480v 60hp motor throwing breaker, now?

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titan1021

titan1021

Senior Member
Location
Northern California
Hey all,
I've recently provided power for my client's latest purchase a used grain mill. The mill has a WEG 3ph 60hp motor and came with a pre-wired starter.
I ran the circuit to the machine and installed a 90amp Siemens BQD series breaker in the load center. We initially tested the machine & starter with just the starter and it seem to run fine. A few days later I installed a 100a non-fused safety switch beside the starter, now when we turn the machine On via the starter it trips the breaker within a second or two. The only other thing that has changed aside from installation of the safety switch is the belts were adjusted. I've checked all the voltages and verified the correct connections, I'm obviously missing something here. Hope someone can point me in the right direction. I've attached a few photos below.
Thank you

Note: The phase to phase voltage on-site is actually 495v
 

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tom baker

tom baker

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My Square D motor data calculator shows a 110 amp branch ckt breaker. Your breaker, called the motor short circuit and ground fault breaker can go larger, but if you have a good ampmeter see if you can get inrush amps. i its tripping that fast its almost a dead short. Pull the belts and see how it starts. Seems like a mechanical issue.
 
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
It seems to me that a 100 breaker would almost certainly trip trying to start a 60HP motor. In most applications the code would allow 200 amp breaker for a 60 HP motor running on 480 volt.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
A 68A motor will draw 600% on startup, that's 408A. On a 90A breaker, that's 450% and according to the trip curve for a Siemens BQD breaker, trip time will be roughly in the 2-4 second range. Checks out...

68.3A FLC x 250% = 170A max breaker (150A or 175A under "next size up" rule). 90A is too small. Starting it unloaded was only good to tell if the motor was defective or not. Starting under load is too much for that breaker. I would go with a 150A to avoid having to do it again. At 270% (408 / 150), trip time will be between 10 and 20 seconds, about what you can get away with on the OL relay anyway, so that should work.
 
titan1021

titan1021

Senior Member
Location
Northern California
Jraef said:
A 68A motor will draw 600% on startup, that's 408A. On a 90A breaker, that's 450% and according to the trip curve for a Siemens BQD breaker, trip time will be roughly in the 2-4 second range. Checks out...

68.3A FLC x 250% = 170A max breaker (150A or 175A under "next size up" rule). 90A is too small. Starting it unloaded was only good to tell if the motor was defective or not. Starting under load is too much for that breaker. I would go with a 150A to avoid having to do it again. At 270% (408 / 150), trip time will be between 10 and 20 seconds, about what you can get away with on the OL relay anyway, so that should work.
Click to expand...

2-4 seconds is right about what we get before the breaker goes. Makes perfect sense. Thank you
 
titan1021

titan1021

Senior Member
Location
Northern California
titan1021 said:
2-4 seconds is right about what we get before the breaker goes. Makes perfect sense. Thank you
Click to expand...

I know you suggest 150a for good reason, do you think a 125a could possibly work in the NGB, HGB, or LGB series. Those are the breakers that I am limited to in the existing panel and they max out at 125.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
titan1021 said:
I know you suggest 150a for good reason, do you think a 125a could possibly work in the NGB, HGB, or LGB series. Those are the breakers that I am limited to in the existing panel and they max out at 125.
Click to expand...
It might work, it might not. It might work some of the time. Best to get something that is going to work for sure.

Maybe you can get some feed through connectors for the bus on the panel board and wire it to a panel mount circuit breaker located in a junction box. Or a fused disconnect.
 
P

paulengr

Senior Member
titan1021 said:
I know you suggest 150a for good reason, do you think a 125a could possibly work in the NGB, HGB, or LGB series. Those are the breakers that I am limited to in the existing panel and they max out at 125.
Click to expand...

All the NHG stuff is doing is changing the instantaneous trip. Your problem is the thermal trip which remains unchanged...check the time current curves. See the following diagram which is the UL specification. Notice the trip times...changing B/C/D manipulates the magnetic trip but that’s not what is getting you.


A 125 A will push the curve out a little and since it’s a fan application you might get it to start and hold. Or not. The following chart is pretty close to your situation except the breaker is closer to a proper (150-175 A) size.


If you can get it to start at all it will always be on the ragged edge and cause problems. It all depends on the fan inertia. If it’s light and starts quick (2-4 seconds) you might be OK. You are tripping though so I’m guessing you aren’t that lucky. As in every time the belts get tightened, it doesn’t get greased, it sat idle for months and starts to seize up even a little which they usually do. Lots of call outs and frustrated customer. This is what you are setting up. Breakers really aren’t meant for motors which is why NEC limits seems very oversized....175 A vs only 68 A running.

Two ways out of this situation. First and most obvious is go to a bigger panel or sometimes you can use a feed through. Second option is replace the starter with a soft start.


Confession: we sell these so I’m biased. We also sell Schneider, Motortronics, and Benshaw and Motortronics heavily private label their soft starts so other brands might be the same thing.

Look at the EMX4e for a fan application. Don’t let the list price scare you. PM me if you are getting crazy prices quoted. If you look at other brands watch out for fake 2 phase soft starts. For instance ABB has two models. One has a bus bar as the middle phase. It works but the unbalanced currents are rough on the motor.

The CSXi looks even cheaper and it is but it requires high speed fuses where the EMX4e just uses standard motor fuses you can buy anywhere so I don’t recommend the CSXi. With a soft start fuses rarely trip since by nature it limits current.

These are electronic starters. Think of it as a 3 phase lamp dimmer because it’s based on the same technology. It gradually brings the motor up to speed while holding the current down so the breaker doesn’t trip. It is not a VFD but for fans and pumps it works very well. With a fan we could set it to limit current to say 250% of name plate. It will take longer to start but it will do it at 170 A instead of 400 A. On 125 A that’s quite a long time before it trips.

It is also easier all the way around. Bearings and windings don’t see the high torque and shock so they last longer.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Jraef said:
A 68A motor will draw 600% on startup, that's 408A. On a 90A breaker, that's 450% and according to the trip curve for a Siemens BQD breaker, trip time will be roughly in the 2-4 second range. Checks out...

68.3A FLC x 250% = 170A max breaker (150A or 175A under "next size up" rule). 90A is too small. Starting it unloaded was only good to tell if the motor was defective or not. Starting under load is too much for that breaker. I would go with a 150A to avoid having to do it again. At 270% (408 / 150), trip time will be between 10 and 20 seconds, about what you can get away with on the OL relay anyway, so that should work.
Click to expand...
Don't we use the table values instead of the marked values for the current?
 
titan1021

titan1021

Senior Member
Location
Northern California
electrofelon said:
Right, 77x2.5 plus next size up=200. if this is a two hundred amp panel, the sub feed or feed through lugs might work well as a solution.
Click to expand...

This is a 200amp panel, it sounds like feed through lugs to a fused disconnect will be the way to go. Would you use RK5 or RK1 fuses.
There are overloads at the starter.
 

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