Star Delta starter vs soft starter

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Hi all,


Got a customer who's asked me to look at a new machine they've gotten in which they're having trouble starting. It's a brand new machine, essentially a grinder with one motor driving three sets of grinding wheels via a hideously complex series of belts and pulleys. The motor is probably (at a guess) around the 30-40kW mark. Essentially, what they're saying is that they have to manually back the top grinding rollers off, start up the machine and run it for a few minutes, then they can stop it, wind the rollers back in, and it'll run fine. All of this screams "mechanical problem" to me, but they've asked me to take a look.


One thing they mentioned is that the main drive has a star-delta starter in it. For a brand new machine, this seems odd - surely a soft starter is about as cheap as all the control gear required for a S/D starter these days, not to mention the enormously reduced complexity. Just wondered if there's any practical reason a star-delta started might be more suited to this application than a soft starter? I suspect it's purely a case of "we've been building them this way for 20 years, if it ain't broke don't fix it", which is backed up by "our other plant has an identical machine and it has no such issues" - I believe the other plant is much older than this one, so it's likely just that the design hasn't been updated in 20 years.

If I were to suggest installing a soft starter, what might I need to watch out for?
 
I can't think of a configuration where an electronic softstarter would increase available starting torque over normal across-the-line or wye-delta (star-delta) starting torque.

Sounds to me that you have a machine that has high torque when cold. Running it a little warms it up enough that the torque comes down into the motor's available range.

You need more motor starting torque or reduced machine cold starting torque. Be absolutely sure you don't have a tight bearing or an alignment (binding) problem. Even a lighter grade of grease or oil sometimes is just enough relief.

Choosing a larger motor is probably the best choice. Everything else leaves you with razor-thin margins for reliable starting.
 
ask them how long they use star?
ask them how much current is in start and delta.
if the weight, or the torque is high slow starting is needed. and yes sometimes a VFD is a good solution, however more complex for the installer.
a softstarter isnot a frequency device so a real VFD is a good solution.
 
I like electronics but it depend where it goes or who use it....IMO: If it need to go on a foreign area, you would better serve the customer with a star/delta, even in 2019....

In some case, you can use a class 20-30 protection to allow for longer starting time on heavier load as long as the motor can handle it.....

Is the voltage at the starter panel remain stable under load? ...(Voltage drop is compensated with higher current.....)

Electronic won't fix mechanic problems....You need to point out what is the real problem at first and then align a solution.

Is it a custom built machine, designed on the corner of a restaurent table and the motor is just too small ?
Is it a well designed machine with something wrong that call for more torque ? Can it be repaired?
Is it a problem because of site condition (Cold weather and grease and lub freeze)
 
It seems that the machine gets stuck when it stops and then a way to release the rollers is to go backwards.

If belts and pulleys allows the motor to go backwards, maybe those sequence of starting backwards first and then forward it could be automated using a VFD and a small PLC or a simply timer.
 
Although the problem is probably due to how they stop the machine, they should cut the supply of material and wait for it to run idle before stopping the motor, then surely next time it would start without problems.
 
Read the manual for the grinder. Maybe you're supposed to be rolls out for startup.
Maybe the product is sticking to the rollers when they leave it stopped with product in it, welding the two rollers together.
Maybe the star delta timer hasn't been set during commissioning by the commissioning engineer.
Maybe when they say "stop the motor and rolls in" they mean "allow the motor to coast, put rolls in, then startup and because it is still at half speed when the star kicks in, the misconfigured SD timer is not an issue.

You can get soft starters with more torque than star delta starters, you just have to current limit to 400% or higher, usually going for the model up. Star delta should be about 75% starting torque/nominal torque, so unless there is product in there, you should be fine.

Soft starters end up with a lot more calls than star delta starters that's for sure.

Soft starters give you voltage when they're off, so there's that. They can fail on, so you either get one with STO, or you put 1-2 contactors upstream depending on your safety performance level required.

Some soft starters, particularly the low kw ones are always on, so they will produce more heat than a star delta. Might mean a bigger enclosure or enclosure cooling to compensate.

Star delta starters though have a relatively high Jerk, so in terms of mechanical life your best bet is a soft starter.

Really though, just read the machine's user manual, the answer will be in there.
 
So after my site visit, I learned a few things:

1. They have some sort of mechanical issue. The machine was definitely, 100% empty of product and yet still would not start when the rollers were wound in. The customer confirms that these rollers should not have to be wound out to start the machine as the manual, machine supplier and evidence of the same machine at the sister plant does not require it

2. The star delta circuit is all correct and operational exactly as it should be, and the timer is set as per the drawings (16 seconds). When the motor does start, it settles down to steady current in star after about 5 seconds, so 16 seconds is plenty

3. Once the motor is started with the rollers wound out, it can be stopped, have the rollers wound back in, and it will then start successfully. This would tend to indicate that there was product present before the initial start - but the line had been completely run out and this machine then run on for several minutes to make sure.

4. They have an even bigger issue, which is that the motor circuit breaker (brand new) does not trip. It's set to 34A, and when I tried to start it initially, it drew 105A on all three phases for a good 5-10 seconds until the upstream 80A breaker tripped. When we did the back rollers off/start/stop/reset rollers/restart, motor currents were as normal

Ultimately, I figured that a soft starter wouldn't help them with either issue that they're experiencing. I've confirmed to them that (with the exception of the faulty breaker, which I'm replacing), there is nothing electrically wrong with the setup, and they've got some sort of mechanical issue at play. Which was ultimately all they wanted from me.

Thanks for the advice!
 
Hmm, you make a good point. The breaker in question is an Eaton, but it has a similar trip curve. I guess the upstream breaker is probably just a standard C curve!

In any case, that's a fail for fault discrimination on the behalf of the supplier!
 

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