange noises and making his lights go dim and the TV set picture flutter.

I went on EBAY the next day and bought a nice TOS Vert VFD. I didn't need to be the one to set him up with a new bit of kitchen ware....
 
Any way, the shared transformer neighbor approached me one day and asked if I new anything about refrigerators. It seemed that his was making strange noises and making his lights go dim and the TV set picture flutter.

When he said "new" anything about 'stead of "knowed" anything about?

Thar were yer HINT as to where THAT deal wuz headed!

:)

I went on EBAY the next day and bought a nice TOS Vert VFD. I didn't need to be the one to set him up with a new bit of kitchen ware....

With TWO of LG's biggest bottom-freezer models, the newer one a "linear inverter" drive?

I don't want to be the one to set MYSELF up with new bits of kitchen ware!

:(
 
I submit you meant they ony pay attention at ALL when in excess of 5 HP?

And few "make you" do anything ... until there is actually some form of chronic deleterious effect on a last few yards, multiple subscriber shared distribution serious enough THEY see it...
.........

Suit yourself.

Read it again, I said what THE RULES are, as written. What they DO about the rules is their problem. If they feel like it, or find a problem, they will refer to those old rules soon enough, because you can bet they never got rid of them.

Most of them wrote the rules back when they actually employed people. Now they have contractors for everything, and I doubt you could get their interest about blinking lights, you'd go on a list to be handled someday.

Back 30 years I got the local powerco to fix their drop for free... Lights in the house blinked when others were turned on, etc. Their lazy-ass linemen had connected a drop neutral to one end of a piece of bare wire, and the other end to the wire from the peckerhead.

What they had not bothered with is that the piece of wire was TWO pieces, and they were just sorta wrapped around each other in the middle. Copper, oxidized to a black color and making virtually no contact. There was probably 20A flowing in the cold water pipe, but I never tested that back then.*

Next day they had a crew out and fixed it without any fuss, no appointment, no BS. They dd not want to get a liability claim for a house fire on account of bad workmanship.

These days, you'd be on a list for a month or more. Neighbor had a big tree limb hanging from her drop, big enough to maybe tear it off her house. They gave her a date six weeks out to deal with it. I went up on the roof and took it off that evening.

* a few years ago a tree branch cut the neutral and I measured 10A in that pipe before I put the kibosh on any hair dryers or stuff that pulled high currents until it was fixed.
 
Suit yourself.

Read it again, I said what THE RULES are, as written.
J? I can find anything I NEED in the details of the rules to support the DAMNDEST of kludges. That's a feature, not a bug.

I've gone over to using a search engine and the net. Day when I could read the entire NFPA it wuz well under 300 pages and we didn't have an internet. Hell - we still had party-line telephones. ISTR NFPA/70 is gittin up around 900 pages now?

Contractors... guy needing a good ground is asking my permission while reaching for the outside hose bibb, South side of the house. Told him go around back, North side, clamp onto one of the ground rods between the HVAC unit and the side door.

Starts his I-know-all-about-grounding-to-water-pipe lectures as he clamps on.
"Suit yerself".

He's back in a few minutes to wiggle his clamp.

Next trip, I asked if he thought it would reduce the effectiveness of a cold water ground if the hose bibb wasn't connected to a water line.

He told me they ALWAYS were connected to a water line, and the water line was ALWAYS grounded ...gives me more i-know-all-about-cold-water-grounds.
Mind, guess he never heard of PEX, either?

Third trip I 'lowed as how he weren't WRONG in the general sense, but...

... since I wuz the very guy who had personally cut the half-inch Copper water line to that partic'lar hose bibb OUT after a leak a good twenty year back and the only "ground" it had was through the brick it was pluggin' a hole in 'case I changed my mind someday-maybe-never...?

He might want to give it up and go around back and clamp onto one of them ground rods!

:D
 
The issue with motor starting isn't so much the continuous loading - sure, a loaded kitchen can draw a lot.

But a 500A startup current, even just for a split second, can do visible light dimming (as mentioned). Neighbours don't like that. It depends on the supply impedance, of course.

