Soft start or motor issues?

[Johnboy2316]

I recently had a service call to a hot mix asphalt plant where they were having trouble with their exhaust fan. Upon arrival I immediately began testing their motors ( 2 200hp motors turning the same pulley connected to a fan). I discovered that one of the 2 motors megged bad so I had it replaced. Their original setup consisted of 2 separate soft starts , one for each motor. It’s been my experience that this type of setup is more prone to problems than a single soft start oversized to 125% of the combined nameplate FLA. I installed a single Toshiba that is good for 500hp or 600A. I am going back later this week to install separate motor overloads for the two motors but for now I just have the drive programmed to protect both motors together. The issue is that from time to time they still go down on “OLC” which means overcurrent at speed. This doesn’t seem right because once the fan starts the combined load stays steady at around 413amps which is far less than the 480amps ( 240amps per motor ) that is the combined fla. on top of that the soft start is programmed at 520amps right now and allows 600% of this for startup. That’s more than enough to get these motors running. I can’t even get them to start at all unless I set the ramp time extremely low ( 5 sec ). If I try to let the motors start slowly , say 30sec , it will go down every time on overcurrent. You can spin the fan pulley by hand with the V-belts removed so at first I didn’t think the issue was mechanical. Now I’m not so sure. It still fails to start sometimes. My current settings are FLA 520 ( because it won’t start at 480 ) , service factor 1.15 , in rush 600% of FLA , ramp time 5 sec. Once it’s running it stays at nice comfortable 413A or so. Does anyone have any ideas? My customer has spent over 30K so far and that doesn’t include lost production. I plan to check the fan itself for buildup and check the fan bearings when I go to install the individual overloads. Please Help.

 

[petersonra]

I don't like trying to have two motors on one shaft without some kind of electronics to make sure the load is being shared correctly.

I wonder what caused the one motor to fail.

 

[Johnboy2316]

Not sure. When I arrived they stated that this was the second motor to have burned up this year and the third soft start to fail. They had cheap WEG brand soft starts. I assumed that the soft starts weren’t set to the same parameters causing the one motor to work harder at startup and eventually causing catastrophic failure. I don’t know for sure but it megged shorted from phase to phase. They had the other motors that had been replaced still laying around so I megged them out of curiosity and they were shorted phase to ground.

 

[Johnboy2316]

What kind of electronics could I use to ensure the load is shared evenly? Individual overloads will keep either motor from overloading but as long as both are under the parameter limit one could be pulling harder than the other. Once I get the individual overloads installed I will be able to see how hard each is working but is there something I could offer to ensure the load is balanced ??

 

[Jraef]

First things first. You say this is for an exhaust fan. Is there a damper on it? If so, is the damper open or closed? On a fan, you want to start it with the damper CLOSED, not open, which is counter to what most people think. But on a fan, load = flow, so no flow = no load on the motor. If there is no damper, they should consider adding one.

If that can’t happen, then what you need to do is to let the soft starter go IMMEDIATELY into current limit, no ramp time at all. On that Motortronics /Toshiba soft start, you want to program the current limit to about 350% at the very least, then set the ramp to zero and the initial torque to about 50% minimum. If it fails to finish accelerating before the OL takes it off line, you may have to bump it to 400% CL, maybe even 450. Make sure you give the motors adequate cooling time between attempts if it does trip on OL, which may be upward of 2 hours between attempts. But do NOT put any ramp time on it. In this case the ramp time is just wasting your thermal capacity of the motor without helping accelerate it. On that starter, it will stay in current limit until the motor accelerates or trios on overload. Once it detects that the motor accelerated, it will go across the line automatically.

Also, I have done literally dozens and dozens of large soft starts with one big starter on two motors connected to the same shaft, it should be fine so long as the motors have these same slip. If not, they will not share the load equally. So that usually means if one motor goes bad, you have to replace them both. That’s never a popular opinion, but it’s the way it is. The only way to do it with disparate motors is to use two VFDs with one set up as a torque follower of the other. That will be more expensive than buying two identical motors.

 

[Johnboy2316]

Thank you. I was reading in the manual and was planning to go to current limit when I go back this week. The damper is 90% closed , that’s as far as it closes for some reason. So, with no ramp time and using current limiting aren’t I basically starting it normally ? I’m other words couldn’t I just go to the across the line feature? It seems like you have done this quite a bit and any advise or input you have will be greatly appreciated.

 

[Jraef]

No, across the line would be 600% current, so if it will start with 350%, that’s about as soft as something like a Wye Delta or Autotransformer starter.

 

[Johnboy2316]

Ok I see what you’re saying. The current limiting will still be soft starting because it will limit the inrush current to 350%. It will just give it a set time at that current to reach speed then engage the bypass contactor. The across the line would be the same as a regular starter and allow the whole 600% inrush. I have a lot of experience with VFDs but am learning these soft starters. Thank you for the advise. I will be sure to post and let you know how the new parameters work.