Motor Controller/VFD/Soft-Start Sizing

[coulter]

Could one of you explain why the overloads did not shut down the soft start?

carl

 

[mayanees]

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jraef.. TX

Carl, overloads...
I don't know the final disposition of the soft-start, nor the specific failure symptoms.
I'll attempt to talk to the tech who did the analysis and upgrade, and report back to this thread with specific drive failure information.
That will take me a few days.
But again Thanks to everyone for the input.
JM

 

[frenchelectrican]

Could one of you explain why the overloads did not shut down the soft start?

carl

I will try my best to expain this part IMO i do not know what the OP did with the control on the VFD contoller if he slect the defalut setting or optioned to leave the overload off for some reason but IMO if they did program this it should wired a exteranal O/L block with N/C concat in tied with PLC control circuit so if the O/L tripped it should shut it down.

starting time most have programed to ramp up and X amount of time to bring up the speed.

but other thing it drawned to my mind that someone did mention about impellers i do not know why if the motor is reverse rotating during shut down time to get this shaft stop rotating and some how the motor can act like generator and back feed the loadside of VFD and fried up the control there.

but really it possible if someone did reprogram the partimter [ settings ] for some reason to bypass the overload setting or set the overload limit way up the point the VFD burnup or other damaged occured

I hope this will help a bit with this info

 

[Jraef]

Frenchelectrician,
I think you missed the fact that this was a soft starter, not a VFD.

An OL relay will trip (assuming class 10) in 10 seconds at 600% current, but maybe 2-3 hours at only 118% current. It is meant to protect the motor, not the controller. At 118% current the soft starter would overheat and the SCRs short. the soft starter should have had a heat sink over-temp switch, but usually by the time that trips it's too late, kind of like an "idiot light".

 

[coulter]

..An OL relay will trip (assuming class 10) in 10 seconds at 600% current, but maybe 2-3 hours at only 118% current. It is meant to protect the motor, not the controller. At 118% current the soft starter would overheat and the SCRs short. the soft starter should have had a heat sink over-temp switch, but usually by the time that trips it's too late, kind of like an "idiot light".

Interesting. This has not been my experience. I've probably worked with six, ranging from 60hp to 350hp, over the last 20 years. Should be firing up a 600hp next year. All were AB SMC, so my experience is limited. All were programmable to trip before damage from overload. The one tech manual I have in front of me cautions one must not exceed the Soft Start Nameplate ratings when setting the motor FLA.

If I am understanding you correctly, most brands of soft starts, can be set up correctly, overloads programmed (or sized) correctly, ocp sized correctly, and can still burn up if the motor bearings start dragging. If so, those sound like poorly designed units.

Without knowing the brand/model and reading the tech manual, I'd be much more inclined to think the unit is question was not set up correctly.

carl

 

[kspifldorf]

Doesn't all softstarts switch to full voltage contactors. what kind of accell times and delays are they set at.

 

[coulter]

My experience is limited to AB SMC. They can be either way - with bypass contactor or without. The voltage ramps can be set as slow as you want. Or one could set for current limit and not set the voltage.

carl

 

[frenchelectrican]

Frenchelectrician,
I think you missed the fact that this was a soft starter, not a VFD.

An OL relay will trip (assuming class 10) in 10 seconds at 600% current, but maybe 2-3 hours at only 118% current. It is meant to protect the motor, not the controller. At 118% current the soft starter would overheat and the SCRs short. the soft starter should have had a heat sink over-temp switch, but usually by the time that trips it's too late, kind of like an "idiot light".

yeah i relized that but really IMO [ i will leave the debate out of the window for now ] soft start is sorta like mofided verison of VFD is only to limit the starting current during the time peroid.

you did make a good point with the Class 10 relay setting i know normally most motors used the class 10 and 20 as well but the soft start device i dont know what class limit they set in and yes i got your point with the heat issuse with this.

Thanks for bringing this up and corret my oversight

Merci, Marc

 

[Jraef]

Although soft starters have internal heat sink temperature sensors (or at least should), most of the lower cost analog units do not have another system of knowing if the unit is putting out too many amps for it's rating like VFDs do. The AB SMC unit is one that does do this, primarily because they use relatively undersized SCRs and they need to protect them. There is another widely popular brand out there right now that are some of the worst in terms of being light duty AND they have no heat sink, yet they do not have any detection of or prevention from over current other than the motor thermal OL circuit, which as I said is there to protect the motor not the starter. They are notorious for field failures, yet because they are cheap and small, people still buy them.