Worse is if you get a bit more volt drop, arc/discharge lights go out. That only has to be a half cycle. Then you have to wait five minutes for them to cool down before they restrike. That's a pain, not to mention safety hazard.

This may be regional, but here in NZ we call the general class of motor starters that reduce the current by reducing the voltage (primary resistance, autotransformer, star-delta, SCR, all that jazz) 'reduced voltage starters' - they still supply mains frequency to the motor, just at less voltage. Only electronic (e.g. SCR) ones are called 'soft' starters - the others all still have sharp steps that give the load quite a jolt.

If all you're trying to do is reduce the peaks, then you *might* be able to set the motors up for a star-delta start. All you need is a set of interlocked contactors and a timer.

For a really simple solution if you're on the edge, make the supply cable longer and thinner to increase the volt drop. Put the gear on the other side of the garage from the panel...
 
For a really simple solution if you're on the edge, make the supply cable longer and thinner to increase the volt drop. Put the gear on the other side of the garage from the panel...

Not a good idea, "permanent" undersizing, no.

But...

More than a few among us have:

- utilized a whole 100-foot still-on the spool of down-sized electrical wire for deliberate Ohmic loss, per your hint. Yah "shunt" it out of the circuit with a contactor.

- Found it handier to utilize a 9 KW "canning" electric range element in series , also shunt-bypassed soon as the starting is done.

Cheap as dirt, more compact than the average spool of wire, and you KNOW they can stand a lot of HEAT with no need of a fan in case they are not already shunted-out before they much get warm at all.

I've got 35 KW and 60 KW around, but those MUST have a lot of air flowing, even with a short duty-cycle conducting at all!

They are common heating elements meant for installation inside HVAC air-handlers right next to the blower fan when a heat-pump finds it too cold outdoors for any gain.
 
Thanks guys, excellent points!

Last night I put a 3% line reactor on the mill. O'scope looking at the incoming 240 / 60 / 1 mains shows less inrush.

Plan on picking up conduit, NEMA boxes, motor starter, fuses and etc for an additional idler. Replacing the "old" breaker, a relatively cheap experiment.

Will likely revamp the drive with a more efficient, inverter duty motor.
 
Thanks guys, excellent points!

Last night I put a 3% line reactor on the mill. O'scope looking at the incoming 240 / 60 / 1 mains shows less inrush.
"Sine Guards" in SERIOUSLY HIGH amps are a drug on the market. So cheap as pulls - or even NOS, never used - that I have three now. See also GE-FANUC.

Series 'em up, and they make more ECONOMIC sense than autotransformers or conventional chokes that cost far more money. Downside is the huge enclosures they need, but Zoro has Weigemann ones DEEP-ASS steel "NO Knockout" cheap enuf and fast.

Plan on picking up conduit, NEMA boxes, motor starter, fuses and etc for an additional idler. Replacing the "old" breaker, a relatively cheap experiment.

Will likely revamp the drive with a more efficient, inverter duty motor.

Don't do that. Not without serious research, anyway. it's cheaper than surprises, yah?

TANSTAAFL! "Tradeoffs" as they are. If yah go VFD, just put a dv/dt or even Sine-guard in the line to protect the old motor.

Whyso?

Most high-e are harder to START than older tech! Some of them a LOT harder! Also harder to protect, sanely. The PHYSICS didn't move. Only the Hoovermint wreck-you-lay-shuns as drove a different "optimizations" set FROM the same dam' menu of the Universe as was there all along. "Compromised" ain't the same as "magical", after all. I did say "TANSTAAFL?"

Plenty of papers online grumbling over that issue if yah have the time for a recce and read.

High-e are not as good as RPC idlers, either, BTW!

The GOOD news is that wreck-you-layshuns have forced a ton of the older tech into the market and depressed the prices of motors and woundy-uppidy-indctorish stuff alike.

Grab it and growl. It still works a treat, and some of it has never even had a wire attached before going off "Standard A" and sold-away to "remaindermen" like NRI for salvage.

2CW
 








 
Back
Top