SMC Flex larger than Motor

Bullzi

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Jun 2012
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Colorado
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Hi guys,
Been handed a new SNAFU on a job. The MCC what provided with a SMC Flex Soft Starter for a 40hp motor but the motor that was installed is a 25hp.

Has anyone dealt with this situation before? Do you see any issues in using a 40hp Soft Starter with a 25hp motor?

Thanks for the help.
 
No issue. What is the 25 HP motor driving. If a centrifugal fan the starter may have been sized up for extreme duty.

As long as the overload is suited for the 25hp motor.

Found two fifty horsepower irrigation pumps a while back that had soft starters but no overload protection. The owner now knows why he was snapping shafts, on locked rotor that is.
 
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It's probably a 150-F60xxx model. They're nominally rated for 60 A, but they can handle motors between 10 and 40 HP.

Especially if it has the "Pump Control Option", it should be fine for running a 25 HP centrifugal pump.
 
Thanks eveyone,
Local Distributor was saying that it "might" not be able to control a motor that small on a soft start. Guess they were looking to sell a new bucket.
Makes me feel a lot better!!
 
That issue is from VFDs, not soft starters. They heard it once and are over-applying the concept. I've tested 600HP soft starters with 3HP motors.

And on VFDs, you don't want to run a motor that is less than 50% of the rating of the VFD, then only if you want to use sensorless vector control mode at really low speeds, because the current sensors in the drive may not accurately pick up the subtle motor current anomalies that the drive needs to see in order to know where the rotor is. But if you use V/Hz control on something like a pump or fan, even that rule no longer applies.

Then again, 25HP is more than 50% of 40HP anyway, so it DOUBLY doesn't apply (if that makes sense...)
 
I will retract my statement about having no issues with a larger soft starter.

If not enough load is applied to the soft starter, the thyristors might not fire correctly. This can cause the motor to chop around (for want of a better term) until a load is applied. Years ago a pump guy who I had supplied a soft start panel for was testing the panel with only a motor connected, no load. It did as I described above. Worked fine once coupled to the pump. Albeit is was a cheap soft starter with only two thyristors. Another guy from from a feed mill had the same problem, cannot recollect what it was driving but it was uncoupled for direction test.

In your case I think you will be ok.
 
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Something that bit me the other day, which might be applicable here...

I was using an SMC-3 (not SMC-Flex, but similar) on a 22kW motor. During dry testing, I wanted to test the sequence without actually running any motors, so we had the motor local isolators off while we test ran the equipment.

If I had the SMC-3 set such that it's auxiliary output was set to it's default "running and healthy" output, everything operated normally. Apply power to start input, auxiliary contact closes, and after a few seconds, the internal bypass contactor comes in.

If, however, I had the SMC-3 set such that it's auxiliary output was set to "at speed", the internal bypass contactor would never come in, and the auxiliary contact would never close.

I phoned tech support, who were able to replicate my issue, and said that if they connected a motor, the starter operated normally - but without a motor connected, it behaved exactly as I described.

I cannot for the life of me work out why changing the function of an auxiliary contact should alter the operation of the internal bypass contactor, but apparently it does. The good news is, I'm absolutely certain that my "fail to start" logic in the PLC functions flawlessly.

The long and short of all this is, if you don't connect a motor to an SMC-3 and set the auxiliary output to "at speed", your bypass contactor will never come in.

It is within the realm of possibility that if you had an extremely undersized motor connected to the SMC-3, it might behave the same way.

And it is also probably within the realm of possibility that the SMC-Flex might display the same behaviour.

By the sounds of it, your motor is not so undersized that you're likely to encounter this one way or the other. But worth knowing about!
 
The code that turns on the bypass contactor is not just a timer. It also must see the current rise to some minimum peak (I think 200%) then fall from that peak to a value that is at least xx% (again, I think 50% of the peak value) so that it knows the motor is done accelerating. This is what prevents someone from attempting to start a motor across-the-Line (DOL) with the bypass contacts, because they are NOT rated to do that. They are rated as “shunt duty” only. All soft starters that have integral bypass contacts do something like this, they have to.

Instability at low phase angles is a hallmark of all of the “2-phase” soft starter designs, it’s another reason to not use those. The ONLY thing they offer is cheapness, in every other way they are not only inferior, they are potentially dangerous to your motor. All the sales hype on some of them “solving” the motor heating problems are just that, hype.
 
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The code that turns on the bypass contactor is not just a timer. It also must see the current rise to some minimum peak (I think 200%) then fall from that peak to a value that is at least xx% (again, I think 50% of the peak value) so that it knows the motor is done accelerating. This is what prevents someone from attempting to start a motor across-the-Line (DOL) with the bypass contacts, because they are NOT rated to do that. They are rated as “shunt duty” only. All soft starters that have integral bypass contacts do something like this, they have to.

That all makes sense - but on the SMC-3 that only seems to apply if the auxiliary contact is set to "at speed". If it's just set to "normal", then without the motor connected and without any current rising and falling, the bypass contactor comes in regardless. That's what had me scratching my head - I'd totally understand if it didn't work at all with no motor connected, for the exact reason you stated above. But for it to happily pull in the bypass contactor on no-load with the starter set to the "out-of-the-box" settings, but not if you modify the function of an auxiliary output, well, that just seems like a screwup.
 
That all makes sense - but on the SMC-3 that only seems to apply if the auxiliary contact is set to "at speed". If it's just set to "normal", then without the motor connected and without any current rising and falling, the bypass contactor comes in regardless. That's what had me scratching my head - I'd totally understand if it didn't work at all with no motor connected, for the exact reason you stated above. But for it to happily pull in the bypass contactor on no-load with the starter set to the "out-of-the-box" settings, but not if you modify the function of an auxiliary output, well, that just seems like a screwup.
Huh, I guess I misinterpreted. Yeah, that doesn't sound right. I wonder if they know? I know some guys on the inside there, I'll see if I can find out if they are aware of it.
 
Well, there's now one guy at the Melbourne tech support office that is aware of it ;)

But I'm definitely curious to know if anyone else at Rockwell is. Let me know what you find out!
 

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